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Sheet Metal Forming Technology Dr. N S Mahesh
M.S Ramaiah School of Advanced Studies - Bangalore
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Session Objectives • At the end of the session the delegates should have understood – Sheet metal cutting and bending – Significance of Forming Limit Diagram (FLD) – The different technologies of tube and sheet hydroforming – Other forming processes
M.S Ramaiah School of Advanced Studies - Bangalore
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Introduction • Products made by sheet – metal forming processes are all around us; they include metal desks, file cabinets, appliances, car bodies, aircraft fuselages, and beverage cans. • Sheet forming dates back to 5000 B.C. when household utensils and jewelry were made by hammering and stamping gold, silver, and copper. • Sheet metal = 0.4 (1/64) to 6 mm (1/4in) thick • Plate stock > 6 mm thick M.S Ramaiah School of Advanced Studies - Bangalore
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Sheet Metal Forming
• Sheet metal forming is a process that materials undergo permanent deformation by cold forming to produce a variety of complex three dimensional shapes. • The process is carried out in the plane of sheet by tensile forces with high ratio of surface area to thickness. • High rate of production and formability is determined by its mechanical properties M.S Ramaiah School of Advanced Studies - Bangalore
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M.S Ramaiah School of Advanced Studies - Bangalore
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Outline of Sheet-Metal Forming Processes
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Classification of sheet metal parts (based on contour) 1) Singly curved parts 2) Contoured flanged parts, i.e., parts with stretch flanges and shrink flanges. 3) Curved sections. 4) Deep-recessed parts, i.e., cups and boxes with either vertical or sloping walls. 5) Shallow-recessed parts, i.e., dish shaped, beaded, embossed and corrugated parts.
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Classification of sheet metal forming (based on operations)
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Sheet-metal Characteristics • Elongation – the capability of the sheet metal to stretch without necking and failure. • Yield-point elongation – Lüeder’s bands on Low-carbon steels and Al-Mg alloys.Lüder’s bands can be eliminated by cold-rolling the thickness by 0.51.5%.
• Anisotropy – Crystallographic and mechanical fibering anisotropy • Grain Size effect on mechanical properties • Residual Stress, Springback and Wrinkling • Testing method – Cupping test – Forming Limit Diagram
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Bending and contouring
(a) Three-roll bender: sometimes does not provide uniform deformation in thin-gauge sheet due to the midpoint of the span localisation of the strain. Often need the forth roll. (b) Wiper-type bender: The contour is formed by successive hammer blows on the sheet, which is clamped at one end against the form block. Wiper rolls must be pressed against the block with a uniform pressure supplied by a hydraulic cylinder. (c) Wrap forming: The sheet is compressed against a form block, and at the same time a longitudinal stress is applied to prevent buckling and wrinkling. Ex: coiling of a spring around a mandrel. M.S Ramaiah School of Advanced Studies - Bangalore
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Bending and Contouring Machines
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Spinning Processes Deep parts of circular symmetry stainless steel, such as tank heads, television cones.
Materials: aluminum and alloys, high strength - low alloy steels, copper, brass and alloys,
• The metal blank is clamped against a
form block, which is rotated at high speed. • The blank is progressively formed against the block, by a manual tool or by means of small-diameter work rolls. Note: (a) no change in thickness but diameter, (b) diameter equals to blank diameter but thickness stays the same. M.S Ramaiah School of Advanced Studies - Bangalore
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Shearing and blanking Shearing The separation of metal by the • A narrow strip of metal is severely movement of two blades plastically deformed to the point where operated based on shearing forces. it fractures at the surfaces in contact with the blades. • The fracture then propagates inward to provide complete separation.
Clearance (normally 2-10% thickness) • Proper -- clean fracture surface. • Insufficient -- ragged fracture surface. • Excessive -- greater distortion, greater energy required to separate metal.
