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WOOD VENEER: LOG SELECTION, CUTTING, AND DRYING

Forest Service U.S. Department of Agriculture

Technical Bulletin No. 1577

Lutz, John F. 1977. Wood veneer : log selection, cutting, and drying. U.S. Dep. Agrie, Tech. Bull. No. 1577, p. 137 Summarizes current information on cutting and drying veneer from many species of wood. Particular emphasis is placed on wood and log characteristics that affect veneer production; techniques for peeling, slicing, and drying veneer; and species involved. KEYWORDS: Peeling, slicing, lathe, slicer, veneer quality, wood species, plywood, decorative panels, containers, thickness, physical properties, mechanical properties, grades. Oxford No. 832.20

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. Stock No. 001-O0O-03723-4.

WOOD VENEER: LOG SELECTION, CUTTING, AND DRYING

by John F. Lutz, Technologist, Forest Products Laboratory, Forest Service, U.S. Department of Agriculture The Laboratory is maintained at Madison, Wis. in cooperation with the University of Wisconsin.

Forest Service U.S. Department of Agriculture

Technical Bulletin No. 1577 January 1978

PREFACE The broad spectrum of veneer cutting and handling for a multitude of uses obviously covers a wide range of operations by many specialists, and involves hard-learned secrets. No one individual can be an expert in all areas—yet his efforts must be in line with those of others in research and industry. In these days of material shortages and pressure on energy sources, it seems doubly important to summarize some of the principles and coordinate the terminology. This bulletin is a view of the art of veneer manufacture as seen by a specialist who spent the last 25 years in research and industry contacts. It represents an attempt to tie together the experiences of many for the benefit of all. Contributions to this web of information have come from literally hundreds of people throughout the United States. The references listed here represent noteworthy contributions, but only a few of them. Harder to document are the thoughts and philosophies that have been shared with the author over the last quarter century. Outstanding among these have been the contributions of other members of the Forest Products Laboratory staff. The research efforts and considered judgment of H. 0. Fleischer,

Curtis Peters, Harry Panzer, Joe Clark, and John McMillen stand out. Other members of the Forest Service have been particularly helpful with information on wood species, especially John Putnam and those involved with surveys of the forest resources. From representatives of the wood industry have come advice, assistance, and encouragement. The contributors are legion, with particular help from Tom Batey of the American Plywood Association and Bill Groah of Hardwood Plywood Manufacturing Association on many phases. In preparing this bulletin, the author relied heavily on three research publications he had written earlier. These three were published as U.S. Department of Agriculture Forest Service Research Papers, by the Forest Products Laboratory. These were: ''Wood and Log Characteristics Affecting Veneer Production,'' by John F. Lutz, USDA Forest Service Research Paper FPL 150, 1971. "Veneer Species That Grow in the United States,'' by John F. Lutz, USDA Forest Service Research Paper FPL 167,1972. "Techniques for Peeling, Slicing, and Drying Veneer," by John F. Lutz, USDA Forest Service Research Paper FPL 228, 1974.

Use of trade, firm, or corporation names in this publication is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval of any product or service by the U.S. Department of Agriculture to the exclusion of others that may be suitable. 11

CONTENTS Page Introduction Wood and log characteristics affecting veneer production Veneer quality as related to end uses Hardwoods or softwoods for veneer Physical properties of wood Mechanical properties of wood Properties of veneer logs

1 ^ 2 ^ -'■^ ^^

Veneer from wood species that grow in the United States

21

Techniques for peeling, slicing, and drying veneer Log storage Bark removal Sawing into bolts or flitches Conditioning wood prior to cutting veneer Veneer cutting equipment Knife and pressure bar on lathe and slicer Conveying and clipping veneer Veneer drying Quality control

29 ^^ ^^ ^^ 34 45 ^^ ^^ '^^ '^^

Veneer yields and volume needed for a plant Veneer yields (rotary cutting) Veneer yields (sliced) Volume of timber needed to set up a veneer plant

87 87 87 88

Literature cited

89

Appendix I—Nomenclature of wood species and veneer

91

Appendix II—Physical properties of U.S. woods for veneer

95

Appendix III—Mechanical properties of U.S. woods for veneer

Ill

Appendix IV—Some processing variables of U.S. woods for veneer

116

Appendix V—Effects of log storage and processing on veneer characteristics

121

Appendix VI—Appearance and suitability of individual U.S. species for various uses of veneer

125

Glossary

133

Index

135

Requests for copies of illustrations contained in this publication should he directed to the Forest Products Laboratory y U.S. Department of Agriculture, Forest Service, P.O. Box 5130, Madison, Wis. 53705.

iii

WOOD VENEER: LOG SELECTION, CUTTING, AND DRYING INTRODUCTION The wood veneer industry uses over a thousand different wood species to make products as diversified as rotary-cut box shook VL inch (6.35 mm) thick to sliced decorative face veneer Vioo inch (0.25 mm) thick. In the United States, the major veneer uses are for structural and industrial plywood components % to 1 inch (9.25 to 25.40 mm) thick and decorative wall panels and furniture parts ?l6 to 1 inch (4.76 to 25.40 mm) thick. With such a wide array of raw materials and final end uses, the field may at first seem overly complex. In part, this may be due to the scarcity of written information summarizing the technical aspects of wood veneer manufacture. This bulletin describes the basic information known about the processes used in manufacture of wood veneer. Wherever possible, the log selection, log heating, veneer cutting, and drying processes are generalized and described as a continuum. To be sure, many individual processing problems are related to specific wood species. However, whenever possible the underlying cause is described and a generalized approach to the problem is suggested. Still, it is impossible to avoid some effects of individual species. In the past, when only a comparatively few species were used for veneer, this was not a great problem. It began to increase, however, as the favored species could not continue to meet increased demands

for veneer. Other U.S. species received closer looks for this product, and species from other countries are being imported into the country in an increasing swell of species, qualities, and quantities. All of this has required more information— information that has been pieced together painstakingly. Material on individual species is compiled for the benefit of the reader in the tables of the Appendix. But, whenever possible, the text of this bulletin tries to present the generalized approach, and for native U.S. species. Common names of wood species are generally given in this publication. But experienced users are well aware of the pitfalls of common names. Therefore, the corresponding official name of the tree from which the wood comes is shown in Appendix I, along with the specific botanical name. The information contained herein comes from Forest Products Laboratory publications, from other research organizations, and from contacts with the veneer and plywood industry. The bulletin is written primarily for people responsible for some part of the veneer manufacturing process. It may also be of interest to others, including those growing trees for use as veneer, for log buyers, users of veneer, and wood technology students.

WOOD AND LOG CHARACTERISTICS AFFECTING VENEER PRODUCTION A successful veneer operation depends on three items: A supply of suitable logs, good processing techniques, and a good sales organization. Most important is an adequate supply of suitable logs. Then to produce suitable

veneer, the logs must have the appropriate wood and log characteristics. The desired wood and log characteristics, in turn, depend on the end uses of the veneer.

VENEER QUALITY AS RELATED TO END USES In this bulletin, veneer is defined as wood the stiffest and strongest and group 5 the least cut Vioo to % inch (0.26 to 6.35 mm) in thickstiff and strong. Properties that are considered ness by a knife, whether by rotary or slicing include bending (modulus of elasticity and methods. Three characteristics of veneer that modulus of rupture), compression parallel and are desirable for all end uses are uniformity of perpendicular to the grain, and shear. thickness, minimum surface roughness, and Classification of species of veneer specified minimum buckle. For decorative face veneer, in Product Standard PS 51-71 for Hardwood control of figure, color, and depth of checks and Decorative Plywood is given in table 3. As into the veneer are important. Other veneer indicated in the table, the classification is based containing natural defects, such as knots, knoton specific gravity. Face veneer for decorative plywood is graded primarily by appearance. holes, splits, and discoloration, can be used as Species for use in wirebound boxes as speciinner plies in many products and as faces of fied in Federal Specification PPP-B-585b are some products like sheathing and container plywood. listed in table 4. The four groups are based on specific gravity and other properties of imporFour broad categories and typical end uses tance in containers such as strength as a beam, of veneer are given in table 1, as well as some resistance to nail withdrawal, shock resistance, wood qualities as they relate to uses of veneer. and tendency to split when nailed or stapled. The classification of species of veneer speciAn indication of the importance, for specific fied in Product Standard PS 1-74, Construction end uses, of all of the wood and log properties and Industrial Plywood, is listed in table 2. The that are discussed in this paper is shown in classification is based primarily on the stiffness and strength of the species. Group 1 woods are table 5.

HARDWOODS OR SOFTWOODS FOR VENEER The reasons for the better bending properMost species can be successfully cut into ties of hardwoods are not definitely known. Two veneer. However, some are much easier to possible explanations are that the hardwoods process than others. Hardwoods, as a class, are have less lignin than the softwoods, and that easier to cut into veneer than softwoods. This lignin in hardwoods is more thermoplastic than probably is because hardwoods can be bent more readily than softwoods (65) ^ All veneer the lignin in softwoods. While construction and industrial plywood is bends severely as it passes over the knife that generally made from softwoods, hardwoods are separates it from a bolt or flitch. Hardwoods, preferred for most other uses listed in table 1. having better bending properties, bend with Good bending properties are particularly useful less damage as checks in the veneer than do softwoods. for some types of furniture.

1 Italicized numbers in parentheses refer to Literature Cited.

PHYSICAL PROPERTIES OF WOOD same category as West Coast Douglas-fir. The Generally, the first information about a speminor southern pines, which have lower specific cies is obtained by a wood taxonomist or wood gravities, did not meet these requirements. anatomist. Working with herbarium material Thus, while not foolproof, specific gravity can and small wood samples, he classifies the spebe used to quickly screen new species for tencies and describes its structure. This informatative classification. tion is valuable for screening species to be conWhile most species can be cut into veneer sidered for use as veneer. Such information is by suitable manipulation of the cutting condioften available from libraries or by contacting tions, it is more difficult to cut wood at the two Federal and State wood research laboratories extremes of the range of specific gravity. Very or wood technology departments of forestry lightweight species tend to cut with a fuzzy schools throughout the world. surface. Dense species require more power to Physical properties of wood of interest to cut and tend to develop deep cracks in the potential veneer producers include specific gravveneer as it passes over the knife. Basswood, ity, moisture content, permeability, shrinkage, with a specific gravity (based on green volume extraneous cell contents, figure, odor, and cell and ovendry weight) of about 0.32, is toward size, type, and distribution. (Values for individthe low end of the range for species that are ual species are given in Appendix II, 'Thysisuccessfully cut into veneer. Hickory, with cal Properties of U.S. Woods for Veneer.") about 0.65, is near the high end. Still, a valuable species like rosewood, specific gravity of Specific Gravity 0.75, can be successfully sliced into face veneer, but this requires suitable heating and limiting Specific gravity or density is easily obtained the cutting to thin veneer. and is often one of the first properties known In gluing, also, the denser the wood the more about a species. As indicated in table 1, it can difficult it generally is to glue (62), be used as a general guide in screening woods Typical specific gravities of woods used for for use as veneer. For example, a wood with construction plywood are 0.41 to 0.55 ; for hardmoderately low specific gravity is preferred wood face veneer 0.43 to 0.65; for core and for use as core and crossbands of decorative crossband veneer of decorative panels from plywood. 0.32 to 0.45; and for container veneer from Detailed information is available about the 0.36 to 0.65 (table 1). Obviously, there are variation in specific gravity of many species, exceptions to these general guidelines. For exand additional data are being collected for other ample, butternut, with a specific gravity of species. Information on the specific gravity of 0.36, is a high-value face veneer. It is suitable wood species can prove commercially valuable. for wall paneling but less suitable where hardFor one example, knowledge of specific gravity ness is a factor, such as the top of a desk. for the various pines proved important in founding the southern pine plywood industry. Green Moisture Content When this industry started, the question was asked if all species of southern pine could be Veneer is often cut from logs soon after the used and still make a product that could be trees are felled. Such bolts or flitches have marketed in the same strength category as essentially the moisture content found in the Douglas-fir for structural softwood plywood. living tree. This moisture content in the wood (Species are placed in various groups for use has a distinct effect on cutting. In general, as structural plywood primarily on the basis of wood with a moisture content above fiber satustiffness and strength; in general, the strength ration but not excessively high is best suited of wood is related to specific gravity.) for cutting into veneer; this makes the wood more pliable than drier wood. In a number of Based on the recorded strength values and studies we found that species with a natural specific gravity records, the major southern uniform moisture content of about 50 to 60 pines—loblolly, longleaf, shortleaf, and slash percent cut well. pine—were permitted to be marketed in the

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Some of the free water is forced out during cutting. This water apparently acts as a lubricant between the wood and the knife and pressure bar and aids the cutting process. The driest wood that we have cut successfully into veneer at the Forest Products Laboratory was a flitch of teak with a moisture content of 25 percent. Like all teak, this flitch had a waxy extractive that probably aided the cutting. We tried cutting even drier wood, but were not successful. This came about because a manufacturer wanted to slice air-dried planks of ponderosa pine into veneer VLG inch (1.50 mm) thick. The wood, which was at about 15 percent moisture content, was heated to about 200° F in water. Continuous sheets of veneer were produced from the flitches but the veneer had pronounced checks on the side that was next to the knife during cutting. After cutting, the veneer sheets immediately curled into tight rolls like window shades, so they were unsatisfactory. Because slicing of the wood at 15 percent moisture content was unsuccessful, we took sapwood air-dried planks from the same shipment, and pressure-treated them with water to a moisture content of over 100 percent. Veneer Vie inch (1.59 mm) thick was then successfully sliced from these planks. In other words, when water is put back into relatively dry wood, the wood can be cut into veneer. Some species have a higher moisture content in one part of the tree than another. For example, the sapwood of Douglas-fir has approximately three times as much water as the heartwood. Butt logs of redwood often have much higher moisture content than upper logs. In addition to requiring long drying times, wood having a very high moisture content is more difficult to cut into veneer than wood of the same species but with a lower moisture content. Examples are some western hemlock (as high as 215 pet), redwood (as high as 245 pet), and Douglas-fir (as high as 130 pet). In normal veneer cutting, the wood is compressed just ahead of the knife. Wood with a very high moisture content can not compress until some water is forced out. As water is relatively noncompressible, it is forced from the wood structure so fast that it ruptures the wood (fig. 1). Commercial experience indicates

M 88966

Figure 1.—"Shelling" or shattering of redwood veneer that was rotary-cut from a "sinker" log. The wood shattered because water was forced out of the wood too fast during cutting.

that high moisture content in "sinker" logs of species like redwood makes them undesirable for veneer because of cutting and drying problems. Likewise, for a long time sapwood veneer of Douglas-fir was not considered A-grade; part of the difficulty was in cutting it into smooth veneer as easily as the heartwood, which has a lower moisture content. Wood may be damaged by freezing if it is stored in a cold climate. For instance, southern pine sapwood was damaged when logs were stored outdoors during the winter in Madison, Wis. Even worse damage was observed in a sweetgum log stored through a winter at Madison when the temperature went from above freezing to as low as -20° F. The end of a bolt cut from this log is shown in figure 2. Ice was found in many of the cracks seen on this end section. Industry reports that walnut logs grown in California and shipped by rail to the East froze when crossing the Rocky Mountains. Veneer cut from those logs was nearly useless due to splits caused by freezing. Moisture content in the tree, then, is generally not a decisive factor in determining

M 84166 F

Figure 2.—Splits and shake in this sweetgrum log were caused by alternate freezing and thawing.

whether wood is suitable for use as veneer. Wood with a very high moisture content is usually more difficult to process than wood having a moderate moisture content such as 50 to 60 percent. On the other hand, it is very difficult or impossible to cut good veneer from wood below the fiber saturation point, approximately 30 percent for all species. Permeability Permeability has a distinct effect on veneer cutting, drying, and gluing characteristics. Sapwood is often more permeable than heartwood of the same species. Bacterial attack in log storage may increase the permeability of wood, thereby changing its cutting characteristics. Wood that is permeable is easier to cut because water is readily forced from the wood; forces that could rupture the wood do not develop. Furthermore, plywood made from veneer that is naturally permeable, such as yellow-poplar, is less subject to "blowout" in the hot press than plywood made from such relatively impervious veneer as spruce. Extremely permeable veneer, such as the sapwood of pine that has been attacked by bacteria, may require a heavy glue spread or changes in gluing techniques to obtain satisfactory bonds.

Shrinkage A small degree of shrinkage is desirable for all wood that is to be cut into veneer. In general, low shrinkage is related to low specific gravity. The low shrinkage of teak and mahogany is one reason these are preferred woods for face veneer. However, even within species having the same specific gravity, a considerable range of shrinkage exists. High shrinkage is undesirable because it; Puts more stress on plywood gluelines with changes in moisture content; may cause cracks in face veneer of crossbanded panels during service; and causes warping unless the crossbanded panels are perfectly balanced. Radial shrinkage is generally less than tangential shrinkage. Consequently, quarter-sliced veneer will often perform better as face veneer or cross band veneer than flat-sliced or rotarycut veneer of the same species. Longitudinal shrinkage may also be a factor in use of veneer. On several occasions we have seen thin decorative plywood panels bow seriously because of the different longitudinal shrinkage characteristics of face and back veneer. Excessive longitudinal shrinkage may be due to short grain, to compression wood in softwoods, or tension wood in hardwoods. Shrinkage is a factor in all veneer uses but perhaps is most important for crossband veneer. Drying conditions may affect the total shrinkage of refractory species like some eucalypts. Wood Structure and Growth Rate In general, it is desirable to have uniform wood structure for ease of cutting, drying, and processing of wood into veneer. The relatively uniform structure, regardless of growth rate, is one reason why diffuse porous hardwoods like yellow-poplar, sweetgum, and yellow birch are such good veneer species. Similarly, softwoods like white pine and Klinki pine are good veneer species. Uniform structure is particularly desirable for crossbands of decorative panels to minimize "telegraphing" of the grain to the face. Such species as Douglas-fir, southern pine, and the oaks have a pronounced difference in density between springwood and summerwood. Assuming other factors are equal, veneer pro-

ducers generally prefer slow-grown wood of such species. In practice this is not always possible; for example, most construction plywood is made from Douglas-fir and southern pine, much of it fast grown. However, veneer from slow-grown logs of these species cuts better, dries with less buckle, and is generally preferred by production personnel. For ease in cutting and drying, veneer logs of such species should have a minimum of six rings per inch. Ponderosa pine growing in the Southeastern United States often has 30 rings or more per radial inch of growth. In tests at the Laboratory, we found this to be excellent wood for cutting into veneer. One of the problems that sometimes occurs with fast-grown softwoods is '^shelling,'' a local separation of the annual rings at the springwood-summer wood boundary (fig. 1). The first few layers of springwood cells are apparently weaker in resistance to shear than cells formed later in the year. Shelling may also occur with slow-grown wood that has soft, weak springwood and high moisture content. Examples are western redcedar and redwood. Shelling is aggravated by use of high compression by the nosebar and by excessive heating of the wood prior to cutting. Fast-grown wood of species such as Douglasfir and southern pine may cause problems in drying, gluing, and finishing {W). The same relationship holds for ring-porous hardwoods like oak. In such woods, it is desirable that the springwood portion of the annual ring be narrow and the summerwood be of moderate density. In other words, the desirable thing is to get as uniform wood structure as possible. Such oak wood cuts well, does not shell readily between rings, and performs well as furniture, paneling, or flooring. Texture Open-grained or coarse-textured woods such as oak and ash have large pores. This is relatively unimportant in veneer cutting and drying but may be important in finishing. A furniture wood with pores larger than those in birch must have the pores filled to get a continuous film of finish. Large pores also affect the appearance of the wood. The size of the pores and the color of the filler used to fill them will

affect the appearance of the finished wood surface. If desired, the filler can be used to accent the figure of the wood. Straight vs. Irregular Grain For ease of veneer processing and for most end uses, straight grain is desirable. Straight-grained wood is easier to cut than irregular grain and the veneer is more likely to remain flat. On the other hand, the market value of certain finished items of irregular grain may be high enough to pay for the extra care needed in handling it. Examples are the curly grain in species like walnut and maple and interlocked grain in mahogany. The curly grain often shows on a flat-cut or tangential surface. Interlocked grain shows as a stripe on quarter-cut or radial surfaces. Identifying irregular grain in logs is discussed further under ''Log Properties." Geneticists are studying the inheritance of interlocked grain in species like red gum. Such information would help in selecting straightgrained trees to breed for lumber and veneer production. Parenchyma Parenchyma cells occur most frequently in wood rays and as concentric bands at the edge of growth rings. These cells are comparatively thin-walled and function primarily for storage of food. They are generally weaker than most other wood cells and so may form zones of weakness when they occur in large bands. Terminal bands of parenchyma in angelique make it difficult to rotary-cut that species without getting a "shelling*' type of failure at the bands of parenchyma. To a lesser extent this same problem occurred when rotary-cutting veneer from Brazil nut (fig. 3). Parenchyma in wood rays may be troublesome when quarter-slicing veneer. The cut will be smooth when the knife moves across the wood in the direction in which the rays run out at the surface being cut. Conversely, when the rays run out at the surface in the direction opposite to the movement of the knife, the cut is rough. In the first instance, the rays are compressed by the cutting action and so cut smoothly. In the second case, the rays are stressed in tension perpendicular to the grain

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Figure 3.—Separation of a parenchyma band in rotarycut Brazil nut veneer. The scale is in inches.

by the cutting action. As they are weak in tension, they split ahead of the knife into the wood and cause a rough surface. This phenomenon of differing roughness of the surface also applies to the orientation of annual rings and fibers (39). Extraneous Cell Contents and Some Effects Cellulose, hemicellulose, and lignin are the primary structural elements of the cell wall. Being polymeric in nature they are essentially insoluble in water and neutral organic solvents. Many other materials may also be present in the wood. They are not part of the wood structure, but they contribute to the wood such properties as color, odor, and resistance to decay. They are grouped under the general heading of extraneous resins, waxes, hard deposits, and the like. Gluing problems have sometimes been attributed to resinous and waxy deposits in the wood. Extraneous materials can generally be removed from the wood by neutral

solvents such as water, alcohol, acetone, benzene, and ether. The range and mixture of extraneous compounds found in wood is very large (28). Many of them have not been fully identified. Further, the amount of extractives varies widely from tree to tree and often within a tree. Therefore, only a few of the extraneous materials that may affect the use of wood as veneer will be discussed here. In general, the extractives constitute only a small percent of the dry weight of the wood. In exceptional cases, however, such as the resin in longleaf pine stumps, the total may be as high as 20 percent. Often the high concentration of extraneous materials that cause diflSculties in processing veneer results from a tree's response to injury. Heavy oleoresin concentrations are often found in southern pine trees that have been tapped for resin. Pitch pockets and blisters are generally considered to be caused by injury to the cambium of trees that secrete oleoresin. The wood contains pockets of oleoresin, which flows readily when the defect is cut open. Fires are reported to stimulate gum production in several species. Insect attack is considered a principal cause of gum spots in black cherry. Wounding of hickories or pecans by cambium-boring insects often results in deposits of calcium carbonate or magnesium carbonate that are hard and large enough to nick a sharp knife. These examples suggest that the percentage of veneer logs free of objectionable concentrations of extraneous materials can be increased in two ways: By selection of tree breeding stock that is resistant to insect attack, and by silvicultural practices that minimize injury to the trees. The terminology concerning extractives is sometimes confusing to nonspecialists in this field. This problem is complicated because most extractives consist of more than one compound. Giim The word "gum" has been used in the past to describe any plant exúdate that feels gummy when fresh and that hardens on exposure to air. In recent years chemists have used the word "gum" specifically for certain types of polysaccarides. True gum is more or less sola-

ble in water and insoluble in nonpolar organic solvents. Arabinogalactan, which may be present in amounts sufficient to interfere with the gluing of veneer cut from butt logs of western larch, is a true gum. Gum spots in black cherry probably consist of true gum and polyphenols, with the polyphenols causing their dark brown color. While a slight amount of gum is permitted in cherry face veneer (7), moderate or heavy concentrations of gum lower the grade. Figure 4 shows the gum that limits use of Brazil nut for veneer. Resin and Oleoresin In contrast to gum, resin denotes materials that are insoluble in water but soluble in neutral organic solvents. Resins occur in ray parenchyma cells of both hardwoods and softwoods. Oleoresin is a mixture of resin and essential oils; it is insoluble in water but soluble in alcohol, alkalies, and most organic solvents. Oleoresin is secreted by vertical and horizontal resin canals in such softwood groups as pine, spruce, Douglas-fir, and tamarack. In hemlock, fir, and redwood, resin canals are normally absent but may be produced by injury to the tree. In veneer cutting, resin is a handicap. It may collect on the pressure bar and encourage chips to jam between the pressure bar and the wood bolt, causing depressions in the veneer. Frozen or solidified resin in knots is very hard and will quickly blunt a sharp knife. Ether-soluble resin occurs in small amounts in many U.S. hardwoods, but generally has little effect on their use for veneer. The relatively large amounts of ether-soluble components found in basswood may explain why this species is more difficult to glue than would be expected from its specific gravity. Resin in core and crossply veneers, such as may occur in the heartwood of cativo and southern pine, is objectionable because it may bleed through the face veneer. Similarly resin in face veneer species like white pine can interfere with furniture finishes. This is particularly true if the end pi'oduct is a TV cabinet, which becomes warm during use. Among the imported hardwoods, vertical and horizontal resin canals are found only in certain species of Dipterocarpaceae. The contents of these canals usually appear white or yellow.

M 136 441

Figure 4.—Gum in a sheet of rotary-cut Brazil nut veneer.

These extractives may be part of the problem in gluing kapur and keruing. Polyphenols Polyphenols can be broadly grouped into tannins and nontannins. Most tannins are of a molecular size generally soluble in water. Polyphenols that are not soluble in water can be removed from wood with polar organic solvents like alcohol or alcohol-benzene. Polyphenols occur in most species and are generally more common in the heartwood than in the sapwood. Color One reason polyphenols are important is because they give wood its typical color. Colored heartwood of decorative face veneer of species like rosewood is much more valuable than lightcolored sapwood. 10

Almost all sapwood is white. This light color is preferred for some face veneer of species like maple. Light-colored wood may also be preferable for containers as it makes a good background for stenciling or other markings. Color is of little importance for construction plywood or for core and crossband veneers. Metal Stain Many polyphenols react with iron and steel in the presence of water to form a blue-black stain. This becomes very obvious and objectionable on face veneer of species like oak and redwood if the wet wood is in contact with iron or steel for even a brief time. Hot wet wood will stain more readily than cold wet wood. Dimensional Stability Nearn (49) showed that many heartwood extractives will partially stabilize the wood dimensionally. One result is that dry, rotarycut heartwood veneer of species like yellowpoplar and Douglas-fir has less end wrinkling and buckle than sapwood veneer cut from the same logs. Flat veneer is easier to handle in plant processing than buckled or wavy veneer.

Hard Deposits The ash content of wood is usually less than 1 percent but in small areas in the wood it can be much greater. The principal inorganic deposits contain calcium, magnesium, or silica. Concentrated minerals have a distinct blunting effect on sharp tools. However, scattered individual crystals of calcium oxalate, which are common in the longitudinal parenchyma and ray cells of many hardwoods, do not obviously affect veneer cutting. Hard deposits that do cause rapid dulling of knives are limited to a few native species such as maple, pecan, and hickory. The ash content in mineral streaks of hard maple is reported to average 5.2 percent and to be high in manganese. Calcium deposits, concentrated in hickory and pecan that is injured by cambiummining insects, will nick a sharp knife. In contrast to continental U.S. species, many tropical hardwoods contain silica. If the silica content exceeds 0.5 percent, it causes rapid blunting of cutting tools. Figure Figure is defined as the pattern produced in a wood surface by annual growth rings, rays, knots, deviations from regular grain such as interlocked and wavy grain, and irregular coloration. Figure is one of the most important characteristics of decorative face veneer. However, for uses of veneer other than decorative face stock, highly figured wood is generally not desired.

Checks in Veneer Checks in the heartwood veneer of rotarycut types are measurably deeper than checks in the sapwood veneer cut under the same conditions. Similarly, high-speed photographs have shown that breaks into the heartwood veneer of yellow birch were more conspicuous than breaks into sapwood veneer cut in the same revolution of the bolt. One possible explanation of these phenomena is that the polyphenols in the heartwood make it less plastic than the sapwood.

Odor Most woods have little odor when dry. Some species, such as cedars, have a pleasant odor that is used to promote the use of the wood. Other woods have a sour or unpleasant odor, particularly if they become damp. Logs stored in a warm climate may develop objectionable odors due to the action of bacteria. The problem is most likely to occur with species that have wide bands of sapwood containing large deposits of starch. Such odors are particularly objectionable in veneer that is to be used for products like food containers or paneling for walls of homes.

Wax A few species of wood have waxy extractives that seem to be an advantage when cutting veneer. Pencil manufacturers recognize this advantage and add wax to incense-cedar pencil blanks to improve the whittling properties of the wood. Conversely, waxy extractives make wood more difficult to glue and finish. Examples of wood that feel waxy to the touch include teak, determa, and cypress. 11

MECHANICAL PROPERTIES OF WOOD Besides physical properties, the information is generally the critical factor for such strucmost generally available about a species is its tural uses as subflooring and roofing. mechanical properties. The most likely sources Modulus of rupture is a measure of the ultiof information on mechanical properties of mate bending strength of the wood. It is of wood are libraries, Federal and State wood interest for containers and for construction research laboratories, and wood technology deplywood. partments of forestry schools. Shear is important in structural applications such as the use of plywood as the web in a box Mechanical properties of particular interest beam. for veneer are strength in tension perpendicuWhen plywood is used as a stressed skin, lar to the grain, hardness, modulus of elasticstrength in compression parallel to the grain ity, modulus of rupture, shear, and compresis important. sion parallel and perpendicular to the grain. Compression perpendicular to the grain is an (Values for individual woods are given in important property when a bearing load is Appendix III, ''Mechanical Properties of U.S. involved, such as a refrigerator on a plywood Woods for Veneer.'') subfloor. A wood strong in tension perpendicular to Referring to end uses listed in table 1, conthe grain is desirable for veneer because it is struction plywood is generally made from softless likely to split in log form, when cutting woods. A major reason is that, for a given into veneer, or in subsequent handling of the specific gravity, softwoods generally have a veneer. higher modulus of elasticity than hardwoods. Hardness is of interest in veneer used for The longer cells and higher lignin content of furniture and flooring, or other places where the softwoods may account for the higher stiffit will receive abrasion and impacts during ness. service. Softwood logs are also more readily available Modulus of elasticity, or stiffness, is imporin large quantity and are less expensive than tant to veneer because stiffness of the plywood veneer-grade hardwood logs.

PROPERTIES OF Selection of species for decorative face veneer is based primarily on the appearance of the wood. Other physical and mechanical properties are important for construction plywood, core and crossband veneer, and container veneer. In addition to the wood properties of various species, their tree and log properties must be taken into account. The average diameter and form of the trees are of obvious interest to any timber user. At one time it was thought that only prime logs, large in diameter and clear of defects, could be used for veneer. While only partially true, this popular concept of an ''ideaF' veneer log nicely introduces the subject of log grades. 12

VENEER LOGS "Ideal" Veneer Log An "ideaF' veneer log is cylindrical in form with the pith in the geometric center of the log end sections. The bark surface of the log and the end sections are entirely free from blemishes. The annual rings on the end sections indicate uniform slow growth so the specific gravity and texture of the wood varies a minimum amount. The grain of the log is straight. The minimum diameter of this ideal log is 14 inches if it is to be rotary-cut, 18 inches if it is to be flat-sliced, and 24 inches if it is to be quarter-sliced. Very few logs meet the criteria of an ideal veneer log. But logs having other characteristics may still be eminently suited and valuable

for veneer. For example, the most obvious exception to this concept is if fancy face veneer is planned ; here irregular grain of a particular type is desired. Function of Log Grades Wood is a natural product and has many variable characteristics. Such characteristics as sweep, log end splits, and knots are among the many factors evaluated when grading a log. Based on all these considerations, the log grader estimates the quality and quantity of veneer that can be produced from the logs. For example, one criterion for a No. 1 Douglas-fir peeler, as given by the official Log Scaling and Grading Rules for five western softwood grading bureaus (58), is that it be suitable for manufacture of clear uniform-colored veneer, to an amount not less than 50 percent of the net scaled content. Log quality used in softwood plants today go from No. 1 peelers to almost any log that can be held by the lathe chucks and turned into veneer. Changing Requirements for Veneer Logs

The same sort of change has occurred in the requirements for hardwood face veneer. At one time such veneer had to be perfectly clear. In recent years such characteristics as small pin knots, insect tracings, and slight stain have been well accepted by the public for prefinished wall paneling, the major use for hardwood plywood. As a result lower grade logs are suitable for manufacture into hardwood face veneer. Veneer Log Grades While there are some formal veneer log grades, many mills have their own local rules for acceptable logs. In their simplest form they specify minimum diameter and length of logs, and the size and number of permissible surface defects, like knots. Harrar (27) has described the frequency and importance of defects in southern hardwood and veneer logs. Grading rules for northern hardwood and softwood veneer logs are published by the Northern Hardwood and Pine Manufacturers Association (52). A guide to Hardwood Log Grading (51) describes a veneer log class. Veneer log scaling and grading of western softwoods have also been consolidated into one set of rules (58).

While plant managers and production foremen would rather work with high-grade peeler logs, the availability of raw material and the changing end uses of veneer and plywood have forced the veneer industry to handle lower grade logs. Improved methods for handling small logs have made it practicable to manufacture veneer from species like aspen, birch, and southern pine with log diameters of 12 inches or less. Equipment developments such as retractable chucks, backup rolls, driven roller bars, and lathe chargers have permitted economic handling of lower grade lots (2), One reason for this switch has been the change in the end use of the veneer. At one time the main end products of the softwood plywood industry were such items as wall paneling and faces for doors. Now the major use is structural C-D grade plywood. Knots as large as 3 inches in diameter and splits as wide as 1 inch can be tolerated in this end product. As a result, the raw material requirements have shifted from peeler grade logs to No. 1 and No. 2 grade sawlogs.

Specific Characteristics of Interest for Veneer Logs The relative importance of any one characteristic in a veneer log depends on the end use of the veneer. For example, figured wood may be desirable for hardwood face veneer but undesirable for core and crossband veneer. A summary of some log characteristics and their relative importance according to the end use is given in table 5. Diameter and Length While it is true that logs as small as 10 inches (25.4 cm) or less are rotary-cut into veneer, this is not the preferred diameter. Other factors being equal, large-diameter logs are preferred for all veneer cutting. Large-diameter logs mean less handling for a given volume of veneer. Furthermore, better quality veneer can be rotary-cut from large-diameter logs than those of small diameter. This is particularly true for thick veneer such as % inch (4.23 mm). 13

Log diameter is even more important for sliced veneer where the width of the veneer is limited to the width of the flitch. The minimum diameter of logs that are used for flat-slicing is about 15 inches (38.1 cm) and for quarterslicing, 21 to 22 inches (53.3 to 55.9 cm). In terms of log length, a species that does not have a bole 8 feet (2.44 m) or longer is of limited value for veneer. Most bolts that are rotary-cut are 8 feet (2.44 m) long, even though shorter bolts are cut for core plies, for furniture, and for containers. Most face veneer slicers are 12 to 16 feet (3.66 or 4.88 m) long. While much of the sliced veneer is used in 8-foot (2.44 m) and shorter lengths, a premium is paid for 12- and 16-foot (3.66 and 4.88 m) lengths.

or compression wood. Sweep limits the number of full-length sheets that can be produced from the log. Sometimes sweep can be minimized by judicious bucking of the logs into bolts for rotary-cutting, but individual bolts must be straight. Slight sweep can be tolerated in logs that are to be sliced, but the flitches should be so sawn that the sweep in the log is perpendicular to the plane of the knife used in slicing. This will permit production of full-length veneers from the start of slicing. Abnormal Wood Logs with the pith off center often have tension wood or compression wood. Both of these forms of abnormal wood shrink more in length than normal wood and so cause buckling of the veneer during drying.

