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45. Ways with Chucks
Jan 02, 2017

THE standard mounting for a chuck on a lathe is a back-plate screwed on the spindle. To change a chuck, you unscrew the mounted one. On many lathes, particularly those employed on production work, this chuck is a large, four-jaw independent one. Taking it off and fitting a small three-jaw type is a procedure that is not always relished; and as Bert, a tool tuner friend of mine, is firmly convinced, there are advantages in holding the smaller chuck in the larger one. Interesting as life always has been to him, there used to be times when he found it exasperating. How often it happened that, as soon as he had put the big chuck on to his lathe, along came the foreman with an emergency job that required the small one. Then one day Bert came across a thick ring of cast iron which he machined on the faces and the periphery. At the finish he had a flange larger than the body of his small chuck, which he mounted using sunken screw. After that, when a small job came along and the large chuck was fitted, he mounted the small one in it, pressing the flange up to the face and turning by the jaws.


Ways with Chucks


This method has the further advantage over a normal mounting for a self-centring chuck in that you can counteract wobble on the work by adjusting the jaws of the independent chuck; whereas, with the self-centring chuck mounted by a back-plate to the spindle, you are forced to use packing or make a split bush. It is a principle that can be recommended for mounting a drill chuck, which is normally fitted by its taper shank in the spindle. Unless such a chuck is a precision type in mint condition, you may find that, mounted in the spindle, it grips small work with a wobble that cannot be corrected. By using a sleeve to mount the drill chuck, as at A, you can adjust the independent chuck so that the work spins true. The sleeve you can machine from mild steel rod, first drilling through, then boring with a tool from the angled topslide. Mounting a self-centring chuck by other means than its orthodox back-plate on the spindle has advantages for machining parts with an off-set. Here the principle differs from that when the work is offset in the independent chuck, for with the jaws of this converging to the centre, there is a limit to the eccentricity that can be obtained on round material.


By mounting the self-centring chuck eccentrically, its jaws centre normally on the work, which is given the required off-set. This is shown at B, where the chuck is on a faceplate. A balance weight should be fitted when the off-set is considerable. You can machine small crankshafts and eccentrics on this set-up. In normal use, a lathe chuck is a revolving vice, the independent type being specially adaptable to work of irregular shape. This indicates possibilities for its use as a special machine vice on substantial drilling machines. A drilling plate, fitted by a threaded stub, as at C, makes a firm base for the chuck. The stub can be an exercise in screw cutting. Using soft material like brass, alu-minium or duralumin. Make the threads a reasonable fit with a smooth finish. For certain milling and shaping jobs, a chuck which is normally mounted by its back-plate must be held securely. Back gear can be engaged as well as single speed. But you usually find that the work is not in the correct position. With the device at D, you can dix a chuck in any position.


Make a pair of blocks, using alu-minium or duralumin, to grip the boss of the back-plate, and mount them between mild steel bars, one each side of the lathe bed, with three bolts made from mild steel rod.To bore soft jaws-with which some self-centring chucks can be fitted, each should be drilled and tapped for a clamp, as at E. Then you can apply pressure to prevent movement. Diagram F shows how you can set up a worn chuck jaw for grinding with a cupped wheel. Fit a tool-maker’s clamp and clock it true with the jaw in the chuck. Mount the jaw on an angle plate on the vertical slide, and clock the clamp true again, as X, before removing it.



ROTARY spindles for driving drills and milling cutters in the lathe vary from the simple fly-cutter arbor, running between centres in a cutter frame, to much more elaborate appliances with specially designed bearings and collet chucks. Each of them is capable of good work within its particular limitations. From experience with most of them, I have attempted to design one which is adaptable and versatile, so that it will cope with the widest possible range of operations on a small lathe. It is intended to be interchangeable with the indexing spindle unit already described, for mounting in either the plain or swiveling quill holder. I have given much thought to the possibility of adapting a single spindle unit for indexing and driving cutters, but it is difficult to combine the most useful features of the indexing spindle, including the use of standard lathe mandrel fittings, with the free running and precision of s high-speed rotary spindle. While we can use the indexing unit as a milling spindle by fitting a driving pulley in place of the change wheel, the type of bearing is not best suited for running at high speed, and its friction is greater than it should be in view of the limited power. I consider that high speed is absolutely essential to the success of rotary milling appliances of this sort. While we may have to run some cutters at low speed, and to introduce reduction gearing to the spindle in order to obtain sufficient torque for driving them, most operations can be carried out more accurately, and with better finish, by light rapid cuts. Even with the most primitive cutter spindles, speeds up to 3,000 r.p.m. or more can be used with advantage, but to stand up to such speeds indefinitely an adequate bearing area and good lubrication are essential.


