How Milling machines work

Milling machines and milling techniques,how to operate milling machine pdf, how to use milling machine step by step,what milling machines are used for, what is milling machine working principle
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Published Date:03-08-2017
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11 Milling machines and milling techniques When you have read this chapter you should understand: • How to identify the main features of a typical horizontal milling machine. • How to identify the main movements of a typical horizontal milling machine. • How to identify the main features of a typical vertical milling machine. • How to identify the main movements of a typical vertical milling machine. • How to select a milling machine appropriate for the work in hand. • The types of milling cutters that are available and their applications. • How to select suitable cutters and how to check for defects. • The correct methods of mounting and holding milling cutters. • The available methods of workholding and setting. • How to use milling machines to produce vertical, horizontal and angular faces and slots. 11.1 Safety Milling machines are classified as especially dangerous machines. In addi- tion to the normal requirements of the Health and Safety at Work Act, these machines are also subject to the Horizontal Milling Machine Regu- lations. Copies of these Regulations are available in the form of a wall chart which is supposed to be hung up near to where such machines are being used. The main danger associated with milling machines is the cutter. There- fore: • Make sure the cutter guard is in place before starting the machine. • Do not remove swarf with a brush whilst the cutter is revolving. • Do not wipe away coolant from the cutting zone with a rag whilst the cutter is revolving. • Do not take measurements whilst the cutter is revolving. • Do not load or unload work whilst the cutter is revolving.Milling machines and milling techniques 343 • Do not put your hands anywhere near the cutter whilst it is revolving. Figure 11.1(a) shows a typical cutter guard as used by skilled oper- ators in toolrooms, prototype workshops and jobbing workshops where the machine is being frequently reset. (a) (b) Figure 11.1 Milling machine guards: (a) toolroom type cutter guard; (b) production type cutter guard Figure 11.1(b) shows a production type guard suitable where only semi- skilled labour is employed to operate the machine. The whole of the cutting zone is guarded and loading and unloading of the workholding fixture takes place safely outside the guard. 11.2 The milling process Milling machines are used to produce parallel, perpendicular and inclined plain surfaces using multi-tooth cutters. These cutters are rotated by the machine spindle, and it is from the plane in which the axis of the spindle lies that determines the name of the machine. The geometry of a single point cutting tool as considered in the previous chapters is shown in Fig. 11.2(a), whilst Fig. 11.2(b) shows how these angles are applied to a milling cutter tooth. The additional secondary clearance angle prevents the heel of the tooth catching on the workpiece as the tool rotates. It also provides chip clearance. Figure 11.2(c) shows an actual milling cutter. Because of the large number of teeth used, the surface produced is vir- tually a plain surface free from ripples. The surface can be improved even further by cutting the teeth with a helix angle as shown in Fig. 11.2(d)344 Engineering Fundamentals Rake angle Rake angle Secondary clearance angle Clearance angle Primary clearance angle (a) (b) (c) (d) Figure 11.2 Milling cutter tooth angles: comparison of cutter angles for a single point cutting tool (a) and ◦ a milling cutter tooth (b); (c) orthogonal cutting (straight tooth) cutter; (d) oblique cutting (30 helical tooth cutter) (photographs reproduced courtesy of Cincinatti Milacron Ltd) instead of straight across the cutter. This also evens out the forces acting on the machine transmission system since one tooth is starting to cut before the previous tool has finished cutting. As for turning, modern practice favours the use of carbide-tipped milling cutters for production milling where high rates of material removal are required or when high strength materials are being machined. These can have brazed-on tips as shown in Fig. 11.3(a) or inserted, disposable tips as shown in Fig. 11.3(b). Nowadays, cutters with disposable carbide and coated carbide tips are widely used on production and even for the prototype machining of high strength, hard and abrasive materials. It would appear from the above comments that the more teeth a cutter has got, the better will be the finish and the faster the cutter will be able to remove metal. This is true only up to a point. For any cutter of a given circumference, increasing the number of teeth reduces the space between the teeth. This makes the teeth smaller and weaker and it also reduces the room for the chips so that the teeth tend to clog easily and break.Milling machines and milling techniques 345 (a) (b) Figure 11.3 Carbide-tipped milling cutters: (a) brazed tip cutter; (b) inserted disposable tip cutter (repro- duced