Log Form

Tension Wood Tension wood (57) is often found in leaning hardwood trees. It is most pronounced in lowdensity species such as cottonwood and aspen. Identifying characteristics in log form include an eccentric pith and silvery, crescent-shaped bands on the log cross section. When tension wood is pronounced, the bands are fuzzy or stringy, because the saw did not cut them cleanly (fig. 5). Tension wood is characterized

For rotary-cutting, it is important that veneer logs have a cylindrical form with the pith in the geometrical center of the log ends. Laboratory and industry tests show that 5 to 6 percent of a typical veneer bolt is lost in rounding it to obtain usable widths of veneer. Taper and Eccentricity Taper is more of a problem than slight eccentricity. Narrow widths of veneer are usable, but short length or fishtails generally are not. Taper also causes short grain in rotary-cut veneer. Such short grain is weak in bending and shrinks excessively in length. It may also lead to bleed-through of the glue in thin face veneer. Logs with pronounced eccentricity result in many narrow pieces of rotary-cut veneer. This veneer tends to be rougher than veneer cut from cylindrical logs because a part of each revolution of veneer is cut against the grain of the annual rings. Eccentric logs are also undesirable because they frequently have abnormal wood (55,57)—tension wood in hardwoods or compression wood in softwoods. Taper and eccentricity may also increase the amount of thick and thin veneer produced. Sweep Sweep or lengthwise curvature of a log is a defect for both rotary and sliced veneer. For one thing such logs often have tension wood

M 7.-. 160

Figure 5.—Tension wood in a cottonwood log is indicated by the arrow.

14

widely. Kubier (36) and others have demonstrated that the wood near the surface of the log is in tension in the longitudinal direction, while the wood near the center of the log is in compression in the longitudinal direction. In the transverse plane or cross section of the log, the wood is in compression near the outside of the log and in tension near the center of the log. In some cases these stresses cause the log ends to split as soon as the log is cut to length. Such an observation should serve as a caution sign when considering a species for veneer. Log End Splits Due to Growth Stresses

Splits that are in the log typically radiate from the pith like spokes of a wheel. When green wood is heated, it expands tangentially and shrinks radially, enlarging these splits (fig. 7). Splits are particularly bad in logs that are to be rotary-cut, because either the bolt is lost completely from splitting during cutting or from the corresponding splits in the veneer. Veneer splits are limiting defects as defined by the product standards for plywood (table 1 and U.S. Department of Commerce {63,64)).

M 28426 F

Figure 6.—Compression wood in a southern pine log is indicated in the outlined area.

by having little of the lignin that stiffens normal fibers. As a result, the wood tends to bend and cling to the knife rather than sever cleanly in veneer cutting. The cutting of tension wood can be improved by using an extra hard knife (such as a 62 to 64 on the Rockwell C-scale) and by keeping the knife very sharp. The wood is sometimes cooled to about 40° F with lowdensity species like basswood to improve the cutting of the softer wood. Compression Wood

Compression wood is typically found in softwood logs that have a pronounced eccentric pith. The crescent-shaped bands are most often found on the wide radius (fig. 6). They are dull, hornlike in appearance, and sometimes have a reddish cast. Compression wood is dense and superficially appears like extra-wide bands of summerwood. Because it is lignified, compression wood cuts well to form a smooth wood surface. However, the stresses in severe compression wood will often cause the green veneer to buckle. The buckle becomes worse in drying and may cause warping in plywood. Pillow (55) gives further information. Growth Stresses Most species of wood have growth stresses. However, the severity of these stresses varies

M 136 337-1

Figure 7.—Splits in the end of a Brazil nut bolt. The splits came from growth stresses in the tree and were greatly enlarged by heating the bolt to 200° F.

15

Log end splits are not quite so serious when the wood is to be sliced. The log can often be sawed to eliminate the major split by making the first saw cut through the split. It is sometimes possible to eliminate other splits if the log is to be quarter- or rift-sliced. Even with careful cutting, some of the stresses in the tree are retained in the flitches. Consequently the flitches tend to bow toward the bark side, particularly during heating. Sometimes flitches are strapped together during heating to reduce this bow. The bow in the flitch that is to be flat-sliced can often be forced out when the flitch is mounted on the flitchtable before slicing. On the other hand, the bow in a quartered flitch is not changed when the flitch is mounted and sliced. Bowed veneer results in considerable loss when the edges of veneer are made parallel by clipping. All in all, a species known to have marked growth stresses will generally yield more veneer by flat-slicing than by quarter-slicing. M 87667 F

Ring Shake

Figure 8.—Knot sequence from the indicator on the bark of Douglas-fir (1) to bolt diameters of 38 inches (2) ; 35 inches (3) ; 30 inches (4) ; 21 inches (5) ; and 17 inches (6).

Ring shake is another undesirable characteristic in logs to be used for veneer. Shake is accentuated by heating in water or steam, and there is no way of eliminating it. To prevent additional damage, plastic clips are sometimes driven across the ring shake to help hold the bolt together during rotary cutting. The plastic can be cut without damaging the knife edge. Use of a roller bar rather than a fixed nosebar is reported to permit an operator to come closer to shake without having the bolt break out. The roller bar exerts less drag on the bolt, so there is less shear force to cause the wood to break at the ring shake. Shake is much more common in old growth than in young trees.

quency of knots is also related to species. Logs of white fir and eastern hemlock, for instance, have many more knots than species like noble fir, longleaf pine, and yellow-poplar. Some species have many knots because the limbs persist for many years. For example, limbs persist on Douglas-fir logs for up to 150 years. In contrast, the limbs of southern pine frequently fall off a few years after they die. One implication is when all logs come from second growth, 100 years or less in age, the southern pines will furnish more knot-free veneer than Douglas-fir. Knot indicators are retained in the bark many years after the limbs have been overgrown. The ability to recognize these indicators is a key factor in accurate grading of logs (37). How an indicator on the surface of a Douglasfir log signaled a serious defect is illustrated in figure 8. The one exception to the degrading effect of knots is decorative veneer of species like western redcedar and white pine. These specialized products call for flitches having sound intergrown knots 1 inch or smaller in diameter. A limited number of knots are permitted and are

Knots Knots are one of the most common and important imperfections in veneer logs. Knots may be sound and intergrown, encased, or decayed. Most encased or decayed knots fall out during the drying of veneer. Knot holes are more limiting defects in standard veneer grades than intergrown knots of the same diameter. In general, there are fewer knots on logs of large diameter, on logs from trees grown in fully stocked stands, and on butt logs. Fre16

desirable in some but not all decorative veneers used as faces of paneling. Epicormic branches and adventitious buds are relatively minor defects that occur on most hardwoods, particularly elm, oak, maple, and sweetgum (37). They are not permitted in clear veneer for some furniture grades but are accepted in many grades of wall paneling.

Straight and Irregular Grain Straight grain is generally considered desirable for veneer logs. A typical commercial veneer log grade will state that deviation from straight grain shall not exceed so many inches per foot of length of log. Spiral grain in the wood is often indicated by spiral grain in the bark. As described under physical properties of wood, straight-grained wood is easier to cut and dry and generally performs better in plywood panels than veneer having irregular grain. The one exception to this rule is for logs suitable for decorative face veneer. In some cases irregular grain is desired because it enhances figure in the veneer cut from the log. The detection of figured wood in standing trees and logs is described in Pillow (56). Essentially the method is based on examining the bark and log end sections for inclination and waviness of the cellular structure of the wood. In some instances this can be detected from the rough outer bark. For example, yellow birch with a smooth bark is generally straightgrained, while that with rough irregular bark often contains curly grain or other grain irregularities. Curly grain may not be apparent in the outer bark but if the outer bark is removed with a draw shave to reveal the soft layers of the inner bark, then the grain pattern can be seen. Figured wood may also be indicated by the shape of splits in the log end surface. If the splits have alternate zig-zag patterns, the wood will almost certainly have a pronounced figure. Another technique is to cut a small section from the log end in a radial direction and then split this piece. The split will follow the grain direction and indicate if it is wavy or curly. Burls on the surface of the log may indicate that the entire log has irregular grain (figs. 9 and 10). Veneer that is figured throughout from small or large burls is often valued for its decorative effects. Other face veneers (table 1) are limited in the size and number of burls that are permitted.

* • 'M if**r

.^i^.

Color In general a uniform color is desirable in veneer logs, but the color desired varies with the end use. Light-colored wood is appropriate for containers as it makes a good background for marking and is psychologically pleasing to

M 91476 F

Figure 9.—A bolt of black gum with many burls on the surface. Veneer cut from this bolt is shown in figure 10.

17

Gum Streaks and Pockets Gum streaks and pockets in hardwood logs can sometimes be seen on log ends. Large gum pockets may be detected as bulges on the log. While a small amount of gum can be permitted in some products, gum is generally regarded as undesirable. Pitch and Pitch Pockets Pitch is found in one hardwood in many softwoods like Douglas-fir, pine, and southern pine. Massed pitch pockets are limiting defects (table 1).

genus and ponderosa pitch and in veneer

Bark Pockets Bark pockets occur in some softwoods and in hardwoods like the oaks and hickories. They may show on the log ends or as overgrowths on the bark (37). Bark pockets are limiting defects for most veneer uses. M 92128 F

Figure 10.—Burls in the sapwood (1) and heartwood (2) of rotary veneer cut from the bolt shown in figure 9. The burls persisted to the 8-inch core of the bolt.

Holes Large holes such as those resulting from a rotted branch stub or a woodpecker nest are major defects in veneer logs. Medium holes up to V2 inch (12.7 mm) in diameter—if extensive—may seriously degrade the log for use as veneer. Such holes may have been made by grubs that tunnel in living trees like oak, or result from tapping sugar maple trees, from bullets, or from an increment borer. Medium-size holes are generally accompanied by severe stain. Pin worm holes made by ambrosia beetles occur in hardwoods like oak and ash and also in various softwoods. This defect can be particularly serious with tropical hardwoods. A few scattered pin worm holes can be tolerated in most veneer uses, but heavy attack seriously degrades the veneer.

the consumer. Maple logs with wide sapwood zones are currently in demand because of the popularity of white face stock. In contrast, the heartwood color is in demand for species like cherry and walnut. Nonuniformity in color between logs can cause problems. For example, the preferred color of walnut is a light gray-brown. Green and purple tinges that sometimes occur in walnut are not wanted because they cause special problems in finishing. Studies at the Forest Products Laboratory have shown that the color of walnut varies with the geographic area in which the trees grow. There is some evidence that the color of walnut heartwood is related to the type of soil in which the tree has grown. In addition, color in veneer can be regulated to some extent by the heating and drying process. When a mixture of species, such as the lauans, is used, the material typically available for faces will display a variety of heartwood colors. Recently some veneer and plywood manufacturers have been using electronic devices to separate such veneer into several groups according to color. This simplifies the finishing process, and aids in marketing the products.

Decay Decay is a severe defect in veneer logs, especially for rotary cutting. If the log center is decayed and soft, the chucks may not be able to hold the veneer bolts securely enough to permit rotary cutting. Slightly or moderately decayed logs can sometimes be cut into veneer, providing the wood is still reasonably firm. The best example of this is Douglas-fir 18

of separating stains for practical purposes is to consider those in the standing tree as opposed to those that may develop after the tree is felled.

that has been attacked by F ornes pini (whitepocket). Millions of square feet of softwood plywood have been made from rotary-cut Douglas-fir veneer containing white-pocket. Sound ñitches for slicing can sometimes be sawn from logs with considerable decay.

Stain in Standing Trees

The terminology concerning stain in standing trees is not well accepted. For example, some authors limit the term mineral stain to small olive or greenish-black discolorations in the heartwood and sapwood of the maples and the gums. Others use the same term to describe brown stains in species like aspen and oak. Still other authors attribute these and other stains in oak and aspen and other hardwoods to oxidation of cell materials and call them oxidation stains. Bacteria have also been reported as associated with various stains in living trees. For purposes of this publication, stains in standing trees will not be separated. Stains are found in both heartwood and sapwood of the living tree and are often associated with injury to the tree such as insect attack or broken branches. In addition to the discoloration, intense areas of stain are more likely to collapse and check during drying. Higher ash content has been found in dark green or black stained maple than in normal bright wood. Some plant personnel report more rapid dulling of tools when cutting such stained wood. Brown stain is common in oak trees growing on upper slopes or ridge tops. Because of the poor growth site, these trees are generally also of poor form and do not supply many potential veneer logs. Oak trees growing on moist soils that may be water-saturated for extended periods are also subject to stain. Logs from such trees may in other respects appear to be of quality suitable for veneer. Stain in standing trees may be sporadic and localized to small streaks or it may occur over broad areas. Consequently, the stain may or may not be visible on freshly cut log ends.

Fire Scars Fire scars are generally obvious on the cross section of a log, and often indicate associated decay. Extensive fire scars make logs of doubtful value for use in veneer. Seams Seams are radial cracks that may or may not be overgrown. They may be caused by wind, lightning, or frost. Seams generally originate at the surface of the log and occur in the standing tree. In contrast, splits due to growth stresses start at the pith and generally do not extend to the surface of the log. As a result, seams are visible on the standing tree but splits are not. As seams occur through the cambium layer they may be overgrown by callus tissue. Splits never have such overgrown tissue. Species that may have seams include oak, ash, maple, beech, and birch. The seriousness of this defect depends on how deeply it penetrates the log and whether it is parallel to the log length or spiralled. A straight seam can be clipped from the veneer with less waste than that caused by a spiralled seam. Bird Peck Bird peck and associated stain is a common characteristic on such species as yellow-poplar and hickory. Bird peck can be detected by characteristic holes in the bark and by strain on the log ends. Logs with this characteristic are generally suitable for core and crossband veneer but may be limited for use as face veneer. Stain The term "stain'' has been used to describe several different phenomena. Causes of some stains are known, such as fungal or contact with iron, while others are still being studied. Further, the severity of some stains is directly related to the amount of sapwood and the environment in which the log is held. One way

Stain that May Develap in Stored Logs or Green Veneer

Four types of stains that may develop in stored logs or during veneer processing are sap stain, mold, oxidative stain, and iron stain. Sap stain is fungal in origin and is most commonly blue in color. It is particularly troublesome in the sapwood of species like sweetgum 19

and southern pine if the logs are stored during periods of warm, humid weather. The color is caused by a concentration of hyphae. For many veneer uses, blue stain is objectionable. It should be controlled by keeping log storage to a minimum and, if necessary, by use of chemical sprays or water sprays. Veneer can be protected by drying the stock as rapidly as possible or by dipping or spraying with an antistain solution if drying is to be delayed. Molds are also fungal in origin, but the color (yellow, brown, red, purple, green, blue, or black) comes mainly from spores of the fungi. Mold is characterized by a downy growth on the surface of the wood. Mild temperature, still air, and abundant moisture promote growth of mold. Under these conditions mold may be a problem in green sapwood veneer that is stored 3 or more days before drying. Control methods are similar to those suggested for blue stain. Oxidative stain is a chemical stain that is thought to be the result of oxidation, sometimes promoted by enzymatic action on certain materials stored in the wood cells. Like blue stain and mold, it develops in the sapwood of logs and green veneer when favorable moisture and temperature exist. It has caused objectionable discoloration of light-colored face veneers of species like birch and maple. In logs, the stain progresses gradually in from the ends during warm-weather storage, so cold-weather storage or reducing storage time is the best method for preventing this stain. Use of a white lead paste end coating, or especially of a waterspray, during storage may materially reduce this stain but will not stop it. Drying the veneer as soon as possible after cutting also reduces the chance of oxidation stain. Concentrated oxalic acid will generally bleach oxidation stain but not fungalcaused blue stain. Tannin and other polyphenols react with iron and steel in the presence of water to form a blue-black stain. This becomes very obvious and objectionable on face veneer of species like oak and redwood if the wet wood is in contact with iron or steel for even a brief time. Concentrated

20

oxalic acid or hydrofluoric acid will bleach out iron stain. These acids must then be flushed from the wood or the stain may reappear. Some references on stains originating during processing logs into veneer include Scheffer (60) and Scheffer and Lindgren (61). Man-Made Defects Other than Holes Man-made defects include stump pull, felling splits, log handling damage, and embedded metal. Stump Pull and Felling Splits

Both these defects cause splits in the veneer cut from the logs. Stump pull is generally obvious as a jagged hole on the long end. Butt logs should be carefully examined as felling splits may close and be difficult to detect. Log Handling Damage

Handling logs with tongs is a needless source of defect. Not only may the tongs put holes in otherwise clear veneer, but they also frequently embed sand or grit that damages the knife used to cut the veneer. Similar problems occur with logs that have dirt or gravel embedded in the outer sapwood when the logs were dropped or damaged on a gravel or cinder surface. Embedded Metal

Buried metal is a serious problem in logs cut from street trees and fence rows. Because barbed wire and nails will generally damage a veneer lathe or slicer knife, many veneer log buyers will not purchase logs that come from along fences or streets. Buried metal may be detected because it has formed a bump on the log. Many veneer mills have magnetic metal detectors for screening all logs and flitches. Soft lead from buck-shot and small arms can be cut without damaging the lathe or slicer knife. However, steel-jacketed bullets or shrapnel such as may be found in timber from a battle zone are very serious defects. Aside from the damage to the knives used to cut the veneer, buried metal often causes extensive stain in the wood.

VENEER FROM WOOD SPECIES THAT GROW IN THE UNITED STATES Information about the use of veneer from various species is given in tables 6 and 7. The specific gravity figures given there help classify the species as in tables 2 to 4. The specific gravity figures for hardwoods can be used with the information in the next section to select a favorable range for heating bolts or flitches prior to cutting veneer. The last four columns of table 6 and 7 rate the species for use in various products. Detailed information on log characteristics, wood characteristics, processing into veneer, and use of the veneer as related to wood species that grow in the United States is given in Appendixes II to VI. Similar but abbreviated information on wood species from around the world is given in the report, "Veneer Species of the World," published in 1976 for the International Union of Forestry Research Organizations. Copies can be purchased from the National Technical Information Service.^

This chapter covers tree species that grow large enough and in sufficient volume in the United States so that they can be considered for veneer. While the use of veneer and plywood is increasing, the timber available in such wellknown veneer species as yellow birch and Douglas-fir has declined. As a result, it is becoming more important to know the potential for making useful veneer from all species that grow in the United States. A number of species have been studied for use as veneer at the U.S. Forest Products Laboratory. In addition, other Government laboratories and universities have published information on veneer species. Still further information is available from the veneer industry. From such sources scattered information has been collected and condensed for this publication. If no published information was available on a species for veneer, the species has been evaluated on the basis of the known physical and mechanical properties of the wood.

2 The National Technical Information Service of the U.S. Department of Commerce is located at 5285 Port Royal Road, Springfield, Va. 22161.

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Table 2.—Classification of species for construction and industrial plywood, PS 1-714. Group 1

Group 2

Apitong !• 2 Beech, American Birch Sweet Yellow Douglas-fir ^ Kapur 1 Keruing i« 2 Larch, western Maple, sugar Pine Caribbean Ocote Pine, southern Loblolly Longleaf Shortleaf Slash Tanoak

Cedar, Port Orford Cypress Douglas-fir ^ Fir California red Grand Noble Pacific silver White Hemlock, western Lauan Almon Bagtikan Mayapis Red lauan Tangile White lauan

Group 3

Maple, black Mengkulang 1 Meranti, red ^' * Mersawa ^ Pine Pond Red Virginia Western white Spruce Red Sitka Sweetgum Tamarack Yellow poplar

Alder, red Birch, paper Cedar, Alaska Fir, subalpine Hemlock, eastern Maple, bigleaf Pine Jack Lodgepole Ponderosa Spruce Redwood Spruce Black Engelmann White

Group 4

Group 5

Aspen Basswood Bigtooth Fir, balsam Quaking Poplar, balsam Cativo Cedar Incense Western red Cottonwood Eastern Black (western poplar) Pine Eastern white Sugar

^ Each of these names represents a trade group of woods consisting of a number of closely related species. 2 Species from the genus Dipterocarpus are marketed collectively: Apitong if originating in the Philippines; Keruing if originating in Malaysia or Indonesia. ' Douglas-fir from trees grown in the states of Washington, Oregon, California, Idaho, Montana, Wyoming, and the Canadian Provinces of Alberta and British Columbia is classed as Douglas-fir No. 1. Douglas-fir from trees grown in the states of Nevada, Utah, Colorado, Arizona and New Mexico is Douglasfir No. 2. * Red meranti is limited to species having a specific gravity of 0.41 or more based on green volume and ovendry weight.

Table 3.—Density categories of the most commonly used species based on specific gravity ranges for hardwood and decorative plywood, PS 51-71 Category A High-density species (0.56 or more specific gravity)

Category B Medium-density species (0.43 through 0.55 specific gravity)

Category C Low-density species (0.42 or less specific gravity)

Ash, commercial white Beech, American Birch, yellow, sweet Bubinga Elm, rock

Ash, black Avodire Bay Cedar, Eastern red ^ Cherry, black

Alder, red Aspen Basswood, American Box elder

Madrone, Pacific Maple, black (hard) Maple, sugar (hard) Oak, commercial red Oak, commercial white Oak, Oregon

Chestnut, American Cypress ^ Elm, American (white, red, or gray) Fir, Douglas ^

Cativo Cedar, Western red ^ Ceiba Cottonwood, black Cottonwood, Eastern

Paldao Pecan, commercial Rosewood Sapele Teak

Gum, black Gum sweet Hackberry Lauan (Philippine mahogany) Limba Magnolia Mahogany, African Mahogany, Honduras Maple, red (soft) Maple, silver (soft) Primavera Sycamore Tupelo, water Walnut, American

1 Softwood.

22

Pine, white and ponderosa ^ Poplar, yellow Redwood ^ Willow, black

Table 4.—Species^ for wirebound boxes as listed in Federal Specification PPP-B-585b

Aspen (popple) Basswood Buckeye Cedar Chestnut Cottonwood Cypress Fir (true firs) Magnolia Pine (except southern yellow) Redwood Spruce Yellow-poplar Willow

Group IV

Group III

Group II

Group I

Ash, white Beech Birch Elm, rock Hackberry Hickory Maple, hard Oak Pecan

Ash (except white ash) Elm, soft Maple, soft Sweetgum Sycamore Tupelo

Douglas-ñr Hemlock Larch, western Pine, southern yellow Tamarack

1 Groupings are based on specific gravity and other properties of importance in container construction. When a group is specified, any species in the group can be used.

Table 5.—Importance of physical and mechanical wood properties and log characteristics as related to manufacture and use of the veneer Property

Construction and industrial plywood

Decorative face veneer

Core and crossband veneer for decorative panels

Container veneer and plywood

A B B B B C A B B B C B B B B C

B B C B A-B B A-B B B B A B A A A-B A

A B B A A B A B B B C A A A B C

B B-C B-C B B-C C B B-C B B A-B B B B B C

Physical property Specific gravity Green moisture content Permeability Shrinkage Close grain Fine texture Straight grain Parenchyma Wax Polyphenols Color of heartwood Dimensional stability Resin Gum Hard deposits Figure

A

Odor

Mechanical property Strength in tension perpendicular to grain Hardness Modulus of elasticity Modulus of rupture Shear Compression perpendicular to grain Compression parallel to grain

B B A A A

B B C C C

B C C C C

B B B A C

A A

B C

C C

B B

23

Comments

Figure is desirable for face „j 1 ; _l-i_ f^veneer and other uses Odor is important for containers used with food.

Table 5.—Importance of physical and mechanical wood properties and log characteristics as related to manufacture and use of the veneer—continued Construction and industrial plywood

Decorative face veneer

Core and crossband veneer for decorative panels

Container veneer and plywood

Cylindrical form Taper Eccentricity Tension wood Compassion wood Sweep Growth stress Log end splits Ring shake Knots Epicormic branches and adventitious buds Burls Color Pitch pockets

A A B B A A B A A B

B B B A B B B B A A

A A B A A A B B A A

B B B B B B B B A B

C B C B

B B A A

B B C A

C B B B

Bark pockets Grub holes Pinworm holes

B B B

A A B

A A C

B B B

Decay

A

A

A

A

Fire scars Frost cracks

B B

A A

A A

B B

Mineral streak Other stains Bird peck Stump pull Felling splits Handling damage Embedded metal Growth rate

C C C A A A A B

A A A A A A A A

C C B A A A A B

C B B A A A A B

Property

Comments

Log characteristic

A—Of major importance B—Of moderate importance C—Of little importance

Í

Pitch in crossbands may bleed through face veneer Heavy pinhole damage will degrade all veneer Some types of decay are permitted in Construction grade plywood Veneer from other parts of the log may be top grade

< These ratings are not hard and fast but are indicative of relative importance of various characteristics.

(

24

Table 6.—Specific gravity and suitability of some U.S. species for various veneer uses ^ Relative suitability Common name

Specific gravity 2

Construction plywood

Decorative face veneer

Inner plies of decorative panels

Container veneer and plywood

HARDWOODS

Alder Nepal Red Ash Black Blue Green Oregon Pumpkin Shamel White Aspen Bigtooth Quaking Basswood American White Beech, American Birch Alaskan paper Gray Paper River Sweet Yellow Buckeye Ohio Yellow Butternut Cherry, Black Cottonwood Balsam poplar Black Eastern Swamp Elm American Cedar Rock Slippery Winged Eucalyptus Hackberry Hickory, pecan Bitternut Nutmeg Pecan Water Hickory, true Mockernut Pignut Shagbark Shellbark Holly, American Honeylocust Koa Laurel, California Locust, Black Madrone, Pacific

0.34 .37

C C

B B

A-B B

A B

.45 .53 .53 .50 .48 .47 .55

B B B C C B B

A A A A B A A

B B B B C B B

A A A A B A A

.35 .35

C C

B B

A A

A A

.32

.56

C C B

C C B

A A C

A A A

.49 .45 .48 .49 .60 .55

B C B B B B

A-B B A-B B A A

B B B B B B

B B B B B B

_

C C C B

C C A A

A A C B

B B C A

—

C C C C

B C C B

B B B B

A A A A

.46 .59 .57 .48 .60 .60 .49

B B B B B B B

A A A A A A-B A-B

B C C B C C C

A A A A A B A

.60 .56 .60 .61

B B B B

A A A A

C C C C

B B B B

.64 .66 .64 .62 .50 .60 .53 .51 .66 .58

B B B B C C B-C C C C

A A A A A A A A B A

C C C C C C B C C C

B B B B C B B-C C B B

—

.33 .36 .47 0.30 .31 .37

25

Table 6.—Specific gravity and suitability of some U,S, species for various veneer uses ^—continued Relative suitability Common name

Specific gravity 2

Construction plywood

Decorative face veneer

Inner plies of decorative panels

Container veneer and plywood

HARDWOODS—continued

Magnolia Cucumbertree Southern Maple Bigleaf Black Boxelder Red Silver Sugar Oak, red Black California black Cherrybark Chestnut Laurel Northern red Nuttall Pin Scarlet Shumard Southern red Water Willow Oak, white Bur Chinkapin Delta post Durand Live Oregon white Overcup Post Swamp chestnut Swamp white White Ohia Persimmon, common Sassafras Silk-oak Sugarberry Sweetgum Sweetbay Sycamore, American Tanoak Tupelo Black Swamp Water Walnut, Black Willow, Black Yagrumo hembra Yellow-poplar

0.44 .46

B B

C C

A A

A A

.44 .52 .41 .49 .44 .56

C B C B C B

A A B B B A

B B C A A B

A A B A A A

.56 .51 .61 .57 .56 .56

B B B B B B B B B B B B B

A A A A B A A A A A A B B

B B B B C B B C B B B C C

B B B B B B B B B B B B B

.81 .64 .57 .60 .60 .64 .60 0.70 .64 .42 .51 .47 .46 .42 .46 .58

B B B B C C B B B B B B C C B B B B B B

B B A A B B B B A A A B A-B B A B B C A B

B C B B C C C C B B B C C C B C B A B C

B B B B B B B B B B B B B B B A A A A B

.46 .45 .46 .51 .34 .26 .40

B B B B C C B

B B B A B-C C B

B B B B B B-C A

A A A B B B A

—

.58 .60

—

.52 .56 .56 .58

—

.60

—

26

Table 6.—Specific gravity and suitability of some U.S, species for various veneer uses ^—continued Relative suitabilityCommon name

Specific gravity 2

Construction plywood

Decorative face veneer

Inner plies of decorative panels

Container veneer and plywood

SOFTWOODS

Cedar AlaskaAtlantic whiteEastern redcedar IncenseNorthern whitePort-OrfordWestern redcedar Cypress Baldcypress Pondcypress Douglas-ñr Coast Interior north Interior west Fir Balsam California red Grand Noble Pacific silver Shasta red Subalpine White Hemlock Eastern Mountain Western Juniper Alligator Rocky Mountain Western Larch, Western Pine Digger Eastern white Jack Jeffrey Knobcone Limber Loblolly Lodgepole Longleaf Pitch Pond Ponderosa Red Sand Shortleaf Slash Spruce Sugar Table-Mountain Virginia Western white Whitebark Redwood

.42 .31 .44 .35 .29 .40 .37

B C C B-C B-C B A-B

B B A B B B A

A A B B B A B-C

A A C B B A B

.42

A-B B

A A

B B

A A

.45 .45 .46

A A A

B-C B-C B-C

B B B

A-B A-B A-B

.34 .36 .35 .37 .40 .36 .31 .37

B-C A-B A-B A-B A-B A-B B-C A-B

C C C C C C C C

C B-C B-C B-C B-C B-C C B-C

A A A A A A A A

.38 .43 .38

B-C B A-B

C C C

B-C B B

A-B A A

.50 .51 .51 .48

C C C A

C C C B

C C C C

C C C B

B-C B-C B-C B B-C B-C A B A B-C B B B B-C A A B-C B-C B-C B-C B B-C A-B

C A-B C A C C C B C C C A B C C C C A C C A C A

C B C B C C C C C C C B C C C C C B C C B C C

B A B A A A B A B B B A A B B B B A B B A A A

.34 .39 .37 .37 .47 .38 .54 .45 .50 .38 .44 .36 .46 .56 .41 .35 .49 .45 .36 .37 .38

27

Table 6.—Specific gravity and suitability of some U.S. species for various veneer uses ^—continued Relative suitability Common name

Specific gravity 2

Construction plywood

Inner plies of decorative panels

Container veneer and plywood

C C

C C

B C B A

B C C C

A A A A A A B B

Decorative face veneer

SOFTWOODS—continued

Spruce Black Blue Engelmann Red Sitka White Tamarack Yew, Pacific

.38

—

.33 .38 .37 .37 .49 .60

B-C B-C B B A-B B-C A-B C

c c

c c

1 Rating of A indicates species is well suited for end product; B, intermediate; and C, generally not well suited for this product. 2 Based on weight when ovendry and volume when green.

Table 7.—Specific gravity and suitability of some imported species for various veneer uses Relative suitability ^ for Common name

Angélique Apitong Avodire Brazil nut Bubinga Caribbean pine Cativo Ceiba Determa Kapur Keruing Klinki pine Lauan Limba Mahogany Mengkulang Meranti Mersawa Muritinga Ocote pine Okoume Paldao Primavera Rosewood Sapele Teak

Specific gravity 2

0.60 .59 .51 .56 .65-.76 .68 .40 .25 .51 .64 .46-.70 .38 .40-.46 .49 .45 .56 .36-.51 .51 .45-.60 .55 .37 .60 .39 .80 .60 .57

Construction plywood

Decorative face veneer

Inner plies of decorative panels

Container veneer and plywood

B A B B B A-B B C C A-B A-B B B B B A-B B B B A B B B-C B-C B B

B-C B A B A C B C C B-C B-C B A A-B A B-C A B B-C C B A A A A A

C B B B-C B-C B-C A-B A-B C C C A B B A B-C A B B B-C A B-C B B-C B B

B-C B B B B B A C B-C A-B A-B A A-B B B B B B B B A B B B-C A B

1 Information primarily from "Properties of Imported Tropical Woods," by B. F. Kukachka, USDA Forest Serv. Res. Pap. FPL 125, 1970 and from "Veneer Species of the World," lUFRO Interim Report, 1976. 2 Specific gravity based on volume when green and weight when ovendry. 3 Rating of A indicates species is well suited for end product; B, intermediate; and C, generally not well suited for this product.

28

TECHNIQUES FOR PEELING, SLICING, AND DRYING VENEER From a given supply of logs, the processor can improve the quality of veneer in two general ways: Handle the wood so that variability is minimized, and carefully select and adequately maintain processing equipment. Handling the wood to minimize variability involves such things as log storage, breaking logs into bolts or flitches, and heating or cooling the wood prior to cutting. Processing equipment involves the basic de-

sign of debarkers, lathes, slicers, veneer conveyors, clippers, and dryers. All of this equipment must be properly maintained, set up, and operated to consistently produce good-quality veneer. The veneer processing techniques described in this bulletin follow the chronological steps in which they occur from log to dry veneer. This is followed by a section on quality control and trouble shooting to minimize veneer defects.