Thickness M.S Ramaiah School of Advanced Studies - Bangalore
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Shearing action in metal
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Clearance
Figure (a) Effect of the clearance, c, between punch and die on the deformation zone in shearing. As the clearance increases, the material tends to be pulled into the die rather than be sheared. In practice, clearances usually range between 2% and 10% of the thickness of the sheet. (b) Microhardness (HV) contours for a 6.4-mm (0.25-in) thick AISI 1020 hot-rolled steel in the sheared region.
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Shearing Figure (a) Schematic illustration of shearing with a punch and die, indicating some of the process variables. Characteristic features of (b) a punched hole and (c) the slug.
M.S Ramaiah School of Advanced Studies - Bangalore
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Punch force • The force required to punch is, basically, the product of the shear strength of the sheet metal by the area sheared. Friction between the punch and the work piece can, however, increase this force substantially. The medium punch force, F, can be estimated from the equation: F = 0.7TL(UTS) • where T is sheet thickness, L is the total length sheared (such as the perimeter of a hole ), and UTS is the ultimate tensile strength of the material. M.S Ramaiah School of Advanced Studies - Bangalore
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Shearing Operations
• Die cutting is a shearing process that consists of the following operations – – – –
Perforating: punching a number of holes in a sheet. Parting: shearing the sheet into two or more pieces Notching: removing pieces (or various shapes) from the edges; and Lancing: leaving a tab without removing any material.
Figure (a) Punching (piercing) and blanking. (b) Examples of various shearing operations on sheet metal. M.S Ramaiah School of Advanced Studies - Bangalore
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Fine blanking • Very smooth and square edges can be produced by fine blanking (a)
(b)
Figure (a) Comparison of sheared edges produced by conventional (left) and by fine-blanking (right) techniques. (b) Schematic illustration of one setup for fine blanking. M.S Ramaiah School of Advanced Studies - Bangalore
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Fine blanked parts used in the automobile industry Fine blanked components in the automatic transmission of a passenger car M.S Ramaiah School of Advanced Studies - Bangalore
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Processes and Equipment • In mass production, punches and dies made of tool steel or sintered tungsten carbide (WC) are used. • Complex geometries can be created in compound dies, while several sheet forming operations are sequentially performed in progressive dies.
M.S Ramaiah School of Advanced Studies - Bangalore
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Forming Equipments 1) Forming presses
2) Dies
3) Tools
Forming machines 1) Mechanical presses 2) Hydraulic presses - energy stored in a flywheel is - slower - acting, longer stroke transferred to the movable slide on the down stroke of the press. -quick - acting , short stroke..
M.S Ramaiah School of Advanced Studies - Bangalore
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Actions of presses (according to number of slides, which can be operated independently of each other) 1) Single - action press - one slide - vertical direction 2) Double - action press • two slides • the second action is used to operate the hold-down, which prevents wrinkling in deep drawing. 3) Triple - action press • two actions above the die, one action below the die. M.S Ramaiah School of Advanced Studies - Bangalore
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Example: Press brake – single action
• A single action press with a very long narrow bed. • Used to form long, straight bends in pieces such as channels and corrugated sheets. M.S Ramaiah School of Advanced Studies - Bangalore
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Tooling Basic tools used with a metalworking press are the punch and the die. • Punch - A convex tool for making holes by shearing , or making surface or displacing metal with a hammer. • Die - A concave die, which is the female part as opposed to punch which is the male part.
Punch and die in stamping
Punches and dies
Die materials: High alloy steels heat treated for the punches and dies. M.S Ramaiah School of Advanced Studies - Bangalore
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Compound dies • Several operations can be performed on the same piece in one stroke of the press. • Combined processes and create a complex product in one shot. • Used in metal stamping processes of thin sheets. Transfer dies •Transfer dies are also called compounding type dies. •The part is moved from station to station within the press for each operation.
