Machining Summary

Draft Document

Machining Summary Lathe Forming and Generating Forming Because a form tool is customized to the profile of the part, a single plunge can machine a complex shape. On CNC machines, this tooling potentially saves time and increases precision Form tools can be expensive so tend to be used where large quantities of a part are required.

Generating This is the use of a single point cutting tool to machine a complex shape It will involve a number of processes such as facing, parallel turning, taper turning and takes much more time than using a formtool

Oblique & Orthogonal Cutting

Turning Operations on the Lathe

Types of Chip Formation & Chip Breaker The type of chip produced depends on the material being machined and the cutting conditions at the time. These conditions include the type of tool used, rate of cutting, condition of the machine and the use or absence of a cutting fluid. Continuous Chip This leaves the tool as a long ribbon and is common when cutting most ductile materials such as mild steel, copper and Aluminium. It is associated with good tool angles, correct speeds and feeds, and the use of cutting fluid.

Continuous Chip Discontinuous Chip The chip leaves the tool as small segments of metal resulted from cutting brittle metals such as cast iron and cast brass with tools having small rake angles. There is nothing wrong with this type of chip in these circumstances.

Discontinuous Chip Continuous Chip with Builtup Edge This is a chip to be avoided and is caused by small particles from the workpiece becoming welded to the tool face under high pressure and heat. The phenomenon results in a poor finish and damage to the tool. It can be minimised or prevented by using light cuts at higher speeds with an appropriate cutting lubricant. Continuous Chip with Buildup Edge

Characteristics of Tool Material For efficient cutting a tool must have the following properties: Hot Hardness This means the ability to retain its hardness at high temperatures. All cutting operations generate heat, which will affect the tool¡¦s hardness and eventually its ability to cut. Strength and Resistance to Shock At the start of a cut the first bite of the tool into the work results in considerable shock loading on the tool. It must obviously be strong enough to withstand it. Low Coefficient of Friction The tool rubbing against the workpiece and the chip rubbing on the top face of the tool produce heat which must be kept to a minimum.

Tool Materials in Common Use High Carbon Steel Contains 1 - 1.4% carbon with some addition of chromium and tungsten to improve wear resistance. The steel begins to lose its hardness at about 250° C, and is not favoured for modern machining operations where high speeds and heavy cuts are usually employed. High Speed Steel (H.S.S.) Steel, which has a hot hardness value of about 600° C, possesses good strength and shock resistant properties. It is commonly used for single point lathe cutting tools and multi point cutting tools such as drills, reamers and milling cutters. Cemented Carbides An extremely hard material made from tungsten powder. Carbide tools are usually used in the form of brazed or clamped tips. High cutting speeds may be used and materials difficult to cut with HSS may be readily machined using carbide tipped tool.

Workholding on Lathe The two main methods of workholding on the lathe are using a 3-jaw or a 4-jaw chuck The 4-jaw chuck can grip square material and can grip material off-centre where a 3-jaw chuck cannot.

Tool Angles There are three important angles in the construction of a cutting tool rake angle, clearance angle and plan approach angle.

Cutting Speed

Where: N = Spindle Speed (RPM) CS = Cutting Speed of Metal (m/min) d = Diameter of Workpiece

Screw Cutting It is a slightly more difficult task than plain turning because it involves accurate setting up of the tool and exact setting of feed in relation to the work rotation.. Fig 10 shows the arrangement in simplified form.

Screw Cutting Set-up

Cutting Fluids The aims in metal cutting are to retain accuracy, to get a good surface finish on the workpiece and at the same time to have a longer tool life.

However during the metal cutting process heat is generated due to:  

the deformation of the material ahead of the tool friction at the tool point

Heat generated due to friction can readily be reduced by using a lubricant. Heat caused by deformation cannot be reduced and yet it can be carried away by a fluid. Thus the use of a cutting fluid will serve to reduce the tool wear, give better surface finish and a tighter dimensional control.

Cutting fluid safety hazards Rancidity – Coolant must be replaced regularly as constant heating and cooling makes it go off quickly Irritant- can be an irritant to the skin and eyes. Contact with skin may cause dermatitis ( skin disease) – Barrier cream is used as a prevention method.

Milling

Plain Milling

Plain milling is the milling of a flat surface with the axis of the cutter parallel to the machining surface. It can be carried out either on a horizontal machine or a vertical machine

End Milling End Milling is the milling of a flat surface with the axis of the cutter perpendicular to the machining surface

Gang Milling Gang milling is a horizontal milling operation that utilises three or more milling cutters grouped together for the milling of a complex surface in one pass.

Straddle Milling In straddle milling, a group of spacers is mounted in between two side and face milling cutters on the spindle arbor as shown.

Up-cut Milling In up cut milling, the cutter rotates in a direction opposite to the table feed. It is conventionally used in most milling operations because the backlash between the leadscrew and the nut of the machine table can be eliminated.

