tools, a tool-holder may be used carrying the same tools as and in a similar manner to Fig. 161, previously described. One disadvantage, however, attends the use of this tool and holder it will not cut a thread the entire depth of the hole, as the end of the holder projects into the hole farther than the point of the tool it

carries.

These holders are placed in the slide-rest in the same manner as the ordinary tools, and the holders may all be used to carry tools shaped to cut either wood or metals or other materials.

The ordinary working threads are usually cut almost to size with a single point tool, and finished with a chasing-tool or tool of many points, held either in the hand or slide-rest. If held in the rest, this tool should be constructed on the spring plan, as such tools produce better work.

Other comparatively exceptional threads are begun and finished by single-point tools: the first cut being taken by a tool rather smaller than the required groove, and the screw finished by a sharp well-ground tool of the right size.

For ordinary threads of fine pitch, whether internal or external, it is a good plan-unless the thread go the entire length of the shaft, or quite through the hole -to turn a groove around the cylinder or in the hole where the thread is to terminate. This groove acts as a landing-place' for the point of the tool, and prevents the point being broken off in withdrawing it from cut.

The same object is effected for square or any shaped threads of coarse pitch by drilling a small hole in the

M

shaft at the spot where the screw groove is to end. If the screw is to have several threads, a hole must be drilled for each. It is also a good plan to turn down a very small portion of the metal at the commencement of the thread, whether internal or external, to the size of the bottom of the threads; this acts as a guide to the workman, as, when the screw-cutting tool reaches this place, the thread is cut to the proper depth.

There are one or two features in the slide-rest which are used mostly for screw-cutting and work of the like nature, to explain the uses of which it will be better to take an example in screw-cutting, and go through the operation with the reader.

Suppose it to be required to make a screw of, say, 10 threads to the inch on a steel cylinder, the screw when cut to be used as a 'hob' or 'master-tap.' By referring to the table, we see that the pitch corresponds with a diameter of 3 of an inch, and if we were making an ordinary tap, that is the size we should turn our steel, but for a master-tap the diameter must be larger by double the depth of thread. The steel should be turned down to this size and to the usual shape for a master-tap. The smaller part to be made the size of the bottom of the thread on the larger.

The turned steel being placed between the centres, take either of the external screw tools, ground to the required angle and shape, and place it in the toolholder of the slide-rest, taking care to have it as nearly as possible at right angles to the line of centres.

The requisite change-wheels being put on in place, throw the nut of the leading-screw into gear, and just move the lathe around by the left hand, whilst screw

ing the surfacing slide towards the work with the right, until the point of the tool just touches the surface.

Unscrew the small screw which fastens the handle h to the slide-rest, let the index-spring rest in one of the divisions of the division-plate on the slide-rest screw, and move the handle h upwards towards the left hand. This will have the effect of withdrawing the tool from against the work. The lathe is then set in motion the reversed way until the motion of the leading screw brings the tool's point at the right-hand end of the work. Now move the slide screw one or two divisions of the plate, bring the handle h back to its usual position, which will throw the tool again inward, and start the lathe in the forward direction. The tool will then traverse along the surface, and cut a spiral line. At the end of its cut withdraw the tool by moving up the handle h, reverse the lathe, and, whilst the sliderest is running back, move in the tool one or more divisions of the index plate. When the tool gets to the end, again reverse the lathe and throw the tool into cut. Proceed in the same manner until the required depth is attained.

To cut a good screw, the tool must be sharp and of the right shape, and especial care be taken that it has plenty of clearance and does not rub at the sides. The lubrication must be plentiful, and the finishing cuts slight.

The tool should always be withdrawn before the lathe is stopped or reversed, and its point must not be allowed to come against the right-hand lathe-centre.

It will generally be expedient to tie the lathe carrier to the driving arm, and when a tool is changed,

care must be taken to re-adjust it so that it will enter the groove already cut.

The amateur will probably imagine that if he were to cut a groove in a cylinder on reversing his lathe the tool would run back in the groove; but this is not the case. He will find on reversing, that the lathe-spindle I will run back some distance before the tool will commence moving; this, as previously explained, is owing to the clearance between the teeth of the wheels, and play of the leading screw, &c., and is termed 'loss of time.'

SELF-ACTING DRILLING, BORING, ETC.

The operations described hitherto under the above head can all be performed with ease in the self-acting lathe; the various motions of the slide-rest, however, afford the means of doing some classes of work which cannot be managed in the hand-lathe. The work in

FIG. 162.

most of these cases is fastened to the saddle of the slide-rest and does not rotate; the boring tool, however, rotates between the lathe-centres. The required traverse is obtained by moving the slide-rest along the bed, and not by any movement of the cutting tool.

The boring bar, Fig. 162, is the instrument in most general use, and very useful and efficient it is. It is placed between the lathe-centres, and is usually driven by a carrier. A glance at the figure will show the reader that it is simply a straight bar of metal-gene

rally steel. It is centred at the ends like a mandril, and has two or three slot holes cut through it to receive various shaped cutters; these cutters are fastened in place by a small wedge driven in the slot at the back of the cutter precisely the same as at Fig. 117. Several cutters are shown in the bar, to give the reader an idea of the manner in which each sort is fastened in proper place for use.

The cutters used in this bar have only two cutting points-frequently only one-but for large holes a stouter bar is used, and the cutters are more numerous, being 4, 6, 8, or any higher number according to the size of the hole. They are arranged round a boss which can be keyed to the bar in any convenient part of its length; the cutters are fastened to this boss by being wedged in a dovetailed groove. This instrument is driven in the lathe in the same manner as the last. Frequently, every alternate groove of the cutter boss is fitted with a piece of hard wood, which, bearing against the side of the hole, steadies the bar, and, by preventing vibration, causes a better hole to be made than when all the grooves are fitted with cutters.

When either of these instruments is used, the work through which the hole is to be bored is fastened to the receiving plate of the slide-rest by bolts, or by any other convenient means, and so that the axis of the required hole shall coincide with the line of lathecentres. The work must of course have a rough hole through it to allow the boring bar to be put in place. The cutter is then set to take the proper depth of cut, and the lathe being set in motion and the handle d depressed, the leading screw will carry the work forward