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Compound and Progressive Die (a)
(d)
(b)
(c)
Schematic illustrations: (a) before and (b) after blanking a common washer in a compound die. Note the separate movements of the die (for blanking) and the punch (for punching the hole in the washer). (c) Schematic illustration of making a washer in a progressive die. (d) Forming of the top piece of an aerosol spray can in a progressive die. Note that the part is attached to the strip until the last operation is completed. M.S Ramaiah School of Advanced Studies - Bangalore
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Progressive forming
washers
• Punches and dies are designed so that successive stages in the forming of the part are carried out in the same die on each stroke of the press. • Progressive dies are also known as multistage dies. Example: progressive blanking and piercing of flat washer. • The strip is fed from left to right. • The first punch is to make the hole of the washer. • The washer is then blanked from the strip. • The punch A is piercing the hole for the next washer. M.S Ramaiah School of Advanced Studies - Bangalore
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Metal sheet used in blanking process
Progressive die Optimize the material usage. Determining factors are 1) volume of production 2) the complexity of the shape M.S Ramaiah School of Advanced Studies - Bangalore
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Sheet metal Bending • Bending is a manufacturing process by which metal can be deformed by plastically deforming the material and changing its shape. • The material is stressed beyond its yield strength but below its ultimate tensile strength. • A process by which a straight length is transformed into a curved length. produce channels, drums, tanks
Principle Types of Bending • Air bending • Bottoming • Coining
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Springback Dimensional change of the formed part after releasing the pressure of the forming tool due to the changes in strain produced by elastic recovery.
Springback is encountered in all forming operations, but most easily occurs in bending M.S Ramaiah School of Advanced Studies - Bangalore
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Air Bending
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Air bending
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Bottoming
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Coining
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Other common types of bending
V-bending M.S Ramaiah School of Advanced Studies - Bangalore
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U-die bending
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Wiping die
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Rotary bending
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Hydraulic flexible forming process
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Hydroforming – Hydroforming is a manufacturing process where liquid pressure is used to form complex shapes – Rolled sheet steels and extruded parts are used to form complex parts using hydroforming – Advantages include weight reduction due to part consolidation, reduced investment in tooling, and reduced number and complexity in production steps
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a
b
c
d
e Faxial
Faxial
f
P a.
Raw tube is loaded into hydroforming dies
b.
Hydroforming press closes
c.
The sealing rods engage the part, seal the ends and fill it with water
d.
Pressure inside the part increases
e.
The sealing rods push the tube into the die (endfeed) and the internal pressure is ramped to its maximum value. The hydroformed part takes on the shape of the die.
f.
Final hydroformed part is removed M.S Ramaiah School of Advanced Studies - Bangalore
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Hydroforming -- exhaust manifold
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HYDROFORMED ENGINE CRADLE Rover 75
A. Roof Headers B. Instrument Panel Supports C. Radiator Supports D. Engine Cradles E. Roof Rails F. Frame Rails. In addition: Cross Car Beams · Headers · A,B,C Pillars · Seat Frames · Roll Bars · Control Arms M.S Ramaiah School of Advanced Studies - Bangalore
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Automotive Hydroformed Parts Automotive radiator supports, Instrument panel beams, Catalytic converter cones and exhaust components, cross members and engine cradles are among the parts currently being manufactured with hydroform technology. M.S Ramaiah School of Advanced Studies - Bangalore
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Hole punching, notching and bending during hydroforming M.S Ramaiah School of Advanced Studies - Bangalore
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Stretch Forming • Stretch forming is widely used in producing automotive body panels. Unlike deep drawing, the sheet is gripped by a blank holder to prevent it from being drawn into the die. It is important that the sheet can deform by elongation and uniform thinning. • During stretch-forming, the steel sheet is subjected to a mixture of elastic and plastic deformation. When the sheet is unloaded, the plastic component of deformation is retained. The elastic component however attempts to neutralize itself; this recovery of deformation is known as springback. M.S Ramaiah School of Advanced Studies - Bangalore
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Stretch forming Forming by using tensile forces to stretch the material over a tool or form block. • used most extensively in the aircraft industry to produce parts of large radius of curvature. (normally for uniform cross section). • required materials with appreciable ductility. • Springback is largely eliminated because the stress gradient is relatively uniform. Stretch forming feasible for aluminum, stainless steel, titanium M.S Ramaiah School of Advanced Studies - Bangalore