LOG STORAGE Veneer logs can be kept in good condition for The sapwood of many species is subject to some time providing the storage conditions are attack by anaerobic bacteria even though the suitable. With poor storage conditions, logs can wood is kept wet. This has caused objectionable deteriorate by drying and cracking of the log odor, particularly in tropical hardwoods like ends and other exposed wood; development of muritinga, ceiba, and cativo. Bacteria may also blue stain, decay, and oxidation stain ; attack by cause excessive porosity in pines like ponderosa insects; cracks and grain separation due to and the southern species. The best way to confreezing and thawing; development of undesirtrol bacterial action is processing felled trees able odor ; and increased porosity due to attack within 1 month or by storing the wood below by bacteria. 40° F. (5° C). Spraying with chemicals may End drying and splits in logs can occur with help, providing the bacteria has not already ensusceptible species like dense hardwoods in one tered the wood. hot, dry, windy day when the sunlight falls Given these many possible problems, what is directly on the log end. End drying is less of a the best procedure for log storage? In general, problem with a species like Douglas-fir stored veneer log storage should be kept to a minimum. in winter in the damp Northwest. Blue stain The first logs into storage should be the first and mold can occur in a week to 10 days on the ones out of storage for processing. Ideal storage sapwood of species like sweetgum and southern conditions would be to end coat and keep the pine stored in humid summer weather in the bark intact on tree-length logs that are either South. Decay generally requires weeks or held at high humidity and a temperature just months to develop. Oxidation stain, which lowabove freezing (34° F or 1° C) or completely ers the value of white sapwood of species like submerged in cold water (34° to 40° or 1° to birch and maple, may occur through the ends 5° C). The next best system would be to keep of unprotected logs stored several weeks during the logs under a roof and all surfaces constantly summer. wet by a water spray. This would be just as Insects like lyctus beetles may attack a log good as the first method, providing the temperawithin hours after felling. To minimize insect ture was between 34° and 40° F (1° to 5° C). attack, logs stored in warm weather should be A common storage method that is generally used within 2 weeks after felling, treated with satisfactory is to keep all log surfaces wet with an approved chemical,^ or stored under water. a water spray but without using a roof. When Freezing and thawing of logs of species such water spray is not feasible, then a chemical as sweetgum and claro walnut may fracture the spray and end coating may permit satisfactory wood so that it is useless for veneer. This is storage. Less desirable methods which are someless of a problem with species grown in northtimes suitable include floating the logs in a ern climates. pond and cold-decking the logs. A much more 3 Check with the local County Agricultural Agent or complete discussion of log storage is given by State Agricultural Experinment Station for approved Scheffer (60). recommendations. 29

BARK REMOVAL veying the material from a mechanical deThe subject of bark removal is one in which barker, as bark may come off in large sheets. two people, both knowledgeable in the field, may In general, softwoods like pine are easier to disagree. The reason is the wide variability in debark than hardwoods like hickory, but there difficulty of removing bark. Three factors that are many exceptions. For example, fall-cut eastmust be considered are: (1) variability of bark ern hemlock is reported to be more difficult to adhesion within a species; (2) variability of debark than northern hardwoods like maple and bark adhesion between species; and (3) type of birch. Other examples of softwoods that are equipment used for debarking. difficult to debark are cypress with a fluted base, western redcedar with stringy bark, and redVariability Within a Given Species wood with very thick bark. The difficulty of Spring-cut logs are easier to debark than fallbark removal of species that grow in the United cut logs of the same species. This general stateStates is shown in Appendix IV. ment is true for all species. Actual measurements of the wood-to-bark bond on several Types of Equipment Used species indicate that this increase of bond Different systems have been used for debarkstrength from spring to fall may be 100 to 200 ing veneer logs, including hand tools, bark percent. saws, water under high pressure, flailing chains, A second factor is the temperature of the and drum debarkers. Some mills have used an wood and bark at the time of bark removal. old lathe to debark and round bolts. At present, Heated wood is much easier to debark. When however, two methods are by far the most comveneer logs were commonly debarked by hand, mon for debarking veneer logs—^the cambioa main reason for heating the logs was to make shear or ring debarker, and the rosser-head bark removal easier. Frozen logs are particudebarker. Combination machines may use either larly difficult to debark. A plant may even install cambio-shear or rosser-head or both. a hot pond to get logs above freezing so they Some factors to consider in choosing a decan be more readily debarked with a mechanical barker, besides the original and operational debarker. costs, include species to be debarked, volume of Another factor in debarking is whether or not wood to be debarked, maximum and minimum the bark has been allowed to dry on the log. diameter of logs to be debarked, importance of Assuming no bacterial action has taken place, fiber loss, pollution, ease of operation, and ease the bark generally adheres more tightly after it of maintenance. has partially dried. In general, the rosser-head debarker has a A fourth factor is the action of bacteria. Logs lower initial cost, lower maintenance cost, is stored in a warm pond or under a sprinkler dureasier to adjust, and is more adaptable for logs ing summer may be subject to attack by bacof a wide range of diameters. The rosser-head teria. Bacteria seem to prefer the inner bark as is generally preferred for debarking rough logs a food source. Consequently, logs stored in a pond and attacked by bacteria may have the of species like hickory, logs that vary widely in diameter, and logs that may be frozen. bark loosened so that it will come off in one big sheet. Such a big piece may jam the bark conThe cambio-shear or ring debarkers are genveyor. Conversely, bacteria attack may make erally preferred by plants processing logs with peeling of bark much easier when using hand relatively uniform diameters and where high tools. production and low fiber loss are important. A typical installation would be in a large southern Species Differences pine plywood plant. Several manufacturers of cambio-shear deIndividual wood species differ in strength of barkers state that, by proper adjustment of tool the bond between the bark and the wood. In one pressure and feed, their equipment can debark study of fall-cut logs, the bark-to-wood bond any species under any conditions, including of quaking aspen was more than 40 percent frozen logs. Similarly, manufacturers of rosserstronger than that of red spruce. head debarkers state their equipment can be Some species like basswood and elm have used to debark any species under any conditions. stringy bark. This becomes a problem in con30

SAWING INTO BOLTS OR FLITCHES It is generally desirable to harvest logs in as long lengths as possible and to saw into bolts or flitches at the veneer-cutting plant. The reasons for doing this include less waste from end drying of the logs, a better opportunity to observe all sides of the log before cutting, availability of skilled labor trained to buck and saw flitches from the logs for the best use, and better mechanical equipment for handling and sawing the logs. The sequences of debarking, bucking into bolts, and heating depends on type of logs, debarking and sawing equipment at the plant, and whether log end splitting is a factor during heating. In general, debarking reduces heating time, as bark is a good insulator. Heating in long lengths reduces waste due to log end splits. On the other hand, bark indicators of hidden defects in the logs may help the sawyer decide where to break the logs for best grade. The bark may also protect the logs during handling. A method sometimes used with hardwoods that tend to end split is to debark in long log lengths, heat in long log lengths, and then buck into bolts just prior to cutting veneer. This method reduces the time required to heat the bolt by eliminating insulation by the bark. Log end splits are confined largely to the ends of the long log and minimized at bolt ends exposed by crosscutting after heating. The process requires a continuous debarker, long heating vats, and equipment to handle long logs. Other disadvantages are that the bark indicators of defects are lost before bucking, and care must be used to prevent the debarked logs from picking up grit during handling. A method used with softwoods like southern pine is to debark in long log lengths, crosscut bolts, and then heat prior to peeling. This requires a continuous debarker but permits heating vats and handling equipment which work with 8-foot and shorter blocks. It is a satisfactory method if end splitting is not a serious problem and the handling equipment is kept clean so the debarked logs do not pick up grit. Large-diameter logs such as old-growth Douglas-fir are sometimes cut to bolt length in a pond, debarked in a machine designed for 8-foot lengths, and then heated or cut at room temperature. The debarking-sawing-heating sequence used

for flitches is generally to buck to length, then saw the flitches, and finally heat the flitches. As flitches are generally a step in producing face veneer, bark indicators are important for cutting logs to length and for producing the flitches. Most or all of the bark is removed in sawing and so does not significantly retard heating. The heated flitches are cleaned and any remaining bark removed with a flitch planer just prior to slicing. Saws Used in Processing Logs to Bolts and Flitches Logs are cut to length of bolts or flitches primarily with large circular saws or with chain saws. In both cases it is important that the log and saw be positioned so the cut is at a right angle to the axis of the log. Logs are generally sawn into flitches with a handsaw or a circular saw. The vertically movable circular saw that is mounted over the log carriage permits sawing logs into thirds as well as halves and quarters. In all cases it is important that the log can be accurately positioned with respect to the sawline and that the sawyer can see both ends of the log. If both lumber and veneer flitches are to be produced, the bandsaw may be advantageous, as generally a smaller saw kerf is produced. What Does the Sawyer Look For? Bolts Factors to be considered in bolts are sweep in the log, end trim, presence of large defects like knots, and the length of the bolts required. If possible, sweep in the log should be minimized as it results in excessive roundup and short grain in the veneer. Thus, even though long bolts are generally more valuable than short bolts, a log with excessive sweep would probably be more valuable if cut into two or more bolts to minimize the sweep. Logs that have been end coated or that have dried and checked should be end trimmed. The cut should be at a right angle to the longitudinal axis of the bolt. Crosscutting with a hand-held saw can result in irregular bolt ends, which in turn can reduce the surface engaged by the lathe chucks and also cause the veneer to vary in length or require excess spurring at the lathe. 31

band veneer. Slicing and stay-log cutting is done primarily to produce decorative face veneer. A stay-log is an attachment for a veneer lathe on which flitches may be mounted for cutting into half-round, back-cut, or rift veneer. Very highquality core and crossband veneer is occasionally produced by quarter-slicing. Small, fast slicers have been used to produce container veneer.

Flitches A log with sweep should be sawn into flitches so the sweep is perpendicular to the plane of the knife used in slicing. This permits fulllength veneers from the start of slicing. A large split or frost crack in a log may be minimized by dividing a log along this longitudinal plane. If possible, knots or other defects indicated in the bark should be trimmed out or be put at one edge or end of the flitch so the defect will occur at the edge or end of the veneer. In general, it is desirable to saw the flitch parallel to the bark and take the taper from the center of the log. This makes for straighter grain and a balanced design in the face veneer. The side of the flitch that is to be the exit side for the knife at the end of the cut should be sloped, with the wide side next to the flitch table to minimize tear-off during slicing. The top and bottom of the back of the flitch should be squared so the slicer dogs can obtain a good grip. The recent developments of remotely controlled extension dogs and a fixture for holding the flitch by vacuum make this precaution less important. Frequently the sawyer preparing flitches for face veneer has the option of sawing the log for lumber. This judgment is generally made after he has sawn through the pith and can see the quality and figure in the wood. If the log has some limitation for slicing, such as ring shake, it may still be possible to recover high-quality lumber.

Rotary Eighty to 90 percent of all veneer is cut by the rotary method (fig. 11-A). The rotary method gives the maximum yield; it results in the widest sheets; knots are cut to show the smallest cross-section; and most juvenile wood and splits are left in the core. Some rotary-cut veneer is used for the decorative eflfect of annual rings or irregular grain, such as that causing "blister'' figure. Flat-Slicing and Half-Round Cutting Flat slicing (fig. 11-F) is done on a slicer, and half-round cutting (fig. 11-B, C) is done on a lathe. Half-round cutting may be done with flitches mounted on a stay-log (fig. 11-C), or by chucking a bolt at one edge rather than at the center, and by having the lathe chucks mounted eccentrically (fig. 11-B). Veneers produced by the flat-slicing and by half-round cutting are similar in appearance. The centers of the sheets are essentially flat-grain while the edges are rift or even quartered material. The half-round method gives slightly wider sheets and a bigger area of flat cutting in the center of the sheet than the flat-slicing method. These two cutting methods show growth rings to advantage. When the grain dips in and out of the sheet, the figure is broadly termed ''crossfire.'* Burls are generally cut by the half-round method and crotches by the flat-sliced method.

Choice of Cutting Direction Some of the ways bolts or flitches are prepared and cut into veneer on a lathe or a slicer are illustrated in figure 11. There are two main directions in which veneer can be cut—parallel to the annual rings (rotary-cut) or parallel to the wood rays (quarter sliced). The other methods fall between these two extremes. Half-round, flat-slicing, and back-cutting all result in cutting parallel to the rings in the center of the veneer and at angles to the rings at the two edges of the veneer sheets. Rift-slicing is a deliberate attempt to cut midway between parallel to the rays and perpendicular to them. The lathe is used to cut practically all veneer used in construction plywood, some decorative face veneer, and most container, core, and cross-

Rift-Cut A quarter section of a log is cut and mounted so that the knife cuts about a 45° angle to the wood rays. This can be done with a stay-log on a lathe (fig. 11-E) or on the slicer (fig. 11-H). The method is used primarily with white oak to produce a figure caused by the wood rays. When the veneer is coarse-textured and the annual rings are not exactly parallel to the edge of the veneer, the figure is called rift-cut. A form of rift-cut that is particularly desirable is comb 32

LATHE

S LI CE R

A, ROTARY (YELLOW BIRCH)

F.

B, ONE'HALF ROUND (RED OAK)

6. OUARTER SLICED (PRIMAVERA)

C. ONE-HALF ROUND (BLACK CHERRY)

H. RIFT SLICED (WHITE OAK)

D, BACK CUT (ROSEWOOD)

I. WHOLE LOG (FLAT SLICED) (ASPEN)

E, RIFT CUT

J.

(WHITE OAK)

FLAT SLICED

(WALNUT)

1. FLAT SLICED 2. BACK CUT 3, OUARTER SLICED M 140 660

Figure 11.—Some of the cutting directions used to obtain different grain patterns in veneer. The species in parentheses are typical of those cut by the method diagramed. The wide dark lines under ''slicer" represent the backboard left at the end of slicing.

33

able than the sapwood. Rosewood is an example of this.

grain. By contrast with the more familiar form, comb grain has fine texture, straight grain, and no broad flakes.

Sawn At one time sawing was a common method of producing veneer, but it is almost obsolete because of the large volume of material lost as sawdust. Sawing does have the advantage that it is not necessary to heat the log or flitch prior to cutting, the two sides of the veneer are essentially the same in quality, and thicker veneers can be produced without developing cracks into the veneer. An example where these advantages are important would be the top or back of a musical instrument, such as the guitar. Species like spruce, oak, cypress, and eastern redcedar are occasionally sawn. Sawn material can be flat-cut, quarter-cut, or rift-cut much the same as when slicing with a knife.

Quarter-Sliced Quarter-slicing (fig. 11-G) produces straight, narrow stripes in straight-grained softwoods like Douglas-fir, redwood, and western redcedar or straight-grained hardwoods like oak and walnut. Quarter-slicing is also done with species having interlocked grain such as mahogany and primavera. This produces a plain stripe or ribbon-grain which reflects light in different directions depending upon the position of the viewer. Plain-stripe is a comparatively broad stripe and not too pronounced. A ribbon stripe has narrower bands and is more highly reflective. When the grain in the wood dips in and out of the sheet, the figure is called a broken stripe.

Figure in Veneer As briefly described under the different cutting directions, the appearance of veneer can be greatly affected by whether the veneer is cut tangential to the annual rings, at a right angle to the annual rings, or somewhere in between. Figures 12 to 15 are examples of the appearance of face veneer.

Back-Cut Back-cutting (fig. 11-D) is done on a lathe with a stay-log, much like half-round cutting, However, instead of cutting from the sapwood side, the cut is from pith side of the flitch. Back-cutting is uncommon and is done where the heartwood is narrow and much more valu-

CONDITIONING WOOD PRIOR TO CUTTING VENEER The moisture content, permeability, and the temperature of wood can have a marked effect on veneer cutting.

Wood Permeability The more permeable wood is to water, the easier it is to cut. But permeability is also largely inherent in the species. Sapwood of some species can be made more permeable by storing in a warm, wet condition so bacteria will attack it. This may make it easier to cut into veneer but it may also affect the odor of the wood and its gluing properties. These disadvantages make it unlikely industry will purposely induce bacterial attack to improve cutting.

Wood Moisture Content Poor cutting results if nearly all cell cavities in the wood are filled with water or if the moisture content is below the fiber saturation point (about 30 percent for all species). Unfortunately, there is little the plant manager can do to drastically change the moisture content in a bolt or flitch. Rapid processing, storage under water, or a sprinkler system will prevent green logs from drying. Logs having very high moisture content cannot be partially dried quickly without developing degrade at the outer portions of the log. Steaming may slightly reduce the time required to dry the veneer.

Wood Temperature The major factor under control of the plant manager is the temperature of the wood when it is cut. This is an area where strong differences of opinion exist among veneer plant managers. For example, a hardwood plant manager 34

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Figure 12.—Rotary-cut yellow birch with the figure caused by annual rings.

Figure 13.—Flat- or plain-sliced black walnut with figure from the annual rings and also a dip in the grain. The dip in the grain is sometimes called cross figures.

in Wisconsin stated that the entire quality control in his plant hinged on proper heating of bolts prior to cutting veneer. He stated that many things depend on whether or not the bolts are properly heated: Smoothness, tightness, and thickness control when cutting the veneer; buckle, splits, and uniform moisture content after drying; and quality of glue bonds. In contrast, a softwood plant manager in Oregon stated that heating of veneer bolts was not worth the cost and he did not want log heating equipment in a plant that was to be built.

Before commenting on these statements, let's examine some of the known effects of heating on green wood. Some Effects of Heating on Green Wood Plasticity Heating green wood makes it more plastic. This fact is easily demonstrated with mechanical tests and is the basis of steam bending of wood. Within the limits used in veneer production, plasticity is not time-dependent; as soon 35

!'

il

1.

f.

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Figure 14.—Rift-sliced white oali. The pencil stripe figure is caused by cutting the wood rays at an angle of about 45°.

Figure 15.—Quarter-sliced primavera. The broken stripe figure is caused by interlocked grain which dips in and out of the sheet.

as green wood reaches a given temperature, it is as plastic as it will get at that temperature. Veneer cut from heated bolts or flitches can be bent with fewer fractures than veneer cut from unheated wood. This effect is more noticeable with dense species and when cutting thick veneer. If a plant is interested in cutting tight, thick veneer from dense species, then heating of the bolts or flitches is an important part of the process.

Hardness

Heating wet wood makes it softer. Hard knots, which if unheated may nick a sharp knife, will often be softened by heating so they can be cut. Heat also softens pitch but does not soften mineral deposits like calcium carbonate and silica. While heating generally aids cutting of dense species, it may oversoften less dense species and result in tearing of fibers and a fuzzy surface 36

steel strap 1 inch (25.4 mm) wide applied by a tool commonly used to strap containers was ineffective in preventing splits at the bolt ends during heating of Brazil nut about 2 feet (0.6 m) in diameter. End splits were reduced in another bolt of Brazil nut heated to 160° F (71° C) by wrapping the ends with i/i>-inch (12.7 mm cable and applying a tensile force of 40,000 pounds (18,000 kg) to the cable. Similar results have been obtained experimentally with red oak. Steel strapping is used on the ends of flitches by some face veneer plants. Plant managers report this reduces splitting. As indicated earlier, the forces that tend to cause end splits during heating are less in flitches than in bolts of the same species.

on the veneer. This phenomenon occurs at different temperatures for different species. In general, if the wood cuts with a fuzzy surface, it is too hot. Dimensional Changes

When green wood is heated, it expands tangentially and shrinks radially. This fact has been verified for both softwoods and hardwoods by a number of researchers. The amount of shrinking and swelling varies with the species. This thermal movement increases with temperature but the rate of increase is slow up to about 150° F (66° C) and then increases more rapidly. Consequently, if a species tends to develop splits through the pith and shake due to heating, a general recommendation is to not heat above 150° F. Tangential expansion and radial shrinkage can occur in flitches without causing end checks or shake. It is, therefore, often possible to use higher heating temperatures with flitches that do not contain the pith than with bolts that do contain the pith.

Color Changes

Heating green logs may darken or lighten the wood. Heating in steam is reported to change color more than heating in water. The color changes may be desirable or undesirable. In general, heating tends to darken sapwood of all species. Similarly, the heartwood of oak, beech, and Port-Orf ord-cedar are darkened by heating. To keep the wood as light in color as possible, minimum heating times and temperatures are recommended for ash, oak, maple, and beech. Heating may also affect later color changes. Wet sapwood of yellow birch may develop orange streaks, thought to be an oxidation stain promoted by enzymes. Adequate log heating tends to inactivate the enzymes and reduce the likelihood of this undesirable orange stain occurring. Similarly, adequate heating of oak sapwood reduces the development of gray-brown oxidation stain. Warm, wet walnut veneer is often held in storage until the sapwood becomes darker from oxidation stain and the heartwood reaches a desirable light gray-brown color. It is then dried to minimize further color changes.

Growth Stresses

When bolts with large growth stresses are heated, the wood at the bolt ends is temporarily weakened in tension perpendicular to the grain ; the growth stresses, together with dimensional changes discussed earlier, may cause star-shaped cracks radiating from the pith at the end of the bolts. The longitudinal growth stresses act primarily at the two log ends. When the wood is heated to a temperature of 180° F (82° C), or higher, 90 percent or more of the growth stresses are relieved. If the wood is heated in long log lengths and then crosscut to bolt lengths, the newly formed bolt ends will have less end splits than would develop if the bolts were cut to length prior to heating. Longitudinal growth stresses tend to cause flitches to bow toward the bark side. This bow may become worse during heating. Bowing can be reduced by strapping the flitches together with the bark side out and allowing the heat to relieve the growth stresses while the flitches are mechanically held flat. A wide, strong strap or heavy chains and bolts must be used as the forces involved are large. Experimental strapping has also been tried to reduce end splits in bolts during heating. A

Strength of Wood

Occasionally a question is raised about whether heating bolts or flitches prior to cutting veneer weakens the dry veneer. As discussed earlier, heating wet wood plasticizes and softens it while it is hot. After drying, the wood cut from heated veneer bolts or flitches has much the same strength as wood cut from unheated controls. 37

However, excessively long heating* and high temperatures can reduce the strength permanently. For example, Douglas-fir and sitka spruce heated for 50 days at 150° F (66° C) in water lost 10 percent of the modulus of rupture of unheated controls. When the same species were heated at 200° F (93° C), the modulus of rupture was reduced about 10 percent in approximately 10 to 12 days. Modulus of elasticity (stiffness) values were affected even less by heating. Heating has a greater effect on hardwoods than on softwoods. In contrast to the 10 to 12 days of heating required for the softwoods, Douglas-fir and sitka spruce, to lose 10 percent in modulus of rupture, it took only 6 to 7 days of heating for a comparable loss in yellow birch. Torque to Turn Bolts

The torque required to turn a bolt into veneer depends on wood density, veneer thickness, bolt diameter, setting of the knife and pressure bar, and wood temperature. In one test, we found that 1/4-inch (6.35 mm) basswood veneer cut at 200° F (93° C) required 42 percent less torque than matched material cut at 35° F (2° C). However, the torque that the bolt end would accept at 200° F (93° C) was 44 percent less than matched material at 35° F (2° C). In other words, heating reduced the cutting force about as much as it reduced the wood's ability to resist spin-out. Bolt heating is sometimes blamed for spinout, or turning of the chucks in the bolt ends. This can happen if bolts are heated at a high temperature for a short time. The bolt ends are then hot and soft, while the inner part of the bolt is cooler and requires a relatively higher force for cutting. The better procedure is to heat the bolts at a lower temperature long enough so each bolt is uniformly hot. This procedure also minimizes splits at the bolt ends that may contribute to break-out of the bolts during rotary cutting.

60° F (16° C) shrank 13.3 percent, while matched veneer peeled at 135° F (57° C) shrank 15.1 percent. The effect was greater the higher the conditioning temperature and the longer the heating time. Drying Time

When sound, green logs with a high moisture content are heated in hot water or steam to 150° F (66° C) or higher, they generally lose 1 to 10 percent of the moisture in the log. This is believed to be caused by air in the cell cavities expanding and pushing out free water. Decayed logs may pick up water during heating in water. It is sometimes thought that warm veneer cut from heated wood will dry faster than veneer cut from bolts or flitches that are not heated. Grantham and Atherton (25) report that Douglas-fir sapwood cut from bolts at about 140° F (60° C) dried 10 percent faster than sapwood from unheated bolts. They found the drying time for Douglas-fir heartwood was the same for veneer cut from heated and unheated bolts. Thin veneer cut from hardwoods generally requires the same drying time whether the bolts or flitches are heated or not. These results are to be expected from the relatively small amount of energy required to heat wood compared to the large amount of energy required to dry it. Warp Veneer cut from heated wood is generally tighter than veneer cut from unheated wood. Tight-cut rotary veneer may tend to reassume the curvature of the bolt more than loosely cut veneer. The tendency of the veneer to curl is also related to the setting of the pressure bar during cutting. If the logs or flitches are heated and then cooled in water, the end grain will pick up water. If the veneer is not spurred at the lathe, this extra water at the ends may affect drying at the ends of the sheets.

Shrinkage

Decay Resistance of Naturally Durable Woods

Heating of softwood veneer bolts or flitches has no detectable eflfect on the shrinkage of veneer cut from them. In contrast, heating bolts or flitches of some collapse-susceptible hardwoods may result in noticeably higher shrinkage of veneer cut from the preheated wood. In one trial, alpine ash veneer peeled at

The heartwood of green Douglas-fir, Alaskacedar, white oak, and true mahogany was heated at 212° F (100° C) for various times from 1 to 48 hours. After 12 hours of heating, the white oak and mahogany were slightly less resistant to decay than similar but unheated 38

logs. The Douglas-fir and Alaska-cedar were not noticeably affected. After 48 hours at 212° F (100° C), all of the woods were slightly less decay resistant than the unheated controls of the same species. The results indicate the heating of wood in normal veneer processing does not degrade decay resistance from a practical point of view.

Conclusions Related to Wood Temperature The improved veneer tightness, smoothness, and color are sufficiently beneficial so that almost all producers of hardwood face veneer heat bolts or flitches prior to cutting veneer. Whether heating has a significant effect on veneer thickness, moisture content after drying, and quality of glue bonds is not well documented. The Wisconsin plant manager was, nevertheless, correct when he stated that heating must be done properly in order to produce high-quality hardwood face veneer. The softwood plant manager in Oregon who did not want heating equipment was producing a very different product. The panels from his plant were to be used mainly for construction such as sheathing. Here, properties of veneer tightness, smoothness, and color are less important. Many western softwood plywood plants make satisfactory construction plywood from unheated veneer bolts. However, research and plant experience indicates that heating pays even for manufacture of construction plywood. This is particularly true if there is any danger that the logs will freeze. Both researchers and industrial veneer producers have found that it is not possible to cut veneer from frozen logs. If the logs do freeze, then the plant without heating facilities will be shut down. Another advantage of heating for this kind of product is that the veneer is tighter cut and as a result a higher percentage of 4-foot-wide and 2-foot-wide sheets is produced. That is, less splitting occurs during handling of the green, tightly cut veneer than when handling loosely cut veneer. In addition, heated knots are softer and result in less knife wear than cutting similar wood from unheated blocks. Grantham and Atherton (25) conclude that heating does pay when cutting veneer for plywood to be used in construction.

Conclusions on Some Effects of Heating Some Benefits

The most obvious effect of heating is that it makes it possible to cut tighter veneer than can be cut from unheated wood. Tighter cutting means greater strength in tension perpendicular to the grain of the veneer and so less splitting of the veneer in handling and less checking of face veneer in service. A second effect of heating is that it softens knots, thereby reducing nicks in the lathe or slicer knife. The sharper knife in turn helps produce smooth veneer surfaces. Other possible benefits of heating include less power to cut equally tight veneer, improvement of color by decreasing oxidation stain in the sapwood, and reduced veneer drying time. Frozen wood cannot be cut satisfactorily into veneer with a knife. Wood bolts or flitches yield veneer of varying quality with varying temperature. The changing temperature may adversely affect the lathe or slicer settings. In general, heating is beneficial when slicing figured face veneer from dense species. Heating is also important if tight veneer is to be produced in thicknesses of % inch or greater. Some Disadvantages

Most disadvantages of heating can be attributed to using too high a heating temperature or too long a heating time. Overheating may cause excessive end splits in bolts of species like oak, fuzzy surfaces on springwood and glossy surfaces on summerwood, shelling or separation on springwood and summerwood during cutting, unwanted darkening of the veneer, increased spinout of bolts by softening the end grain, or increased shrinkage. But the heating temperature must be very high and the heating time very long to affect the strength and durability of the wood.

Time Required to Heat Veneer Bolts and Flitches Most investigators agree upon some points about the time required to heat veneer bolts and flitches, but other points are controversial. First, let us examine the points that are generally accepted. 39

bient temperature. This is particularly true if logs of high moisture content may be frozen at some part of the year. While ice conducts heat faster than water, the heat required to melt the ice can result in longer heating time (11). When heating frozen wood of species with a low moisture content like Douglas-fir heartwood, the heating times are shorter than for frozen logs with very high moisture content like western hemlock.

Generally Accepted Points Uniform Final Temperature

The bolt or flitch should be heated long enough so that temperature of the wood from the start of cutting to the end of cutting varies no more than 10° F (6° C). To achieve this goal, the heating time must be sufficiently long and the heating medium (steam or hot water) must circulate freely to all surfaces of bolts and flitches.

Effect of Grain Direction

Effect of Diameter

End grain heats about 2V2 times as fast as side grain. The rate of heating in the tangential and radial directions is about the same. Because most flitches and bolts are long compared to their cross sections, heating through side grain generally is the controlling factor. Faster end-grain heating probably means knots heat faster than surrounding clear wood. This is fortunate as one of the reasons for heating is to make the knots soft enough so they will not turn the edge of the lathe or slicer knife.

The time required to heat a large-diameter bolt or flitch is much longer than the time required to heat one of small diameter and generally increases with the square of the diameter. For example, while a bolt 1 foot (0.3 m) in diameter might be heated in 14 hours, a bolt of the same species 2 feet (0.6 m) in diameter would require about 60 hours. This example is from the report by Fleischer (20) for wood having a specific gravity of 0.50, an initial wood temperature of 60° F (16° C), a temperature of the water used to heat the bolt of 150° F (66° C), and the final temperature at a 6-inch (15 cm) core of 140° F (60° C). The time required to heat nonfrozen logs increases approximately as the square of the log diameter (11).

Variability of Heating

The rate of heating the flitches, even of the same species, is somewhat variable due to irregular shapes, differences in specific gravity, and defects like cracks. Therefore, heating schedules cannot be precise. In general, the schedules should be developed for the largest bolts or flitches that are to be heated, starting from the lowest ambient temperatures in the log storage area. The most common problem in heating veneer logs is insufficient vat capacity to adequately heat the logs or flitches under all operating conditions.

Effect of Temperature Gradient

The greater the difference in temperature between the wood and the heating medium, the faster the heating rate. As the wood approaches the temperature of the heating medium, the rate of heating becomes very slow. As a result, when selecting heating schedules, it is generally practical to aim for a core temperature 10° F (6° C) lower than the temperature of the heating medium. Some veneer plants use an equalizing period at the end of the heating cycle to take advantage of the faster heating with a large temperature gradient and still end up with relatively uniform temperature throughout the block (25). A limitation to this practice is the bolt end splitting that may occur with the high initial temperature.

Controversial Points Effect of Heating Medium

MacLean (i6) reported that water heats wood 5 to 10 percent more slowly than steam. He found the slowest rate of heating in air at low humidities, but the rate was increased as the humidity was increased. In contrast, Feihl (11) found that hot water heats as fast or faster than steam. Feihl points out that this apparent conflict may be due to the experimental conditions. MacLean was using steam at 212^ F (100° C) and higher, while Feihl used a steam-air mix at a temperature generally below 200° F (93° C).

Total Temperature Change Required

The colder the wood, the longer the heating time required to bring it to the desired cutting temperature. In other words, the heating capacity of a plant should be figured for the worst winter conditions rather than the average am40

Some commercial plants inject steam into a vat and at the same time spray hot water over the bolts or flitches. In addition to adding heat to the wood, the hot water spray prevents drying and checking. A commercial modification of the steaming hot-water spray method is to blow steam through an alkaline water solution. Salts added to water will raise the boiling point slightly. These few degrees change in temperature would not seem to be important for conditioning veneer logs. This would appear to be mildly alkaline because.we know strong alkali solutions will break down wood structure. However, in typical heating cycles, the alkali would not penetrate more than a fraction of an inch in most wood species.

attributes it to either higher specific gravity of the wood or higher moisture content in the log. Log End Splits Many veneer plant operators believe rapid heating increases end splits. Meriluoto {Jf.8) reports that heating frozen birch at less than 5° C (41° F) until the wood was above 0° C (32° F) resulted in less end splits than when heating frozen bolts in water at 14° C (57° F). A few trials at the U.S. Forest Products Laboratory with nonfrozen bolts showed little difference between end splits in bolts heated slowly and matched bolts put directly into water that was at the desired final temperature. While slow heating may slightly reduce end splits, the maximum heating temperature seems to be more important. The higher the heating temperature, the larger the end splits.

Effect of Differences in Moisture Content and Specific Gravity

Duration of Heating at Constant Temperature Some researchers have reported that longtime heating using a low temperature has the same effect in conditioning wood for cutting veneer as shortime heating at a high temperature. Experiments at the Forest Products Laboratory in Madison indicate this is questionable. Duration of heating up to several days does not affect the plasticity and hardness of the wood. This in turn means excessively long heating periods do not improve the tightness or smoothness of the veneer compared to shortterm heating to the same final temperature. Heating longer than necessary to bring the wood to the desired cutting temperature may darken the wood and increase shrinkage.

MacLean H6) found wood that is well below 30 percent moisture content heats more slowly than green wood, but differences in moisture content above about 30 percent had no important effect on the rate of heating. All veneer cutting, of course, is done with wood at a moisture content of 30 percent or higher. For practical purposes, MacLean is suggesting that green wood of any given species will heat at about the same rate at any moisture content above 30 percent. In contrast, he found that the rate of heating of wood varied inversely with the specific gravity {J^6), Although the heat conductivity of wood increases with the increase in specific gravity, the diffusivity (a measure of the rate of temperature change) decreases as specific gravity increases. In other words, the lighter woods will heat to a given temperature more rapidly than the heavier woods, although the heavier ones are better conductors of heat. Feihl {11) found that the rate of heating is related to the specific gravity of the total log (that is, the wood and the water). He reported that sinker logs require longer heating time than logs that float one-third out of water, and that logs that float one-half out of water require less time to heat than logs that float one-third out of water. In general, Feihl and MacLean agree heavier logs require longer heating. MacLean attributes the longer heating time to higher specific gravity of the wood; Feihl

Conclusions on Time Required The most common difficulty in heating veneer bolts and flitches is insufficient vat capacity. The single largest factor in the required heating time is the diameter of the bolt or flitch to be heated, with required heating time increasing directly with the square of the log diameter. Good estimates of the required heating time for unfrozen logs can be made from Forest Products Laboratory Report No. 2149 (20). FeihFs report (11) can be used to estimate the heating time for frozen and unfrozen wood. A special heating cycle may be appropriate if the color of the wood is important. 41

220

200

180

160

140

120 LEGEND: /—BASSWOOD 2-ASPEN, QUAKING 3-COTTONWOOD, WESTERN

100-

4-YELLOW - P'OPLAR 5- SWEETGUM 3

TUPELO

6-WALNUT, BLACK 80-

-

7-BIRCH,

YELLOW

8-MAPLE, SUGAR 9- OAK, NORTHERN RED 10-BEECH, AMERICAN 60-

~

II-OAK, WHITE 12 - HICKOR Y, SHA GBA RK

40V

0.30

0.35

O 40

0.45

0.50

SPECIFIC

0.55

0.60

0.65

GRAVITY M 145 394

Figure 16.—Favorable temperature range (area between heavy lines) for cutting veneer of hardwood species of various specific gravities. Points show favorable temperatures for the individual hardwood species indicated. The data apply to the rotary cutting of veneer Vs inch thick, of straight-grained wood, free of defects such as knots or tension wood (''soft streaks").