Down-cut Milling In down cut milling, the cutter rotates in the same direction as the table feed. This method is also known as Climb Milling can only be used on machines equipped with a backlash eliminator or on a CNC milling machine. This method, when properly treated, will require less power in feeding the table and give a better surface finish on the workpiece

Dividing Head A dividing head is a tool that is used to divide a circle into equal divisions. It can be used to move a workpiece at set intervals to help to produce a number of similar cuts. It is used for cutting square and hexagonal ends on bars and for cutting gear teeth on a gear. With a gear ratio of 40:1, 40 turns equals 360, 20 turns equals 180, 10 turns equals 90 and so on.

Slab Milling For heavy cutting of large and flat surfaces. Cutter is mounted on an arbour Done on horizontal Milling Machine

Examples of Vertical Milling Cutters

End Mill – (4 cutting edges) Used for standing milling – cannot drill holes in work piece as the cutting edges don’t meet at the tip

Slot Drill – (2 cutting edges) Used for standard milling and can also drill holes and take out closed slots as cutting edges meet at the tip Roughing Cutter – Used when removing a large amount of material, does not give a good finish

Ball Nosed Cutter – Used for milling a curved bottomed slot

Dovetail Cutter – Used for milling dovetail slides on a workpice

T-Slot Cutter – Used for milling T-Slots on drill and milling machine tables

Measurement & Inspection Direct and Comparative measurement Direct measurement is taken using a measurement device which gives an exact reading, vernier calipers, micrometer etc. Comparative measurement is taken using a go - no go gauge measuring the ability to fall within a set tolerance

A Go-NoGo gauge (or Go/no go) refers to an inspection tool used to check a workpiece against its allowed tolerances Plug Guages Used for checking tolerance in slots and holes and thread pitch tolerance in threaded holes Go side must fit and No-Go must not fit for part to conform

Snap Guage Used for checking tolerance bands on external diameters. The part must slip past outer shoe but not inner one to conform

Telescopic Gauges Used to measure a gap or internal diameter that may be hard to reach with a conventional device. Guage is locked to size and measured using a micrometer

Note - Micrometer is accurate to .001mm while Vernier Callipers is accurate to .002mm Slip Guages

Interference fit / Clearance fit

Tolerances in engineering It is virtually impossible for Industry to consistently manufacture size perfect components. In machining there is a number of factors which influence the accuracy of components, such as machine setup, machine reliability (wear and tear), cutting tool imperfections, operator skill level ect. Because of all of the above, making batches of exact sized parts is unobtainable. As a solution the manufacturer works with their customer to devise tolerance bands for the dimensions of each part. These tolerance bands allow some movement in component size while skill keeping components within a working tolerance for the customer. For example a customer may request a pin to be manufactured with a diameter of 20mm. However, on further investigation a pin with a diameter in the region of 19.95 – 20.05 may be acceptable to the customer. This would lead to the manufacturer making the pin to a tolerance of ±.05

(Examples of Tolerances) Engineers Component drawing showing permitted tolerances

Precise measurement can be inaccurately taken for a number of reasons  operator error  poorly calibrated measurement device  incorrect room temperature (19 degrees is ideal)

Precision Grinding Grinding process:    

Referred to as abrasive machining Cutting element is the Grinding Wheel Rotates at very high speeds Work is feed into the rotating wheel

Grinding Wheels are made up of 2 components 1. The abrasive grains or Grit 2. The Bond

Importance of Bond:     

While grinding the abrasive grains become worn The bond should then allow these worn grains to break away. This exposes new sharp grains underneath This process is known as Self Dressing A grinding wheel selected correctly will always self dress and cut properly

The main faults which can develop are: 1. Loading 2. Glazing Loading:  Wheel becomes loaded when small particles get lodged between the grains.  Wheel does not cut effectively  Surface finish is affected Glazing:  Grinding Wheel will have a shine off it  Grains on the wheel become worn don’t cut anymore

Dressing a grinding wheel Self dressing: When abrasive grains break away from the bond to expose new grains underneath. Process continues...... Abrasive grains can also break along cleavage planes to produce very sharp edges. Similar to breaking a smooth sandstone in pieces. Dressing Wheel: A diamond traversed across the wheel will correct any irregularities

There are 3 main grinding processes: 1. Surface Grinding 2. Cylindrical Grinding 3. Internal Grinding

Internal Grinding

Workholding for grinding (Surface Grinding)  Main form of workholding in surface grinding is the Magnetic Chuck.  Uses high powered magnets to hold the workpiece in position.  It’s advantage is no part of the device gets in the way of the process.  Before using the magnetic chuck you must consider the following:  The work must be magnetic  Work must be flat to be in contact with chuck  Surface of work and chuck must be clean  Magnetic chuck can be switched on and off using a lever.  Method of switching is called flux diversion

The Pedestal Grinder

The pedestal grinder can be bench or floor mounted and is used for small jobs in the workshop. This grinder allows the operator to hold the workpiece so does not have the same level of precision as the other grinding processed discussed Two of its main functions would be: 1. Grinding tungsten tips for TIG welding 2. Grinding tool angles on HSS lathe cutting tools The wheels on a pedestal grinder are dressed using a special grinding wheel dresser