courtesy of Richard Lloyd (Galtona) Ltd) When choosing a milling cutter for a particular job, the spacing (pitch) of the teeth should be kept as wide as possible for a given class of work in order to provide adequate strength and chip clearance. Thus coarse pitch cutters should be used for roughing out robust work as Cutter they have more efficient material removal characteristics and are more rotation economical in the cutting power required. Finer pitch cutters should be Workpiece used with light cuts where fragile work is involved and a fine finish is required. Chip Direction of feed 11.2.1 Up-cut or conventional milling (a) This is shown in Fig. 11.4(a). You can see that the work is fed towards the cutter against the direction of rotation. • This prevents the work being dragged into the cutter if there is any backlash in the feed mechanism. Cutter Workpiece • Unfortunately this technique causes the cutting edges to rub as each rotation tooth starts to cut and this can lead to chatter and blunting of the Chip cutting edge. Direction of feed • The cutting action tends to lift the work off the machine table. (b) • For safety this is the technique you should always adopt unless Figure 11.4 Chip formation your instructor advises you to the contrary because he or she knows when milling: (a) up-cut milling; that your machine is equipped to operate safely using the following (b) down-cut milling technique.346 Engineering Fundamentals 11.2.2 Down-cut or climb milling This is shown in Fig. 11.4(b). Here you can see that the work is fed into the cutter in the same direction as the cutter is rotating. Safety: The climb milling technique can be used only on machines fitted with a ‘backlash eliminator’ and which are designed for this technique. If it can be used safely this technique has a number of advantages, particularly for heavy cutting operations. • The cutter does not rub as each tooth starts to cut. This reduces the risk of chatter and prolongs the cutter life. • The cutting forces keep the workpiece pressed down against the machine table. • The action of the cutter helps to feed the work forward and takes most of the load off the feed mechanism. 11.3 The horizontal The horizontal milling machine gets its name from the fact that the axis of spindle milling machine the spindle of the machine, and therefore the axis of the arbor supporting the cutter, lies in a horizontal plane as shown in Fig. 11.5. The more important features and controls are also named in this figure. Figure 11.5 Horizontal spindle milling machineMilling machines and milling techniques 347 11.3.1 Basic movements and alignments of a horizontal spindle milling machine The basic alignments and movements of a horizontal milling machine are shown in Fig. 11.6. The most important alignment is that the spindle axis, and therefore the arbor axis, is parallel to the surface of the worktable. The depth of cut is controlled by raising the knee and table subassembly. The position of the cut is controlled by the cross-slide and the feed is provided by a lead screw and nut fitted to the table and separately driven to the spindle. Unlike the feed of a lathe which is directly related to the spindle speed and measured in mm/rev, the feed of a milling machine table is independent of the spindle speed and is measured in mm/min. Overarm Steady 90° = = bearing 90° = = Spindle = axis 90° Table Saddle Column Knee 90° 90° Base Figure 11.6 Horizontal spindle milling machine: movements and align- ments The horizontal milling machine can produce surfaces that are parallel to the worktable as shown in Fig. 11.7(a). It can also produce surfaces that are perpendicular to the worktable as shown in Fig. 11.7(b). The use of a side and face is shown in Fig. 11.7(c). It can be seen that for this latter cutter the depth of cut is limited by the relative diameters of the cutter and the arbor spacing collars. 11.4 The vertical spindle The vertical milling machine gets its name from the fact that the axis milling machine of the spindle of the machine, and therefore the axis of the cutter being used, lies in the vertical plane as shown in Fig. 11.8. The more important features and controls are also named in this figure. 11.4.1 Basic movements and alignments of a vertical spindle milling machine The basic alignments and movements of a vertical milling machine are shown in Fig. 11.9. The most important alignment is that the spindle axis, and therefore the cutter axis, is perpendicular to the surface of the348 Engineering Fundamentals Machine column Spindle and arbor axis Spacing collars Arbor (a) Machine Face mill or column shell-end mill Spindle and arbor axis (c) (b) Figure 11.7 Surfaces parallel and perpendicular to the worktable (horizontal milling machine): (a) use of a slab mill to machine a surface parallel to the milling machine table; (b) use of a face mill or a shell-end mill to machine a surface perpendicular to the milling machine table; (c) use of a side and face milling cutter to machine a surface perpendicular to the milling machine table – the depth of the perpendicular surface is limited by the relative diameters of the cutter and spacing collars worktable. The depth of cut is controlled by raising the knee and table