42

chambers. With a good system it is possible to keep the temperature in the vat to within 2° to 4° (l"" to 2° C) of the desired temperature. A system that works well with hot water vats is to pump the water from one vat to another. After heating one vat, the hot water is pumped to a second vat which has just been loaded with unheated logs. The process is repeated and the hot water goes from tank to tank. Only enough heat is added to maintain the temperature of the water. Some mills strap a number of bolts or flitches together so they can be handled as a bundle. This practice is all right provided the flitches are separated by stickers to allow for circulation of the hot water to all wood surfaces. Another method that has been used commercially is to move the bolts or flitches progressively through a hot water or steam tunnel. This practice has the merit of straight-line production. To be successful, the bolts or flitches should be about the same diameter, the heating time should be long enough to ensure heating to the core of the bolts or center of the flitch, and the heating medium should circulate so that all surfaces of the bolts or flitches are heated the same amount. The temperature at the core of larger bolts should be checked by drilling a hole in the middle of the core as it comes from the lathe. The hole should be 1 to 2 inches deep and just large enough to accept the thermometer. The thermometer should be inserted immediately and the temperature recorded. If the cores of the large-diameter bolts are within 10° F (5° C) of the temperature in the vat, the smaller bolts will also be adequately heated. Proper heating will aid in producing tightly cut veneer of uniform thickness. Underheating will result in less tight veneer and may result in excessive handling splits and variation of veneer thicknesses. Overheating may cause large end splits in the bolts, spin-out, fuzzy veneer, and shelling of the grain. An example of the heating times required for bolts of different diameters is given below. More detailed information and tables are given

Hot Water Versus Steam Heating Producers of hardwood face veneer generally prefer hot water vats while many softwood construction plywood operations use steam chambers. Recently heating- by hot water has become the preferred method for all types of veneer plants. The rate of heating in the two systems is about the same, assuming that both are properly operating. The actual temperature throughout the vat can be controlled more accurately with water than with a steam-air mix. End drying is never a problem when heating in water vats, while it can be a problem in steam chambers if the relative humidity is not kept high. For workers, steam chambers are safer, as a fall into a hot water vat is generally fatal. In terms of manpower, one man with a lift truck can load and unload steam chambers for a large plant while two or more men are generally needed to operate hot water vats. Heating Suggestions with Hot Water or Steam Debark logs prior to heating. Heat in tree lengths or the maximum length possible. Segregate logs by diameter so the larger diameter logs can be given the needed longer heating time. Heat U.S. species at the temperature suggested in Appendix IV. For unfamiliar hardwoods, use the heating temperature indicated for the specific gravity of the wood (fig. 16). The heating tanks should be arranged for circulation of steam or hot water so heat can flow easily to all sides of the bolts or flitches. Steam should not impinge directly on the ends of logs, bolts, or flitches. The temperature in the vats should be recorded at half-hour or shorter intervals. Heating should preferably be controlled by automatic valves on the steam lines, regulated by heat sensors in the heating chamber. Temperature-sensing devices should be placed in several locations in the vats or steam chambers. These in turn should automatically control the heating of the water vats or steam

in (11,20),

43

Examples : Heating time as related to bolt diameter.— (Green 8-ft-long bolts with a specific gravity of 0.50, an initial temperature of 70° F (21° C), water or air-steam vat temperature of 150° F (66° C) and a final temperature at a 6-in. (15 cm) core of 140° F (60° C).) Bolt diameter Required heating time (in.) (cm) (h) 12 31.5 14 24 63 60

have hold-downs that will keep the logs under water during heating. The doors on steam chests or covers on water vats should be tight and preferably be insulated. In many commercial operations as much as heat is lost to the atmosphere as is used to heat the wood. Some Other Methods of Heating Veneer Logs and Flitches Some methods other than hot water or steam, or steam-air mixtures below 212° F, have been used on a small scale commercially or tried in the laboratory. These include heating in steam under pressure, electrically heating the wood, and forcing hot water or steam longitudinally through the wood.

Heating time as related to final core temperature,— (Green 8-ft-long bolts with a specific gravity of 0.50, an initial temperature of 70° F (21° C), water or air-steam vat temperature at 150° F (66° C), and various final temperatures at a 6-in. (15 cm) core.) Final Required Bolt core heating diameter temperature time (in.) (cm) (°F) (°C) (h) 24 63 140 60 60 24 63 120 49 34 24 63 100 38 22

Heating in Steam Under Pressure A few veneer mills heat veneer bolts in steam under pressure. This shortens the heating time as there is a bigger differential between the starting temperature of the wood and the temperature of the heating medium. However, there is no special change at a temperature of 212° F (100° C). In going from 210° to 220° F (99° to 104° C) the reduction in heating time is comparable to the time in going from 200° to 210° F (94° to 99° C). The disadvantages of a short heating cycle in steam under pressure include the very large temperature gradient from the surface to the core of the bolts and excessive bolt end splits.

Heating time as related to initial ivood temperature.— (Eight-ft-long bolts with a specific gravity of 0.56, a green moisture content of 80 pet, various initial temperatures, water or air-steam vat temperature of 150° F (55° C), and final temperatures at 4-in. (10 cm) core of 140° F (60° C).) Initial Required Bolt wood heating diameter temperature time (in.) (cm) (°F) (°C) (h) 12 31.5 0-18 27 12 31.5 40 4 21 12 31.5 70 21 16

Electric Heating Electrical methods have been used experimentally to heat bolts or flitches in an attempt to reduce the heating time required with water or steam. In one set of experiments, electrodes were placed at each end of a bolt or flitch and an electrical current sent through the wood by as high as 5,000 volts. Because the wood acted as a resistor, it was heated. This method is fast but has not been accepted commercially due to nonuniform heating. The electrical current follows the path of the least resistance, which may be wet streaks, cracks, or mineral streaks in the wood. These areas overheat and the other parts of the bolt or flitch are underheated. High frequency has also been used experimentally to heat veneer bolts. High frequency

Construction of Steam and Hot Water Vats Most heating vats or stegm chambers are made from reinforced concrete. The vats should be constructed so that good circulation of the heating medium can be attained. The steam pipes should not be placed so live steam will impinge directly on the logs ends. Steam blowing directly on the log ends overheats them and accentuates log end splits. If logs that float are to be heated in hot water, the tanks should 44

inally through the wood structure (35). The experimenters report that heating time was reduced to minutes and that satisfactory veneer was cut from bolts heated this way. The method requires that the wood be permeable and that a cap be attached to each bolt.

tends to overheat the wetter parts of the wood, and is much more expensive than heating in steam or water. Forcing Hot Water or Steam Longitudinally Through Wood Short beech veneer bolts have been heated experimentally by forcing hot water longitud-

VENEER CUTTING EQUIPMENT wear plates and mechanisms for taking up slack or play when it occurs. Similarly, it is desirable to have hydraulically operated dogs on slicers and hydraulically operated chucks on a lathe. Any tendency of the wood work piece to come loose in cutting is automatically corrected as hydraulic pressure resets the dogs or chucks. Another source of unwanted movement of the lathe or slicer is heat distortion. The use of A-frames with screw takeups on the nosebar casting is one method of correcting for this. Another desirable feature is a means of keeping the lathe or slicer at a uniform temperature during setup of the knife and pressure bar and during cutting. An added benefit is the reduction of blue stain caused by the reaction of iron or steel with wet wood. Keeping the knife and pressure bar warm reduces condensation and so reduces the staining. The heart of any lathe or slicer is the knife and pressure bar. The machine should permit rapid change of the knife and bar and easy adjustment of the clearance angle of the knife and the lead and gap between the knife and nosebar. If these adjustments are diflScult to make, the operator will make as few adjustments as practical. Consequently, the machine will produce poorer quality veneer than would be produced on easily adjustable equipment. Retractable dogs on slicers and retractable chucks on lathes permit secure holding of large wood flitches and bolts; when the dogs or chucks are retracted they permit continuous cutting to thin backboards or small-diameter cores. Recent development of the vacuum table permits fast loading of flitches and cutting to a thin backboard. However, the flitch back should

In selecting veneer cutting equipment, it is important to remember the forces involved in cutting. In one rotary-cutting study (^5), calculated loads were as high as 200 pounds per inch of knife and 500 pounds per inch of pressure bar. Pictures comparing early lathes and modern lathes indicate that experience has dictated the desirability of more rigid lathes. A lathe or slicer operator never has trouble because the equipment is too rigid, but excessive movement of machine parts is a common problem. If smooth, tight veneer of uniform thickness is to be produced, it is better to have a lathe or slicer that is stronger than necessary rather than to have one that is underdesigned. Some face veneer slicers are made so excessive pressure cannot be applied to the flitch. The knife and bar carriage is not fastened on the ways of some horizontal slicers. Thus, if the total force against the flitch exceeds the weight of the knife and pressure bar carriage, the carriage is lifted from the ways. Similarly, on vertical slicers, the knife and bar assembly is not held on the half-bearings that allow the knife to be offset on the upstroke of the flitch table. If there is excessive nosebar pressure, thin veneer sheets are produced and eventually the flitch will not clear on the upstroke. At this time, the knife and bar carriage will be lifted slightly from the half-bearing. When the carriage falls back, the noise alerts the slicer operator that he has too much nosebar pressure. There is no mechanism such as this for lathes. Excessive nosebar pressure can progressively build up until the bolt spins out of the chucks, the motor stalls, or some part of the lathe breaks. Any moving part on a lathe or slicer is subject to wear. Consequently, preloaded antifriction bearings are a good investment as well as 45

be wide, flat, and smooth to maximize the holding power of the vacuum table. The lathe drive is often a separate item allowing the purchaser to specify the type desired. Some options include a steam engine, a.c. motor with a speed changer, d.c. motor with a motor-generator set, and hydraulic motor. In all cases it is desirable to be able to increase spindle speed as the block diameter decreases to keep the cutting speed constant. Hancock and Hailey (26) describe lathe drives in some detail.

Cutting Action on Lathe and Slicer Similarities of Lathe and Slicer The knife and pressure bar are very similar on both the lathe and slicer and perform the same function. Cross sections of a lathe (fig. 17) and slicer (fig. 18) illustrate the position of the knife and bar in the two machines. Terminology used to describe the knife and pressure bar on lathes and slicer is shown in figure 19. The knife severs the veneer from the bolt or flitch. The knife bevel angle is about the same for knives on a lathe or slicer. The knife used on a lathe may be slightly more hollow ground.

BOLT

CHUCK KNIFE ANGLE

M 140 657

Figure 17.—Cross section of a veneer lathe having a fixed pressure bar.

46

M 140 656

Figure 18.—Cross section of a vertically operating veneer slicer.

Continuous cutting is advantageous because it means more production with a given cutting velocity, wider sheets of veneer, and a more uniform cutting condition. Full rotary cutting is approximately tangential to the annual rings and knots are exposed at their smallest cross section. In full rotary cutting, there is no impact at the start of cutting or tearofF at the end of cutting as may occur when slicing or cutting with a stay-log.

The pressure bar on both the lathe and slicer compresses the wood, with maximum compression ideally occurring just ahead of the knife edge. This compression reduces splitting of the wood ahead of the knife, reduces breaks into the veneer from the knife side, and forces the knife bar assembly against the feed mechanism, thereby helping control veneer thickness. For both the lathe and slicer, the pressure bar is, therefore, important in controlling the roughness, depth of checks, and thickness of the veneer. The slicer has a fixed nosebar while the lathe may have a fixed nosebar or a rotating roller bar.

Advantages of Slicer A main advantage of the slicer is that it permits sawing the log into flitches to present the most decorative grain pattern. As the veneer sheets are kept in consecutive order, figured veneer can be readily matched. Flitches can be heated with less danger of end splits developing than in comparable bolts being heated for rotary cutting. Sliced veneer is always cut from a flat surface, and most veneer is used on a flat

Advantages of Lathe Logs to be cut into veneer on a lathe need to be crosscut to the desired bolt length, but they do not need to be processed through a sawmill prior to cutting veneer. After roundup of the bolt, the lathe cuts a continuous strip of veneer. 47

KNIFE AND FIXED BAR

KNIFE AND ROLLER BAR M 144 168

Figure 19.—Knife and pressure-bar terminology. Symbol A B C D E F G H I J K

Alternate Term

Preferred Term Knife angle Knife bevel angle Clearance angle Lead Pressure bar bevel Gap Exit gap Nosebar compression angle Knife surface next to wood work piece ** Knife surface next to wood veneer ** Length of knife bevel

Knife pitch Knife sharpness angle Vertical opening * Pressure bar sharpness angle Horizontal opening * Restraint Bar angle

* Satisfactory for vertically operating lathe or slicer but is misleading for horizontally operating slicers. ** The term knife face is sometimes applied to J by knife manufacturers and to I by lathe operators. To reduce ambiguity, this terminology is suggested.

48

cut. Then as the veneer is cut, it separates into pieces the same width as the spacing of the knives on the back-roll. Since the scoring knives cut slightly deeper than the veneer thickness, they generally leave a light score mark on the tight side of the next piece of veneer. The back-roll lathe is, therefore, better suited for cutting thick container veneer than thin decorative veneer. All lathes are generally equipped with spur knives so veneer can be cut to one or more lengths while it is being peeled.

surface. By contrast, rotary veneer cut from a curved surface must be flattened for most uses. The disadvantage of cutting from a curved surface becomes more pronounced with thicker veneers cut from small-diameter bolts. Sliced veneer is cut with a draw motion across the knife, while rotary veneer is cut with no draw motion. Theoretically, the draw cut should aid cutting. However, recent experiments at the U.S. Forest Products Laboratory indicate that the effect of the draw cut on smoothness, tightness, and veneer thickness is relatively unimportant. Veneer as long as 16 feet is produced on a slicer while most rotary-cut veneer is 10 feet or shorter. The flitch on a slicer is backed by the flitch table while support for a veneer bolt may be provided by a backup roll.

Some General Comparisons of Veneer Cut on the Lathe and Slicer In general, the greatest yield is obtained by rotary cutting. Half-round, flat-slicing, or back cutting provide intermediate yields; and the least yield is obtained by quarter- or rift-slicing. The smoothest and tightest veneer can be produced by quarter- or rift-slicing, followed by rotary cutting ; the roughest and loosest veneer is produced by flat slicing, half-round, or backcutting. Differences in roughness are due to the effect of wood structure orientation (S9), While slicing and rotary cutting involve some differences and inherent advantages, good-quality veneer can generally be produced by either method. The quality of the end product is determined more by the log quality, the heating of the bolts or flitches, and the setting of the knife and pressure bar than by differences in the cutting method.

Advantages of Cutting with Stay-Log on Lathes The stay-log makes it possible to produce veneer on a lathe, similar in appearance to sliced face veneer (fig. 11-C). The advantages of stay-log cutting on the lathe are very similar to the advantages of slicing. The flitches can be selected for appearance of the grain and consecutive sheets can be matched for decorative faces. Sheets cut with the stay-log are generally wider than sheets cut on the slicer. For example, half-round veneer cut with a stay-log would probably be slightly wider than flat-sliced veneer cut from the same log. Veneer cut with a stay-log is taken from a curved surface in comparison with veneer that is sliced from a flat surface. Veneer cut with stay-log may be up to 10 feet in length.

Undesirable Movement of Wood and Machine Parts Knife and pressure bar settings are meaningful only if the wood is held securely in the lathe or slicer and if the machine parts have a minimum of play.

Back-Roll Lathe A modification of the rotary lathe is the back-roll lathe (fig. 20). It cuts the veneer ribbon to preset widths and so replaces a clipping operation. This special type of lathe has ways that carry the knife-bar head-blocks extended out on the log side of the lathe. On the extended ways, a frame is mounted to carry the back-roll. The entire mounting is fed toward the log by feed screws at the same rate at which the knife is fed. Knives mounted radially in the back-roll make an impression into the veneer bolt slightly deeper than the thickness of the veneer being

Undesirable Movement of Wood on Lathe Bolts are held by chucks in a lathe. In general, the larger the chucks the more securely the bolt is held. The chucks transmit the torque needed to cut the veneer and also must resist the tendency of the bolts to ride up on the knife. The spurs on the chucks should, therefore, be designed not only to transmit power to turn the bolt but also to keep it from shifting from the spindle center. The best spur configuration is not well established. Some mills prefer half 49

SIZED VENEER

M 140 658

Figure 20.—Back-roll lathe.

circles; others, star-shaped spurs and a ring around the circumference of the chuck. In practice, the spurs sometimes become battered and bent and may collect wood debris. For best performance, they should be in their original shape and clean. The chucks and spindle ends should be tapered for a positive secure fit. The pressure used to set the chucks in the bolt ends depends on the wood species, heating, and chuck size. Generally, enough pressure is used to indent the spurs at least three-fourths their length into the bolt ends. Square-cut bolt ends allow a more uniform grip than bolts that are end trimmed on a bias. The wood in contact with the spurs receives fluctuating loads during cutting, which may cause the bolt to become loose in the chucks. On older lathes, the operator must watch for this and further indent the spurs if any looseness of the bolt is observed. Newer lathes have hydraulic chucking. A relatively high pressure is used to set the chucks and then a lower pressure is maintained hydraulically to insure the spurs remain seated during cutting. If too

high hydraulic end pressure is used during cutting, the wood bolt may bend when it reaches a small diameter. Another modern solution to holding the bolts more securely is the use of retractable chucks. Larger chucks and spindles hold the bolt at the start of peeling ; they are retracted during peeling, allowing smaller inner chucks and spindles to hold and drive the bolt until the final core diameter is reached. A modification of this is sequentially retractable chucks such as 5-inch (13 cm) inner chucks, with one 8-inch (20 cm) outer chuck on one end and one 12-inch (30 cm) outer chuck at the other end. The bolt is first driven with the 12-and 8-inch chucks. At a bolt diameter of about 14 inches (35 cm), the 12inch chuck is withdrawn and the bolt is then driven with one 8- and one 5-inch chuck. At a diameter of about 10 inches (25 cm), the 8inch chuck is withdrawn. Cutting is continued with the two 5-inch chucks driving the bolt to the final core diameter. To obtain maximum recovery, bolts are turned to as small a diameter as practical. The 50

bolt is loaded as a beam by the knife and pressure bar. Its resistance to bending is directly related to the cube of the radius of the bolt. At small bolt diameters, an unsupported bolt bends in the middle away from the knife. The bolt becomes barrel-shaped and the veneer ribbon wrinkles in the middle. To overcome this problem, backup rolls have been built to support the bolt during cutting. Some early backup rolls operated with a fixed pressure against the bolt. But this caused problems. The cutting force fluctuates during peeling, and a fixed pressure against the bolt surface sometimes increased rather than reduced bowing of the bolt. Improved backup rolls fix their position geometrically to keep the bolt cylindrical. One method of doing this is a servo-system with a follower at the end of the block that signals adjustments of pressure on the backup roll. Another method (22) is to have this backup roll positioned mechanically by the feed mechanism so the bolt remains a cylinder. When properly made and operated, backup rolls permit cutting bolts 8 feet long (2.44 m) to a final core diameter of about 4 inches (10 cm).

The wear problem with feed screws is greatly reduced by a ball feed screw drive. Motion of the carriage for the pressure bar and knife is obtained by ball bearings turning a ball screw. This movement by rolling friction means less wear than for sliding friction with an acme screw and nut. Most production lathes develop some play between the knife frame and the bar frame. The amount of movement depends on the looseness in the lathe and the amount of pressure exerted against the bar during cutting. To detect and correct for this play, dial gages should be mounted at each end of the lathe with the gage on the knife frame and the sensing tip against a bracket on the bar frame. These gages should be zeroed after setting the gap or horizontal opening. Any play will show on the gages as a reading other than zero and the original gap or horizontal opening restored by adjusting the nosebar until the gages read zero. Walser (67) describes a method to preload the pressure bar assembly to improve accuracy when setting the veneer lathe. Play can also affect the lead or vertical opening. This is less common than play in the gap or horizontal opening. Again, dial gages can be mounted to detect and guide correction of the

Undesirable Movement or Play in Lathe Machine Parts

play.

All movable parts must have some clearance, and wear increases this clearance. Many lathes have built-in methods of taking up slack as wear progresses. However, it is not uncommon to find that production lathes have developed excessive wear and looseness or play in the mechanism. Some specific areas to check are spindle sleeves and bearings, feed screws, headblock or knife-angle trunnions, nosebar eccentric, and blocks under screws used to change the lead (vertical adjustment) of the pressure bar. The greatest wear is likely to be in the spindle sleeves and bearing, with the next largest amount in the feed screws and movable parts of the nosebar assembly. Some modern lathes minimize these problems by using preloaded roller bearings for the spindles and an air cylinder to keep the knife bar always against one side of the feed screw. In addition, some lathes have replacable wear surfaces for the ways.

Other things being equal, the greater the overhang of the spindles the more spring in the cutting system. This is most noticeable when short bolts are cut on a long lathe. If both short and long bolts are to be cut on the same lathe, the lathe should be equipped with spindle steady rests.

Spindle Overhang

Heat Distortion of Lathe

Bolts that have been heat-conditioned prior to cutting may cause the knife and pressure bar to distort. It is generally agreed that heating causes the knife to rise in the middle, decreasing the lead. Heat may cause the pressure bar to drop or move in a horizontal plane, depending on the lathe. On some lathes, one method of correcting for these changes is to adjust the pull screws on the A-frame built over the pressure bars for this purpose. A better solution is to heat the knife and pressure bar to the expected operating condition prior to 51

the final fitting (setting) of the knife and bar. Some lathes have had heating elements built in them to prevent heat distortion. Another good practice is to store sharpened knives in a warm area so they are at the same temperature they attain during cutting. Feihl and Godin {H) suggest heat distortion can also be controlled by continuous cooling of the knife bed and the pressure bar bed. However, they and others indicate heating the knife and bar works better than cooling, particularly for long lathes.

Undesirable Movement or Play in Slicer Parts Play can develop in all moving parts such as feed screws, offset mechanism, flitch table ways, and knife-bar carriage ways. Most modern slicers have means of taking up slack in these parts. A regular maintenance schedule should be followed. Feed by Pawl and Ratchet

Some slicers advance the knife by a pawl and ratchet for each stroke. This is highly accurate providing the same number of teeth are advanced each stroke, there is little play in the feed mechanism, and there is no overtravel of the carriage. The number of teeth advanced each stroke should be checked several times before and during actual cutting. The brake on the shaft which advanced the knife each stroke should be adjusted so there is no overtravel.

Undesirable Movement of Wood on Slicer The wood flitch is generally held against the bed on a vertical or horizontal slicer with dogs. In some vertical and all horizontal slicers, gravity helps hold the back of the flitch against the flitch bed. However, in the most common vertically operating face veneer slicers, the flitch is cantilevered from the bed and dogging is very important. Heated flitches may be bowed or twisted. Very often this bow or twist can be removed by forcing the flitch flat against the flitch table and dogging it securely. Here oversized dogs are useful at the start of the cutting. A recent development has been retractable dogs, which are extended for maximum holding power at the start of slicing and then automatically retracted when the slicing cut approaches the dogs. Older slicers had the dogs set by screws. After intermittent cutting, the flitch would often become loose, so the slicer would have to be stopped and the dogs reset in the wood. Modern slicers have hydraulic dogs which maintain good contact with the flitch throughout cutting. The hydraulic cylinders actuating the dogs have check valves to prevent the flitch from shifting during slicing. A recent practice is to glue valuable flitches such as walnut to an inexpensive backboard and then slice to the glueline. Special glues and gluing techniques are used to bond the hot wet flitches to the backboards. Another innovation is to hold the flitch against the table with a pattern of vacuum cups. The flitch back should be wide, smooth, and flat or the flitch may break loose from the table during cutting.

Feed to a Stop Plate

Some slicers feed by moving the previously cut surface against a stop plate. The surface of the flitch and of the stop plate must be free of splinters or other debris and the flitch must be advanced flush to the stop to produce veneer of uniform thickness. Offset on Vertical Face Veneer Slicers

The offset mechanism on modern slicers is hydraulically operated and does not generally require attention once the cam is set to retract the knife at the bottom of the stroke. The amount of offset is adjustable and should be large enough to insure clearance of the flitch on the upstroke. Excess offset should not be used as it may induce slight vibration to the knife. The knife and bar carriage pivot on half bearings for the offset. Since the half bearings are not held at the top, if the flitch fails to clear on the upstroke, the knife bar carriage may be lifted from the half bearings. Similarly, high nosebar pressure cannot be used without danger of unwanted movement of the knife carriage on the half bearings. As with the lathe, it is desirable to have dial gages mounted at each end of the slicer with the gage on the knife frame and the sensing tip against a bracket on the bar frame. The gages are particularly useful for returning to the 52

previous setting after the bar has been retracted to hone the knife.

the slackness in the lathe will be taken out by the time the veneer is wide enough to use. This veneer will be more uniform in thickness than veneer cut just after the pressure bar has been closed. Some slicer operators set to cut tight veneer and run into a gradual buildup of the flitch face with respect to the knife due to cutting veneer thinner than the feed. Eventually, the knife carriage will vibrate due to excessive pressure against the knife and pressure bar. The operator will then throw off the feed for one stroke, cutting a thick shim and continue to cut. This is poor practice as consecutive sheets cut after each shim are gradually changing in thickness. Better practice is to change the pressure bar setting (larger lead or gap) so that a constant full thickness veneer will be cut.

Heat Distortion of Slicer

Since face veneer slicers are generally longer than lathes, heat distortion of the knife and bar may be more of a problem. As on the lathe, the heated knife rises in the middle and the pressure bar drops. The pull screws on the A-frame on the casting holding the bar can compensate for movement due to heat. A better solution, and one that is built into modern slicers, is a means of heating the knife and bar prior to fitting them, and then keeping these parts continually warm. This not only greatly reduces any changes in the knife-bar setting due to cutting hot flitches, but also reduces condensate and the iron-tannate stain that results when iron or steel particles come in contact with wet wood.

Eflfect of Speed of Cutting on Veneer Quality When Knospe (SS) reviewed some of the veneer cutting literature in 1964, he concluded that cutting speed has a minimal influence on the quality of veneer. Recent unpublished work by A. 0. Feihl indicates that for practical purposes this is true within the speeds of about 100 to 500 feet (30 to 150 m) per minute. However, at least two studies (6,JfS) have shown that the strength of the veneer in tension perpendicular to the grain decreases with an increase in cutting speed. Lower strength in tension perpendicular to the grain is generally caused by deeper checks into the veneer. In addition, high cutting speed with wood species having a very high m.oisture content may increase the incidence of mashed grain and shelling. In summary then, the cutting speed does not seem to be a critical controlling factor for most veneer production. However, if optimum veneer tightness and smoothness are important, it may pay to use a moderately slow cutting speed. When slicing %-inch and thicker veneer, there may be a slight vibration of the slicer due to the impact at the start of the cut. Inclining the length of the flitch 3° to 5° from the long direction of the knife lessens this impact as the cutting starts at one corner of the flitch. A slower speed also reduces the impact at the start of each cut.

Dynamic Equilibrium on Lathe and Slicer Many have observed that the first sheets from a flitch on the slicer and the first few revolutions of veneer from a bolt on the lathe are thinner than the nominal knife feed. Hoadley (29) studied this phenomenon with a knife and pressure bar mounted on a pendulum dynamometer. He attributed the thin first cuts primarily to compression of the wood beyond the thickness of cut, followed by springback after the cut. With the same advance, both the compression and springback became progressively larger until a full thickness chip was produced. Hoadley called this dynamic equilibrium. Later studies on both an experimental and commercial lathe at the Forest Products Laboratory Hi) indicated that the thin cuts were due mainly to takeup of slackness in the lathe. Veneer cut from a small, more rigid experimental lathe reached full thickness quicker than veneer cut on a 4-foot-long commercial lathe. When the pressure bar was against the wood, it tended to force the bolt and knife in opposite directions. When the bar was retracted and the knife alone engaged the bolt, the knife and bolt were drawn together. As a result, opening the bar (for example, to clear a splinter) during cutting results in large changes of veneer thickness on a lathe that has slackness. In contrast, if the pressure bar is kept closed from the start of cutting, then much of 53

KNIFE AND PRESSURE BAR ON LATHE AND SLICER Type of Knife Selecting the Knife Most veneer knives are supplied the full length of the lathe or slicer. However, two- and three-piece knives are sometimes used with a special clamping arrangement so they can be ground and set as a unit. If one section is damaged, it can be replaced without replacing the entire knife. The hardness of the knife should be specified and can readily be tested. A soft knife can be easily honed and is tough but also wears rapidly. A hard knife is diflftcult to hone, is more likely to chip if it hits something hard, but holds a sharp edge much better. Most rotary veneer plants prefer a knife with a Rockwell hardness on the C scale of 56 to 58. Knives for face veneer slicers are often 58 to 60 on the Rockwell C scale. To keep as sharp an edge as possible when cutting low-density woods like basswood, a knife with a Rockwell hardness of 60 to 62 may even be used. Bevel angle, wedge angle, and sharpness angle all refer to the angle that results from the intersection of the two surfaces which form the knife edge. This and other terminology used with the knife and pressure bar are shown in figure 19. The knife bevel angle may vary from about 18° to 23°. The smaller the angle, the less the veneer is bent as it is cut and hence the tighter the veneer. In contrast, the larger the bevel angle the stiffer the blade and the better the edge can withstand impact. More care must be taken when grinding the smaller bevel angles as the knife tip is more likely to heat than when grinding a knife to a large bevel angle. An 18° bevel angle may be used to slice properly heated flitches of eastern redcedar while a 23° bevel angle is often used to rotary cut bolts of unheated softwoods. Many veneer knives are ground to a bevel angle of 20°or 21°. Some lathe and slicer operators prefer to measure the length of the knife bevel rather than the knife bevel angle (fig. 19). Some relations of knife thickness, knife bevel angle, and knife bevel length follow :

The knife represents the largest maintenance cost in cutting veneer and consequently it is worthwhile to use good purchasing specifications and take care in grinding and setting the knives in the lathe or slicer. What should be specified when ordering a knife for the lathe or slicer? The length of the knife and presence or absence of slots and their spacing will be determined by the equipment on which the knife will be used. Other factors such as depth, thickness, hardness, insert or solid, and the grind can be specified. In addition, the percent carbon and other components of the steel could be specified. However, the exact components of the knife steel are generally not published by knife manufacturers. As a result, most veneer plant managers deal with a reputable knife manufacturer and specify only the size, shape, hardness, and whether they want an insert or solid blade. An ideal knife should have maximum stiflfness, toughness, corrosion resistance, and wear resistance. The most common knife thickness for lathes is % inch (16 mm), and for face veneer slicers, % inch (19 mm). Thinner knives such as V2 inch (13 mm) are sometimes used on the lathe; they are less expensive but also less stifli. The European horizontal slicers may use a knife ^%2 inch (15 mm) in thickness, supported with a blade holder. In general, the veneer knife should be thicker when cutting thick veneer. When cutting thin veneer, thinner knives can be used if they are properly supported. The choice of an inlaid knife or one hardened throughout may depend on the end product. Hardwood face veneer is generally cut with an inlaid knife. The mild steel used for backing is stable and easy to grind. It can be readily drilled so that the knife can be held firmly when back grinding. The highly refined hardened tool steel insert is generally of highest quality for cutting wood. Knives that are hardened throughout reportedly may stand up better when cutting hard knots. They are sometimes, but not always, used in plants producing construction plywood. 54

Knife Thickness

Knife Bevel Angle

Inch 1/2 (0.500)

Degrees

5/8 (0.625)

3/4 (0.750)

18 19 20 21 22 23 18 19 20 21 22 23 18 19 20 21 22 23

it possible to cut abrasive wood longer between honings. A microbevel about 0.015 inch wide is often applied at the edge of the knife to make the included angle about 30° (10). If a tough knife could be made from tungsten carbide ground to a 20° included angle, this should be a good material for cutting wood containing silica or calcium carbonate crystals. The third method of knife wear is corrosion as described by Kivimaa (SI ) and by McKenzie and McCombe (^7). Acetic acid and polyphenols in some woods react with the steel knife and corrode it. This reaction makes the common blue iron stain that is so objectionable on face veneer as well as causing wear of the knife. Kivimaa (SI) found that knife wear was greatly retarded by putting a positive potential on the wood work piece and a negative potential of 1,500 volts on a planer knife. Later at Madison we put a positive charge of 300 volts on a rigid pressure bar on a lathe 4 feet (1.2 m) long and a negative charge on the knife. The charge greatly retarded blue stain from the knife as compared to the stain that developed on oak veneer when the lathe was stopped momentarily without a charge to the pressure bar. However, a shallow brown stain occurred on the veneer next to the knife. In addition, blue stain from the tool steel pressure bar became worse. When a stainless steel pressure bar was used, the blue stain was nearly stopped next to the bar but the shallow brown stain again occurred on the wood next to the tool steel knife. Ralph Scott, a research chemist at the U.S. Forest Products Laboratory, checked the wood next to the knife (negative terminal) and found it to be a strong base (pH 10 to 12). Apparently hydroxyl ions were released at the negative terminal and formed a base that turned the wood brown. Another difficulty with running 300 volts direct current from the pressure bar to the knife was that sap forced from the bolt ends made a short and the arc caused a big crater in the knife at this point. A third problem was that the stain was spotty over the 4-foot (1.2 m) length of veneer, indicating the electric current took the path of least resistance and so was not acting uniformly to reduce stain and knife wear. McKenzie and McCombe (Í7) successfully rotary-cut bolts 4% inches (12 cm) long with

Knife Bevel Length Inch 1.618 1.536 1.462 1.395 1.335 1.280 2.023 1.920 1.827 1.744 1.668 1.600 2.427 2.304 2.193 2.093 2.002 1.919

The ground surface is generally slightly concave to make the knife easier to hone. For the lathe, the recommended hollow grind is 0.002 to 0.004 inch (0.05 to 0.10 mm) while slicer knives generally have a hollow of 0.001 to 0.002 inch (0.025 to 0.05 mm). The flatter grind for a slicer knife means less chance for the flitch to rub against the heel of the knife and stain the wood. More hollow can be used on a lathe knife as the bolt surface curves away from the ground surface of the knife. However, the hollow should not exceed 0.004 inch (0.10 mm) as this weakens the knife edge. While the details of the knife bevel can be changed by grinding at the veneer producing plant, the knife should be ordered as it will be used to eliminate an extra grinding. Knife Wear Knife wear apparently takes place by three methods: Impact, abrasion, and corrosion. Impact and abrasion are mechanical phenomena while corrosion is chemical in nature. Mechanical impact is most obvious when a hard object, such as a small piece of gravel, chips the knife edge. Damage due to mechanical impact may also occur when the knife hits hard, unheated knots. Such knots may turn the extreme edge of the knife. Woods containing 1 percent or more of silica or calcium carbonate are abrasive and rapidly wear a rough edge on a veneer knife. Use of a tough tool steel rather than a brittle steel may help reduce the damage due to mechanical impact. Use of a microbevel (10) or back bevel reduces the chance of damage due to impact and may make 55

the knife held at a negative potential of 60 volts with respect to the nosebar. They report that knife wear was reduced 60 percent. In spite of the difficulties in applying a positive electrical potential to the bolt or flitch and a negative potential to possibly both the knife and pressure bar, the method does look technically interesting. An alternative would be development of stainless knives than can hold an edge sharp enough for good veneer cutting.

which the grinding wheel traverses, the knife bed should be adjusted until it is parallel to the ways. To maintain even wear of the ways, the grinding wheel should traverse the entire length of the grinder even when grinding short knives. The surface of the knife that goes against the grinder bed must be checked for bumps or other rough spots that will prevent the knife from lying perfectly flat. If necessary, the back of the knife should also be ground to restore a plane surface. (See "Back Grinding.") Heat can cause metal to expand and deform. The grinder and knife should therefore be kept at as uniform a temperature as possible during grinding. An example of poor practice was a grinder set near a radiator. During summer the knife bed was straight. However, in winter with the radiator on, the grinder bed was heated on one side and warped enough to result in unsatisfactory grinding. Similarly, the water used to cool the grinding wheel and knife should be at room temperature and be recirculated. A stream of water with synthetic coolant should be directed against the grinding stone '¥1 inch ahead of where the stone contacts the knife edge during grinding. Godin (2i) considers overheating of the knife tip the most serious problem in grinding and lists four main causes: (1) Too heavy a cut; (2) inadequate cooling; (3) clogged grinding wheel; and (4) too hard a grade of grinding wheel. Heating is less likely to occur if the knife edge is pointed up and engages the grinding wheel first during grinding. A feed of 0.0003 to 0.0005 inch (0.0008 to 0.012 mm) is suggested for each complete traverse of the wheel. At the FPL we like to dress the wheel and use a very fine feed for the last one or two traverses of the sharpening. This helps give a fine surface. Some manufacturers polish the knife by multiple passes without feeding. The smooth edge reportedly aids good veneer cutting. Care must be used with this technique or the grinding wheel may rub, heat, and weaken the knife tip. Another cause of an irregular edge is dubbing at the two ends of the knife. The most likely causes are looseness in the grinding wheel spindle bearings, excessive end play, and slack in the feeding mechanism. However, even a

Grinding Veneer Knives The purpose of grinding is to restore a straight, sharp, tough edge. If these three requirements are kept in mind, they may help guide good grinding practice. In order to grind a straight edge, it is necessary to start with a rigid level grinder. The most satisfactory veneer knife grinders have a fixed bed for mounting the knife and a traveling grinding wheel. The abrasive may be a solid cup wheel or a segmented wheel. Some operators prefer the segmented wheel because it requires less dressing and replacement segments are less expensive than a new cup wheel. A magnetic chuck makes it faster to set the knife for grinding. A V-belt drive in place of gears reportedly reduces chatter marks on the knife. The knife bed as well as the ways on which the grinder moves must be rigid, straight, and parallel to one another. The ways are generally hand scraped for accuracy when the grinder is made. The ways should have wipers to keep them clean in use. The accuracy of the ways can be measured in the veneer plant by traversing them with a dolly holding a gage. A special telescope with a measuring crosshair is leveled like a transit and then sighted on the gage on the dolly. The dolly is moved along the ways and any deviation from a straight line can be recorded. If the ways are not straight, they must be straightened at the factory. After the ways have been determined to be straight, they are used as a reference to determine if the knife bed is straight and parallel to the ways. This can be readily done in the veneer plant by indexing with a surface gage, such as a dial indicator, from the grinding wheel carrier which moves on the ways. If the knifebed is not parallel to the ways on 56

grinder in good mechanical condition may slightly round the ends of the knife. This may not be a problem as the end inch or two of the knife generally does not engage the wood when cutting veneer on a commercial lathe or slicer. If it is important to have the knife straight to the extreme ends, then dummy knife sections 4 to 6 inches (10 to 15 cm) can be attached to the knife bed at the two ends and in line with the knife being ground. Sections of a discarded knife can be used for this. The dummy sections absorb the heavier cut at the start of each traverse of the wheel and the main knife is not dubbed at the ends.

knife develops a heavy wire edge, the grinding wheel can be stopped and the wire edge removed while the knife is still clamped in the grinder. A few more passes of the wheel will create a new fine wire edge that can be easily removed by honing. After the wire edge is removed, the edge is finished by lightly honing with a finetextured stone that has been stored in kerosene. More detailed suggestions for grinding and honing veneer knives are contained in Canadian Forestry Service Publication No. 1236 {2Í). Secondary Knife Bevels

When a sharp knife ground to a bevel angle of about 21° is first put in the lathe or slicer, it is easily nicked by a knot or other hard substance. These nicks are removed by honing the knife in place on the lathe or slicer. After several bolts or flitches are cut, the knife edge wears slightly and this, plus the honing, makes the extreme edge more resistant to damage. This condition is sometimes called a work-sharp knife. When examined under a microscope, the edge is seen to be slightly rounded so it is probably closer to 30° to 35° than to 21° at the extreme tip. Such a knife will remain sharp and do a good job of cutting for several hours if no very hard material is hit. For other steel knives used to cut wood, such as planer knives, the smaller the bevel or sharpness angle, the faster the knife wears. The rate of wear goes up much faster if the bevel angle or sharpness angle is less than 30° to 35°. This wear phenomenon is apparently the same for veneer knives. Realizing this, the veneer industry has long had a practice of putting a back bevel on the knife. This strengthens the knife edge and is commonly used with knives installed on core lathes for peeling unheated softwoods. Kivimaa and Kovanen {32), Feihl {10), and others have studied the use of a precision microbevel put on either side of the knife. They report that a second bevel can be honed on either or both sides of the knife, and that the final included angle of 30° or 35° with a microbevel 0.010 to 0.020 inch in width greatly improves the strength of the knife edge. At least one commercial grinder has a separate grinding wheel that can grind a microbevel at the same time the main bevel is being ground. Some slicer operators use a two-bevel knife.

