IT will always be found difficult to turn the handle of the slide-rest so evenly that the work shall be left uniformly smooth and true. If the tool rests too long on one spot a deeper cut is the result, which, although very shallow, shows as a line upon the work; and this always occurs, again and again, when the slide-rest handle receives an intermittent motion, which, more or less, it nearly always does. Hence, for one reason, a self-acting motion is superior to that produced by hand, and this is easily managed by the aid of gearing or cog-wheels, and, with slightly less certainty, by means of cords and pulleys, generally connected to the mandrel through the medium of an overhead shaft.

But another reason for the adoption of self-acting Lathes is that in a factory one man can attend to two or three instead of one - a matter of economy that cannot be disregarded. And, thirdly, this mechanism enables a workman to cut screws of any desired pitch, say from one to sixty threads in an inch, with certainty; and also coarser screws called spirals, having perhaps but. one complete turn in six inches. Apart, therefore, from such other uses as the self-acting Lathe may be occasionally put to, the above are ample reasons for its general use where much and varied work is carried on. The general form of these Lathes may be gathered from the accompanying engravings of Lathes, numbered 15 and 16 in the catalogue of the Britannia Company, of Colchester (Plates XL and XII.). A screw runs the whole length of the bed in front, of a quarter pitch - i.e., of four threads to the inch. This passes through a nut in the apron of the slide-rest, just behind the straight handle. The rest is free to move along the bed from end to end, the edges of the bed being planed to a V shape, and being embraced by guide-bars similarly planed, and adjustable to take up wear. Turning the main or "leading" screw, as it is called, in one direction, causes the rest to travel from right to left, and reversing the motion causes it to return. This is the main principle of all self-acting Lathes.

PLATE XL-SMALL SELF-ACTING AND SCREW-CUTTING LATHE.

But it is also evident that means will be needed to stop the traverse at any given point, without having at the same time to stop the Lathe. This is accomplished by sawing in half the nut through which the screw passes, and attaching each half to a short, vertical slide working in guides. The handle seen in front carries a plate with two spiral grooves in it, in which pins standing out from the halves of the nut engage, so that by turning the handle on one side the two halves are separated in an instant, leaving the screw free; while turning it back closes the nut upon the screw, and causes the slide-rest to resume its motion. The slide-rest is placed upon a saddle which fits to and slides upon the bed; and it is very often so made that the whole upper part can be removed, leaving the saddle free, so that work can be clamped upon it for boring or planing (if the Lathe has a planing head fitted to it). To the left is seen a large cogwheel, attached to the screw, and by which, through other changeable wheels and pinions, the rate of motion of the screw can be varied, so as to give to the slide-rest either a quick or slow motion at pleasure. Two small wheels (seen on the left of the headstock below the mandrel) are fitted on the same plate, which is centred on a pin; so that either can be made to gear with the small pinion on the mandrel. This serves to reverse the motion of the screw, while the flywheel still continues to revolve in the same direction as before. Bolted to the back of the saddle, but removable at pleasure, is a die stay, or support for long work liable to bend under the tool. Being carried along the bar as the turning proceeds, and being all the time close to the tool, it supports the work perfectly, and quite removes all tendency in it to deviate from its proper position. A drip-can, with soap and water, is also similarly mounted, to lubricate the work.

The action of the screw causes a very slow movement of the saddle and its rest, and, therefore, when the cut is finished, and it becomes necessary to run back the rest, to commence a new cut, means are required for doing this with rapidity. For this purpose there is a rack attached to the front of the Lathe bed, into which a pinion gears, which has its bearings near the half-nut slides, in the apron of the saddle. The handle to this is the long one seen in front. The half-nuts being opened, to free the rest and its screw, a few turns of the handle carry the tool back again to its starting-point, ready to commence a fresh cut. The Lathes illustrated have friction wheels to the crank axle, which is straight, a slotted crank being attached outside the left standard, and this is worked by an anti-friction chain motion passing to the treadle. The pile of cog-wheels seen in front, twenty-two in all, are for attaching to a stud, on a radial arm, which is mounted concentrically with the screw, and can be clamped in position after the gear wheels are arranged on screw, mandrel, and stud. These are necessary for automatic screw-cutting. It is evident that, with a given set of wheels and pinions - geared, on the one hand, with the mandrel, and, on the other hand, with the screw - any desired rate of speed can be obtained between the two; thus, while the screw makes one turn, and draws the slide-rest and tool ¼in., the mandrel may be made to revolve either once or twice, or a dozen times, according to the sizes of the wheels and pinions used, as will be fully explained in the chapter on screw-cutting. The slide rest can, therefore, be arranged for merely turning a smooth surface - which is, in reality, an infinitely fine screw - or it can be made to travel so fast in proportion to the speed of the mandrel as to cut a coarse spiral.