Rotary Spindle For Small Lathe



Opposed-cone Bearing

The bearing which has long been favoured for high speed and precision, both for light lathe and milling appliances, is the double opposed-cone type, in which radial loads are taken on acute-tapered journals, and end thrust loads on integral obtuse-angled collars. But this is a difficult bearing to machine and fit properly, as the two angular surfaces must register exactly and simultaneously in their bush. If this is done properly, it is one if the worst. Some attempts ever produced; if not, it is one of the best machine tool bearings ever produced; if not, it is one of the worst. Some attempts to use it (not only by amateurs) have been utter failures. When a tapered journal bearing is used with an independently adjustable thrust surface, construction can be simplified. I have experimented with tapered bearings of varying angle from 5 to 30 degrees included angle, and with different thrust bearings, including axial and angular-contact ball races. Standard races of suitable dimensions for a compact milling spindle are apparently unobtainable, but a plain thrust ring, opposed to a fairly acute-angled journal, has been found to give excellent results for all but the highest loads. This is the bearing which I recommend. The spindle is designed to take standard 8 mm. collets, obtainable ready made for use in watch and instrument lathes; the essential dimensions for fitting them are shown, and the particulars for other sixes of collet can be found in the ME Handbook. For the sleeve bearing, which is turned externally to the same size as the bearing of the indexing mandrel, cast iron is the best material, but bronze or other good bearing metal is also suitable. If extra length is allowed for chucking, both outside and inside may be machined at one setting. Back centre support can be used for the outside. In the boring of both the parallel and tapered surface, a fixed steady on the outer end of the sleeve will be found helpful. It is essential that these surfaces should be as smooth and accurate as possible. After the sleeve has been parted off, it is reversed for counter boring and internal screw cutting.


Getting Concentric truth

For concentric truth in the second setting you may use a plug mandrel with a taper to match the bore, and a parallel pilot end may be used, or the sleeve may be held in the chuck and clocked over the outside to the highest possible accuracy. With either method, the fixed steady can again be applied to support the end while you are machining. I recommend 24 t.p.i. thread as for other screwed parts, but other pitches can be used if they are more convenient. Coarse threads should be avoided, as they increase the risk of error in concentricity. A clearance recess provided behind the thread allows the tool to run out into space, and eliminates the danger of digging in. While the spindle is intended to be made of high tensile steel, a good machining quality mild steel will give reasonable wear. In machine tool practice, spindles are usually hardened and ground, but these processes are not generally for the amateur. Open-hearth casehardening involves the risk of distortion. Chrome deposition to a depth of not more than 1 or 2 thou will provide a hard surface without distortion, but if after-treatment of any kind is to be avoided your best course is to use steel of the highest quality, preferably an alloy or high carbon steel.Preliminary roughing of the spindle may be carried out between centres, after which the spindle may be chucked at the large end for drilling and boring, with a fixed steady again used on the projecting end, and the chuck end clocked for exact truth. You should drill undersize and finish to size with a D-bit. If you do not have a specially long drill, to reach the depth of nearly four inches, a shorter drill, turned down on the shank, and brazed into a length of tube, will serve still better, as the tube can be used to inject cutting lubricant. Great care is needed in centring and starting the drill, as any initial error will worsen as the depth of hole increases. The D-bit, which is easily made from silver steel rod, has a natural tendency to produce a true bore, but it will not correct a badly drilled hole. A chamfer should be provided at the mouth of the hole for centring.


After drilling the deep hole reverse the spindle, with the same precautions to ensure truth at both ends, and drill and bore it to fit the collets. The important point to observe is the boring of mouth of the hole to the correct size and angle and to perfect smoothness. To mount the spindle for finishing the outside, turn a piece of material in the chuck to form a dummy collet, with the pilot end a press fit in the spindle bore. With the spindle mounted on this, and supported by the back centre at the small end, the concentric truth of the bore is assured. In the external machining, the fit of the parallel and tapered journals is extremely important. The parallel journals can be finished by lapping, but this process is not satisfactory for the taper bearing, which must be machined to provide a perfect contact over the full length of its mating surface in the sleeve. You can check with marking colour. After getting the most accurate tool finish possible, you may remove the final high spots by a dead-smooth Swiss file. The exact end location of the spindle in the sleeve is less important that a perfect surface fit. Leave the screw-cutting on the tail end of the mandrel, and the fitting of the tail bush and driving pulley, to the last.