Back Grinding

After a knife is used, it may wear unevenly on the side where the veneer passes through the throat between the pressure bar and the knife. It may also be bent by excessive local pressure as from a knot or chip buildup. This can be detected by placing a straightedge at a right angle to the cutting edge. If this surface is not flat, then grinding the side of the knife that goes next to the bolt or flitch will not result in a straight edge. The solution is to grind a flat surface on the veneer side of the knife. The grinder bed is tilted ^/2° to 3° toward the knife and the knife is ground to produce a bevel % to 1-% inches long. A magnetic chuck on the grinder facilitates this grinding. Otherwise, the knife body must be drilled and tapped not more than 12 inches apart so the knife can be mounted securely for back grinding. Some modern grinders are equipped with two grinding wheels so the face and back of the knife can be ground at the same time. Haning Knife

The knife should be ground only enough to obtain a thin wire edge the length of the knife. The wire edge is removed by careful honing with a stone on one side of the knife, then the other. The stone should be medium grain and medium to soft in hardness. The stone should be saturated with kerosene. Some operators use one stone and others use two stones, one on each side of the knife simultaneously. In either case, each pass of the stone cuts at the base of the wire edge and bends it away from the stone. After several passes, most of the wire edge will fall off. Honing is continued until all of the wire edge is removed. If a badly nicked 57

the template indicates level. The same adjustment is then made at the other end of the knife. If the span is short and the knife deep and stiflf, the knife height should be the same across the lathe. However, with longer knives, particularly those that have been ground so they are not so deep, the knife may sag in the middle. One way of checking this is to level a transit with a telescope about 20 feet (6 m) from the lathe and swing it from one end of the knife to the other. The knife edge should be in line with the crosshairs along its length. If the knife sags in the middle, it should be raised with the leveling screws near the center of the knife. Once the knife edge is true, some operators make scribe marks on the lathe so they can reposition knives with precision. Another method is to measure the extension of the knife from the top of the knife bed. To speed up knife changes, some lathes have knife holders. After grinding, the knife is preset to the desired height in the holder, and the holder quickly bolted in place in the lathe. Some plants in effect preset the knife by pouring babbit metal at the bottom edge of the knife after each grind. The depth of the knife is thus kept constant and the knife can then be placed on the height-adjusting screws without changing them. Sag in the knife can also be checked with a tautly stretched fine wire. If there is wear in the spindle bearings, the bolt will ride up during cutting, taking up the play. To compensate for this, the knife edge is sometimes set above the spindle centers the same amount as the play in the spindles. This results in the knife edge being at the spindle centers during cutting. After the knife is set to the spindle centers, the knife angle is adjusted. In general, the side of the knife that contacts the bolt is approximately vertical (tangent to the surface of the bolt). Such a knife is said to have an angle of 90°. If the knife leads into the bolt 2°, the knife angle is 92° and the clearance angle 2°. A lathe knife can also be set with a negative clearance. A knife angle of 89° means the knife has 1° negative clearance. Most lathes are built so the knife angle can be made to change automatically with the bolt diameter. The objective is to keep the width of the knife surface that rubs against the bolt

The main bevel is 19° and the second bevel is 21°. Grinding of the second bevel is continued until the length of the second bevel is about V2 inch. When cutting, this is the only part of the knife that rubs against the flitch, and so the two-bevel knife reduces stain. Some operators like the two-bevel knife and others do not. Setting Knife Information on setting the knife and bar in a lathe assumes that the knife frame and bar frame of the machine are in proper alinement with the center of rotation of the spindles. Similarly, it is assumed that the knife and bar ways on the slicer are level and perpendicular to the flitch ways. It is further assumed that there is a minimum of play in the moving parts of the lathe or slicer and that the machine parts are at the same temperature they attain in use. If these conditions are not met, the careful setting of the knife and bar on the static machine may be changed so much in the dynamic cutting condition that poor quality veneer will be produced. Feihl and Godin (15) describe methods of checking the basic alinement of lathes. Setting the Knife in the Lathe and Slicer A correctly ground flat knife with a straight cutting edge is the first requirement. If a knife holder is used, it must also be clean and flat. A clean, flat bed on the lathe or slicer is the second requirement. (If these conditions are not met, it is difficult or impossible to correctly set the knife.) The knife or knife and knife holder is then set on the two end adjusting screws. The clamping screws are tightened by hand so that the knife is flat against the bed but free to move. To this point, the procedure is the same for the lathe and the slicer. Setting the Lathe Knife

After the knife is resting on the two end adjusting screws on the lathe, the knife edge is raised until it is level with the center of the spindles. This can be facilitated by using a template consisting of an accurately machined wood block cut out at one end to one-half the diameter of the spindle. The cutout end rests on the spindle and the other end on the knife edge. The height of the knife is then adjusted until a spirit level on the back of 58

To prevent these problems, some lathe operators increase the angle of the knife until a corrugated veneer surface results. They then reduce the knife angle gradually until the corrugations disappear and use this knife angle for cutting. For best results, we recommend determining and recording the knife angles that are satisfactory and using an instrument for measuring this angle when the knife is set. Instruments for measuring the knife angle are described by Fleischer (19), Feihl and Godin (15), Fondronnier and Guillerm (21), and Dokken and Godin (9). While all are suitable, the French design (21) (fig. 21) and the Canadian design (9) are easily read. If the knives are all ground the same, they can be interchanged on a lathe or slicer without changing the knife angle or clearance angle. However, if the knives are ground so the bevel or sharpness angle is as little at 1-2° different, the cutting can be altered significantly. Consequently, we recommend the knife angle be

about the same when cutting a bolt of a large diameter as at a small diameter. For example, when cutting at a bolt diameter of 3 feet (91 cm), the knife angle may be 91° ; at a diameter of 6 inches (15 cm) the angle may be 89° 30'. The means of changing the knife pitch varies with different lathes. Feihl and Godin (15) describe several methods that can be used to properly set the pitch ways. The lathe manufacturers should be consulted for recommended procedure for use with their lathes. In general, lathe operators use less lead into the bolt (lower knife angles) when cutting lowdensity woods than when cutting thick veneer. For example, Fleischer (17) suggests a knife setting of 90° 30' when cutting Viö-inch (0.8 mm) yellow-poplar (low-density wood) and 90° 45' when cutting Mw-inch (0.8 mm) yellow birch (high-density wood). Fleischer shows a pronounced eifect of veneer thickness on the best knife setting. For Vmo-inch (0.25 mm) birch, he recommends a knife setting of 92°, for y32-inch (0.8 mm) 90° 45', for Mn-inch (1.6 mm) 90° 15', and for Vs-inch (3.2 mm) veneer 90°. These settings are for log diameter from 20 to 12 inches (50 to 30 cm). When the correct knife angle is being used, the knife side next to the bolt will show Vic to VH inch (1.6 to 3.2 mm) of bright rub below the knife edge. If the correct knife angle is not used, the veneer may show this. Too high an angle causes the knife or bolt to chatter and results in a corrugation on the veneer and the bolt surfaces. The waves are closely spaced with three or more waves per inch of veneer width. Too low a knife angle results in too much bearing on the knife, forcing it out of the ideal spiral cutting line. When the force on the knife builds up, it may then plunge into the bolt, resulting in thick and thin veneer with waves a foot or more apart. Some lathe operators use low knife angles, as the heavy bearing of the knife against the bolt tends to smooth the surface of the veneer. Lathe and knife manufacturers do not like this practice because the pressures on the face of the knife may become so great that the knife will be bent and the knife failure blamed on the knife manufacturer. Low knife angles also require more power for turning the bolt and cause more stain and wear to the lathe.

M 130 i)3il

Figure 21.—Instrument of French design for measuring the knife angle. It is held by magnets to the face of the knife, the bubble is centered, and the knife angle is read on the vernier.

59

into the veneer. It compresses the wood just ahead of the knife and so allows the knife to cut rather than split the veneer from the bolt or flitch. This helps control rough surfaces and checks into the veneer. By keeping a force between the knife carriage and the flitch or bolt, the pressure bar takes up slack in the machinery always in the same direction and so aids control of the veneer thickness. There are two common types of pressure bars —the fixed pressure bar and the roller pressure bar.

checked with an instrument after each knife change. Setting the SHcer Knife

Setting the knife in the sheer is similar to setting the knife in a lathe except that the position of the knife edge in a sheer is set by the extension of the knife from the bed. The sheer knife edge should extend above the knife bed just enough so the ground face of the knife clears the bolts that hold it against the knife bed. In other words, the knife should extend as little as possible and still make certain the flitch will clear. On vertical face veneer slicers, this distance is about 2 inches. Like the lathe knife, the slicer knife should rest on the two end adjusting screws. The knife is then brought against the bed and any sag in the middle is removed with the height-adjusting screws near the middle of the slicer. Since slicer knives are often longer than lathe knives, this adjustment is more critical on the slicer. A taut ñne wire can be used as a guide to determine sag in the knife or, if the pressure bar bed is known to be straight, it can be used as a guide. A pressure bar that has been ground uniform in thickness is brought up against the pressure bar bed. The bottom of the pressure bar can then be used as a reference to determine if there is a sag in the slicer knife. Once the knife edge is determined to be straight, the knife is bolted firmly in place and all of the adjusting screws are brought in contact with the bottom of the knife. The knife angle of the slicer is relatively easy to set compared to the lathe knife. Since all cutting is from a flat surface, the knife angle does not change with flitch diameter. Further, the knife must lead into the flitch so the heel of the knife does not rub hard against the flitch. Experimentally, we have found that a sheer knife angle from 90° 20' to 90° 30' (about V2° clearance angle) can be used to slice wood from Vioo to % inch (0.25 to 6.3 mm) in thickness from both low-density and highdensity woods. Like the lathe knife, the angle of the slicer knife should be checked with an instrument each time a knife is replaced.

Fixed Pressure Bar Two factors to consider when selecting a fixed pressure bar are its stability and wear resistance. The most common metals are tool steel, steinte, and stainless steel. The tool steel bar is relatively stable, machines easily, and is relatively inexpensive. A stellite bar is more expensive, harder to grind, and less stable. However, the stellite bar will wear many times longer than the tool steel. Stainless steel is easier to grind than stellite and, like stellite, does not stain the veneer. The fixed bar is generally ground to a bevel angle of about 74° to 78°. As the wood bolt or flitch approaches the fixed bar in the lathe or slicer, the wood is compressed along a plane 12° to 16° from the motion of the wood. When cutting ^As inch (0.9 mm) or thinner veneer from dense hardwoods, the bar should be ground to a sharp edge. The edge of the bar is generally slightly eased or rounded when cutting thicker veneer from low-density woods or woods subject to rupture on the tight side of the veneer from rubbing against the bar. Various researchers recommend an edge radius of about 0.015 inch (0.3 mm). But Fleischer (17) reports rounding the bar to Vs-inch (3.2 mm) radius did not improve the smoothness of western hemlock veneer and may be disadvantageous. Roller Pressure Bar The roller bar is the second major type of pressure bar. In U.S. practice, the bar is commonly of bronze, generally % inch (15.9 mm) in diameter if it is a single bar and %> inch (12.7 mm) in diameter if it is a double roller bar. The single roller bar is driven directly while the double roller bar is driven with a

Pressure Bar The pressure bar is important for controlling thickness, smoothness, and depth of checks 60

backup roll. Two advantages of the double roller type stand out: (1) The drive roller can be larger so there is less breakage of the rollers, and (2) the knife and pressure bar can advance very close to the chucks, permitting peeling to smaller diameter cores than with a single roller bar. The drive chain for a single roller bar may protrude up to 1 inch beyond the surface of the roller bar. Roller bars are generally lubricated with 1 percent vegetable oil mixed in water and introduced through holes in the cap that holds the bar.

position of the bar with respect to the knife is fixed if any two of the three openings are fixed. For example, if the lead and gap are set, this also automatically sets the exit gap. Which two are chosen for setting the knife and bar should depend on the ease with which the openings can be measured and on how the knife and bar can be adjusted on a specific lathe or slicer. Examples of how these three openings are interrelated for different veneer thicknesses and different settings are given in tables 8 through 11.

Comparison of Fixed Bar and Roller Bar The fixed bar is the simplest and most commonly used pressure bar. It is used exclusively on slicers and is by far the most common bar used to cut hardwoods on a lathe. The roller bar is more common in the United States for cutting West Coast softwoods and has occasionally been used to cut eastern softwoods and hardwoods. The fixed bar can be used to cut veneer of any thickness. The %-inch (15.9 mm) diameter roller bar cannot be set to cut veneer much thinner than Vio inch (1.6 mm). Most veneer peeled with the aid of a roller bar is used in construction plywood and is yi2 inch (2.1 mm) or thicker. In general, it is easier to set a fixed bar precisely than a roller bar. A major advantage of the driven roller bar is that it requires less torque to turn a bolt; this in turn means less spinout of the bolts at the chucks and less breakage at shake and splits in these bolts. Another advantage of the roller bar is that it pushes through small splinters that otherwise may jam between a fixed bar and the bolt and degrade the veneer.

Setting Fixed Pressure Bar on Lathe (by Lead and Gap) When the knife edge and the pressure bar edge are ground straight, it is much easier to set the bar. These two edges must be straight and as perfectly alined as posible for precision veneer cutting. All the precautions suggested under knife grinding should also be used when grinding a new edge on a fixed pressure bar. The bed for the bar and the nosebar cap should be clean and straight. The bar is inserted between the bed and the cap and the nosebar locking screw tightened just enough to hold the bar against the bed but loose enough so the bar can be moved without bending it. The bar should extend from the supporting casting only a minimum amount so it is a rigid as practical. After the knife is set, the bar is moved toward the knife with adjusting screws at the two ends of the bar until the bar is about %2 inch (0.8 mm) behind the knife edge. Setting Lead

The nosebar bed on most lathes has adjusting screws at the two ends that allow the entire bed to be raised or lowered, increasing or decreasing the lead of the nosebar edge with respect to the knife edge. The amount of lead (vertical opening) is adjusted primarily for the thickness of veneer being cut. Some lathe operators set the lead one-third of the thickness of veneer being cut. Fleischer (17) suggests there is a straight-line relationship with a lead of 0.0005 inch (0.12 mm) when cutting Moo inch (0.25 mm) and a lead of 0.030 inch (0.8 mm) when cutting Vs-inch (3.2 mm) veneer. Some settings using a variable lead that depend on veneer thickness are shown in table 9. Certain lathes made in Germany do not

Setting Pressure Bar The information on setting the pressure bar, like the information on setting the knife, assumes the lathe or slicer is in good mechanical condition with a minimum of looseness in moving parts. The knife, pressure bar, and surrounding metal parts on the lathe or slicer should be at the approximate temperature they will attain during cutting. Cross sections of the knife with a conventional fixed bar and a roller bar are shown in figure 19. Three openings between the knife and the bar are indicated—the lead, gap, and exit gap. With any knife-bar combination, the 61

Table 8.—Lathe settings with a fixed bar and a constant lead Feed (veneer thickness) In. 0.010 .032 .042 .0625 .100 .125 .1875 .250

Mm 0.25 .81 1.07 1.59 2.54 3.17 4.76 6.35

Lead In. 0.030 .030 .030 .030 .030 .030 .030 .030

Gap Mm 0.76 .76 .76 .76 .76 .76 .76 .76

In. 0.009 .029 .038 .056 .090 .112 .169 .225

Exit gap Mm 0.23 .74 .97 1.42 2.29 2.84 4.29 5.71

In. 0.019 .038 .046 .063 .095 .115 .168 .221

Mm 0.48 .97 1.17 1.60 2.41 2.92 4.27 5.61

I Fixed bar, knife bevel 20°, knife angle 90° (0° clearance), lead 0.030 in. (0.76 mm), and gap 10 pet less than feed.

Table 9.—Lathe settings with a fixed bar and a variable lead ^ Feed (veneer thickness) In. 0.010 .032 .042 .0625 .100 .125 .1875 .250

Mm 0.25 .81 1.07 1.59 2.54 3.17 4.76 6.35

Lead In. 0.005 .010 .012 .017 .024 .030 .043 .056

Gap Mm 0.13 .25 .30 .43 .51 .76 1.09 1.42

In. 0.009 .029 .038 .056 .090 .112 .169 .225

Exit gap Mm 0.23 .74 .97 1.42 2.29 2.84 4.29 5.71

In. 0.010 .031 .040 .058 .093 .115 .173 .230

Mm 0.25 .79 1.02 1.47 2.36 2.92 4.39 5.84

1 Fixed bar, knife bevel 21°, knife angle 90° (0° clearance), 1 ead changing with veneer thickness (13),, and gap 10 pet less than feed.

Table 10.—Lathe settings with a roller bar and a fixed lead ^ Feed (veneer thickness) In. 0.0625 .100 .125 .1875 .250

Mm 1.59 2.54 3.17 4.76 6.35

Lead In. 0.085 .085 .085 .085 .085

Gap Mm 2.16 2.16 2.16 2.16 2.16

In. 0.056 .090 .112 .169 .225

Exit gap Mm 1.42 2.29 2.84 4.29 5.71

In. 0.062 .094 .114 .167 .220

Mm 1.57 2.39 2.90 4.24 5.59

1 5/8-in.-diameter roller bar, knife bevel 20°, knife angle 90° (0° clearance), lead 0.085 in. (2.16 mm), and gap 10 pet less than feed.

Table 11.—Lathe settings with a roller bar and a variable lead ^ Feed (veneer thickness) In. 0.0625 .100 .125 .1875 .250

Mm 1.59 2.54 3.17 4.76 6.35

Lead In. 0.068 .075 .079 .089 .100

Exit gap>

Gap Mm 1.73 1.90 2.01 2.26 2.54

In. 0.056 .090 .112 .169 .225

Mm 1.42 2.29 2.84 4.29 5.71

' 5/8-in.-diameter roller bar, knife bevel 2V, knife angle 90° (0° clearance), gap equal exit gap equal 10 pet less than feed.

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In. 0.056 .090 .112 .169 .225

Mm 1.42 2.29 2.84 4.29 5.71

have a lead or vertical-opening adjustment. This distance is built in the lathe to be about 0.020 inch (0.5 mm). It coincides with the lead suggested by Fleischer for cutting veneer about Vu inch (2 mm) thick. All agree that the bar edge should be set above rather than at or below the knife edge. It is also generally agreed that the distance the bar is set to lead the knife must be the same at all points along the knife edge. The common method of checking this opening is to insert a feeler gage of the proper thickness in the lead (fig. 22) between the knife edge and the bar. When the feeler gage is perpendicular to the ground face of the knife, the opening is the same as the thickness of the gage. After the bar is set this way at both ends, it should also be checked at other intervals along the knife. Some lathes have push-pulls so the bar can be warped locally to make the lead or vertical opening uniform across the lathe. However, if the knife and bar are ground straight and the knife bed and bar bed are also straight, any local adjustment of the lead should be minimal. Use of a feeler gage may

slightly nick the blade. It is, therefore, good practice to lightly hone the knife after setting the lead. Setting the Gap

The second bar adjustment is the gap or horizontal opening. This is the distance from the leading edge of the pressure bar to a plane extended from the ground surface of the knife. Some experienced operators like to bring the edge of the bar to the same plane as the knife edge. Then by feeling with the thumb, they can tell if there are any spots where the bar is ahead or behind the knife edge. These local spots are brought in line with the push-pull screws at the back of the bar. Once the bar is "fit" to the knife, it is retracted to give the desired opening or gap and clamped. We prefer to use instruments to help make this critical setting. Two such instruments are described by Fleischer (19) and Feihl and Godin (15). Both are essentially dial-micrometer depth gages that use the ground surface of the knife as a reference and measure to the edge of the bar. To automatically position the measuring pin, Fleischer (,19) suggests that

M 139 942

Figure 22.—Adjusting the lead of the pressure bar with a feeler gage. The lead of the bar is moved until a feeler gage of the desired thickness is at a right angle to the face of the knife when the gage is inserted in the opening between the knife and the bar.

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M 139 940

Figure 23.—Measuring the gap between the knife and pressure bar edge. Measurements are chalked on the nosebar casting and any deviations greater than 0.001 inch removed with the push-pull adjustment of the bar.

rected by the push-pull screws at the back of the bar. For accurate cutting, the gap should be within ±0.001 inch (0.025 mm) at all positions. The actual value of the gap will depend on the thickness of veneer and somewhat on the species being cut. A figure commonly quoted is for the gap to be 20 percent smaller than the thickness of veneer being cut. Experiments at the U.S. Forest Products Laboratory indicate this results in high compression of the wood by the nosebar. It would only be used when cutting thin veneer from an easily compressible species that is resistant to damage by scraping the nosebar over the wood surface. It is possible the 20 percent figure may have been derived from measurements on lathes that had some looseness or play and not correcting for the looseness. When the pressure bar is set as described earlier we have found a compression of 10 to 15 percent to be good for cutting veneer from i/io to Vs inch (1.6 to 3.2 mm) thick. Twenty percent compression may be satisfactory when

the instrument rest on the top of the pressure bar and on the ground face of the knife (fig. 23). While one man holds the instrument in contact with the knife and the movable sensing pin against the leading nosebar edge, a second man advances the bar until the correct gap or horizontal opening is indicated. When advancing the bar, the adjustment should always be made to take the play out of the adjusting screws. First the two ends are checked. If they do not indicate the same opening, then they must be brought to the same position with the adjusting screws at each end of the pressure bar bed. Assuming the knife and bar were ground straight and were not warped when mounted on the lathe, the gap should now be the same across the lathe. However, since this is one of the critical lathe settings, we routinely check the opening or gap at 4-inch intervals along the bar. The value of each reading is chalked on the casting holding the pressure bar. Any gradual bends or humps in the bar are then plainly visible. Local deviations are cor64

cutting thinner veneer. Higher compression (smaller gap or horizontal opening) may result in tighter veneer; it may also cause the veneer to be thinner than the knife feed and cause damage to the tight side, such as shelling of the grain on susceptible species like western redcedar and redwood. The advantage of using instruments to measure the knife angle and pressure bar settings is that the setup can be readily duplicated. When experience shows that a certain setting is good for cutting a given thickness of veneer from a given species at a given temperature, then the information should be recorded and the exact processing conditions duplicated when this item is produced again.

gap or horizontal openings between the knife and bar to be between 0.029 and 0.032 inch (0.725 and 0.800 mm). In effect, the bar is then compressing the wood just ahead of the knife edge 0.004 to 0.007 inch (0.1 to 0.175 mm). Face veneer producers sometimes set the bar to compress the wood only 0.001 or 0.002 inch. When slicing thicker veneer such as Vs (0.125) inch (3.25 mm), the bar may be set to leave a gap of 0.115 inch (2.95 mm), or 0.010 inch (0.25 mm) less than the feed. As with the lathe, more compression (slightly smaller openings) can be used when cutting low-density woods than when cutting highdensity woods. Setting Roller Pressure Bar on Lathe (by Lead and Gap) The roller bar is most commonly used when rotary-cutting western softwoods ¥12 to ^AG inch (2.1 to 4.8 mm) in thickness. It is not suitable for cutting veneer thinner than Mc inch (1.6 mm). The reason is that the pressure should be applied against the bolt just ahead of the knife edge. When cutting veneer thinner than i/iß inch (1.6 mm), a roller bar set at a fixed bar lead would over-compress the veneer after it is cut by restricting the throat between the roller bar and the knife. This restraint may cause the veneer to jam and break. In industry practice, %-inch- (15.9 mm) diameter roller bars are generally set with a lead of V16 (0.062) inch (1.6 mm) or more. From theoretical considerations and laboratory experiments, Feihl, Colbeck, and Godin (13) recommended a roller bar lead or vertical gap of 0.085 inch (2.16 mm) when cutting Douglasfir Vio to 1/4 inch (2.54 to 6.35 mm) in thickness. They also describe an instrument for measuring the lead of a roller bar. Lathe settings for several veneer thicknesses using a fixed lead are shown in table 10. The gap is set much the same as with a fixed bar. That is, good results are obtained by compressing the wood ahead of the knife about 10 to 15 percent of the veneer thickness. This varies with species, wood density, and veneer thickness as discussed under the fixed pressure bar. The gap or horizontal opening can be set and checked with a depth gage reading to 0.001 inch (0.025 mm).

Setting Fixed Pressure Bar on Slicer (by Lead and Gap) The slicer bar is ground and set by the same method as described for setting the fixed bar on the lathe. The difference comes in the actual value of the settings. On the lathe, the lead or vertical opening may be set at various openings such as 0.010 inch (0.25 mm) for %o-inch (0.5 mm) veneer to 0.030 (0.75 mm) for Vs-inch (3.2 mm) veneer. On the slicer, the lead or vertical opening is generally set at about 0.030 inch (0.75 mm). We have cut veneer of satisfactory quality from Vioo to % inch (0.25 to 6.3 mm) in thickness with this lead. A smaller lead such as 0.020 inch (0.5 mm) can be used when cutting i/is-inch (0.9 mm) and thinner veneer. However, this smaller lead may result in more splinters breaking off at the end of the cut and more chance that splinters will become jammed between the knife and bar, causing rub marks on the veneer. Not as much pressure can be applied with the nosebar on a vertical operating face veneer slicer as can be applied on a lathe. The knife and pressure bar rest on half bearings, permitting the knife and bar to be offset to clear the ñitch on the upstroke. If the pressure bar is set for excessive pressure against the flitch, it will cause the knife and bar carriage to rock on the half bearing; the result is poor veneer and possibly damage to the slicer. When slicing ^As-inch (0.036-in.) (0.9 mm) veneer, we have found the range of satisfactory 65

Setting Roller Pressure Bar (By Gap and Exit Gap) Collett, Brackley, and Gumming (7) suggest that lathes having a roller bar be set by gap and exit gap. They comment that, for veneer thicknesses from Mo to % inch (2.54 to 6.35 mm), the literature indicates that the gap and exit gap can be the same. This simplifies the recordkeeping as only one value needs to be recorded for each veneer thickness of each species. They recommend use of a depth gage to measure the gap and a feeler gage to measure the exit gap. The amount of compression they suggest at both the gap and exit gap is 10 to 20 percent of the veneer thickness. Table 11 shows some settings where the gap and exit gap are the same. Setting Fixed Pressure Bar (By Lead and Exit Gap) Lead and exit gap are suggested by Fondronnier and Guillerm (21) as the openings to be measured when setting a lathe with a fixed bar. They list the lead changing in a regular manner with veneer thickness as follows : Veneer Thickness (in.) 0.039 .078 .118 .157 .197 .236

(mm) 1 2 3 4 5 6

the bar. They report that the method eliminates play in the horizontal mechanism; provides a direct measure of pressure against the bar and so gives the operator good control of the setting; and finally that the veneer produced was equal in quality to veneer produced with a bar set to fixed stops. The method is being tried commercially. Possible Ways to Generalize Setting of Lathe and Slicer Optimization of veneer peeling or slicing may require different knife and pressure bar settings for each specific cutting situation. However, it would be convenient to have one knife setting that could be used to cut veneer of any species into any thickness from ¥32 to % inch (0.8 to 6.3 mm). Similarly, it would simplify pressure bar settings if one lead could be used for cutting all veneer. From an examination of the literature and our own experience, it is possible to do this. Generalized Knife Settings The knife settings specified in figure 24 are broadly applicable, and may be particularly valuable as a starting point for cutting unfamiliar species. The knife should be ground to a 21° bevel with 0.002-inch (0.05 mm) hollow grind. The knife angle can be set to 90° 30' or, stated another way, with %° clearance angle. For lathes having an automatic change of knife angle with change in bolt diameter, the knife can be set at 90° 30' when it is 12 inches (30 mm) from the spindle center. This knife setting can be used to cut veneer V¿2 to % inch (0.8 to 6.3 mm) in thickness from any species on the slicer or on the lathe from bolt diameters of 24 inches (60 cm) to a 6-inch (15 cm) core.

Lead or Vertical Opening (in.) (mm) 0.020 0.5 .024 .6 .028 .7 .031 .8 .035 .9 .039 1.0

They suggest the exit gap should be 10 to 20 percent less than the veneer thickness. Further they recommend that feeler gages be used to measure both the lead and exit gap. Setting Gap by Pressure Rather Than to Fixed Stops During rotary cutting of veneer, the force against the pressure bar may vary as much as from 10 to 500 pounds per lineal inch (178 to 8,900 kg/m) of contact with the wood (45), Feihl and Carroll (12) adapted a research lathe to allow the bar to float and maintain the gap by pressure delivered by a cylinder and piston acting against the bar frame. In other words, they set the lead to stops but allowed the gap to be determined by the force against

Generalized Setting of a Fixed Pressure Bar The pressure bar should be ground to have an included angle to 75°. This results in the woodwork piece being compressed along a plane approximately 15° from the cutting direction. The edge of the bar that contacts the wood should be rounded to an edge having a radius of about 0.015 inch (0.3 mm). 66

KNIFE AND FIXED BAR

KNIFE AND ROLLER BAR M 144 168

Figure 24.—Knife and pressure bar settings of general applicability are specified in terms of the diagram. These settings might be used to cut veneer from 1/32 to V^. inch in thickness. Symbol A B C D

Symbol

Generalized Settings

E F

Knife angle = 90° 30' Knife bevel = 21° with 0.002-inch hollow grind Clearance angle = 30' (i/2°) Lead = 0.030 inch for fixed bar or 0.085 for %-inch-diameter roller bar

G H

67

Generalized Settings Pressure bar bevel = 75° Gap = 90 percent of veneer thickness (10 pet compression) Exit gap = Gap = 90 percent of veneer thickness (roller bar) Nosebar compression angle = 15° (fixed bar)

The lead of the fixed pressure bar ahead of the knife edge can be 0.03 inch (0.75 mm) for both the lathe and the slicer. The gap from the edge of the pressure bar to the plane of the ground face of the knife can be 90 percent of the thickness of the veneer being cut. Veneer Vs2 to V^ inch (0.8 to 6.3 mm) in thickness and of various species can be cut with these fixed pressure bar settings (fig. 24).

Summary of Generalized Lathe and Slicer Settings Suggested ''universal" lathe and slicer settings—listed in figure 24—are not optimum settings, but they should permit cutting veneer of moderate quality from any species into any thickness from V32 to % inch (0.8 to 6.3 mm). (The roller bar is not satisfactory for use when cutting veneer thinner than Vw in. (1.6 mm).) In general, excluding the extreme ranges of specific gravity, one species of wood acts much like another and the veneer cutting process does not change abruptly within the range of thickness from y32 to % inch (0.8 to 6.3 mm). The settings listed with figure 24 will generally result in a moderately tight cut. If tighter and smoother veneer is desired, smaller openings between the knife and pressure bar may be used. Lathes having automatic pitch adjustment could be set to have a knife angle of 91° at a bolt diameter of 36 inches (91 cm) and a knife angle of 89° 30' at a bolt diameter of 6 inches (15 cm). Ideally, the rate of change of the knife pitch should be greater at the smaller diameters. A smaller fixed pressure bar lead such as 0.020 or 0.015 inch (0.5 to 0.4 mm) can be used for cutting Vio-inch (1.6 mm) and thinner veneer.

Generalized Setting of Roller Pressure Bar The generalized settings for lathes with a roller pressure bar are for cutting veneer Vir, to % inch (1.6 to 6.3 mm) in thickness. The lead of the roller bar should be 0.085 inch (2.16 mm). That is, the center of the 5.8-inch- (15.9 mm) diameter roller bar should lead the knife edge by 0.085 inch (2.16 mm). The comparable figure for the fixed bar is 0.030 inch (0.75 mm) (fig. 24 and tables 8 and 10). An Alternate Generalized Setting of Roller Pressure Bar Collett, Brackley, and Gumming (6) describe setting a roller bar with the gap and exit gap equal. As with the rigid bar, a generalized setting would be to have the gap and exit gap both 90 percent of the thickness of the veneer being cut (fig. 24 and table 11).