subassembly or, for some operations raising or lowering the spindle. For maximum rigidity, the spindle is normally raised as far as possible. The position of the cut is controlled by the cross-slide and the feed is provided by a lead screw and nut fitted to the table and separately driven to the spindle. As for horizontal milling, the feed of a vertical milling machine table is independent of spindle and is measured in mm/min. Vertical milling machines produce surfaces parallel to the worktable by means of face milling cutters mounted directly on the spindle end as shown in Fig. 11.10(a). Compared with the rate of metal removal that can be removed with a slab or roller mill on a horizontal machine, larger surfaces can be covered in one pass at greater rates of material removal with a face mill on a vertical spindle machine. Surfaces perpendicular to the worktable are produced by the side of an end milling cutter as shownMilling machines and milling techniques 349 Spindle axis Face mill (a) Figure 11.8 Vertical spindle milling machine Spindle axis Spindle axis Spindle axis End mill = Head Table = 90° 90° 90° Saddle Knee Column 90° Base (b) Figure 11.10 Surfaces parallel and perpendicular to the work- Figure 11.9 Vertical spindle milling machine: movements and align- table (vertical milling machine): ments (a) use of a face mill to machine surfaces parallel to the work- table; (b) use of an end mill to in Fig. 11.10(b). Since the cutter is supported as a cantilever by its shank machine surfaces perpendicular alone, the load that can be put on it is limited and only relatively low to the machine table rates of material removal can be removed in this way.350 Engineering Fundamentals 11.5 Types of milling Although side and face milling cutters and slab (roller) milling cutters cutters and their are usually associated with horizontal milling machines, and end mills, applications slot drills and facing cutters are normally associated with vertical milling machines, any cutter can be used with either machine given a suitable toolholding device. For the time being, however, we will consider the cutters and the surfaces that the produce in conjunction with the machine with which they are most usually associated. 11.5.1 Horizontal milling machine cutters Figure 11.11 shows some different shapes of milling cutter and the sur- faces that they produce. When choosing a milling cutter you will have to specify: Rotation Rotation Feed Feed (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) Figure 11.11 Horizontal milling machine cutters and the surfaces they produce: (a) slab milling cutter (cylin- der mill); (b) side and face cutter; (c) single-angle cutter; (d) double equal-angle cutter; (e) cutting a V-slot with a side and face mill; (f) double unequal-angle cutter; (g) concave cutter; (h) convex cutter; (i) single and double corner rounding cutters; (j) involute gear tooth cutter • The bore of this must suit the arbor on which the cutter is to be mounted. In many workshops one size of arbor will be standard on all machines and all the cutters will have the appropriate bores. • The diameter of the cutter. • The width of the cutter to suit the work in hand. • The shape of the cutter. • The tooth formation. 11.5.2 Vertical milling machine cutters A selection of milling cutters suitable for a vertical milling machine is shown in Fig. 11.12 and some typical applications are shown in Fig. 11.13. Note that only slot drills can be used for making pocket cutsMilling machines and milling techniques 351 Ball-nosed Dovetail End mill Slot drill slot drill cutter T-slot cutter Woodruff Corner Face milling cutter cutter rounding cutter Figure 11.12 Typical milling cutters for vertical spindle milling machines Rotation Rotation Feed Feed (a) (b) (c) This recess would have to be machined with a slot drill which is the only cutter that will work from the solid A This ‘blind’ keyway would have to be sunk with a slot drill B This recess can be cut with an end mill since cutter can work in from edge of blank. A slot drill could also be used (d) (e) (f) Initial slot machined to size with a suitable slot mill Woodruff key Table Woodruff feed cutter T-slot cutters have peripheral and face cutting teeth unlike the woodruff cutter which only cuts peripherally (g) (h) Figure 11.13 Vertical milling machine cutters and the surfaces they produce: (a) end milling cutter; (b) face milling cutter; (c) slot drill; (d) recess A would need to be cut with a slot drill because it is the only cutter that will work from the centre of a solid; recess B could be cut using a slot drill or an end mill because it occurs at the edge of the solid; (e) this blind keyway would have to be sunk with a slot drill; (f) dovetail (angle) cutter; (g) T-slot cutter; (h) Woodruff cutter352 Engineering Fundamentals from the solid. All the other cutters have to be fed into the workpiece from its side as they cannot be fed vertically downwards into the work. When choosing a cutter you will need to specify: • The diameter of the cutter. • The length of the cutter. • The type of cutter. • The type of shank. Some cutters have solid shanks integral with the cutter for holding in a chuck, whilst other cutters are made for mount- ing on a separate stub arbor. Some large face milling cutters are designed to bolt directly onto the spindle nose of the machine. 