Drawbar for Collets

For standard pull-in collets, a drawbar and handwheel will be required. They are simple to make, and can be altered according to convenience. The essential details and dimensions are shown in the drawing. As the thread in the end of the drawbars used n the clock tool trade of the collet, you may have to make a special tap unless you can get one of the drawbars used in the clock tool trade and adapt it for length. A hollow drawbar is not generally needed for a milling spindle, but there are occasions when it may be useful.

To ensure concentric truth on the end face and inside and out, turn, screwcut and bore at one setting. It should be a running fit on the spindle, but without any tendency to bind when it is adjusted and locked up; a little extra clearance is better than excessive friction. The flange of the bush, and also the edge of the lock nut, may be notched to take a C-spanner, as for the adjusting collars of the indexing spindle. There is no real need to limit the diameter of these parts, as the spindle assembly is inserted in the quill holder from the reverse end, but it is just as well to maintain some uniformity. The side faces of the lock nut should be machined true with the thread. This applies also to the spindle lock nuts; you can make them hexagonal for convenience in adjusting them with a thin spanner. For the thrust collar, I suggest an oil-hanrdening steel, but mils steed; if it can be case-hardened without distortion, is quite suitable. The collar can be machined from bar and parted off at one setting, with the entering end of the hold slightly chamfered, so that it will be sure to go right home against the shoulder on the spindle. At this point it should be a light press fit, but the rest of the surface may be slightly eased done for convenience in fitting. After being hardened, the collar should be lapped true on both the front and back surfaces, and checked for parallelism with a micrometer. A piece of plate glass, smeared with fine carborundum or emery paste, is suitable for these flat lapping operations.The driving pulley is another component which is subject to modification; the details and dimensions shown are suitable for general purpose. It will take either in. round belting (Whiston’s plastic belting is specially suitable) or endless V-belt of in. section. The angle of 40 degrees ‘inclusive, with the bottom of the groove clear of the belt, gives the most efficient belt grip possible. Many pulleys are made with too obtuse a groove angle, allowing the belt to bottom, with excessive slip and loss of efficiency. The pulley is mainly driven by friction between the two lock nuts ‘on the spindle, but a driving pin is also fitted (after end locations are adjusted). It consists of a 4 BA steel screw with the end turned down to fit a in. hole in the spindle. You had better drill the tapping hole in the pulley, and counterbore it to in. diameter, before the groove is fully turned, or you may find it difficult to start the drill. A hole is also drilled in the taper part of the spindle to take the collet locating pin, which should be a light driving fit, and of such a length that it neither projects above the journal surface, nor fouls the keyway in the collet when or the spindle itself, and though more elaborate keying arrangements are often used I have not found them necessary.

An oil hole is drilled in the sleeve bearing and tapped to take a short 2 BA grubscrew, so that you can close it against the entry of dirt after you have fed in oil. It should not be tapped right through; there is then no risk that the screw will foul the spindle. In the assembling of the unit, the spindle is first inserted in the sleeve and the thrust collar threaded on the small end; it may be pushed truly home by being screwed in the tail bush. Bring the spindle into its taper seating so that it ends to jam, and them adjust the bush so that it is just barely free again, and lock it in position. Put the front locking nut and the pulley on the spindle and adjust the nut to working clearance, when the back nut can be locked against the pulley. The adjustments should be made with the spindle dry, but when the work has been completed oil can be fed in to fill the space around the relieved part. A light low-viscosity spindle oil, such as Shell Vitrea (as recommended by Myford for lathe bearings) is suitable. After the spindle has been run in until the oil no longer comes out black, some slight readjustment, in the same order as before, may be necessary. You will probably find it better to dismantle the parts and examine them for high spots-perhaps more than once.


