Using Centre Lathe Chucks

Before you begin using the lathe it is a good idea to try a number of ‘dummy runs’ so you become familiar with the controls. You should practise changing spindle speeds by altering pulley belts or using the gearbox and learn how to engage the back gear, if one is fitted. It is also important to get the feel of the carriage and slide controls so you are quite certain of the direction the tool will move when you turn the handles. If your lathe has a micrometer dial fitted to both the cross and top slide (compound slide) handles, you should know how much tool movement results when you turn the handles through one division — usually 0.02mm (or 0.001in). This is important to obtain the correct depth of cut. Finally, practise engaging and disengaging the automatic feed to the carriage by the carriage engagement lever (or half-nut lever).

Plain turning

The quickest and easiest method of turning work on a lathe is to use the three-jaw chuck ; a great many jobs can be completed using this chuck. You may have to turn long work between centres — However, if your work is long and you only need to carry out turning operations on the ends of the work, you can use a three-jaw chuck as long as the work will fit inside the lathe spindle. Never attempt to work on bars held in a chuck if they project out of the left-hand end of the lathe spindle; this can be dangerous.

Fitting chuck

It is essential you make sure the threads on the lathe spindle nose and in the back of the chuck are perfectly clean. If the threads are clean, you can easily screw the chuck onto the spindle nose .until it is firmly seated. Always screw the chuck on by hand; if you use the driving mechanism to turn the spindle screw into the chuck, you not only risk injury but you may also find it impossible to remove the chuck.

Removing chuck

During use, the chuck will tighten up on the spindle nose. To remove it, lock the spindle by engaging the back gears but leaving the back gear pin in position (this pin locks the driving pulley to the lathe spindle). Place a metal bar or a stout timber batten between the jaws of the chuck and loosen the chuck by levering downwards. Finish removing the chuck by hand and place a board across the bed of the lathe in case the chuck slips as it becomes free. If the chuck is very difficult to free, you may have to place a hardwood block on the rear shear of the lathe bed and, with the back gear engaged, by means of the belt rotate the spindle backwards to bring one of the jaws into contact with it. Never use the chuck key as a lever to loosen or tighten a chuck.

Placing work

To place work in a chuck, it must be held firmly and not have excessive overhang — although the working length must, of course, project from the chuck. For this reason you should not use the three-jaw chuck for work which has irregular surfaces or which, because of its large diameter or short length, cannot be firmly gripped by the jaws of the chuck. The overhang permissible will depend upon the rigidity of the workpiece; but as a guide, work should not project from the chuck more than about 100mm (or 4in). Use both hands to tighten the chuck key, but don’t use more leverage than this to tighten the jaws of the chuck. If the work has to overhang the lathe by a large amount, you can support the projecting end with the dead centre in the tailstock.

With the work in position, set the lathe to rotate at about 100 revolutions per minute and check it is running true. Adjust the belts or gears to obtain the correct cutting speed for the particular material you are using and the diameter of work you are turning.

Setting tool

Mount the tool post and the tool on the top compound slide. Most plain turning requires a right-hand knife tool; it is essential the tool point is set to the correct height. The cutting edge should be at right-angles to the length of the work; the correct approach angle is obtained when the tool is set in the tool post so the length of the tool is at right-angles to the centre line of the lathe. If you have a set of tools, you can experiment as you gain experience using the lathe with slight variation of approach angles provided by the standard tools you own.

Facing cut

To take a facing cut, use the cross slide and top slide handles to bring the tool point against the face of the work and tighten the saddle clamp, if one is fitted. It may be necessary to set the tool at a slight angle to the work for some facing. Draw the tool clear of the work with the cross slide handle and use the top slide to set the required depth of cut. Start the lathe and slowly feed the tool into the work, keeping the the side of the work. Draw the tool back, using the top slide handle, and set the depth of cut by means of the cross slide handle — if it has a dial.

Once you have prepared work for a sliding cut, you can either take a long cut which may require moving the whole carriage in the direction of the work, using the carriage handle; or you can engage automatic feed and allow it to move the carriage, and hence the tool, forward. If you use automatic feed, be sure you are alert to disengage the feed at the end of the required length of cut. Alternatively you can take a short cut by moving the tool forward using the top slide; but several trial cuts may be needed to ensure the top slide is set true.

After you have taken the first cut, bring the tool back along the length of the work until it is clear; you can then adjust the cross slide to determine the depth of the second cut. Remember the diameter of the work is reduced by twice the depth of the cut; the last cut taken should be a fine one if you require a good finish. Check the diameter at intervals, using outside calipers, vernier calipers or a micrometer. Stop the lathe before checking, not only for safety, but also to avoid undue wear of your measuring instruments.

Tool point steady and using a coolant if the work-piece is of steel or aluminium. As the tool reaches the centre of the work it will generate a flat surface. If a small ‘pip’ of metal is left at the centre, the tool point was not at the correct height and will have to be reset.

Sliding cut

Once the end of the work has been faced you can mark out the length of sliding cut necessary. A simple method is to move the tool point along the work to the limit of the sliding cut which is to be taken; by bringing the tool point up against the work in this position and rotating the chuck by hand, you can scribe a line around the circumference of the work. If you chalk the work in this area before scribing, the line will show up more clearly. Bring the tool back to the end of the work and use the cross slide handle to bring it into contact with

Taper cuts

You can make taper cuts using the three-jaw chuck by swivelling the top slide to an appropriate angle. The length of taper you can cut is limited by the travel of the top slide; on smaller lathes this is likely to be between 25 and 50mm (1 and 2in). To cut a longer taper you will have to turn between centres.