11.6 Cutter mounting Safety: Make sure the machine is electrically isolated before attempt- (horizontal milling ing to remove or mount arbors and cutters. machine) 11.6.1 Long arbor For most milling operations on horizontal spindle milling machines the cutters are mounted on a long arbor as shown in Fig. 11.14(a). One end of the arbor has a taper for locating in the spindle nose of the milling machine. It also has a slotted flange that registers with the driving dogs on the spindle nose. This arrangement provides a positive drive to the Spacing collars Cutter Taper to locate arbor Driving dog (a) (b) Taper to fit spindle nose Register for driving dog Arbor register tapped to receive draw bolt (c) Figure 11.14 Horizontal milling machine arbor: (a) long arbor for horizontal milling machine; (b) milling machine spindle nose; (c) taper register of arbor to fit spindle noseMilling machines and milling techniques 353 arbor and no slip is possible. Details of the spindle nose are shown in Fig. 11.14(b) and details of the taper on the arbor end are shown in Fig. 11.14(c). The taper of a milling machine spindle nose is not self- holding like the morse taper of a drill shank. Milling machine arbors have to be held in place by a threaded drawbar that passes through the whole length of the spindle. Tightening the drawbar into the end of the arbor pulls it tightly into the spindle nose. The outer end of the arbor is supported in a steady. The steady itself is supported by the milling machine overarm as shown in Fig. 11.15. The forces acting on a milling cutter when it is removing metal rapidly are very great. Therefore the cutter arbor must be adequately supported and the cutter correctly positioned to avoid inaccuracies, chatter and, at worst, a bent arbor. In Fig. 11.15(a) the cutter is incorrectly mounted. There is excessive overhang from the points of support. Overarm Arbor Cutter Excessive Excessive Intermediate Overhang reduced Steady overhang overhang steady to a minimum (a) (b) (c) Figure 11.15 Correct use of overarm steady: (a) bad mounting; (b) and (c) good mounting In Fig. 11.15(b) the overarm and steady bearing have been repositioned to provide support as close to the cutter as possible. Also the cutter itself has been mounted as close to the spindle nose as possible. Thus any overhang has been reduced to a minimum and the cutter is supported with the maximum rigidity. Sometimes the shape and size of the work prevents the cutter being mounted close to the spindle nose. Figure 11.15(c) shows how an addi- tional, intermediate steady can be positioned on the overarm to support the arbor immediately behind the cutter. This again reduces the overhang to a minimum. 11.6.2 Mounting cutters on a long arbor The following description assumes that the machine has been left in a clean condition without a cutter on the arbor but with the spacing collars in position on the arbor and the locknut only finger tight to prevent it and the collars from getting lost. • Remove the locknut from the spindle end and slide the bearing bush and the spacers off the arbor. • Carefully clean the arbor and check for scoring or other damage. Report any such damage to your instructor/supervisor. In severe cases of damage the arbor may have to be replaced.354 Engineering Fundamentals • Estimate by eye the position of the cutter from the size and shape of the work and the position of the cut and slide as many collars onto the shaft as are needed to ensure the cutter will be in the correct position. • Inspect the cutter for blunt cutting edges, chipped teeth and damage to the bore. If these or any other defects are found, return the cutter to the stores to be exchanged for one in good condition. • Clean the sides of the cutter and its bore and slide this onto the arbor as shown in Fig. 11.16(a). Milling cutter teeth are very sharp, particularly at the corners. Protect your hands by wearing leather gloves or holding the cutter in a thick cloth wiper. Key Keyway Align key and keyway Spacing collars by sighting Cutter (b) (a) Figure 11.16 Mounting a cutter on a long arbor: (a) keying the cutter to the arbor – length of key is greater than the width of the cutter, any portion of the key that extends beyond the cutter is ‘lost’ in the spacing collars which also have keyways cut in them; (b) tightening the arbor nut – the steady must be in position when tightening or loosening the arbor nut to prevent bending the arbor • Insert a key into the keyway of the arbor to drive the cutter. This prevents the cutter slipping and scoring the arbor. Also, if the cutter stops rotating whilst the table feed is engaged, the arbor will be bent. Although you will see people not bothering with a key, so that they just rely on friction to drive the cutter; this is not good practice for the reasons already mentioned. • Slide additional spacing collars onto the arbor as required to bring the bearing bush in line with the steady bearing. These spacing collars should be kept to a minimum to avoid excessive overhang and to ensure maximum rigidity as previously mentioned. • Position the overarm and the steady