With the 8 mm. collets, the spindle will take cutter shanks or arbors up to in. dia., which is large enough for most operations within its range. You may fit larger arbors, with extension for overarm centre support, by machining the driving end to match the collets, and locking them in with the drawbar. The smaller collets are useful for holding dental burrs and similar cutters, and single-point end mills (virtually D-bits) which can be made in a few minutes from high-speed or silver steel rod. At least one arbor should be provided with a cross hole to hold fly-cutters for cutting clock wheels and pinions. Grinding wheels can be used to a limited extent, but for really effective grinding with tiny wheels much higher speeds-10,000 to 20,000 r.p.m.-are necessary, and these are almost impossible to obtain except with speciaised–and very expensive-motorised appliance.Rotary cutter spindles all involve problems in the provision of a suitable drive, especially when they are used in all sorts of odd locations and angles. A good deal of practical information on all kinds of drives can be found in the ME Handbook Milling in the lathe, which covers every aspect of appliances and methods.



43. Lathe Milling
Dec 05, 2016

A centre lathe with vertical slide and angle plate is equipped for many operations that would otherwise require a universal milling machine. For you can make single-point tools perform the same operations as multiple-tooth cutters by reducing the rate of feed so that a gig-in cannot occur. You can make the tools from round and square bits and pieces of silver steel rod, to mount in holders in chucks or to fit in the taper in the lathe spindle. The cost in cash is small. If time presses, and you must keep to bare essentials, there are several easy-to-make holders that can be made from mild steel bar. They can be case-hardened or left soft. For frequent use, you should case-harden them before they become dented and worn. According to need, you can set the vertical slide squarely or at angles on the cross-slide. The angle plate you can mount on the vertical slide, flat or sloping. To put on cuts, you have the screws for the cross and vertical slides. Any of these screws can be used to move work past a rotating cutter; and so, besides making set-ups at compound angles, you can feed work in any of three planes at right-angles. Diagram A shows some typical operations which are performed by running tools in the chuck with the work mounted on the vertical slide. For boring A1, you can sometimes use an ordinary boring tool in the independent chuck.   


Lathe Milling


If the hole is small, you can make a round-shanked tool from silver steel rod and mount it in a holder. In each case, set the tool by adjusting it in the chuck jaws. Fix the vertical slide and cross-slide by the screws to their gib pieces, or wedge the slides with packing to prevent movement. Feed the work by the saddle. For facing A2, you can fit a round tool in a reamed hole in a rectangular mild steel holder, fixing it through a grub screw. Adjust the circle swept by the tool by setting the tool and the holder. Apply cut to the work from the saddle, locking this to the bed or retaining its position with lead-screw nut. Feed the work with the cross-slide or vertical slide. For slotting or grooving A3, you can use an end-mill in a holder in the chuck, setting the tool to run concentrically. Apply cut again from the saddle, and feed the work by the cross-slide or vertical slide. The ordinary end-mill has two cutting edged so that the end of the tool appears as at B1. To re-sharpen it, you need an accurate square-edged grinding wheel. When this creates a problem, the solution is to make an end-mill with a single cutting edge, B2. Then you can re-sharpen it on any flat face on a grinding wheel. The single-edged end-mill will do the same job as a double-edge type when it is run fast with slow feed for the work. You can make a holder to mount round-shank tools in the independent chuck as at B3. Chunk two pieces of rectangular mil steel bar; face and centre the end and drill through undersize. Finish the bore with a standard drill or a reamer. The abutting edges of the pieces you can ease with a file for the holder to grip the shanks firmly.



To keep the halves of such a holder together, turn the ends circular and machine the grooves for circlips, B4 Make the circlips from coils of springs. A shank pushes into the holder with a friction grip. Another holder for a chuck is shown at C, upper diagram. In this the tool is clamped by a grooved cotter made from a bolt. Fit the bolt first. Drill and ream the hole for the tool. Then cut the head off the bolt. The holder at C, lower diagram, has a taper shank to fit in the lathe spindle. The rear end can be tapped for a draw-bolt. You slit the other end with a saw to line WX for clamping. A holder for square tools, diagram D, consists of a centre block with side-plates YZ fixed by countersunk screws. 



ON most components, diameters are more important than lengths as their accuracy controls the working fit of parts. A piston must be a nice running fit in its cylinder, to retain pressure but not to seize. A housing must be a precise interference fit for a ball race, to avoid excessive force for fitting-or to prevent the ball race from working loose.