Positioning Bolts and Flitches For maximum yield of rotary veneer, it is essential that bolts be chucked in the geometric center. If the bolts are chucked eccentrically as little as ¥2 inch, the recovery of veneer can be reduced significantly. H. C. Mason, an industry consultant, stated in 1972 that use of boltdiameter-measuring instruments and a minicomputer controlling a lathe charger to precisely center the bolt in the chucks, will result in at least a 7-percent increase in recovery of veneer for a typical Douglas-fir veneer plant. The way a flitch is mounted on the slicer table has little effect on yield, but it can aflfect the smoothness of the veneer (S9), An eccentric flat-cut flitch should be dogged with the pith toward the start of the knife cut. A quartered flitch should be turned 180° when the cut approaches the true quarter. These and related phenomena are discussed in detail in (39).

Generalized Setting of the Gap by Pressure Feihl and Carroll (12) report that pine veneer that is Vio to VG inch (2.5 to 4.2 mm) in thickness can be cut satisfactorily with the pressure on a floating roller bar of about 60 pounds per linear inch (1.070 kg/m) of bar contacting the wood bolt. They further conclude : ''It is not impossible that in some mills (when all species are fairly similar and veneer thicknesses are in the same range) it would be practical to use one pressure setting."

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CONVEYING AND CLIPPING VENEER A German machinery manufacturer recently announced a system to reel sliced veneer by first applying string to the ends of the veneer sheets as they come from the slicer. The string then "leads'' the veneer onto the reel where it can then be stored before unreeling into a dryer.

Conveying Veneer from Lathe As veneer comes from the lathe, it may be manually pulled out on a table, but more generally it is moved to long trays in line with the clippers or is reeled. The tray system is most common in both softwood and hardwood plants. As the veneer comes from the lathe, a short tipple directs unusable veneer to a waste conveyor. Usable veneer is directed into one of the trays with belts synchronized to the lathe speed. After one tray is full, the veneer is broken or cut, and the veneer directed to another tray. This must be done carefully to prevent the veneer ribbon from being folded and split. The second mechanical means of conveying veneer from the lathe is with a reel. The reel system works best with Vs-inch (3.2 mm) and thinner hardwods cut from sound bolts. Like the tray system, the first unusable veneer is directed to a waste conveyor. Then the usable roundup is collected on a short tray or table. Finally, when a sound ribbon veneer comes from the lathe, it is tacked to a reel and the veneer reeled up as it is peeled. The speed of the reel is synchronized with the lathe. The veneer is reeled with the loose side out. Combination tray and reeling is popular with some plants peeling species like lauan. The better grades are cut into thin face stock and reeled. Lower grades are cut into thicker core stock and conveyed on trays.

Clipping Green Veneer Veneer stored on trays is fed to one or more clippers. In a typical installation, with six trays from a lathe, three trays would feed to one clipper and the other three to a second clipper. A modern clipper has some sensing and measuring device so veneer can be clipped to nominal 4-foot (1.2 m), 2-foot (0.6 m), or random widths. Random widths may be generated when defects such as knots and splits are clipped from the veneer ribbon. An accurate sensing device coupled with the clipper soon pays for itself by greater yields of usable veneer. The green veneer is then sorted by widths, grades, and possibly by sapwood and heartwood in preparation for drying. Reeled veneer is stored in racks and unreeled just ahead of the clipper. The clipping operation is much the same as that described for veneer stored on trays. One limitation of reeled veneer is that, if it is cut from hot bolts, it should be clipped before the veneer cools and sets in a curved shape. Flitches of green sliced veneer sometimes have defects clipped out or are trimmed before drying. Packs about Vi-inch (6.3 mm) deep are clipped together as a book. The green clipping saves drying of material that will not be used.

Conveying Veneer from Slicer It is important to keep the sliced veneer sheets in consecutive order. In many plants, two men turn the veneer over as the sheets come from the slicer and stack them consecutively with the loose side up. In some cases, a short conveyor takes the veneer from the slicer to a position where it is more convenient to stack it. Some European plants automatically convey the sliced veneer to a veneer dryer. Dryer capacity should be sized for the wood veneer species, thickness, and production rate of the slicer.

Clipping Dry Veneer Veneer on trays or on reels is sometimes fed to the dryer in a continuous ribbon. As the veneer comes from the dryer, it is clipped to size. This system reportedly results in less waste and split veneer. One dryer manufacturer states that drying of a continuous ribbon will result in at least a 4-percent increase in recovery of dry veneer.

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VENEER DRYING An essential part of the veneer-producing process is to dry the veneer. The amount of this drying varies widely. Products that require a minimum of drying—such as bushel baskets and fruit containers—may bring the veneer below a moisture content at which it will mold (about 20 pet). On the upper extreme is drying of softwood veneers that are to be glued with a phenolic hot-press glue, in which case the veneer must be 5 percent or lower in moisture content. In between are such products as decorative face veneer, generally dried to 8 to 10 percent moisture content, and commercial hardwood veneers that are to be glued with a urea glue, in which case 6 to 8 percent moisture content is desirable in the veneer. In all cases, a major criterion is to dry the veneer at the lowest total cost. Because most veneer operations are set up in a straight-line production system and the production from the lathe and slicer is very high, it is generally necesary to have a fast drying system. Dried veneer should: (1) Have a uniform moisture content; (2) be dried without buckle or end waviness; (3) be free of splits; (4) be in good condition for gluing; (5) have a desirable color; (6) have a minimum of shrinkage; (7) avoid collapse and honeycomb; and (8) have a minimum of casehardening. (Veneer is casehardened when the outer layers are in compression and the center or core is in tension.)

veneer sheets, which tend to dry faster than the bulk of the sheet. It may also be a factor in curly-grained or other figured veneer where at least partial end grain is exposed on the broad surface of the veneer. As these areas dry faster than surrounding straight-grain areas, they can cause stresses and buckling in the veneer sheet. The difference in drying rates between radial and tangential surfaces is small but may show up. Quarter-sliced veneer will take slightly longer to dry than rotary-cut veneer of the same thickness, and flat-sliced veneer may dry slower on the near-quarter edges than in the flat-grain area at the center of the sheet. The moisture in the veneer naturally affects the total drying time, as expressed in several ways. Veneer from butt logs may have higher moisture content than top logs. For example, the difference in moisture content of the heartwood of redwood from different logs may be as much as 2 to 1. Furthermore, the wetter heartwood veneer requires significantly longer drying time than drier heartwood of the same species. Comstock (8) indicates that density of the veneer may be another factor in total drying time. The denser wood heats more slowly than less dense wood and requires more total calories to heat and dry. The differences between the sapwood and heartwood may be factors with some species and not with others. Bethel and Hader (3) report that the sapwood of sweetgum will dry 25 to 30 percent faster than the heartwood of sweetgum. The difference is attributed to the difference in permeability of the sapwood and the heartwood. This same phenomenon has been observed at the U.S. Forest Products Laboratory when drying veneer of túpelo and other hardwoods like overcup oak. In contrast, Comstock (8) reports that drying time in a jet dryer does not depend on whether the veneer is heartwood or sapwood. Similarly, there is a lack of agreement on the effect of species on veneer drying. Fleischer (18) found that redwood and sweetgum heartwood dried at a slower rate than yellow-poplar heartwood. Bethel and Hader (3) also found differences in the drying of different species. Comstock (8) and Fleischer (18) indicate that veneer drying is controlled to a large extent by

Some Veneer Properties That Aflfect Drying Factors that affect drying of veneer include both the wood itself and the drying conditions. An obvious factor is the thickness of the veneer. Thicker veneers dry more slowly than thin veneers. A modification of this is variation in veneer thickness from the nominal thickness. Commercial %-inch (3.2 mm) veneer will often vary ±0.008 inch (0.2 mm) or more in thickness. The thicker portions of the veneer take longer to dry than the thinner portions and contribute to a nonuniform final moisture content. A second factor is the grain direction on the surface of the veneer. End grain dries several times faster than tangential (fiat) grain. Endgrain drying is significant at the ends of all 70

matched material dried in kiln. The least buckled will be veneer dried between flat hotplates. Temperature and drying time are factors that can affect the rate of drying. For example, Vg-inch (3.2 mm) heartwood of Douglas-fir dried at 250° F (121° C) may require 20 minutes in the dryer. The same kind of veneer dried at 320° F (160° C) may dry in 10 minutes. Increasing the drying temperature to 400° F (204° C) may reduce this drying time to about 6 minutes. Douglas-fir heartwood veneer has been dried in 2-I/2 minutes by using a drying temperature of 550° F (288° C). Such a high drying temperature may, however, lead to problems in gluing the veneer. Another factor which is universally agreed to affect the drying rate is the air velocity across the veneer surface. In loft drying, air movement is very slow from convection currents. Veneer dried in a kiln might be subject to air velocities of several hundred feet per minute. This higher air velocity, together with the higher temperatures used in the kiln, greatly accelerates the drying. Prior to 1960, most mechanical veneer dryers had air circulation either in the longitudinal direction of the dryer or across the width of the dryer. Typical air velocities in such dryers were about 600 feet (180 m ) per minute. Most mechanical dryers made after 1960 have the air impinging directly onto the face of the veneer through slots or orifices. The air velocity is in the range of 2,000 to 10,000 feet (600 to 3,000 m) per minute. This very high air velocity tends to break up any boundary layer at the veneer surface and greatly improves heat transfer. As a result, with a given dryer temperature, thin veneer will dry about one-third faster in a jet dryer than in a mechanical dryer having longitudinal or cross circulation air movement. The fastest heat transfer is by conduction. In general, with a given dryer temperature, veneer dried between heated platens requires less drying time than veneers in a dryer that depends on air circulation to transfer the heat. The drying occurs fastest when the metal cauls are perforated to allow moisture to escape while maintaining high heat transfer from the hot plates. The roller conveyor or wire-mesh conveyor in conventional mechanical veneer dryer aids in the drying by transferring heat by conduction

the rate of heat transfer to the veneer. Fleischer qualifies this by saying that this controlling factor is a function of veneer thickness and also to some degree of veneer species. Comstock (8) states that differences between species and between hardwood and sapwood are not important independent variables aside from their effect on the veneer density and moisture content. He developed a general equation for the time required to dry veneer in a jet dryer. He was, therefore, interested in generalities that could be used for any given species. Bethel and Hader (3) concluded that the drying rate of veneer may be controlled by moisture diffusion. From the literature then, it appears that the rate of heat transfer to veneer is an important factor in the rate of veneer drying. However, diffusion, at least in part, controls rate of drying in %-inch (3.2 mm) and thicker veneer of the impermeable species such as sweetgum heartwood. Reaction wood—tension wood in hardwoods and compression wood in softwoods—shrinks more longitudinally than typical wood of the the same species. As a result, sheets of veneer containing streaks of tension wood or compression wood tend to buckle during drying. Do breaks (knife checks) in the veneer during cutting have any effect on drying? Experiments at the Forest Products Laboratory do not show any difference in the drying rate of i/ißor Vs-inch (1.6 or 3.2 mm) loosely cut and tightly cut sapwood veneer of sweetgum and yellow birch dried at 200° to 350° F (93° to 177° C) with an air velocity of 600 feet (180 m) per minute. The loosely cut veneer was easier to flatten after drying. Some Dryer Conditions That Can Affect Veneer Drying In general, dryers are operated to hold the veneer flat and transfer as much heat as possible to the veneer during drying. The importance of holding the veneer flat can be judged by comparing matched sheets of veneer dried with various amounts of restraint. In general, buckle will be greatest in the veneer hung from the ends and allowed to dry at ambient room conditions. Next will be veneer restrained by stickers and dried in a kiln. Veneer dried in a mechanical dryer with a roller or wire-mesh conveyor will buckle less than 71

to the veneer surface. Some investigators have reported that the heat transfer from the rolls may be as much as 20 percent of the total heat transferred to the veneer. This heat transfer from the rolls is veryobvious when comparing the drying rates of veneer through an essentially empty dryer and one in which the conveyor is full of veneer. In the full dryer, the rolls are cooled by the wet veneer and the required drying time for a given final moisture content increases. This means the first veneer through an empty dryer will emerge much drier than veneer coming from a full dryer. If the drying time is set according to the first veneer through the dryer, the time will be too short, and veneer coming from a full dryer will be much higher in moisture content. The relative humidity in a kiln can be used to control the final moisture content of the veneer. The relationship of wet-bulb and dry-bulb temperatures to the final equilibrium moisture content of the wood is shown in figure 25. The ability to control the final moisture content of the veneer is one of the main advantages of the dry kiln. Most veneer is dried in mechanical dryers at temperatures above 250° F. At these higher temperatures, Fleischer reports that relative humidity has no effect on the drying rate (IS). As a matter of interest, the calculated equilibrium moisture content of wood in saturated steam at 220° F (104° C) is about 11 percent. At 240° F (116° C) it is about 5 percent. Recent experiments show that veneer steamed at 220° to 240° F in a kiln or in a hot press will come to the desired final moisture content. Drying veneer to a controlled final moisture content should reduce degrade, reduce shrinkage, and provide a superior surface for gluing.

most satisfactorily with a restraint weight of about 5 pounds per square foot (24 kg/m-) when drying thin face veneer. In a roller dryer the rollers are generally hollow tubes which rest directly on the veneer. Both the roller conveyor and the wire-mesh conveyor can contribute to drying by conducting heat directly to the surface of the veneer. Longitudinal, crosscirculation, and impingement air movement are used in these progressive dryers. The method most commonly used in new veneer plants today is the jet dryer with the air impinging on the veneer surface at velocities of 2,000 to 10,000 feet (600 to 3,000 m) per minute. Some veneer is dried in progressive kilns. These kilns are operated at temperatures below 212° F (100° C) and, consequently, the relative humidity and equilibrium moisture content of the veneer can be controlled. Control of the final moisture content and production of veneer that is easily glued are two of the main advantages of the progressive kiln. Some products, like baskets, are assembled from green veneer and then dried. Usually heated tunnels with conveyors are used to dry veneer to about 20 percent moisture content to prevent mold. A few veneer plants use progressive platen dryers. Many users of face veneer redry their veneer in a platen dryer. A rather unique face dryer made in Germany consists of perforated drums, with a partial vacuum inside the drums. The vacuum holds the veneer against the heated drum and reportedly works satisfactorily with relatively thin veneer. The dryer does not seem well adapted for veneer thicker than y2s-inch (0.9 mm). An all-infrared dryer has been used commercially on the West Coast, but its use was discontinued because of high drying costs. Recently banks of gas-fired infrared heaters have been placed at the green end of a few dryers used with softwood veneer for construction plywood. They boost the temperature to reduce the drying time of thick sapwood veneer. Similarly, high-frequency and microwave energy have been used as a part of drying systems to equalize moisture content at the end of the drying cycle. These methods have not been generally used because of high equipment and power costs (59),

Types of Veneer Dryers By far the most common veneer dryer is the direct-fired or steam or hot water-heated progressive conveyor type. The roller conveyor is used most commonly with rotary-cut veneer. A wire-mesh conveyor is used for drying continuous ribbons of rotary-cut veneer and for sliced and half-round veneer. It permits feeding the veneer sidewise so that the sheets can be kept in sequence for matching, in contrast to the roller dryer where the sheets are fed endwise. The wire-mesh conveyor is reported to work 72

Figure 25.—Lines of constant equilibrium moisture content.

73

Drying veneer between perforated cauls in a hot press has been shown experimentally (30) to be a fast way to dry flat veneer. Veneer Drying Emissions A factor of current interest is veneer dryer emissions and whether they contribute to air pollution. Recent studies indicate the opacity of the plume from veneer dryers ranged up to 82 percent with an average of 21 percent (1), Opacity is judged visually by qualified raters. Rating is in 20-percent increments similar to the Ringelmann Smoke Scale. The State of Oregon passed a law in 1972 limiting opacity of plumes from existing veneer dryers to 20 percent and from new dryers to 10 percent. The opacity of the plume can be reduced by using stack velocities over 2,000 feet (600 m) a minute. While this may pass the opacity limitation, it is costly because it results in a large heat loss. Also, it does not cut down on pollution. Another approach is to filter the stack gases at high velocity through a fiberglass mat. This system can reportedly reduce the average opacity to 5 percent or less (5). Still another approach is to recirculate the air in direct-fired dryers through a heated duct at 1,200° F. In one-half second the hydrocarbons are incinerated and visibility of stack emissions reduced accordingly (5). Heat of combustion of the hydrocarbons is recovered by a heat exchanger to lower the total fuel needed to operate the system. Applied Drying Suggestions for Mechanical Dryers Dry the veneer as soon as practical after cutting to minimize end splits, oxidation stain, mold, and blue stain. This is particularly important for light-colored wood. To minimize drying time, operate the dryer at the maximum temperature consistent with good glue bonds and wood color. In general, this will be about 400° F (204° C) at the green end and 360° F (182° C) at the dry end of the dryer. If gluing or veneer color are problems, lower the dryer temperature. Decreasing the dryer temperature by 100° F (38° C) (for example, from 350° to 250° F (177° to 121° C) ) will approximately double the drying time. 74

Keep the dryer vents as nearly closed as practical. This will reduce the energy consumed and reduce veneer dryer emissions. If condensation and haze in the building become troublesome, open the vents the minimum amount needed to correct the problem. In general, operate the dryer with the maximum air circulation possible. It may sometimes be necessary to reduce the air velocity to prevent overdrying and splitting of very thin veneer. Keep the dryer as full of veneer as possible. Dryer schedules should be based on a full dryer operating at a steady temperature and air movement. Segregate green veneer by required drying time. The green veneer sorts should be by veneer thickness, species, and—for many softwoods—by sapwood and heartwood. Doubling the veneer thickness will more than double the drying time. Sapwood of species like Douglasfir requires about twice as much drying time as heartwood veneer. Heartwood and sapwood of many hardwoods dry in about the same time. Veneer containing both sapwood and heartwood or wet streaks in the heartwood should be dried on the sapwood schedule. The veneer drying time should be regulated by the kind of veneer being fed in the green end. It is tempting for the dryer operator to change the drying time from the dry end, depending on whether the emerging veneer seems too wet or too dry. If he does, there may be a constant shifting of drying times and a corresponding shifting in the average moisture content of the veneer out of the dryer. A better method is to carefully determine the proper time to dry veneer of a given thickness, species, and sapwood or heartwood and use this schedule when similar veneer is dried again. Even when the best dryer schedules are maintained, there will be a range of moisture content in the emerging veneer. Consequently, it is very desirable to have a constant electronic check of the moisture content in the veneer. Veneer having wet spots can be pulled separately. After standing overnight or longer, the veneer can be rechecked for high moisture content and wet pieces redried. If automatic moisture-detection equipment is not available, then the veneer out of the dryer should be checked regularly with a hand-oper-

ated moisture meter. When such meters are calibrated for a given species and make firm contact on cool veneer, they are quite accurate from about 6 to 15 percent moisture content. An experienced dryer operator can sometimes tell in general the veneer is drying by subjective methods. When veneer is being overdried, static electricity makes the dryer snap and pop. Overdried veneer may be hotter to touch and in extreme cases may be darkened. Underdried veneer will be cool to touch, there

will be less noise from static electricity, and the veneer may be more free of end waviness and buckle. All veneer should be cooled and held flat as it comes from the dryer. Cool veneer is less likely to buckle and will not contribute to precure of gluelines. The dried veneer should be neatly stacked on flat skids and the top of the pile weighted. Flitches of sliced veneer should be promptly strapped in flat crates.

QUALITY CONTROL Undried Veneer The quality of veneer is affected by log quality, by the care used in storing the logs or flitches, by heating the wood prior to cutting, and by the mechanical condition, setup, and operation of the lathe or slicer. Quantitatively five factors should be checked at regular intervals : Stain, uniformity of thickness, roughness of the veneer surface, breaks in the veneer, and buckle or other distortions of the veneer. Control of Stain Stain on veneer may be due to fungus, oxidation, or contact of the wet wood with iron or steel. Blue stain is the most common fungus stain that occurs readily in the sapwood of most species if unprotected logs are stored during warm weather. The best control is rapid processing of the logs or storage of the logs under water or under a water spray. If water or water spray is not available, end coating the logs is beneficial. Oxidation stain is generally a yellow or tan stain that may penetrate from the ends of unprotected logs during summer storage. Like fungus stain, it can be prevented by rapid processing of the logs or by storing logs under water or under a water spray. End coatings are also helpful. Oxidation stain may also occur on the surface of veneer sheets between the time they are cut and dried. A common example is the yellow stain that may develop on birch or maple sapwood. The stain is sometimes compared to the

browning of a freshly cut surface of an apple. Enzymes, moisture, favorable temperatures, and air are factors in this color change. Probably the best way to control this stain is to dry the veneer promptly after cutting so the surface is dried before oxidation takes place. Holding wet veneer over a weekend is likely to cause stain on susceptible wood species. Another control method is to heat the logs sufficiently to inactivate the enzymes present in the wood. This generally means heating the logs for 2 days at 160° F or higher rather than limiting heating to overnight. We have been told that running the veneer through boiling water as soon as it is cut may prevent the stain. When wet wood comes in contact with iron or steel, it reacts to form a blue-black stain. The stain becomes worse the longer the contact and the hotter the wood. It may be particularly prevalent on woods like oak that have a high tannin content, and is very noticeable on lightcolored wood like the sapwood of maple. Such stain is not particularly important for uses like construction plywood but is very objectionable on decorative face veneer. Control methods include keeping the knife and pressure bar as clean as possible; heating the knife and pressure bar to reduce condensation; lacquering the knife and pressure bar so that only the extreme edges have exposed steel that can stain the wood ; using stainless metals for the pressure bar and knife ; using a double bevel on the slicer knife so the heel of the slicer knife cannot rub against the flitch; using a greater knife angle (more clearance) so the heel of the slicer knife does not contact the flitch ; and using less nosebar pressure. 75

M 139 943

Figure 26.—Micrometer for measuring veneer thickness to 0.001 inch.

For quality-control purposes, it would probably pay to have a comparator such as described by Bryant, Peters, and Hoerber (4). The size of the anvil or contacting surface should be about 1/2 inch (12.7 mm) in diameter and the weight on the top anvil about 0.66 pound (300 g). When checking thickness of heavy veneer, we have found an air-operated cylinder with adjustable contact pressure and anvils about 2 inches (5 cm) in diameter to be fast and accurate (fig. 27). The tolerance permitted in green veneer will depend in part on the end use. For exacting end uses, this tabulation may be a guide: The lathe or slicer will need to be in very good condition and set up and operated with

Control of Veneer Thickness Uniform veneer thickness is desirable for production of high-quality glue bonds in plywood, for minimizing show-through of the core, and for producing panels to a specified thickness. Since uniform veneer thickness is so important, it should be checked on a regular basis. As a minimum, at the green end, the foreman and the lathe or sheer operators should have hand micrometers that read to 0.001 inch (0.025 mm) (fig. 26). They should be encouraged to check veneer thickness at the start of each shift, at each knife change, after any change in thickness being cut, and randomly at other times. 76

Veneer Thickness 1/4 % Vv, %:> Mi4

(in.) (0.250) ( .125) ( .062) ( .031) ( .016)

(mm) 6.3 3.2 1.6 .8 .4

Tolerance (in.) ±0.00 ±.004 ±.003 ±.002 ±.001

when the pressure bar is contacting the wood, the knife carriage and the wood work piece are forced apart. To minimize the production of thin veneer at the start of cutting, the lathe should have tight-fitting parts; the pressure bar should be closed from the start of cutting and throughout the cutting; and moderate nosebar pressure should be used. This is discussed in more detail by Lutz, Mergen, and Panzer (44). Another cause of variable veneer thickness is an improper setting of the knife angle or knife pitch. If the pitch is too low, the veneer is thick and thin in waves, the crest of which may be 1 or more feet apart. Feihl and Godin {16) report, "This defect is particularly pronounced in winter when veneer is cut from logs that are not adequately heated and contain some frozen wood. When such logs are peeled with a low knife angle, the frozen parts tend to produce thin veneer and the thawed parts thick veneer." The corrective measures are to heat the logs to a uniform temperature and to change to a higher knife angle (greater clearance angle),

(mm) ±0.127 ±.102 ±.076 ±.051 ±.025

care to produce veneer that will consistently meet these specifications. Many commercial operations run with tolerances approximately double those listed. Control of Thickness of Veneer Cut on Lathe

The most common fault in veneer thickness is thin veneer for the first few revolutions of veneer cut on the lathe. The major cause of this thin veneer is looseness in the moving parts of the lathe. A secondary cause is deflection of the wood by the pressure bar beyond the knife edge (29). Further, when the knife alone is contacting the wood, the knife carriage and the wood work piece are pulled together. In contrast,

M 139 941

Figure 27.—An air-operated device for measuring veneer thickness. The pressure on the anvils can be easily changed to suit the species and thickness being measured.

77

A number of investigators (4) have found that wood having high moisture content is more susceptible than drier wood to being cut thinner than the knife feed. An example is the tendency of Douglas-fir sapwood veneer to be thinner than heartwood veneer when cut with the same lathe settings. One solution is to use less nosebar pressure when cutting sapwood of conifers than when cutting heartwood. Wood having high moisture content, such as southern pine sapwood, tends to be thinner than would be expected from the knife feed when cut at fast speed and with high nosebar pressure (ÍS), Slower cutting speed or less nosebar pressure should result in better thickness control. Shake, heart checks, or splits in the log, and soft centers that allow the bolt to move in the chucks can cause irregular veneer thickness. These unwanted thickness variations are related to specific bolts and do not occur on sound bolts. Larger chucks and continuous end pressure help when cutting bolts with soft centers or with large end splits. Misalinement of the pressure bar and knife may cause a thickness variation from one end to the other end of the veneer sheet. If the bar moves back at one end of the lathe, the gap or horizontal opening is wedge-shaped. As a result, the emerging sheet of veneer is thick and short at the edge cut with the large gap, and thin and long at the edge cut at the smaller gap. The veneer coming from the lathe runs in the direction of the thicker veneer and the bolt takes a conical shape. The corrective measure is to aline the bar parallel to the knife. Then check for play in the nosebar assembly. Movement of the pressure bar during cutting may be greater at one end than the other and so cause misalinement (16). Misalinement of the lead of the pressure bar with respect to the knife may also cause this phenomenon but it is less likely to occur and relatively less important than misalinement of the gap. A conical-shaped bolt may also be caused by a much larger overhang of one spindle than the other. The remedy is to center the bolt endwise with respect to the knife. Similarly, if the knife edge is not parallel to the axis of the spindle, a conical bolt will be generated. The correction is to adjust the nut

of one of the feed screws of the lathe carriage until the knife frame is parallel to the axis of the spindles (15). Misalinement of the knife and bar may cause barrel-shaped bolts and veneer that is thicker at the edges than in the middle. This may be caused by closing of the bar lead and gap at the center of the lathe due to heat expansion when cutting hot bolts. It can best be corrected by heating the knife and bar prior to setting up the lathe. Alternately, the lathe can be equipped with a cooling system or the nosebar frame may have a yoke and pull screw. A barrel-shaped bolt may also be caused by bending of the bolt in the lathe. This is most likely to occur when cutting long bolts to a small diameter. Use of a backup roll can prevent bending of the bolt during peeling. Control of Thickness of Veneer Cut on the Sheer

The pressure bar is generally bolted into position on the slicer and the flitch is backed up with a steel table. Consequently, the veneer cut on the slicer may be more uniform in thickness than veneer cut on the lathe. Since most veneer cut on a slicer is Míj-inch (1.6 mm) or thinner, this also makes thickness control less of a problem than with thicker rotary-cut veneer. Even so, the first few sheets cut on a slicer may be thinner than nominal thickness. The cause is primarily play in the feed mechanism and the flitch table. As with the lathe, it may also be due to compression of the wood beyond the knife edge by the pressure bar (29). A warped flitch that is not held securely against the flitch table by the dogs may also result in thin veneer. Having all slicer parts closefitting, the flitch securely held against the flitch table, and using moderate nosebar pressure should minimize these sources of nonuniform sliced veneer. Less common reasons for nonuniform veneer include heat distortion of the knife and pressure bar that results in veneer cut from near the center of the slicer to be thin. Heating the knife and pressure bar prior to setting up the slicer is the best way to overcome this problem. Yokes and pull screws on the pressure bar holder can also be used to help correct the alinement of the pressure bar to the knife edge. A nonuniformly heated flitch may also result in nonuniform veneer thickness. 78

M 141 666

Figure 28.—An instrument for measuring roughness of wood surfaces by moving a stylus across the rough surface. The insert shows the type of trace the instrument records.

A slicer that indexes the previously cut surface against a stop plate may produce uneven veneer if splinters or other debris come between the flitch and the stop plate. Slicers having a pawl and ratchet feed must have the same number of teeth advanced every stroke. If the mechanism is not set carefully, an incorrect thickness may be produced. Similarly, if the feed index train is not braked, momentum may carry the knife carriage beyond the desired index. Splits or shake in flitches can cause uneven veneer thickness. These thickness variations do not occur with sound flitches.

veneer can cause gluing problems, require excessive sanding, and cause ñnishing problems. Measuring the roughness of wood surfaces is a complex problem. Peters and Mergen (5^) described a stylus trace method they developed for measuring wood surfaces (fig. 28). Earlier Lutz (38) described a light-sectioning method for measuring roughness of rotary-cut veneer (fig. 29). Northcott and Walser (50) have published a visual veneer roughness scale which in turn was obtained by measuring depressions on the surface of the veneer samples with a dial micrometer. For research, the stylus trace method, the light-sectioning method, and the dial micrometer give values for comparative purposes. For mill use, a visual veneer roughness scale is probably more useful. Actual veneer samples that have been measured for

Control of Veneer Roughness Like nonuniform veneer thickness, veneer roughness is undesirable for all end uses. Rough 79

surface roughness in the laboratory could be kept near the lathe or slicer for visual comparison with the veneer being produced. The orientation of the wood structure (39) and the growth rate of softwood trees (iO) affect the smoothness of knife-cut veneers. When cutting against the grain of the wood fibers, annual rings, or wood rays, the wood tends to split ahead of the knife and into the wood work piece, causing depressions on the tight side of the veneer. The annual ring effect is most pronounced when rotary-cutting fastgrown softwoods at small core diameters. The ray effect is pronounced when quarter-slicing goes beyond the true quarter. Cutting against the fibers occurs around knots, with curly grain and with interlocked grain. The thicker the veneer, the more likely the veneer will be rough. It is sometimes possible to mount the flitch or

bolt to minimize cutting against the grain (39). Probably the best control is to adjust the nosebar to increase the pressure just ahead of the knife tip and so reduce splitting ahead of the knife. Proper heating of the wood and use of a sharp knife also help reduce this roughness. Another type of roughness is a fuzzy surface. It is most common on low-density hardwoods like cottonwood that contain tension wood. Overheating of any species may also cause fuzzy surfaces. Control may include log selection to avoid tension wood, cutting the wood at as low a temperature as is practical, and keeping the knife sharp. An extra hard knife will keep a sharp edge longer than a soft knife and can be used with low-density woods. Use of a slightly eased fixed nosebar edge and continuous flushing of the surface between the wood and the nosebar with cold water may also help.

M 141 667

Figure 29.—An instrument for measuring veneer surfaces by light sectioning. The insert shows what is seen through the magnifying glass of the instrument.

80

Corrugated veneer with three or four waves per inch of veneer is generally associated with too high a knife angle. Feihl and Godin (16) report corrugated veneer can also be caused by cold or dry wood and by setting the knife edge too low. Other causes are too much overhang on the spindles, cutting to a small core without adequate support for the core, and wood bolts that become loose in the chucks. Corrective measures are obvious from the stated causes.

Shelling or separation of the springwood from the summerwood may occur when rotarycutting or flat-slicing both softwoods and hardwoods that have a relatively weak zone between the springwood and summerwood. Hemlock, true firs, western redcedar, and angelique are species that may develop shelling. Overheating of the wood, too much nosebar pressure, too sharp a nosebar, or a dull knife may contribute to shelling. Shattering of the veneer surface is somewhat like shelling and may occur with wood having a high moisture content and low permeability. For example, Douglas-fir sapwood and sinker redwood bolts may develop shattered veneer surfaces if cut at high speed and with high nosebar pressure. Apparently water in the wood is compressed so fast that it ruptures the wood structure to escape. Lower nosebar pressure and slower cutting speed reduce the occurrence of shattered veneer surfaces. Nicks on the knife edge or pressure-bar edge may cause scratches on the veneer. Scratches from the knife occur on both the tight and loose side of the veneer while scratches from the pressure bar occur only on the tight side of the veneer. These scratch marks are so common that they can often be used to distinguish onehalf-round from flat-sliced veneer. The scratches on the half-round veneer are at a right angle to the length of the sheet while those on flat-sliced veneer are at some acute angle corresponding to the draw of the slicer. Careful examination of the veneer, followed by honing the knife and pressure bar when necessary, will minimize these scratch marks. This is particularly important for decorative face veneer. The scratches may take more stain than surrounding wood even if the sanded wood appears to be free of scratches. Grain raising is occasionally seen on softwood veneer cut from wood having a dense summerwood and much less dense springwood. Excessive pressure from the nosebar overcompresses the springwood. After the veneer is cut, the springwood recovers, resulting in raised grain. The corrective measure is to reduce the nosebar pressure. Feihl and Godin (16) report that bulging of knots in the core is related to raised grain and they suggest increasing the knife angle as well as decreasing the nosebar pressure as means of correcting this fault.

Control of Cracks or Breaks into the Veneer Breaks into the veneer may be on the side of the veneer that is next to the knife or on the side next to the pressure bar during cutting. By far the most common are small cracks that develop on the side of the veneer next to the knife. They may be caused by splitting ahead of the knife edge or by bending the veneer as it passes the knife after it is cut. The terms tight and loose side of the veneer refer to this phenomenon, with the loose side being the side that has the checks. These small breaks are also known as knife checks, lathe checks, or slicer checks. Less prevalent but perhaps more serious are breaks on the bar side or tight side of the veneer. Three samples are grain separation, lifted grain, and cracks approximately perpendicular to the veneer surface. Loosely cut veneer is weak in tension perpendicular to the grain. As a result, it may develop splits or break readily during handling, thus lowering the grade of the veneer. Deep checks in face veneer may also contribute to surface checks in furniture or other finished panels. On the other hand, loosely cut veneer may develop more wood failure than tightly cut veneer. As a result, veneer is sometimes cut loosely on purpose to increase the wood failure when the plywood is evaluated by the standard plywood shear test. Three methods have been used to measure looseness of veneer. One method is to pull 1-inch- (2.54 cm) long veneer samples apart in tension perpendicular to the grain on a suitable test machine (fig. 30). Because of variability, a minimum of about 30 samples should be tested to obtain a value for a given cutting condition. The values obtained can be compared with values for matched sawn and planed pieces of the same size. 81

A second method of evaluating veneer checks is to apply an alcohol-soluble dye to the checks by brushing it on the dry veneer surfaces or by dipping the end of the dry veneer in the dye. The dye penetrates into the checks. The depth of checks as a percentage of the veneer thickness can be estimated from scarfed sections of the samples (fig. 31). The method works very well with relatively impermeable veneer such as Douglas-fir heartwood where the dye is generally confined to the checks; it is less satisfactory with permeable veneer such as southern pine sapwood due to overall penetration of the dye into the wood. A third method is to flex the veneer across the grain. Tightly cut veneer is suffer than loosely cut veneer. Two factors are most important in minimizing depth of checks on the loose side of the veneer. They are adequate heating of the wood and use of adequate nosebar pressure. Factors that may increase checking are logs that have partially dried and use of a knife bevel much greater than is commonly used.