To taper-turn in the chuck, you need only to set the top slide at an angle to the centre line of the lathe. When you have set it to the correct angle, the cutting procedure is the same as for a straight sliding cut; set the depth of cut by means of the cross slide and move the top slide to feed the tool into the work. If you are cutting short tapers or bevels in this way, it is advisable to lock the carriage to the lathe bed. To produce very accurate tapers, angles have to be calculated and set to minutes of a degree; this may not be possible without a good quality vernier protractor.

It is likely you will be matching the taper to an existing internal taper, in which case you should turn the taper down to about 0.75mm (0.030in) oversize. Coat it with engineer’s blue and either bring the internal taper onto the external one or remove the chuck and bring the external taper into the internal one; don’t remove the work from the chuck. At any point where the engineer’s blue is removed, the taper needs turning down more. Even ifthe top slide has been set with a vernier protractor, you may have to adjust the setting after you have carried out this test.

Taper ratio

Tapers are quoted either as the amount of taper per unit of length — the taper ratio (for example 1 in 20) — or by quoting the included angle of the taper (for example 2 degrees 52 minutes). To find the taper ratio, measure the perpendicular distance between the large and small diameters of the taper and divide this by the difference between the diameters.

To set the top slide over, you will need to know the angle — which is half the included angle of the taper. To find this, halve the taper ratio and use a table of tangents (or a scientific calculator) to find the angle whose tangent is equivalent to this halved ratio. For example; if the taper is 1 in 20, halving

the ratio gives 1 in 40 which equals 0.025. From a table of tangents, a tangent of 0.025 corresponds to an angle of 1 degree 26 minutes — half the included angle of the taper.

Boring cuts

To bore a hole on the lathe, you will need a hole in the work which the tool can enter. If you are boring a solid bar, face the end of the bar as already described; mount a drill chuck in the tailstock to hold a centre drill bit and drill the bar. Replace the centre bit with a drill bit 0.5mm (0.020in) smaller than the finished diameter of the bore; drill the hole by advancing the tailstock barrel. After the hole is drilled to the required depth, remove the bit, move the tailstock back and set up the boring tool.

Use a boring tool or a boring bar with a tool at its tip. Set the tool up in the tool post so the point of the tool will move parallel to the lathe centre line — you can set the tool point very slightly above the centre line of the lathe. Only take light cuts because the boringtool has a tendency to bend. Set the depth of cut by using the cross slide and advance the tool into the work by means of the carriage. Boring tools need to be sharp; if you need to bore a large hole. Resharpen the tool before you take the light finishing cut.

Use inside calipers frequently to check the hole size; always measure the inside diameter in more than one place to ensure the hole is circular. Remember the diameter of the hole increases by twice the depth of cut you take; make sure the tool is well lubricated during boring, otherwise it is likely to overheat.

Internal tapers

You can bore taper holes in much the same way as plain holes. The angle at which the top slide is set over is calculated as for an external taper; once you have done this you can follow the same process as for boring a parallel hole, but using the top slide. If you intend to bore a tapered hole and turn an externally tapered shaft to fit inside it, bore the hole first since there may be difficulty in measuring the angle of the taper to a high degree of accuracy and the internal taper may not be exactly as designed because of the flexibility of boring tools. If the two do not match up, it is not too difficult to adjust the external taper to match the hole you have bored. As the external taper is cut you should, from time to time, check the fit by placing the taper into the bored hole.

Warning

The latter precaution is a general principle which you should adopt in all metal cutting work ; it is impossible to replace metal which has been machined away. Check your dimensions frequently; if work is oversize, you should continue to take very light cuts until you arrive at the finished size. Another precaution which helps to maintain accuracy is to plan your turning operations before you begin, so all the work is completed without removing work from the chuck. Errors are almost inevitable if work is removed from the chuck during turning operations.

Parting-off

Once the work has been turned and bored if necessary, you will need to cut it from the bar. One way to do this is to stop the lathe and saw through the work with a hacksaw; make sure the blade of the hacksaw does not come into contact with the lathe bed and damage the slideways. When the work has been sawn off, reverse it and face the sawn end.

A more difficult method, which requires much practice, is to use a recessing or parting-off tool. Mount the tool in its holder and set it at right-angles to the work so its point is at exactly centre height; you can set it at slightly below centre height to avoid digging in. Set the lathe to run at one speed below the normal turning speed and feed the tool evenly into the work to ensure it is cutting continuously. As the tool begins to finish the cut, the end of the work is unsupported; if the work is hollow, you can place a bar through this end, supported in the tailstock. If the work is solid, however, you may need to use the forefinger and thumb of your right hand to support the end lightly as the tool finishes its cut, or to catch the end as it falls. Warning The parting-off operation is dangerous because you may be tempted to use your hand near the moving parts of the lathe. It is better to use a hacksaw to cut through the work when the lathe has stopped.

The parting-off operation is also the one in which the tool is most likely to jam in the work; since it is usually the last operation, this can ruin a piece of work on which you have spent a lot of time. For these reasons, it is strongly advisable to practise using the parting tool and develop methods and techniques of supporting work before the tool is used to finish an important piece of work.

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