Accuracy is implicit for these diameters, and for many others; and so we exercise the greatest care in machining them, and check the sizes frequently with micrometers and gauges as they approach the finished dimensions. On the other hand, we may take most of our length dimensions from a rule-which is quite satisfactory if we are careful; but when our attention relaxes? Or our judgment varies, we fall into error, or find that we have an accumulation of mistakes. I was reminded of this recently when I turned up an old letter. The correspondent has made a small engine, which I had designed. He said that there must be an error in the length dimensions on drawing, because he had made all the parts to size but the piston was not in its correct position at top dead centre. In fact, he had made an accumulation of small errors, as was proved by adding and subtracting relevant dimensions on the drawing. They cancelled correctly, and so it was suggested that the solution to his problem was to alter the dimension to the crown of the piston.This method is quite acceptable for “one-off” jobs, all parts of which which can be made to suit each other, providing that the errors are kept within bounds.  


Machining Lengths Accurately


To rely on it as a principle is not advisable, as it does not help to improve our standards of workmanship. On the contrary, we should aim at precision-which is not difficult to achieve on length dimensions. On a lathe which has a micrometer collar to its topslide screw, facing tools can be fed from one face to another, and so short lengths can be machined within two or three thou of dimensions. On a lathe without this collar, and lacking one for its leadscrew as well, other methods must be adopted, as shown at diagrams A, B and C. For a small step to a deep shoulder, as at Al. you can often use the shank of a twist drill to set the tool for the final facing-cut from the end of work. Alternatively, standard silver steel or mild steed rod can be used. A lathe tool will serve as well, or piece of flat mild steel. By another method, as at A2, end-gauges are placed to jaw of chuck. You make the gauges by facing a steel rod to a micrometer. For an inside step or recess, as at B, a gauge consisting of a piece of suitable material can be placed to the inside face to locate the tool for the final cut from the end. When a lathe has a stop on the bed, standard material can be used between it and the saddle to provide settings for both inside and outside steps.



Sometimes, for a short repetition run on a centre lathe, it pays to make a setting gauge. I made one for machining a number of unions, from a large steel washer and three screws and nuts setting the dimensions by micrometer and depth gauge. Diagram C shows the principle of device. A short screw 1 set the facing rool from the flange to the end of the union. Another short screw 2 set the parting tool for the thickness of flange; length. By a similar principle, you can employ a stop and a gauge to set a tool for facing work to length on a mandrel, as at D. Use faced angle iron with a piece of rectangular material bolted to it. Tighten the bolt with the two held down on a flat surface. If you make the clamped work carrier which I described last week, you can use it with an end gauge, as at E, to locate a tool for facing the shoulders of a shaft between centers. To locate a long shaft in the chuck, make a reference line on it ans aet this to the chuck jaws with a flexible rule, as at F. Then you can use gauges, as at A2, to the end of the shaft.  



41. Self Feeding Facing Tool
Oct 23, 2016

In meeting the needs of a small workshop, a lathe may perform many operations that are work for specialised machines in a large machine shop. It is not unusual for features on components of a single model or piece of equipment to require the maker’s lathe to function as planer, shaper, grinder, milling machine and boring mill-as well as in its ordinary capacity. Much ingenuity is necessary in setting up and machining components-and one may regret the lack of facilities provided by specialised machines, particularly when one is working near the full capacity of the lathe. An occasion of this sort occurs when there is need to face a large surface or series of bosses on a component which cannot be swung on a lathe principle, or moved across the bed past a tool or cutter in the manner of work on a milling machine. What is needed then is the feature of the special head on a boring mill by which a tool can be swept, not on a fixed radius, but in a flat spiral. Given this, the work can be fixed either to the slotted saddle or to the bed of the lathe-which is used to maximum capacity.



Self Feeding Facing Tool



Many operations can be performed, of course, with a tool in the chuck or on the faceplate, sweeping on a fixed radius; but to machine a flat surface with it, cross feed is required, and is not possible at full capacity, though it is another matter when, as now indicated, the tool besides being rotated is fed on an increasing or diminishing radius.The easiest way of providing this self-feed for a tool is an arrangement as at A, with the feedscrew turned by a star wheel from a peg on the lathe bed, as at B. Simple guides for the tool block can be from flat material, attached to the faceplate (or to a large chuck backplate), as at C and D. A right-hand thread on the screw draws the tool block towards the centre, retracting the tool from the radius giving the large circle X. To return the tool, a loose handle can be fitted to the feed screw-or it can be rotated with a screwdriver via a slot in its end. For a fixed components, depth of cut must be provided by setting the tool further out of its block. It can be done without special provision for the type of work performed, although a taper-ended screw from the end of the block to the bottom of the tool would give fine adjustment.