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Stretch forming equipment
Using a hydraulic driven ram (normally vertical). • Sheet is gripped by two jaws at its edges. • Form block is slowly raised by the ram to deform sheet above its yield point. • The sheet is strained plastically to the required final shape. Examples: large thin panel, most complex automotive stamping involve a stretching component. M.S Ramaiah School of Advanced Studies - Bangalore
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Deep drawing
The metalworking process used for shaping flat sheets into cupshaped articles. Pressing the metal blank of appropriate size into a shaped die with a punch Examples: bathtubs, shell cases, automobile panels. M.S Ramaiah School of Advanced Studies - Bangalore
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Deep drawing • It is best done with double-action press. • Using a blank holder or a holddown ring
• Complex interaction between metal and die depending on geometry. • No precise mathematical description can be used to represent the processes in simple terms.
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As the metal being drawn, • Change in radius • Increase in cup wall
Metal in the punch region is thinned down biaxial tensile stress. • Metal in the cup wall is subjected to a circumference strain, or hoop and a radial tensile strain. • Metal at the flange is bent and straightened as well as subjected to a tensile stress at the same time. M.S Ramaiah School of Advanced Studies - Bangalore
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Redrawing Use successive drawing operations by reducing a cup or drawn part to a smaller diameter and increased height – known as redrawing. Examples: slender cups such as cartridge case and closed end tubes. 1) Direct or regular redrawing : smaller diameter is produced by means of a hold-down ring. The metal must be bent at the punch and unbent at the die radii see Fig (a). Tapered die allows lower punch load, Fig (b).
2) Reverse or indirect redrawing : the cup is turned inside out the outside surface becomes the inside surface, Fig (c). Better control of wrinkling and no geometrical limitations to the use of a hold down ring.
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Formability • Formability is an important property for strip steels used for beverage cans, automotive panels, domestic appliance casings, etc. • Unfortunately, "formability" cannot be simply defined using a single material property, but is dependent on the particular forming process (e.g. deep-drawing, stretch forming, etc.) and on the sheet thickness.
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• The most appropriate measure of formability for stretch forming is the strain hardening exponent, or n value.
σ = kεn
• where k is a constant. • A high value of n is desired if the strip is to show good stretch formability. • Hot rolled strip steels have values of n ~ 0.1, whereas cold-reduced Al treated low carbon strip steels have n values of 0.2 - 0.25.
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Circle Grid Analysis • The plastic deformation during forming can be measured by circle grid analysis (CGA). • A sheet of metal is prepared for CGA by etching a circle grid onto the surface. Plastic deformation in the steel during the forming operation causes the circles to deform into ellipses. The amount of plastic strain at each circle can be calculated from the major and minor diameters of the ellipses. • The major strain is always positive, the minor strain can be positive or negative (or zero). M.S Ramaiah School of Advanced Studies - Bangalore
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Nakazima Tests and Forming Limit Diagrams • CGA is used in the Nakazima test to construct forming limit curves. • These give us useful information about which combinations of major and minor strains are likely to result in failure for a material of a given thickness
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Nakazima Tests and Forming Limit Diagrams (cont) A series of blank test pieces of different width are etched with a grid of circle The sheets are then deformed in a hemispherical shell test until splitting occurs
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Nakazima Tests and Forming Limit Diagrams (cont)
The narrowest strip (A) has deformed uniaxially whilst the wide strip (C) has deformed biaxially. Strips of intermediate width will deform under a range of stress states M.S Ramaiah School of Advanced Studies - Bangalore
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Nakazima Tests and Forming Limit Diagrams (cont) The dimensions of the unbroken ellipses closet to the splits are measured and the major and minor strains calculated
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These values are then plotted on a graph of major strain on the y-axis, against minor strain on the x-axis Values from samples of intermediate widths are also added A curve is then drawn through the data points “FLC” At strain values above this curve, the material for this thickness will fail
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A parallel curve drawn 10% below the FLC is usually given. For strains below this, the material will be safe from failure Strain levels within the marginal zone should normally be avoided as small variations in steel stock, lubrication conditions etc could lead to the possibility of splitting
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Factors Affecting Stretch-Formability • All forming limit curves for the grades of steel used in automotive bodies have essentially the same shape. The difference is their vertical position on the diagram, which is determined by the strain hardening exponent, n, and thickness of the steel. • Steels with a high FLD0 have a better formability than those with a low FLD0. What would be the consequence on formability by using thinner, higher strength steels with a lower n value?
