I need tell you no more about turning a bar of iron in the lathe, because the above directions apply in all cases; but if you have to turn the face of a piece of metal that is carried in a chuck of some kind, you should always work from the middle towards the edge, and if the graver is used, its bevelled face will lie towards you during the process. Take care to chuck the metal very firmly, for it is most annoying to have it suddenly leave the chuck or shift its position after you have been at the trouble of turning part of it truly. In such case it is very difficult to replace it exactly as it was before, and all your work has in consequence to be gone over again. When taking the final cut, or before, if you like, dip the end of the tool into water, or soap and water, and see the effect. The surface turned in this way will be highly polished at once, and the tool will cut with much greater ease, so that a large, clean shaving will come off. When using a slide-rest, you will find it always better to keep water just dripping upon the work and point of the tool; but there is a drawback, nevertheless, to this plan, for, as might be expected, it makes a mess and rusts the lathe, and sometimes the work as well, so the water must be constantly wiped off it.
THE SLIDE-REST.
I shall now pass on to describe a mechanical arrangement called a slide-rest, of which there are two separate and distinct forms, one for metal and one for ornamental turning in ivory and hard wood. The ornamental work that can be done I shall pass by for the present, because few boys are provided with the costly apparatus required, and I am rather addressing those young mechanics whose tastes incline them to model machinery and to practise the various operations of mechanical engineering on a small scale. To such a slide-rest is an almost necessary addition to the lathe, for there is a great deal of work which, I may say, cannot be done without it; for instance, boring the cylinders of engines (except small ones of brass), turning the piston-rods and various pieces which require to be accurately cylindrical and of equal size, perhaps for the length of a foot or more. Hand-work has accomplished something in this way in olden days, but the inability of workmen to advance beyond a certain standard of perfection with hand-tools alone, became such a hindrance to the manufacture of the steam-engine, as improved by Watt and others, that had not Maudsley, Naysmith, and others developed the principle of the slide-rest and planing machine, we should not yet have lived to see those gigantic engines which tear along like demon horses with breath of fire, at the rate of sixty miles or more in as many minutes. So likewise would various other machines, which now appear absolutely necessary to supply our various wants, have stood in their primitive and imperfectly developed forms; for it is necessary, before constructing a machine, to have the means of turning cylindrical parts truly, and producing perfectly level plates where required.
The object of a slide-rest is to provide means for holding a tool firmly, and giving it a power to traverse to and fro and from side to side, so that by the first we may be able to cause such tool to approach or recede from the work, and by the second we may cause it to move in a perfectly straight line along its surface from end to end. This is accomplished in the following manner:—The drawing being a representation of one of the first machines constructed for the purpose. A rectangular frame, A, of iron is carried by a pair of strong uprights, B B, fixed to the sole-plate, C, by which it is attached by a bolt to the bed of the lathe. Lengthwise of this frame runs a screw, prevented by collars from moving endwise, but which can be turned round by the winch-handle, D. Thus a nut through which this screw passes, and which only has endwise motion, will, when the latter is turned by its handle, traverse it from end to end in either direction, according as the screw may be turned from right to left or the contrary. This nut is attached to the under part of a sliding-plate, E, which has a part projecting between the sides of the frame, and also two others on its outside, by which it grasps the same with great accuracy, and is prevented from any shake or play as the whole with the nut is made to traverse to and fro along the frame.
Lengthwise of this sliding-plate, that is, in a direction the opposite to that of its own traverse, are two bars bevelled underneath, fixed exactly parallel to each other, which are so arranged to guide the cross traverse of another plate with chamfered edges to fit the bevels of the guide bars. This second plate has on its upper surface two clamps which secure the tool. It is plain, then, that by this arrangement the two required movements are attained, the lower plate sliding along in one direction parallel with the lathe-bed, and the other across it. In the original rests, this upper plate with the tool was moved by hand, and in the modern rest for ornamental turning (which this was also constructed for) the same is done, but a hand-lever is added for the purpose.
But although a similar arrangement is needed for metal, it is plain that the top plate should have a more easily regulated motion, and that we should be able to advance the tool as near the work as may be desired, and then to retain it securely at that distance while giving the necessary movement in the direction of the length of the object to be turned. The method of effecting this is at once suggested by the screw and nut of the lower part, and by merely adding to the top a similar arrangement, the desired end is at once attained.
Fig. 54.