Rate of feed of the tool is governed by the t.p.i. on the screw which is moved 1/5 turn for every revolution of the faceplate. Divide 1,000 by 5(200), and the answer thus obtained by the t.p.i., gives the feed per revolution in thou. With a screw of 26 t.p.i. which it is convenient to use, the feed is just under 0.008 in. per revolution. This is satisfactory for a facing operation if the tool has a small flat at its cutting edge. Back gear for slow speed should, of course, be used. The tool block is mounted to its flat base by countersunk screws; and the two are drilled and tapped at the joint line for the feedscrew. If adjustment is required to give a good working fit for the screw, it can be provided by carefully facing (draw-filing) the bottom of the tool block. Drawfiling can likewise be the method of fitting the guides to the flat base of the tool block, to ensure the firm support that is essential to obviate chatter in machining. To the same end, the extension of the tool from its block should be a minimum. Any or all of three features in the drive will safeguard it from jamming if the lathe is left running. The feedscrew may be given a plain portion on to which the thread in the block can run. The star wheel is gripped between locknuts, admitting of lip under heavy thrust. And the peg for the wheel is set in a plate which is tightened just to hold in normal working.



40. Cutting Discs and Rings
Oct 05, 2016

CUTTING a disc or ring neatly from sheet material invariably requires swinging either the tool or the material in a circle. Merely marking the material and cutting it with scissors or metal shears produces inferior results, even on outside edges, and the difficulties are obviously increased with inside edges. Successive cuts with scissors or shears are never really uniform, and contours are a series of deviations and flats approximating to circles. In addition, if a material is soft, like thick sheet cork, it can be forced out of shape and chipped at the edges by local pressure. For material like cardboard, fibre, composition sheeting, thin plywood, and even soft metal, the type of washer cutter at A is very suitable With the blade set to radius and the centre point pressed well down, the tool can be turned in regulated cuts. Except on thin material which can be severed at a single rotation, the first cut should be moderately heavy to leave an indentation to help as a guide for following cuts. A firm backing should be provided by way of a flat board.


Where, owing to the thickness of the material, several turns must be made, the centre can be worn and the blade will wander from the original cut. To prevent this, a pad can be used at the centre, such as a piece of flat steel with a small hole for the point. A piece of square steel with the corners turned down can be used to grip the material when it does not matter if the material is marked. Working from both sides is advantageous in cutting a disc or ring from plywood; and it is helpful to drill a centre hole into which a small bush can be fitted, such as a short piece of tubing, to take the point of the tool. Cuts from opposite sides then meet at the centre without risk of wander. Where centre marking is not possible a piece of material with the centre can be temporarily stuck on; and in the case of glass, a suction cup with a moulded-in screw can be usefully employed ( such as is used to fix demisters to windscreens ). The screw locates an arm with a turned-up end to which a glazier’s diamond can be clipped, as at B. Adjustment is not usually necessary to cover a range of sizes; and having clipped the diamond tool to the arm, the radius can be marked and the bole drilled.To break out the circle afterwards, straight cuts can be made at the corners, as at C, so pieces X can be taken off. Then pieces Y will substantially break away. Fragments left can be “nibbled” and crumbled off with flat-nosed pliers, and the circle finished by careful free-hand grinding edgewise on a wheel.



Cutting Discs And Rings


For soft, easily deformed materials, a thin sharp blade is required. There is probably nothing better than piece of razor blade. Using this, a tool can be as at D, the portion of blade held by a screw and washer to the turned-down end of the arm, which should be slotted for setting with locknuts to radius on the stem. This last can be mild steel stud, with a flat handle at the top, and pointed at the bottom by turning down and filing in the lathe, then case-hardened. Alternatively, it can be drilled and a gramophone needle fitted.For sheet metal, cutting on the lathe is generally best with 2 backing provided by plywood or board. In large sizes this can be fixed to the faceplate with metal screws with countersunk heads. The sheet to be sut can then be attached with waste portions, using small bolts or wood screws. A narrow V-tool, light cuts and use if back gear are essential. It is advisable for the centre of the sheet to be held up by a pad from the tailstock or when severed, it may swing off centre and jam. A friction grip on this principle between wood pads is sufficient for edge-machining a small disc, as at E, a steel washer taking the trust of the tailstock centre; and with concavo-convex faces on metal pads, as at F, a transparent disc can be shaped, machined and sprung into the bezel of an instrument as a substitute glass.



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