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Forming Limit Diagram for a Car Door Panel
A car door panel is shaped by stretch forming. Major and minor strains at different points across the surface is calculated using CGA M.S Ramaiah School of Advanced Studies - Bangalore
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Forming Limit Diagram for a Car Door Panel (contd)
The strains are plotted on the FLD at different points across the surface using CGA. All the points on the FLD fall within the ‘safe’ area, and no failure (splitting or necking) is expected M.S Ramaiah School of Advanced Studies - Bangalore
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Forming Limit Diagram for a Car Door Panel (contd)
The material (thickness, n value) and process variables (lubrication, die geometry etc) are changed, and some of the points now fall above the FLD. In this case splitting would expect to occur M.S Ramaiah School of Advanced Studies - Bangalore
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Forming limit diagram for different materials
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Defects in formed parts
Springback problem Edge conditions for blanking
Crack near punch region
• Local necking or thinning or buckling and wrinkling in regions of compressive stress. • Springback tolerance problems. • Cracks near the punch region in deep drawing minimised by- increasing punch radius, lowering punch load.
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Defects in formed parts • Radial cracks in the flanges and edge of the cup due to not sufficient ductility to withstand large circumferential shrinking. • Wrinkling of the flanges or the edges of the cup resulting from buckling of the sheet (due to circumferential compressive stresses) solved by using sufficient hold-down pressure to suppress the buckling. • Surface blemishes due to large surface area. Ex: orange peeling especially in large grain sized metals because each grain tends to deform independently use finer grained metals. • Mechanical fibering has little effect on formability. • Crystallographic fibering or preferred orientation may have a large effect. Ex: when bend line is parallel to the rolling direction, or earing in deep drawn cup due to anisotropic properties.
Earing in drawn can
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Defects in formed parts
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Stretcher strains or ‘worms’ (flamelike patterns of depressions). Associated with yield point elongation. • The metal in the stretcher strains has been strained an amount = B, while the remaining received essentially zero strain. • The elongation of the part is given by some intermediate strain A. • The number of stretcher strains increase during deformation. The strain will increase until the when the entire part is covered it has a strain equal to B. •
Stretcher strain in low-carbon steel
Solution: give the steel sheet a small cold reduction (usually 0.5-2% reduction in thickness). Ex: temper-rolling, skin-rolling to eliminate Relation of stretcher strain to stress yield point. Relation of stretcher strain to stress strain curve. M.S Ramaiah School of Advanced Studies - Bangalore
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Summary The session covered following aspects of sheet metal forming – Sheet metal cutting and bending- types and methods – Dies for sheet metal punching, blanking etc., – The different technologies of tube and sheet hydroforming – Significance of Forming Limit Diagram (FLD) – Case studies on forming limit of metals – Defects in formed parts
Thank you M.S Ramaiah School of Advanced Studies - Bangalore
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