/

Assuming proper heating schedules are being used as described earlier, the temperature through the flitch or bolts should be relatively uniform. One way to check the bolt temperature is to drill a %-inch- (6.3 mm diameter hole radially an inch or two (2.5 to 5 cm) deep at the center of the cores remaining after cutting veneer from large- and small-diameter bolts. A thermometer should immediately be inserted in the hole and the temperature recorded. This temperature should be within 10° F (5° C) of the desired temperature for good cutting. This method is recommended over measuring the temperature at the surface of the bolt, as the surface temperature of a heated block changes very fast when it is exposed to air. If the measured temperature is not satisfactory, the heating schedules should be rechecked and the actual temperatures in various positions in the heating vat should be monitored with thermocouples throughout the heating cycle. Nosebar pressure was described in detail earlier. For quality control, perhaps the most useful procedure is to be certain that the lathe or slicer settings are made with instruments, and that gages are mounted on the equipment to show any unwanted movement of the nosebar with respect to the knife edge during cutting. With good veneer species like yellow birch and yellow-poplar, it is possible to cut veneer as thick as Vs-inch (3.2 mm) with no visible checks on the knife side of the veneer. Grain separation is similar to shelling and is a failure of wood between annual rings. The defect may not be noticed in the green veneer but later causes trouble when the plywood made from the veneer is bent as for a boat hull. Two species that have developed the defect are okoume and lauan. The cause is related to relatively weak zones in the wood and is generally considered to be due to setting the bar with too much lead and too small a gap. If suspected, it may be detected in dry veneer or plywood by tapping with a coin or stroking with a stiff brush. The void causes a different noise than the noise that comes when tapping or brushing sound veneer. Lifted grain is a separation of large groups of fibers in figured veneer like curly birch (16). It is serious because such areas cannot be

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APPENDIX III- -MECHAÎVICAL PROPERTIES OF U.S. WOODS FOR VENEER Seven mechanical properties—tension perpendicular to the grain, hardness, modulus of elasticity, modulus of rupture, compression parallel to the grain, compression perpendicular to the grain, and shear—are given in this Appendix. The figures for tension perpendicular are taken from green material while the others are for wood at 12 percent moisture content. Tension perpendicular is important during cutting when the wood is green while the other

mechanical properties are most important for use of veneer in the dry conditions. Most of the mechanical properties listed here came from the Wood Handbook. In some cases, the information is from universities or from foreign laboratories. For up-to-date Canadian and U.S. values, it is suggested the reader check American Standards for Testing Materials D 2555.

MECHANICAL PROPERTIES OF U.S. WOODS FOR VENEER Common name

Tension perpendicular to grain (green)

Hardness (side)

Modulus of elasticity

Modulus of rupture

Compression parallel to the grain— maximum crushing strength

Compression perpendicular to the grain— fiber stress at proportional limit

Shear parallel to grain— maximum shearing strength

Lh/in.^

Lb

1,000 Lh/in.^

Lh/in.^

L6A'w.2

Lh/in.^

Lh/in.^

12 percent moisture content

UNITED STATES HARDWOODS

Alder Nepal Red Ash Black Blue Green Oregon Pumpkin Shamel White Aspen Bigtooth Quaking Basswood American White Beech, American Birch Alaskan paper Gray Paper River Sweet Yellow Buckeye Ohio Yellow Butternut Cherry, Black

—

—

—

5,820

440

1,080

12,600 13,790 14,100 12,700 11,060 12,800 15,400

5,970 6,980 7,080 6,040 5,690

760 1,420 1,310 1,250 1,460

1,570 2,030 1,910 1,790 1,720

7,410

1,160

1,950

1,430 1,180

9,100 8,400

5,300 4,250

560 370

1,080 850

410

1,460

8,700

4,730

370

990

720

1,300

1,720

14,900

7,300

1,010

2,010

200

—

840 760 910

—

1,900 1,150 1,590

13,800 9,800 12,300

7,510 4,870 5,690

830 750 600

1,420 1,340 1,210

430 430

1,470 1,260

2,170 2,010

16,900 16,600

8,540 8,170

1,080 970

2,240 1,880

— —

— —

430 570

490 950

360 460 690

960 1,170 1,700

510 590

1,020 1,380

8,500 9,800

590

850 1,290 1,200 1,160 990 860 1,320

1,600 1,400 1,660 1,360 1,260 1,660 1,770

310 230

420 350

280

390 490

—

590 590 770

—

—

—

380

—

—

.—.

— 1,170 1,180 1,490

111

—

—

— 7,490 8,100 12,300

—

— 4,170 5,110 7,110

MECHANICAL PROPERTIES OF U.S. WOODS FOR VENEER—continued Common name

12 percent moisture content

Tension perpendicular to grain (green)

Hardness (side)

Modulus of elasticity

Modulus of rupture

Compression parallel to the grain— maximum crushing strength

Compression perpendicular to the grain— fiber stress at proportional limit

Shear parallel to grain— maximum shearing strength

Lh/in:^

Lh

1,000

Lh/in,'

Lh/in.'

Lh/in.^

Lb/in.^

UNITED STATEIS HARDWOODS—continued

Cottonwood Balsam poplar (Balm of Gilead) Black Eastern Swamp Elm American Cedar Rock Slippery Winged Eucalyptus Hackberry Hickory, pecan Bitternut Nutmeg Pecan Water Hickory, true Mockernut Pignut Shagbark Shellbark Holly, American Honeylocust Koa Laurel, California Locust, Black Madrone, Pacific Magnolia Cucumbertree Southern Maple Bigleaf Black Boxelder Red Silver Sugar Oak, red Black California black Cherrybark Chestnut Laurel Northern red Nuttall Pin Scarlet

160

300

1,100

6,800

4,020

370

790

270 410

350 430

1,270 1,370

8,500 8,500

4,500 4,910

300 380

1,040 930

590 690

830 1,320 1,320 860 1,540 1,330 880

1,340 1,480 1,540 1,490 1,650 2,200 1,190

11,800 13,500 14,800 13,000 14,800 15,600 11,000

5,520 6,020 7,050 6,360 6,780 8,200 5,440

690 950 1,520 820 1,020

1,510 2,240 1,920 1,630 2,370

890

1,590

1,790 1,700 1,730 2,020

17,100 16,600 13,700 17,800

9,040 6,910 7,850 8,600

1,680 1,570 1,720 1,550

1,960 1,850 2,800

19,200 20,100 20,200 18,100 10,260 14,700 13,300 8,000 19,400 10,450

8,940 9,190 9,210 8,000 5,540 7,500 7,300 5,640 10,180 6,880

1,730 1,980 1,760 1,800 920 1,840

1,740 2,150 2,430 2,110 1,710 2,250

1,130 1,830 1,310

1,860 2,480 1,810

—

640 850

—

630

__ — 680

1,580 1,810 1,820

—

—

— —

1,970 2,140 1,880

—

—

—

780 770

1,020 1,580 850 1,270 1,700

—

—

2,220 2,260 2,160 1,890 1,110 1,630 1,570 940 2,050 1,230

440 610

700 1,020

1,820 1,400

12,300 11,200

6,310 5,460

570 860

1,340 1,530

600 720

850 1,180

1,450 1,620

10,700 13,300

5,950 6,680

750 1,020

1,730 1,820

560

—

950 700 1,450

1,640 1,140 1,830

13,400 8,900 15,800

6,540 5,220 7,830

1,000 740 1,470

1,850 1,480 2,330

700 800 690 770 750

1,210 1,100 1,480 1,130 1,210 1,290

1,640 990 2,280 1,590 1,690 1,820

13,900 8,700 18,100 13,300 12,600 14,300

6,520 5,640 8,740 6,830 6,980 6,760

930 1,160 1,250 840 1,060 1,010

1,910 1,470 2,000 1,490 1,830 1,780

800 700

1,510 1,400

1,730 1,910

14,000 17,400

6,820 8,330

1,020 1,120

2,080 1,890

680 930

—

—

112

—

—

MECHANICAL PROPERTIES OF U.S. WOODS FOR VENEER—continued Common name

Tension perpendicular to gram (green)

Hardness (side)

Modulus of elasticity

Modulus of rupture

Compression parallel to the gram— maximum crushing strength

Compression perpendicular to the gram— fiber stress at proportional limit

Shear parallel to grain— maximum shearing strength

Lh/in,'

Lh

1,000 Lh/in,'^

Lh/in.'

Lh/in}

Lh/in,^

Lh/in.''

12 percent moisture content

UNITED STATES HRDWOODS—continued

Oak (cont.) Shumard Southern red Water Willow Oak, white Bur Chinkapin Delta post Durand Live Oregon white Overcup Post Swamp chestnut Swamp white White Ohia Persimmon, common Sassafras Silk-oak Sugarberry Sweetgum Sweetbay Sycamore, American Tanoak Teak Tupelo Blackgum Swamp Water Walnut, Black Willow, Black Yagrumo hembra Yellow-poplar

480 820 760

1,060 1,190 1,460

1,490 2,020 1,900

10,900 15,400 14,500

6,090 6,770 7,040

870 1,020 1,130

1,390 2,020 1,650

800 730

1,370 1,190

1,030 1,420

10,300 12,600

6,060

1,200

1,820

—

—

—

1,040 940 730 790 670 860 770 950 1,200 520

1,970 1,100 1,420 1,510 1,770 2,050 1,780 2,370 2,010 1,120

18,400 10,320 12,600 13,200 13,900 17,700 15,200 18,300 17,660 9,030

8,900 6,530 6,200 6,600 7,270 8,600 7,440 8,900 9,170 4,760

2,840 1,710 810 1,430 1,110 1,190 1,070 1,400 1,990 850

2,660 2,020 2,000 1,840 1,990 2,000 2,000 2,360 2,160 1,240

540

2,680 1,660 1,190 1,360 1,240 1,620 1,360 2,090 2,300 630 930 960 850

630

770

1,140 1,640 1,640 1,420

9,900 12,500 10,920 10,000

5,620 6,320 5,680 5,380

1,000 620 560 700

1,280 1,600 1,680 1,470

960

1,130

1,820

13,900

7,900

1,410

1,320

570

810

1,200

9,600

5,520

930

1,340

600 570 430

880 1,010 450 320 540

1,260 1,680 1,010 1,090 1,580

9,600 14,600 7,830 6,490 10,100

5,920 7,580 4,100 3,490 5,540

870 1,010 430 270 500

1,590 1,370 1,250

1,130 800

— —

—

510

—

—

1,190

UNITED STATES SOFTWOODS

Cedar AlaskaAtlantic whiteEastern redcedar IncenseNorthern whitePort-OrfordWestern redcedar Cypress Baldcypress Pondcypress

330 180 330 280 240 180 230

580 350 900 470 320 560 350

1,420 930 880 1,040 800 1,730 1,120

11,100 6,800 8,800 8,000 6,500 11,300 7,700

6,310 4,700 6,020 5,200 3,960 6,470 5,020

620 410 920 590 310 620 490

880 850 1,080 860

300

510

1,440

10,600

6,360

730

1,000

113

—

MECHANICAL PROPERTIES OF U.S. WOODS FOR VENEER—continued Common name

12 percent moisture content

Tension perpendicular to grain (green)

Hardness (side)

Modulus of elasticity

Modulus of rupture

Compression parallel to the grammaximum crushing strength

Compression perpendicular to the grain— fiber stress at proportional limit

Shear parallel to gram— maximum shearing strength

Lh/in.'^

L6

1,000 Ld/in.^

Lh/in.^

Lh/in.^

Lh/in.^

Lh/in.^

UNITED STATES SOFTWOODS—continued

Douglas-fir Coast Interior north Interior south Interior west Fir Balsam California red Grand Noble Pacific silver Shasta red Subalpine White Hemlock Eastern Mountain Western Juniper Alligator Rocky Mountain Western Larch, Western Pine Digger Eastern white Jack Jeffrey Knobcone Limber Loblolly Lodgepole Longleaf Pitch Pond Ponderosa Red Sand Shortleaf Slash Spruce Sugar Table-Mountain Virginia Western white Whitebark Redwood Big tree

300 340 250 290

710 600 510 660

1,950 1,790 1,490 1,820

12,400 13,100 11,900 12,600

7,240 6,900 6,220 7,440

800 770 740 760

1,130 1,400 1,510 1,290

180 380 240 230 240

400 500 490 410 430

1,230 1,490 1,570 1,720 1,720

7,600 10,400 8,800 10,700 10,600

4,530 5,470 5,290 6,100 6,530

300 610 500 520 450

710 1,050 910 1,050 1,180

300

400 480

900 1,490

7,100 9,800

4,330 5,810

490 530

1,020 1,100

230 330 290

500 740 540

1,200 1,320 1,640

8,900 11,200 11,300

5,410 6,840 7,110

650 1,030 550

1,060 1,230 1,250

,160

650 720

6,700 8,310

4,120 5,340

1,380 890

1,042 1,065

330

830

1,870

13,100

7,640

930

1,360

250 360 260

380 570 500

1,240 1,350 1,240

8,600 9,900 9,300

4,800 5,660 5,530

440 580 790

900 1,170 1,210

270 260 220 330 280 280 310 300 380 320 400 270 320 400 260

430 690 480 870 620 740 460 560 730 690 1,010 660 380 660 740 370

1,170 1,800 1,340 1,990 1,430 1,750 1,290 1,630 1,410 1,760 2,060 1,230 1,200 1,550 1,520 1,510

9,100 12,800 9,400 14,700 10,800 11,600 9,400 11,000 11,600 12,800 15,900 10,400 8,000 11,600 13,000 9,500

5,290 7,080 5,370 8,440 5,940 7,540 5,320 6,070 6,920 7,070 9,100 5,650 4,770 6,830 6,710 5,620

580 800 610 960 1,010 1,120 580 600 1,030 810 1,020 730 480 980 910 440

800 1,370 880 1,500 1,360 1,380 1,130 1,210 1,100 1,310 1,730 1,490 1,050 1,200 1,350 850

260

480

1,340

10,000

6,150

700

940

—

114

MECHANICAL PROPERTIES OF U.S. WOODS FOR VENEER—continued Common name

Tension

12 percent moisture content

dicular to gram (green)

Hardness (side)

Modulus of elasticity

Modulus of rupture

Compression parallel to the gram— maximum crushing strength

Compression perpendicular to the grainfiber stress at proportional limit

Shear parallel to gram— maximum shearing strength

L6/tn.î

Lb

1,000 Lh/in.^

Lh/in.^

Lh/in.'

Lb/in.'

Lb/in.^

UNITED STATES SOFTWOODS—continued

Spruce Black Blue Engelmann Red Sitka White Tamarack Yew, Pacific

100

520

1,530

10,300

5,320

530

1,030

240 220 250 220 260 450

390 490 510 480 590 1,600

1,300 1,520 1,570 1,340 1,640 1,350

9,300 10,200 10,200 9,800 11,600 15,200

4,480 5,890 5,610 5,470 7,160 8,100

410 470 580 460 800 2,110

1,200 1,080 1,150 1,080 1,280 2,230

115

APPENDIX IV—SOME PROCESSING VARIABLES OF U.S. WOODS FOR VENEER Ease of bark removal is based on fall-cut wood debarked by machine. The conditioning temperatures are those suggested for cutting veneer about h inch thick. The recommended temperatures for rotary cutting take into account the tendency of the species to develop splits at the ends of the bolts during heating. For slicing, the recommended temperature will often be 10° to 20° F higher than for peeling because splitting is less of a problem when heating flitches for slicing. The last columns are rated on an A, B, and C scale. A indicates that the speciñc property is basically favorable for use as veneer and C indicates that the particular property may be a problem in utilizing the species for veneer. For example, an A rating for log splitting due to heating indicates the species is little affected by heating while a C rating indicates that log end splits are a major problem with this species. The A, B, and C ratings for drying times are comparative. The time required to dry veneer varies widely with species and with the type of dryer being used. For this reason, rather than give specific times for a specific dryer, drying times are given in comparison with other species—yellow birch for hardwood veneer and Douglas-fir for softwood veneer.

116

Yellow birch was selected as "typical" for hardwood veneer because this is a well-known veneer species and one on which FPL had much drying data. Besides, the sapwood and heartwood of yellow birch take about the same time to dry. Our data show that no other hardwoods dry much faster than yellow birch. In contrast, several hardwood species require considerably longer drying time than yellow birch. So drying time ratings for hardwoods are either B or C For softwoods, the comparison is based on the drying of sapwood or heartwood of Douglasfir. The sapwood of Douglas-fir takes significantly longer drying time than the heartwood. The quality and recovery of veneer from all species is sensitive to the setting of the knife and pressure bar. However, acceptable veneer can be cut from some species with a wider range of settings than can be tolerated by other species. An A rating for sensitivity to settings of the knife and pressure bar indicates the species tolerates a wide latitude in machine setting; a C rating indicates the species cuts well only within a narrow range of machine settings. Under defects in drying, an A rating means a species is relatively free of the characteristics listed, while a C rating means the veneer from the species is subject to this particular drying defect.

SOME PROCESSING VARIABLES OF U.S. WOODS FOR VENEER Common name

Ease of bark removal by machine 2

Suggested conditioning temperature Rotary

Sliced

Sensitivity Aggrato setting vation of— of log split- Knife Presting sure due to heating bar

Drying time

Defects in drying

Sapwood

Heartwood

Buckle

Splits

Collapse

B B

B B

A A

A A

A A

UNITED STATES HARDWOODS

Nepal Red Ash Black Blue Green Oregon Pumpkin Shamel White Aspen Bigtooth Quaking Basswood American White Beech, American Birch Alaskan paper Gray Paper River Sweet Yellow Buckeye Ohio Yellow Butternut Cherry, Black Cottonwood Balsam poplar Black Eastern Swamp Elm American Cedar Rock Slippery Winged Eucalyptus Hackberry Hickory, pecan Bitternut Nutmeg Pecan Water Hickory, true Mockernut Pignut Shagbark Shellbark

1 2

100-140 80-140

140-160 120-160

2 2 2 2 2 2 2

120-140 140-160 140-160 140-160 140-160 140-160 140-160

140-160 160-180 160-180 160-180 160-180 170-180 160-180

1 1

40-70 40-70

3 3

A B

A A

A A

— — — —

B

B

B

A

— — — —

— — — —

— — — —

B

— — — —

— — — —

— — — —

— — — —

B B

A B

B B

B B

B B

B B

B B

A A

40-70 40-70

A A

B B

A A

C C

C C

B B

A A

B B

40-70 40-70

40-70 40-70

A A

C C

B B

B C

B C

A A

A A

A A

1

160-180

180-190

B

B

B

B

B

B

A

A-B

2 2 2 2 2 2

140-160 120-140 120-140 120-140 140-160 140-160

160-180 140-160 140-160 140-160 160-180 160-180

A

B — B — B B

1 1 2 2

40-70 40-70 70-90 120-140

—

2 2 2 2

B

B

A

B

B

B

—

—

—

—

B B B

A A B B

B A A A

A A A-B A-B

— —

— —

— —

— —

B B

C B

B A

A A

C C C C

C C C C

C C C C

B B B B

C C C C

B B B B

C C C C

C C C C

C

B

A

—

—

—

B B

A A

B B A

B B A

C C B

C C B

—

—

—

B A

B A

B A

C C C C

B B B C

B B B B

B B B C

C C C C

B B C B

B B B B

A A A A

C C C C

B B B B

B B B B

B B B B

C C C C

B C B B

B B B B

A A A A

—

—

B B B B

B B B B

A — B B B B

40-70 40-70 100-200 150-170

A A A B

— —

— —

C B

C B

— B B

40-70 40-70 40-70 40-70

40-70 40-70 40-70 40-70

A A A A

B B B B

B B B B

2 2 2 2

120-140 160-170 160-170 120-140

B B B B

B B B B

2 2 1

160-170 140-160 120-140

150-170 190-200 190-200 180 then 150 190-200 180-200 140-160

B C A

3 3 3 3

160-180 160-180 160-180 160-180

190-200 190-200 170-180 190-200

3 3 3 3

160-180 160-180 160-180 160-180

190-200 190-200 190-200 190-200

B

117

—

c c

A

SOME PROc:ESSING VARIABI.ES OF U.S. w-OODS. FOR VENEÎÎR 1—continued Common name

Ease Suggested of conditioning bark temperature removal Rotary by Sliced machine ^ op

AggraSensitivity vation to setting of log of-- split- ting PresKnife due to sure heating bar

Drying time Sapwood

Defects in drying

Heartwood

Buckle

Splits

Collapse

B B

A B

B B

A A

C

B B B

A A A

op UNITED STATES HARDWOODS —continued

Holly, American Honeylocust Koa

2 3

—

150-160 140-160 140-160

170-180 180-190 160-180

B B

B A

B B

B B

3 3

150-160 160-180 150-160

190-200 180-190 180-190

B B B

B B B

B B B

—

—

C

C

C B B

1 1 1

70-120 70-120 70-120

120-140 120-140 120-140

A A A

A A A

A A A

— — —

— — —

A A A

A A A

A A A

2 2 2 2 2 2

80-120 160-180 80-120 100-140 80-120 160-190

120-140 170-190 120-140 130-150 120-140 170-190

B B

—

A B

—

A B

—

B B

—

B B

—

B B

—

B B

—

A B

—

B B A-B

A A C

A A C

C C B

C C B

A B A-B

A B B

A A A-B

2

140-160

180-200

C

B

B

C

C

A

B

A

2 2 2 2 2 2 2 2 2 2 2 2

140-160 140-160 140-160 140-160 140-160 140-160 140-160 140-160 140-160 140-160 140-160 140-160

160-180 180-200 180-200 180-200 180-200 180-200 180-200 180-200 180-200 180-200 180-200 180-200

C C C C C C C C C C C C

B B B B B B B B B B B B

B B B C B B B B B B B B

C C C C C

C C C C C

B A A B B

B B B C B

A B A C B

2 2 2 2 2 2 2 2

140-160 140-160 140-160 140-160 160-170 140-160 140-160 140-160

180-200 180-200 180-200 180-200 200-210 180-200 180-200 180-200

C C C C C C C C

B B B B B

B B B B B

2 2 2 2

140-160 140-160 140-160 170-180

180-200 180-200 180-200 200-210

C C C B

— 2 2 1 1

150-200 100-120 150-160 120-140 120-140

190-200 120-150 170-180 140-160 140-160

1

120-140

150-160

Jüaiirei,

California Locust, Black Madrone, Pacific Magnolia Cucumbertree Southern Sweetbay Maple Bigleaf Black Boxelder Red Silver Sugar Oak, red Black California black Cherrybark Chestnut Laurel Northern red Nuttall Pin Scarlet Shumard Southern red Water Willow Oak, white Bur Chinkapin Delta post Durand Live Oregon white Overcup Post Swamp chestnut Swamp white White Ohia Persimmon, common Sassafras Silk-oak Sugarberry Sweetgum Sycamore, American

C

—

—

—

B B

B B

B B B B

B B B C

C

C

—

— — c — c c c c — — c — — c c — c c

— — c — c c c

— — A —

— — B

—

— — — —

A A A

B C C

B C C

c — — c c — c c — c c

— — —

— — —

A

B C

B

B

B

—

—

—

A

A

B

— c — c

B

A

A

c

B

—

C

A

118

A

B

— — B — —

— C —

— — — — — — C —

—

—

A A B

B B B

B B A

B

—

B

—

—

—

A

B

B

c

C-B

B

B

A

—

B

— c — c

A

— A

SOME PROCESSING VARIABLES OF U.S. WOODS FOR VENEER i—continued Common name

Sensitivity Aggrato setting vation of— of log split- Presting Knife sure due to heating bar

Suggested Ease conditioning of temperature bark removal — Sliced by Rotary machine ^

Drying time Sapwood

Defects in drying

Heartwood

Buckle

Splits

Col-

UNITED STATES HARDWOODS—continued

Tanoak Teak Tupelo Blackgum Swamp Water Walnut, Black

1 2

150-160 190-200

1 1 1 2

Willow, Black Yagrumo hembra Yellow-poplar

3 2 1

120-140 120-140 120-140 180 then 150 40-70 50-80 70-120

180-190 200-210

C B

B A

B B

C C

C C

B A

C A

C A

150-160 150-160 150-160 180 then 150 40-70 70-80 120-140

A A A B

A A A B

A A A B

C C C B

C C C B

B B B B

A A A A

B B B A

B A A

B B A

B A A

C B B

C

— B

B B A

B B A

A B A

UNITED STATES SOFTWOODS

Cedar AlaskaAtlantic whiteEastern redcedar IncenseNorthern whitePort-OrfordWestern redcedar Cypress Baldcypress Pondcypress Douglas-fir Coast Interior north Interior south Interior west Fir Balsam California red Grand Noble Pacific silver Shasta red Subalpine White Hemlock Eastern Mountain Western Juniper Alligator Rocky Mountain Western Larch, Western

3

120-140

140-160

B

A

B

B

B

A

A

A

2

60-100

100-130

A

A

B

B

B

A

A

A

2 3

140-160 70-120

160-180 70-120

B A

C B

B B

B

—

A C

B A

B A

A —

2 3

120-140 120-160

140-160 140-160

B B

C A

C B

— B

C B

A A

B A

B A

3

140-160

160-180

B

C

C

B

C

A

B

B

3 3

60-120 60-120

120-140 120-140

A A

B B

C C

C C

C C

A A

B B

A A

1

60-140

140-180

A

B

B

B

B

A

B

A

1

60-140

140-180

A

B

B

B

B

A

B

A

1

60-140

140-180

A

B

B

B

B

A

B

A

1

60-140

140-180

A

B

B

B

B

A

B

A

1 1 1 1 1 1 1 1

70-130 70-150 70-150 70-150 70-150 70-150 70-130 70-150

120-150 130-160 130-160 130-160 130-160 130-160 120-150 130-160

B B B B B B B B

B B B B B B B B

B B-C B-C B-C B B-C B B-C

B B B B B B B C

C C C B-C B-C C C C

B B B B B B B B

B B B B B B B B

A A A A A A A A

2 2 2

120-160 120-160 120-160

160-180 160-180 160-180

B B B

B B B

C C C

B B B

C C C

B B B

B B B

A A A

3

140-160

160-180

B

C

B

B

A

B

C

A

3 3 3

140-160 140-160 140-150

160-180 160-180 160-180

B B B

C C B

B B B

B B B

A A C

B B A

C C B

A A A

119

SOME PROCESSING VARIABLES OF U.S. WOODS FOR VENEER i—continued Common name

Suggested Ease of conditioning bark temperature removal Rotary Sliced by machine ^

Aggravation of log splitting due to heating

Sensitivity to setting of— Knife

Drying time Sapwood

Defects in drying

Heartwood

Buckle

Splits

Collapse

B B

B B

B B

A A

B

C B C B B B B B B B B B C

A B B B-C B B B B A B B B B B A

B B B B-C B B B B B B B B B B B

A A A A A A A A A A A A A A A

Pressure bar

UNITED STATES SOFTWOODS—continued

Pine Digger Eastern white Jack Jeffrey Knobcone Limber Loblolly Lodgepole Longleaf Pitch Pond Ponderosa Red Sand Shortleaf Slash Spruce Sugar TableMountain Virginia Western white Whitebark Redwood Big tree Spruce Black Blue Engelmann Red Sitka White Tamarack Yew, Pacific

]L ]L ]L ]L 1L ]L ]L 1L 1L ]L ]L ]L 1L ]L ]L ]L ]L ]L

60-140 70-120 70-120 60-140 60-140 60-120 120-160 60-140 120-160 120-160 120-160 60-140 70-120 120-160 120-160 120-160 120-140 60-120

140-180 120-140 120-140 140-180 140-180 120-140 160-180 140-180 160-180 160-180 160-180 140-180 120-140 140-180 160-180 160-180 140-160 120-140

A A A A A A A A A A A A A A A A A A

B B B A B C B A B B B A B B B B B B

B B B A B B B A B B C A B B B B B B

]L L 1L ]L Í5 ÍI

120-160 120-160 60-120 60-120 70-160 70-160

160-180 160-180 120-140 120-140 160-180 160-180

A A A A B B

B B B C B B

B B B B C C

B B B B C C

B B C B C C

B B A B A A

B B B B C C

A A A A A A

]L ]L ]L ]L ]L :L íI

70-120 70-120 70-120 70-120 70-120 70-120 140-160 160-180

120-140 120-140 120-140 120-140 120-140 120-140 150-160 180-200

A A A A A A B

C C C C C C B B

B B B B B B B B

B B B B B B B

B B B B B B C B

B B B B B B B C

B-C B B B-C B B-C B B

A A A A A A A A

—

1 A, species property very suitable for veneer; B, intermediate; and C, less desirable for veneer. 21, species relatively easy to debark; 2, intermediate to debark; and 3, difficult to debark.

120

B B

— B B B B B B B B B B B B B B B

—

—

APPENDIX V- -EFFECTS OF LOG STORAGE AND PROCESSING ON VENEER CHARACTERISTICS Most information in Appendix V is again based on the A, B, and C scale, and expresses relative ratings. Information in the columns head '^Relative freedom from veneer characteristics originating in log storage and processing'' involves a highly variable set of data. All these characteristics are at least to a degree under the control of the processor.

An A rating would indicate that the wood is résistent to development of a particular characteristic even under a wide range of processing conditions. A C rating indicates that the wood is highly susceptible to this particular characteristic and should indicate caution in processing to keep this specific characteristic to a minimum.

EFFECTS OF LOG STORAGE AND PROCESSING ON VENEER CHARACTERISTICS ' Relative freedom from veneer characteristics originating in log storage and in processing

Common name

Sap stains

Mold

Iron stain

Oxidative stain

SurfaceI irregularities

Bacteria Odor

Extreme permeability

Fuzzy

Shelling

Rough

B A

B B

A A

A A

A

A A A A A A A

B B B B B A B

UNITED STATES HARDWOODS

Alder Nepal Red Ash Black Blue Green Oregon Pumpkin Shamel White Aspen Bigtooth Quaking Basswood American White Beech, American Birch Alaskan paper Gray Paper River Sweet Yellow Buckeye Ohio Yellow Butternut Cherry, Black Cottonwood Balsam poplar Black Eastern Swamp Elm American Cedar Rock Slippery Winged

B A

B B

B B

B B B B B B B

B B B B B B B

B B B B B A B

B B

C C

B B A

C C

B A

— — — —

— — — —

A C

A A

— A A

A A A A A A A

A A

B B

C C

— —

C C

A A

B B

B B B

A A B

C C B

A A A

A A A

C C A

A A A

A A B

B A A A A A

B B B B B B

A B B B B B

B C C C B B

— —

—

A

A A

B — A A A A

B A A B

B — B B B A

— —

—

— — B C

C C B B

—

— —

— —

— —

— —

A A

— B A

A A

C A

A A

A A

B B B B

C C C C

A A A A

B B B B

C C C C

—

C C C C

A A A A

B B B B

A B A A B

A A A A A

A A A A A

B B B B B

B B B B B

A A A A A

A

121

A

B — A A — A A

— — —

— A

—

B B

—

—

B

B

B

—

—

—

B B

B B

B B

—

—

—

EFFECTS OF LOG STORAGE AND PROCESSING ON VENEER CHARACTERISTICS i—con. Relative freedom from veneer characteristics originating in log storage and in processing

Common name

Sap stains

Mold

Iron stain

Oxidative stain

Surface irregularities

Bacteria Odor

Extreme permeability

Fuzzy

Shelling

Rough

UNITED STATES HARDWOODS—continued

Eucalyptus Hackberry Hickory, pecan Bitternut Nutmeg Pecan Water Hickory, true Mockernut Pignut Shagbark Shellbark Holly, American Honeylocust Koa Laurel, California Locust, Black Madrone, Pacific Magnolia Cucumbertree Southern Maple Bigleaf Black Boxelder Red Silver Sugar Oak, red Black California black Cherrybark Chestnut Laurel Northern red Nuttall Pin Scarlet Shumard Southern red Water Willow Oak, white Bur Chinkapin Delta post Durand Live Oregon white Overcup Post Swamp chestnut Swamp white White Ohia Persimmon, Common Sassafras

B C

B C

C B

C C

A A

A A

A B

A B

B B

B B B B

B B A B

B B B B

A A B A

A A A A

A A A A

A A A A

A A A A

C C C C

B B B B C A A B A A

B A B B

B B B B

A B A A

A A A A

A A A A

A A A A A A A A A A

C C C C A B B B B A

—

—

A A A

A A A

A A A A A A A A A A

C C

C C

B B

A A

A A

A A

B B B B B B

C C C C C C

A A

A A

A A

A A

B B

A A A A A A A A A A A A A

C C C C C C C C C C C C C

A A A A A A A A A A A A A B

C C C C C C C C C C C B A C

—

—

—

—

B A

A B C B B

A

A B

B B B C B

B B

C C

A A

A A A A A C

B B B B B B

A A A A A A A A A A A A A A A A A A A A A A A A A A B

—

— A

—

—

—

—

—

A A A

A A A

A A B

A A A

B B B

C C C C C C C C C C C C C

A A A A A A A A A A A A A

A A A A A A A A A A A A A

A A A A A A A A A A A A A

A A A A A A A A A A A A A

B-C B-C B-C B-C B-C B-C B-C A-C B-C B-C B-C B-C B-C

C C C

A A A A A A A A A A A A A

A A A A A A A A A A A A A

A A A A A A A A A A A A A

A A A A A A A A A A A A A

B-C B-C B-C B-C B-C B-C B-C B-C B-C B-C B-C B B

c c c c c c c c B C

—

122

—

—

—

—

—

EFFECTS OF LOG STORAGE AND PROCESSING ON VENEER CHARACTERISTICS i—con. Relative freedom from veneer characteristics originating in log storage and in processing

Common name

Sap stains

Mold

Iron stain

Oxidative stain

Surface irregularities

Bacteria

Fuzzy

Shelling

Rough

A B A A A

A A A A A

B A A A A

A A B C B

A B B B

A A A B C B B

A A A A A A A

A B A A B A A

—

A A A A A A A

A A A A A A A

A B A B B A B

A A A B C A C

A B B B B A B

Odor

Extreme permeability

UNITED STATES HARDWOODS—continued

Silk-oak Sugarberry Sweetgum Sweetbay Sycamore, American Tanoak Teak Tupelo Black Swamp Water Walnut, Black Willow, Black Yagrumo hembra Yellow-poplar

A C C B B A A

A C C C B A A

B C B A A C B

A C B C A C A

B B B A C C C

B B B B C B C

A A A C B B A

C C C B C B B

B C A A A

A B B C

UNITED STATES SOFTWOODS

Cedar AlaskaAtlantic whiteEastern redcedar IncenseNorthern whitePort-OrfordWestern redcedar Cypress Baldcypress Pondcypress Douglas-fir Coast Interior north Interior south Interior west Fir Balsam California red Grand Noble Pacific silver Shasta red Subalpine White Hemlock Eastern Mountain Western Juniper Alligator Rocky Mountain Western Larch, Western

B B A B

A C A A A A A

A A A A A A A

B B B C B B C

B B

B B

B B

B B

B B

B B

B B

C C

B B

A A A A

A A A A

B B B B

A A A A

A A A A

A A A A

A A A A

B B B B

B B B B

A A A A A A A A

A A A A A A A A

A A A A A A A A

A A A B A A A A

B B B B B B B B

B B B B B B B B

B B B B B B B B

B B B B B B B B

B B B B B B B B

B B B

B B B

B B B

B B B

B B B

B B B

B B B

C C C

B B B

A A A A

A A A A

B B B B

A A A A

A A A A

A A A A

A A A A

A A A B

B B B B

__ B

123

EFFECTS OF LOG STORAGE AND PROCESSING ON VENEER CHARACTERISTICS i—con. Common name

Relative freedom from veneer characteristics originating in log storage and in processing Sap stains

Mold

Iron Stain íi^*^"i^^

Oxida4-'ï'«'r^x Live stain

Bacteria Odor

Surface irregularities

Extreme permeability

Fuzzy

Shelling

Rough

UNITED STATES SOFTWOODS —continued

Pine Digger Eastern white Jack Jeffrey Knobcone Limber Loblolly Lodgepole Longleaf Pitch Pond Ponderosa Red Sand Shortleaf Slash Spruce Sugar Table-Mountain Virginia Western white Whitebark Redwood Big tree Spruce Black Blue Engelmann Red Sitka White Tamarack Yew, Pacific

C B B C B B C B C C C C B C C C C B C C B B-C A A

B B B B B B C B C C C B B C C C C B C C B B A A

A A A A A A A A A A A A A A A A A A A A A A C C

B B B C B B A B A A A C A A A A A C A A C B B B

B B B B B B B B B B B B B B B B B B B B B B A A

C B B C B B C B C C C C B C C C C C C C C B A A

A B

B B

B B

A B C A B A A A A B A A A A B A A B C B B

A B B B A B B B A B B B B B B B B B B C C

B B B B A B B B B B B B B B B B B B B B B

B B B B B B A

B B B B B B A

A A A A A A B

A A A A A A A

—

—

—

—

A A A A A A A A

A A A A A A A A

C C C C C C B A

B B B B B B B A

B B B B B B B B

1 A, good—species resists development of undesirable characteristics under a wide range of operating conditions; B, species intermediate in resistance and C, poor—species susceptible to this undesirable development.