bearing as shown in Fig. 11.16(b) and tighten their clamping nuts. • It is now safe to tighten the arbor locknut. This must be tightened or loosened only with the steady in position. This prevents the leverage of the spanner bending the arbor. • Set the machine to a moderate speed and start it up. Out-of-true run- ning can result from a warped cutter, incorrect grinding and lack of cleanliness in mounting the cutter. If it runs out of true, switch off the machine, remove the cutter, check for cleanliness and remount. • If the cutter still runs out, seek the assistance of your instructor.Milling machines and milling techniques 355 11.6.3 Straddle and gang milling These techniques are more associated with production milling than with toolroom and prototype work. However, since they are associated with the use of horizontal milling machines they are included here. Straddle milling Straddle milling is used to machine two sides of a component at the same time as shown in Fig. 11.17(a). Solid spacing collars are used to take up most of the space between the cutters and an adjustable collar is used for the final adjustment. Gang of cutters Side Spacing Side and collars and face face cutter cutter Arbor Work Work Vice Vice (a) (b) Figure 11.17 Straddle and gang milling: (a) straddle milling; (b) gang milling Gang milling Gang milling is even more ambitious and involves milling all the sides and faces of the component at the same time as shown in Fig. 11.17(b). To maintain the correct relationships between the cutters, they are kept together as a set on a spare mandrel and are all reground together when they become blunt. 11.7 Cutter mounting Safety: Make sure the machine is electrically isolated before attempt- (vertical milling machine) ing to remove or mount arbors and cutters. 11.7.1 Stub arbor Figure 11.18(a) shows an ‘exploded’ view of a stub arbor and a shell end milling cutter. The cutter is located on a cylindrical spigot and is driven positively by dogs. It is retained in position by a recessed bolt. To356 Engineering Fundamentals Retaining Stub arbor Shell and mill screw (a) Retaining bolts Inserted teeth Face mill arbor (bolt-on type) Face mill body (b) Figure 11.18 Use of stub arbors: (a) shell end mill; (b) face mill maintain the correct fit, the spigot and register must be kept clean and the cutter must be tightened onto the arbor so that there is no movement between the cutter and the arbor during cutting. Figure 11.18(b) shows a small face mill and its arbor. In both cases the arbor is located in the taper bore of the spindle nose and it is retained in position by the threaded drawbar that passes through the length of the machine spindle. Note: Stub arbors and their associated cutters can also be used on horizontal spindle A D milling machines. Another type of stub arbor is shown in Fig. 11.19. This allows the B cutters normally associated with a horizontal milling machine to be used on a vertical spindle milling machine. Because the stub arbor is supported C only at one end, it is not as rigid as the horizontal milling machine arbor and this restricts the size of the cutter that can be used and the rate of metal removal that can be employed. E Vertical milling Figure 11.20 Collet chuck for Clamps machine spindle screwed-shank solid end mills: A – main body of collet chuck; Stub arbor the locking sleeve B positions the collet C and mates with the taper nose of the collet to close the Side and collet on the cutter shank; the face milling collet is internally threaded to cutter prevent the cutter E being drawn out of the chuck whilst cutting; V-blocks the male centre D anchors the shank end of the cutter and Figure 11.19 Stub arbor for use with cutters normally associated with ensures true running horizontal milling machinesMilling machines and milling techniques 357 11.7.2 Collet chuck Basically a collet is a hardened and tempered steel sleeve with a parallel bore on the inside and a tapered nose on the outside. It is slit at regular intervals around its circumference so that it can close onto the shank of the cutter when the outer sleeve is tightened. Concentric tapers are used to ensure true running and to compensate for wear. Figure 11.20 shows a section through a typical collet chuck. • The shank of the cutter has a threaded portion at its end that screws into the rear end of the collet. This prevents the forces acting on the flutes of a cutter with positive rake from drawing the cutter out of the collet. • The hardened and ground conical centre serves to locate the rear of the cutter and also to act as an end stop and prevents the cutter and the collet being pushed up into the chuck body. 11.8 Workholding The work to be machined on a milling machine may be held: • In a machine vice. • Clamped directly onto the machine table. • Clamped to an angle plate that is itself clamped to the machine table. • In a milling fixture for production work. To save time pneumatic clamping is often employed. In this chapter only the first two methods will be considered. 11.8.1 Machine vice (plain) Figure 11.21(a) shows a plain machine vice. It has two sets of fixing holes so that it can be set with its jaws either parallel to the travel of the machine table as shown in Fig. 11.21(b), or it can be set with its jaws perpendicular to the travel of the machine table as shown in Fig. 11.21(c). To facilitate setting, the underside of the vice body has slots machined in it both parallel and perpendicular to the fixed jaw. Tenon blocks can be secured into these slots. The tenon blocks stand proud of the slots so that they also locate in the T-slots of the machine table as shown in Fig. 11.21(d). To maintain positional accuracy of the vice: • Check the tenons are a close slide fit in the tenon slots in the vice body and also in the T-slots of the machine table. • Check that the tenons are clean and free from burrs and bruises. • Clean the tenon slots and insert the tenon blocks, securing them with socket head cap screws.358 Engineering Fundamentals (a) (c) Vice base Tenon block T-slot (b) (d) Figure 11.21 Mounting and setting a plain machine vice: (a) plain machine vice; (b) vice set with jaws parallel to T-slots; (c) vice set with jaws perpendicular to T-slots; (d) use of tenon block to align vice with the T-slots • Check that the T-slots in the machine table are clean and free from burrs and bruises. • Lower the vice carefully onto the machine table and locate the tenons in the appropriate T-slot. • Secure the vice to the machine table with suitable T-bolts. Ordinary hexagon bolts should not be used as their heads do not fit properly and they can work loose. • The vice should now be ready to hold the work. 11.8.2 Machine vice (swivel base) If the machine vice has a swivel base as shown in Fig. 11.22(a), or it is a plain vice without tenons, then it will have to be set either parallel orMilling machines and milling techniques 359 (a) Vertical slide Verdict type dial test indicator (DTI) Magnetic base Parallel strip (b) Figure 11.22 Setting swivel base machine vices and plain vices without tenons: (a) swivel base machine vice; (b) setting a machine vice360 Engineering Fundamentals perpendicular to the worktable with the dial test indicator (DTI) as shown in Fig. 11.22(b). 11.8.3 Direct mounting Work that is too large to hold in a vice or is of inconvenient shape can be clamped directly to the machine table as shown in Figs 11.23(a) and 11.23(b). Sometimes the shape of the casting or forging is such that a jack or wedge is required to level the work ready for cutting. The example shown in Fig. 11.23(c) shows that the opposite end to the clamp is supported on a packing piece. There will, of course, be clamps at both ends of the workpiece. Sometimes castings are slightly warped but not sufficiently to allow the use of jacks and wedges. Thin packing and pieces of shim steel should be inserted under the casting to remove any ‘rock’ and to provide support under the casting where clamps are to be used. Tightening clamps down onto an unsupported part of the casting could cause it to crack. Work Work (a) (b) (c) Figure 11.23 Holding larger work: (a) use of clamps; (b) use of table dogs; (c) levelling work Angle plates as described in Section 7.3.2 can also be used for locating and supporting work on the milling machine table.Milling machines and milling techniques 361 11.8.4 Dividing head (simple indexing) Sometimes you will need to make a series of cuts around the periphery of a component; for example, when cutting splines on a shaft or teeth on a gear wheel. Such an operation requires the work to be rotated through a given angle between each cut. This rotation of the work through given angles between the cuts is called indexing. Figure 11.24 shows a simple (direct) dividing head. The index plate locates the spindle of the head directly without any intermediate gearing. In the example shown there are only two rows of holes for clarity. In practice there would be many more rows to give a bigger range of possible spacings. Plunger Index plate 12-hole Carrier circle Plunger Body Spindle arm Work (End elevation Centre showing the 8-hole index plate) Catch plate circle Worktable Figure 11.24 Simple dividing head ◦ For example, we can index through 120 between the cuts so as to give us three equally spaced slots. We would use the 12 hole circle in the index plate since this is divisible by three, and we would move the plunger arm through a distance of four holes between the cutting of each slot. If we had wanted four slots we have a choice, we could have rotated the work through three holes in the 12 hole circle between each cut, or we can rotate the work through two holes in the eight hole circle. The result would be the same. Figure 11.25(a) shows a typical component where three equally spaced slots are to be cut. • The blank would be turned and bored to size ready for milling. • The blank would then be mounted on a mandrel and supported between the dividing head and its tailstock as shown in Fig. 11.25(b). • The work is centred under the cutter and cutting takes place. • For rigidity, cutting should take place towards the diving head and towards the ‘plus’ end of the mandrel so that the blank cannot work loose. • You can either complete each slot before indexing to the next one, or you can index from slot to slot for each increase in the depth of cut so that all the slots are finished together.

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