124

APPENDIX VI—APPEARANCE AND SUITABILITY OF INDIVIDUAL U.S. SPECIES FOR VARIOUS USES OF VENEER The last five columns of the Appendix VI table in a sense summarize all the data. An A rating indicates the species is well suited for the indicated product. A B rating indicates the species is moderately well suited for this prodEnd Use

uct, and a C rating indicates the species is generally not suited for the particular end product. In making these classifications, the following broad criteria were considered :

Typical Specific Uses

Desirable Veneer Qualities

Construction plywood

Building construction as subfloor, wall sheathing, roof sheathing, concrete forms, and overlaid panels.

High stiffness and strength, moderate weight, and readily glued

Decorative face veneer

Prefinished decorative wall panels, furniture, flush doors, kitchen cabinets, and case goods

Attractive figure and color, moderately hard, and readily glued

Inner plies for decorative panels

Inner plies for prefinished wall panels, furniture, flush doors, kitchen cabinets, and case goods

Low weight, low shrinkage, straight grain, fine uniform grain, and easily glued

Container veneer and plywood

Wirebound boxes, bushel baskets, paper-overlaid veneer, cleated panel boxes, and plywood-sheathed crates

High in stiffness, shock resistance, and resistance to splitting, light color, free from odor and taste, and moderate in weight

In some instances additional end uses and comments are listed under "other."

125

APPEARANCE AND SUITABILITY OF INDIVIDUAL U.S. SPECIES FOR VARIOUS USES OF VENEER Common name

Clear Figure of veneer veneer 1 Rotary- and flat-sliced Quarter- and rift-sliced

Relative suitability for—2 Construction plywood

Decorative face veneer

Inner plies of decorative panels

Container veneer and plywood

Other

UNITED STATES HARDWOODS

Alder Nepal

Faint growth ring. Large rays slightly darker than background do

Red Ash Black

Blue Green Oregon Pumpkin Shamel

B B B B A

White

A-B

Aspen Bigtooth

B

Conspicuous growth ring, occasional burls and cross figure .do. .do. .do. .do. Pronounced parabolas from the wide growth rings. Occasional pin knots Conspicuous growth ring, occasional burls and cross figure Faint growth ring

Scattered large flakes from wood rays Occasional large flakes

C

B

B

B

Distinct not conspicuous growth ring, occasional burl

B

A

B

A

do do do do Distinct stripe from growth rings. Faint crossbar

B B C C B

A A A B A

B B B C B

A A A B A

Distinct not conspicuous growth ring, occasional burl

Occasional cross figure, silky luster

A

do

A

Quaking Basswood American White Beech, American

A A B

Faint growth ring do Faint g owth ring

Plain, fine texture do Numerous small flakes up to 1/8 inch in height

Birch Alaskan paper

C

Faint growth ring pattern. Slow grown. Many knots and burls Distinct not conspicuous growth ring, occasionally wavy do do do do

Too small to quarterslice Generally plain. Occasionally wavy

B

Gray

C

Paper River Sweet Yellow Buckeye Ohio

B — A A —

Yellow Butternut

— C

A-B

.do

C C B

C C B

A A C

A A A

A-B

do do do do

B B B B

A-B B A A

B B B B

B B B B

Faint growth ring, close grain

Plain

do Faint to moderate growth ring, very lustrous

do Plain; the figure is due to color and luster

C C

C A

A C

B C

126

Underlayment plywood ....do....

Plywood flooring

APPEARANCE AND SUITABILITY OF INDIVIDUAL U.S. SPECIES FOR VARIOUS USES OF VENEER—continued Common name

Figure of veneer Clear veneer ^ Rotary- and flat-sliced Quarter- and rift-sliced Construction ply wood

Relative suitability forDecorative face veneer

Inner plies of decorative panels

Container veneer and ply wood

UNITED STATES HARDWOODS—continued

Faint growth ring, occasional burl, pin knots, and gum spots common

Light colored small ray flecks, satiny luster

B

B B B B

Faint growth ring do do do

Plain

C C

B

Faint growth ring Distinct growth ring stripe with fine wavy pattern within each ring do do Faint growth ring Conspicuous growth stripe ring with fine wavy pattern within each ring Distinct growth ring do stripe Faint growth ring Distinct growth ring stripe with fine wavy pattern within each ring Faint growth patterns. Ribbon grain. Occasional crossbar. Occasional crossbar. Many pin knots Many pin knots Distinct not conspicConspicuous growth uous growth stripe, ring fine sparkle from small rays

Cherry, Black

Cottonwood Balsam poplar Black Eastern Swamp Elm American Cedar Rock

B B

Slippery Winged

Eucalyptus

B

Hackberry

B

Hickory, pecan Bitternut

C

Nutmeg Pecan Water Hickory, true Mockernut Pignut Shagbark Shellbark Holly, American

C C C

Honeylocust

A

C C C C C

.do. .do. .do.

c c

B

B C C B

B B B B

A A A A

B B B

A A

C C

A A

B

A

B

A

B

A

C

A

B

A-B

C

B

B

A-B

C

A

B

Distinct not conspicuous growth ring, almost always straight grain do do do

Faint growth rings, fine rays, occasional dark stripes

B

do do do

B B B

A A A

C C C

B B B

do do do do Very close grain, almost no visible pattern Conspicuous growth ring

do do do do Very plain uniform texture

B B B B C

A A A A A

C C C C C

B B B B C

B

B-C

Koa

Irregular grain, dark streaks

Laurel, California

Faint growth ring, occasional burl or blisters

Distinct not conspicuous growth ring, occasional mild cross figure Curly, wavy grain, fiddle-back dark streaks Mixture of plain and highly figured due to mottle, stumps, and burls

127

C

B-C C

Other

APPEARANCE AND SUITABILITY OF INDIVIDUAL U.S. SPECIES FOR VARIOUS USES OF VENEER—continued Common name

Clear Figure of veneer veneer 1 Rotary- and flat-sliced Quarter- and rift-sliced Construction ply wood

Relative suitability for—2 Decorative face veneer

Inner plies of decorative panels

Container veneer and ply wood

UNITED STATES HARDWOODS—continued

Locust, Black

C

Madrone, Pacific

B

Distinct growth ring, dark streaks associated with borer holes Faint growth ring, close grain, figure due to pigment changes in heartwood

Magnolia Cucumbertree Southern Maple Bigleaf

A A B

Distinct not conspicuous growth ring

C

B

C

B

Bland figure is limited to color changes in the heartwood

C

A

C

B

Faint growth ring do

Plain

B B

C C

A A

A A

Faint growth ring, occasional burls, blister, curly, and quilted Faint growth ring, occasionally curly, wavy, birdseye Faint growth ring, close grain like the maples Faint growth ring, occasionally curly or wavy, often with pith flecks do Faint growth ring, occasionally curly, fiddle-back, birdseye, wavy

Most plain, occasionnally curly and wavy

C

A

B

A

Most plain, occasionally curly and wavy, small dark rays Plain

B

A

B

A

B

B

C

B

Most plain, occasionally curly and wavy, small dark rays

B

B

A

A

do do

C B

B A

A B

A A

B

A

B

B

B B B B B B B B B B B B

A A A B A A A A A A B B

B B B

C

B B B B B B B B B B B B

B B B B

B B A A

B C B B

B B B B

do

Black

A

Boxelder

C

Red

B

Silver Sugar

B A

Oak, red Black

C

Conspicuous growth ring, rotary-cut veneer has a watery figure with great contrast

C B B C B B C B B B C C

do do do do do do do do do do do do

Pronounced flake on the true quarter and a narrow flake when rift cut; distinct not conspicuous growth ring stripe do do do do do do do do do do do do

B B B B

do do do do

do do do do

California black Cherrybark Chestnut Laurel Northern red Nuttall Pin Scarlet Shumard Southern red Water Willow Oak, white Bur Chinkapin Delta post Durand

128

c

B B C B B B

c

Other

APPEARANCE AND SUITABILITY OF INDIVIDUAL U.S. SPECIES FOR VARIOUS USES OF VENEER—continued Common name

Figure of veneer Clear veneer 1 Rotary- and flat-sliced Quarter- and rift-sliced Construction ply wood

Relative suitability for—^ Decorative face veneer

Inner plies of decorative panels

Container veneer and ply wood

c

C

B B

C C

B B

B B B B B B

B B A A A B

c c

B B B C

B B B B B B

Other

UNITED STATES HARDWOODS—continued

Oak, white (cont.) Live Oregon white

C C

Overcup Post Swamp chestnut Swamp white White Ohia

B C B B B B

Persimmon, common

C

Distinct not conspicuous growth ring

Occasional ribbon due to interlocked grain

C

A-B

C

B

Sassafras

—

C

B

C

B

Silk-oak

A

Distinct not conspicPronounced growth uous growth ring ring Faint growth ring pat- Moderate-sized ray flakes lead to the tern name "lacewood" Distinct not conspicConspicuous growth uous growth stripe, ring fine sparkle from small rays Plain Faint growth ring Faint growth ring, oc- Distinct not pronounced ribbon occasionally irregular casionally irregular darker streaks darker streaks

B

A

B

B

B

B

C

A

B B

C B

A B

A A

B

A

B

A

B

B

C

B

B

A

B

B

B

B

B B A

B B B

Sugarberry

Pronounced ray flakes Pronounced flake on the true quarter and a narrow flake when rift cut; distinct not conspicuous growth ring stripe do do do do do do do do do do Faint growth ring pat- Poorly defined ribbon grain tern. Occasional burls

Moderate growth ring Conspicuous growth ring, rotary-cut veneer has a watery figure with great contrast

Sweetbay Sweetgum

A A

Sycamore, American

B

Faint growth ring

Tanoak

B

Plain, occasional burls

Teak

A

Moderate growth rings, dark irregular streaks, occasional burls

Tupelo Black Swamp Water Walnut, Black

Pronounced reddish flakes up to 1/4 inch in height Inconspicuous wood rays and occasional burls Faint growth stripe. dark irregular streaks, sometimes mottled, fiddleback or curly grain

Faint growth ring A A B

Distinct not pronounced ribbon, low luster do do do do Distinct not conspicInconspicuous growth uous growth ring, stripe, occasional occasional wavy and burl, crotch, curly cross figure

129

B B B

A A B

Face for plywood flooring Laminated golf club heads

APPEARANCE AND SUITABILITY OF INDIVIDUAL U.S. SPECIES FOR VARIOUS USES OF VENEER—continued Common name

Clear Figure of veneer veneer 1 Rotary- and flat-sliced Quarter- and rift-sliced Construction ply wood

Relative suitability for—2 Decorative face veneer

Inner plies of decorative panels

Container veneer and ply wood

Other

UNITED STATES HARDWOODS—continued

Willow, Black Yagrumo hembra

B A

Yellow-poplar

A

Faint growth ring Plain, moderate-sized vessels Faint growth ring

Plain, fine texture Plain

C C

B-C C

B B-C

B B

Plain

B

B

A

A

Toy airplanes

UNITED STATES SOFTWOODS

Cedar AlaskaAtlantic white

B

Faint growth ring

None

B

B

A

A

C

None

C

B

A

A

Faint growth rings. Spike knots included sapwood

C

A

B

C

B-C

B

B

B

B-C B

B B

B A

B A

Incense-

C

Distinct, not conspicuous growth ring Distinct growth ring, many knots, streaks of white sapwood alternating with purple-red to dark red heartwood Faint growth ring

Northern white Port-OrfordWestern redcedar

C A

do do

B

Distinct, not conspicuous growth ring

do

A-B

A

B-C

B

B

Conspicuous irregular growth ring

A-B

A

B

A

B

do

Distinct, not conspicuous growth ring stripe do

B

A

B

A

Distinct, not conspicuous growth ring stripe do do do

A

B-C

B

A-B

A B A

B-C B-C B-C

B B B

A-B A-B A-B

Faint growth ring stripe Distinct, not conspicuous growth ring stripe do do Faint growth ring stripe Distinct, not conspicuous growth ring stripe do

B-C

C

C

A

A-B

C

B-C

A

A-B A-B A-B

C C C

B-C B-C B-C

A A A

A-B

C

B-C

A

B-C

C

C

A

Eastern redcedar

Cypress Baldcypress Pondcypress Douglas-fir Coast Interior north interior south Interior west Fir Balsam

B-C

A-B

Conspicuous growth ring

B B B

do do do

C

Distinct, not conspicuous growth ring Conspicuous growth ring

California red

B-C

Grand Noble Pacific silver

C B C

do do do

Shasta red

B-C

do

Subalpine

C

Conspicuous growth ring

Faint growth ring stripe do do

130

Small boat parts Cedar chests

Decorative knotty faces and etched veneer

APPEARANCE AND SUITABILITY OF INDIVIDUAL U.S. SPECIES FOR VARIOUS USES OF VENEER—continued Common name

Figure of veneer Clear veneer 1 Rotary- and flat-sliced Quarter- and rift-sliced Construction ply wood

Relative suitability for—_2 Decorative face veneer

Inner plies of decorative panels

Container veneer and ply wood

A-B

C

B-C

A

B-C

C

B-C

A-B

B A-B

c c c

B B

A A

C

C

c c

C C B

Other

UNITED STATES SOFTWOODS—continued

do

C

do

C

Distinct, not conspicuous growth ring do do

Faint growth ring stripe do do

C

Distinct growth ring, many knots, mixed white sapwood and light red-brown heartwood

Too small to quarterslice

C

Rocky Mountain Western Larch, Western

C C B

do do Conspicuous growth ring

do do Distinct, not conspicuous growth ring stripe

C C A

B

C C C

Pine Digger

C

Distinct, not conspicuous growth ring Faint growth ring

Faint growth ring stripe None

B-C

C

C

B

B-C

A-B

B

A

Distinct, not conspicuous growth ring do do Faint growth ring Conspicuous growth ring

Faint growth ring stripe do do None Distinct, not conspicuous growth ring stripe Faint growth ring stripe

B-C

C

C

B

B B-C B-C A

A C C C

B C C C

A A A B

B

B

C

A

A

C

C

B

B-C B B

C C A

C C B

B B A

B

B

C

A

B-C

C

C

B

A A B-C B-C B-C

C C C A C

C C C B C

B B B A B

White Hemlock Eastern Mountain Western Juniper Alligator

c B

Eastern white

B

Jack

C

Jeffrey Knobcone Limber Loblolly

B C C B

Lodgepole

C

Longleaf

B

Pitch Pond Ponderosa

C B B

do do Distinct, not conspicuous growth ring

Red

B

do

Sand

B

Conspicuous growth ring

Shortleaf Slash Spruce Sugar Table-Mountain

B B B A C

do do do Faint growth ring Conspicuous growth ring

Distinct, not conspicuous growth ring; faint "pocked" appearance Conspicuous growth ring

Distinct, not conspicuous growth ring stripe do do Distinct, not conspicuous growth ring stripe Faint growth ring stripe Distinct, not conspicuous growth ring stripe do do do None Distinct, not conspicuous growth ring stripe

131

Decorative knotty faces

Decorative knotty faces

APPEARANCE AND SUITABILITY OF INDIVIDUAL U.S. SPECIES FOR VARIOUS USES OF VENEER—continued Common name

Clear Figure of veneer veneer 1 Rotary- and flat-sliced Quarter- and rift-sliced Construction ply wood

Relative suitability for—2 Decorative face veneer

Inner plies of decorative panels

Container veneer and ply wood

B-C B B-C A-B

C A C A

C B C C

B A A A

B

A

C

A

B-C B-C B B A-B

C C C C B

C C C C B

A A A A A

B-C A-B

C B

C C

A B

C

A

C

B

Other

UNITED STATES SOFTWOODS—continued

Virginia Western white Whitebark Redwood

C A C A

do Faint growth ring do Distinct, not conspicuous growth ring; occasionally wavy and burl Distinct, not conspicuous growth ring

do None do Faint growth ring stripe; occasionally wavy and burl

Big tree

A

Spruce Black Blue Engelmann Red Sitka

C C C C B

Faint growth ring do do do do

None

White Tamarack

C C

do Conspicuous growth ring

Yew, Pacific

C

Mild growth ring figure

do Distinct, not conspicuous growth ring stripe Not quarter-sliced

Faint growth ring stripe do do do do

Decorative etched veneer faces

Aircraft parts

1 An A rating indicates veneer logs of the species tend to have a high percent of clear wood, a C rating indicates a low percent of clear wood, and a B is intermediate. 2 A, indicates species is well suited for end product; B, intermediate; and C, generally not well suited for this product.

132

GLOSSARY Annual growth ring,—The layer of wood growth put on a tree during a single growing season. In the temperate zone the annual growth rings of many species (e.g., oaks and pines) are readily distinguished because of differences in the cells formed during the early and late parts of the season. In some temperate zone species (black gum and sweetgum) and many tropical species, annual growth rings are not easily recognized. Bird peck.—A small hole or patch of distorted grain resulting from birds pecking through the growing cells in the tree. In shape, bird peck usually resembles a carpet tack with the point towards the bark; bird peck is usually accompanied by discoloration extending for considerable distance along the grain and to a much lesser extent across the grain. Birdseye.—Small localized areas in wood with the fibers indented and otherwise contorted to form few to many circular or elliptical figures remotely resembling birds' eyes on the tangential surface. Sometimes found in sugar maple and used for decorative purposes; rare in other hardwood species. ßolt,— (l) A short section of a tree trunk; (2) in veneer production, a short log of a length suitable for peeling in a lathe. Burl,— (1) A hard, woody outgrowth on a tree, more or less rounded in form, usually resulting from the entwined growth of a cluster of adventitious buds. Such burls are the source of the highly figured burl veneers used for purely ornamental purposes. (2) In lumber or veneer, a localized severe distortion of the grain generally rounded in outline, usually resulting from overgrowth of dead branch stubs, varying from 1/2 inch to several inches in diameter; frequently includes one or more clusters of several small contiguous conical proturberances, each usually having a core or pith but no appreciable amount of end grain (in tangential view) surrounding it. Cellulose,—ThQ carbohydrate that is the principal constituent of wood and forms the framework of the wood cells. Closed sicZe.—Side of veneer not touching knife as it is peeled from log (also called tight side of veneer). Com6i^ram.—Veneer cut at about a 45° angle to the wood rays. The rays show as narrow, straight stripes on the face of the veneer. White oak is commonly sliced to produce combgrain face veneer. Compression wood,—Wood formed on the lower side of branches and inclined trunks of softwood trees. Compression wood is identified by its relatively wide annual rings, usually eccentric, relatively large amount of summerwood, sometimes more than 50 percent of the width of the annual rings in which it occurs, and its lack of demarcation between springwood and summerwood in the same annual rings. Compression wood shrinks excessively lengthwise, as compared with normal wood. Crossband,—To place the grain of layers of wood at right angles in order to minimize shrinking and swelling; also, in plywood of three or more plies, a layer of veneer whose grain direction is at right angles to that of the face plies. Crossfire,—Figure in fancy face veneer caused by the grain of the wood dipping in and out of the face of the veneer sheet. Crotch veneer,—Veneer cut from fork of tree to provide pleasing grain, figure, and contrast.

Density,—As usually applied to wood of normal cellular form, density is the mass of wood substance enclosed with the boundary surfaces of a wood-plus-voids complex having unit volume. It is variously expressed as pounds per cubic foot, kilograms per cubic meter, or grams per cubic centimeter at a specified moisture content. Diffuse-porous wood.—Certain hardwoods in which the pores tend to be uniform in size and distribution throughout each annual ring or to decrease in size slightly and gradually toward the outer border of the ring. Dubbing.—The extra heavy cut that may occur at the ends of a lathe or slicer knife when it is ground. This rounds the ends of the knife and is undesirable. Taking up slack in the parts of the grinding machine or use of short dummy knife sections at the ends of the knife during grinding will reduce or eliminate dubbing. Earlywood.—The portion of the annual growth ring that is formed during the early part of the growing season. It is usually less dense and weaker mechanically than latewood. Equilibrium moisture content.—The moisture content at which wood neither gains nor loses moisture when surrounded by air at a given relative humidity and temperature. Extractive,—Substances in wood, not an integral part of the cellular structure, that can be removed by solution in hot or cold water, ether, benzene, or other solvents that do not react chemically with wood components. Fiber saturation point,—The stage in the drying or wetting of wood at which the cell walls are saturated and the cell cavities are free from water. It applies to an individual cell or group of cells, not to whole boards. It is usually taken as approximately 30 percent moisture content, based on ovendry weight. Figured veneer,—General term for decorative veneer such as from crotches, burls, and stumps. Flitch,—A portion of a log sawn on two or more faces —commonly on opposite faces, leaving two waney edges. When intended for resawing into lumber, it is resawn parallel to its original wide faces. Or, it may be sliced or sawn into veneer, in which case the resulting sheets of veneer laid together in the sequence of cutting are called a flitch. The term is loosely used. Gum.—A comprehensive term for nonvolatile viscous plant exudates, which either dissolve or swell up in contact with water. Many substances referred to as gums, such as pine and spruce gum, are actually oleoresins. Hardwoods.—Generally one of the botanical groups of trees that have broad leaves in contrast to the conifers or softwoods. The term has no reference to the actual hardness of the wood. Heartwood.—The wood extending from the pith to the sapwood, the cells of which no longer participate in the life processes of the tree. Heartwood may contain phenolic compounds, gums, resins, and other materials that usually make it darker and more decay resistant than sapwood. Latewood.—The portion of the annual growth ring that is formed after the earlywood formation has ceased. It is usually denser and stronger mechanically than earlywood.

133

Lignin,—The second most abundant constituent of wood, located principally in the secondary wall and the middle lamella, which is the thin cementing layer between wood cells. Chemically it is an irregular polymer of substituted propylphenol groups, and thus no simple chemical formula can be written for it. Mineral streak,—An olive to greenish-black or brown discoloration of undetermined cause in hardwoods. Moisture content,—The amount of water contained in the wood, usually expressed as a percentage of the weight of the ovendry wood. Mold.—A fungus growth on wood products at or near the surface and, therefore, not typically resulting in deep discoloration. Mold discolorations are usually ash green to deep green, although black is common. Oleoresin.—A solution of resin in an essential oil that occurs in or exudes from many plants, especially softwoods. The oleoresin from pine is a solution of pine resin (rosin) in turpentine. Parenchyma,—Short cells having simple pits and functioning primarily in the metabolism and storage of plant food materials. They remain alive longer than the tracheids, fibers, and vessel segments, sometimes for many years. Two kinds of parenchyma cells are recognized—those in vertical strands, known more specifically as axial parenchyma, and those in horizontal series in the rays, known as ray parenchyma. Peel,—To convert a log into veneer by rotary cutting. Pitch streaks.—A well-defined accumulation of pitch in a more or less regular streak in the wood of certain conifers. Plywood.—A composite panel or board made up of crossbanded layers of veneer only, or veneer in combination with a core of lumber or of particleboard bonded with an adhesive. Generally the grain of one or more plies is roughly at right angles to the other plies. Pressure bar Fixed.—A bar on a lathe or slicer set to compress the wood just ahead of the knife edge. Roller,—Used on some lathes in place of a fixed pressure bar and performs the same function. Quarter-slicing,—A method of cutting face veneer nearly parallel to the wood rays. If the rays are large, as in oak, then they are prominent in the face veneer. Quarter-slicing also shows interlocked grain to advantage in species like mahogany. Reaction wood.—Wood with more or less distinctive anatomical characters, formed typically in parts of leaning or crooked stems and in branches. In hardwoods this consists of tension wood and in softwoods of compression wood. Resin,—Inflammable, water-soluble, vegetable substances secreted by certain plants or trees, and char-

134

acterizing the wood of many coniferous species. The term is also applied to synthetic organic products related to the natural resins. Resin ducts,—Intercellular passages that contain and transmit resinous materials. On a cut surface, they are usually inconspicuous. They may extend vertically parallel to the axis of the tree or at right angles to the axis and parallel to the rays. Short-grain,—Term used for cross grain as when end grain is exposed on face of veneer. Showthrough,—Term used when effects of defects within a panel can be seen on the face. Sliced veneer,—(See Veneer,) Softwoods,—Generally, one of the botanical groups of trees that in most cases have needlelike or scalelike leaves, the conifers; also the wood produced by such trees. The term has no reference to the actual hardness of the wood. Specific gravity,—As applied to wood, the ratio of the ovendry weight of a sample to the weight of a volume of water equal to the volume of the sample at a specified moisture content (green, air-dry, or ovendry). Stain.—A discoloration in wood that may be caused by such diverse agencies as micro-organisms, metal, or chemicals. The term also applies to materials used to impart color to wood. Straight-grained wood,—Wood in which the fibers run parallel to the axis of the piece. Tension wood,—A form of wood found in leaning trees of some hardwood species and characterized by the presence of gelatinous fibers and excessive longitudinal shrinkage. Tension wood fibers hold together tenaciously, so that sawed surfaces usually have projecting fibers, and planed surfaces often are torn or have raised grain. Tension wood may cause warping. Texture,—A term often used interchangeably with grain. Sometimes used to combine the concepts of density and degree of contrast between springwood and summer wood. Veneer,—A thin layer or sheet of wood. Rotary-cut veneer,—Veneer cut in a lathe which rotates a log or bolt, chucked in the center, against a knife. Sawed veneer,—Veneer produced by sawing. Sliced veneer,—Veneer that is sliced off a log, bolt, or flitch with a knife. Veneer checks,—When wood is cut into veneer with a knife, checks often form on the side of the veneer next to the knife. In general, checks tend to be deeper in thick veneer of dense wood than in thin veneer of lowdensity wood. Also called knife checks, lathe checks, and slicer checks. Veneer clipper,—Machine for cutting veneers into desired sizes.

INDEX techniques, 29, 74 temperatures, 74 time, 38, 74,117 veneer, 70

Abnormal wood, 15 Adventitious buds, 17, 24 Appearance, 125 Back grinding, 57 Back-roll lathe, 49 Bacterial action, 29,121 Bark pockets, 24 Bark removal, 30,117 Bird peck, 19, 24 Bolts for veneer, 31, 51, 68 Botanical names, 91 Box shook, 1,125 Bucking into bolts, 31 Buckle, 3, 83,117 Burls, 17, 24 Bushel baskets, 5,125

Eccentricity, 14, 24 Electric heating, 44 Embedded metal, 20, 24 End uses, 4,125 Epicormic branches, 17, 23 Extractives, 9, 23 Extraneous cell content, 9 Faces, 4 Felling splits, 20 Figure, 11,17, 23, 34,129 Fine texture, 23 Fire scars, 19 Flat-slicing, 32, 49 Flitches for veneer, 32, 68 Flush doors, 5, 125 Function of log grades, 13 Furniture parts, 2, 5

Case goods, 5,125 Checks in veneer, 11 Chucks, 49, 58 Cleated panel boxes, 5, 125 Clipping veneer, 69 Close grain, 23 Color, 10,17, 24, 95 Common names, 91, 95 Compression parallel, 23, 111 Compression perpendicular, 23, 111 Compression wood, 15, 24 Concrete form, 4,125 Conditioning wood, 34, 117 Construction plywood, 4,125 Container plywood, 22,125 Conveying veneer, 69 Core, 5,125 Cracks, quality control, 81 Crossband, 5, 125 Cutting : back cut, 34, 49 direction, 32 equipment, 45 flat-slicing, 32, 49 half-round, 32, 49 quarter-sliced, 34, 49 rift-cut, 32, 49 rotary, 32, 45 sawn, 34 slicing, 45 speed, 53 stay-log, 49 Cylindrical form, 24

Generalized settings, 66 Grain effects, 8,17, 40, 95 Grinding : veneer knife, 56 back grinding, 57 Growth rate, 7, 24 Growth stresses, 15 Gum, 9, 23 Gum streaks and pockets, 19 Half-round cutting, 32, 49 Handling damage, 24 Hard deposits, 11, 23 Hardness, 23, 111 Hardwoods, 2 Heat distortion, 51 Heating : benefits, 39 bolts and flitches, 31, 44 color changes, 37 decay resistance, 38 dimensional changes, 37 disadvantages, 39 drying time, 38 effects, 34 growth stresses, 37 hardness, 36 hot water, 40, 42 plasticity, 34 rate, 41 shrinkage, 38 steam, 40, 42, 44 strength, 37 time required, 40, 41 torque, 38 variability, 40 warp, 38 Hot water heating, 42

Debarking, 30,117 Decay, 24 Decorative plywood, 3, 125 Core, 4, 125 Crossband, 4, 125 Defects in drying, 117 Diameter effect, 40 Dimensional stability, 11, 23 Dryer : emissions, 74 fires, 86 types, 72 Drying:

Ideal veneer log, 12 Individual species, 91, 95, 111, 116, 121, 125

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Industrial plywood, 4 Inner plies : 4 case goods, 4 flush doors, 4 furniture, 5 wall panels, 4 Irregular grain, 17, 23

construction, 4, 22, 125 industrial, 5, 22, 125 Plywood-sheathed crates, 5, 125 Polyphenols, 10 Prefinished panels, 5 Properties of veneer logs, 11 Pressure bar: flxed, 60, 65 generalized setting, 66 lead for lathe, 61 lead for slicer, 61 roller, 60, 65 setting, 61, 117 setting gap, 63 terminology, 48 Processing variables, 116, 121

Kitchen cabinets, 4 Knife: angle, 48, 59 back grinding, 57 bevel, 48, 55 generalized settings, 66 grinding, 56 honing, 57 secondary bevels, 57 selection, 54 setting, 58,117 slicer, 60 terminology, 48 wear, 55 thickness, 55 type, 54 Knots, 16, 24

Quality control: buckle, 83 casehardening, 86 checks or cracks, 81 collapse, 86 color, 86 honeycomb, 86 shrinkage, 86 stain, 75 veneer roughness, 79 veneer thickness, 75 Quarter-sliced, 34, 49

Lathe : advantages, 47, 49, 69 back-roll, 49 cutting action, 45 dynamic equilibrium, 53 operation, 45 stay-log, 49 Log: breakdown, 31 characteristics, 24 diameter eccentricity, 14 end splits, 15, 24 grades, 13 handling damage, 20 processing, 31 requirements, 13 splits, 29, 41 storage, 29, 121

Requirements for veneer logs, 13 Resin, 10, 23 Resistance to splitting, 24 Retractable chucks, 50 Rift-cut, 32, 49 Ring shake, 16, 24 Roof sheathing, 4 Rotary cutting, 32, 47, 49, 87 Sawing into bolts, 31 Scars, 24 Seams, 19 Shake, 16 Shear, 23, 111 Shelling, 6, 8, 121 Shrinkage, 7, 23, 95 Slicer: advantages, 47, 49 dynamic equilibrium, 53 heat distortion, 53 offset, vertical face, 52 mechanism, 45 parts movement, 52 pawl & rächet, 52 stop plate, 52 wood movement, 52 yields, 87 Slicing techniques, 29, 45, 49 Species : appearance, 125 bark removal, 30 classiñcation for plywood, 22 density ranges, 22, 95 individual, 30, 91 log storage, 121 nomenclature, 91 processing variables, 116, 121 properties, 23 specific gravity, 25

Mechanical properties, 12, 23, 111 Metal stain, 11 Mineral streak, 24 Modulus of elasticity, 23, 111 Modulus of rupture, 23, 111 Moisture content, 3, 6, 23, 34, 41, 73, 84, 95, 111 Mold, 121 Movement, undesirable: wood, 49, 51 machine parts, 49, 51 Names, 91, 95 Odor, 11, 23, 29 Oleoresin, 10 Overlaid panels, 5 Paper-overlaid veneer, 5 Parenchyma, 8, 23 Peeling techniques, 29 Permeability, 7, 23, 95 Physical properties of wood, 3, 23, 95 Pitch pockets, 24 Plywood : block flooring, 4, 125

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suitability, 125 United States, 21, 25, 91, 95, 111, 116, 125 Species nomenclature, 91 Specific gravity, 3, 23, 25, 41, 42, 95 Specific uses, 125 Spindles, lathe, 50 Spinout, 38 Splits, 24, 117 Spur configuration, 50 Stains, 19, 24, 29, 121 Stay-log, 49 Steam heating, 42 Storage of logs, 29 Straight grain, 17, 23 Stresses, growth, 15 Stump pull, 20, 24 Subfloor, 4 Suitability for use, 125 Surface roughness, 121 Sweep, 24

conveying and clipping, 69 cutting, 1, 4, 45 decorative face, 4, 125 dryers, 74 drying, 70 figure, 11, 17, 23, 34, 129 flitches, 32, 68 gluability, 4 hardwoods, 22, 91, 121 lathe, 49, 69 properties, 70, 95, 111 quality, 2, 4, 75 roughness, 2, 121 slicer, 49, 69 softwoods, 22, 91, 121 species, 91 stiffness, 23, 111 strength, 23, 111 thickness,, 2, 76 uses, 4, 125 volume, 87 Veneer logs: characteristics, 13, 30 diameters, 13 form, 14 grades, 13 length, 13 properties, 13 sweep, 14 taper, 14 Veneer plant requirements, 88 Veneer yields, 87 Volume for plant, 87

Taper, 24 Temperature : constant, 41 final, 34, 39, 40 gradient, 40 storage, 29 total change, 40 Tension perpendicular. 111 Tension wood, 15, 24 Terminology, 48, 67 Texture, 8 Thickness, 2, 76 Timber requirement, 88 Torque, 38 Tree names, 91

Wall panels, 4, 125 Wall sheathing, 4, 125 Wax, 11, 23 Wirebound boxes, 5, 125 Wood: conditioning, 34, 117 movement in cutting, 49 permeability, 7, 23, 34, 95 physical properties, 3, 95 species, 4, 22, 91 suitability for veneer, 125 temperature, 34

Undesirable movement, 49, 51 Uniformity of thickness, 2, 76 Veneer : appearance, 125 buckle, 3, 83 characteristics, 121 checks, 11 color, 17, 24, 95

"l^ us GOVERNMENT PRINTING OFFICE: 1978 O-24S-770

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