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.
The actual construction of such rest varies somewhat, but Fig. 54, H, shows it in its most ordinary form. The lower part is, of course, to be clamped down securely to the lathe-bed, there being a projection below which is made to fit accurately between the bearers similar to that beneath the poppits. This projection secures the correct position of the rest, of which one frame or plate will travel lengthwise of the bed, while the other will move exactly at right angles to it. But in the compound slide-rest, which is very general, there is also a third circular motion, by which the upper part can be set at any angle with the lower, instead of being permanently fixed at right angles to it. By this the tool can be made to approach the work more and more as it passes along it; and it will therefore cut deeper at one end of its traverse than at the other. The result will be that what is thus turned will not be a true cylinder, but a cone, i.e., it will be larger at one end than the other, although otherwise smooth and even.
We are thus provided with the most valuable addition to the lathe ever devised by mechanics, and it is no longer a question of the strength and skill of the workman whether we can produce a perfect piece of work, but simply of the accuracy with which the lathe and rest are constructed, and of the form and condition of the tools to be used. The latter are not exactly like those ordinarily used, although the principle of the cutting angles already laid down needs to be adhered to even with more unfailing attention than that required for the correct formation of hand-tools. Moreover, it is plain that—here we shall no longer feel whether the tool is working as it ought to do—we shall be unconscious of the precise amount of strain that is being brought to bear against its edge, and if it is by chance working in a bad position, at a wrong angle, we cannot re-adjust it in a moment as we could a hand-tool by a slight movement of the fingers or wrist.
Hence you will see at once how very important it is that tools for the slide-rest should be shaped with the most rigid adherence to correct principles; and, further, that they should be so fixed in the slide-rest as to meet the work at the precise angle, and at the height exactly suited to the material of which it is composed. As regards the latter point, it may be taken as an almost invariable rule that the work should be attacked on its axial line (that is, a line that would run from end to end of it dividing it lengthwise into equal parts, or, as it would commonly be named, its middle line). If the tool meets it above this, it is most likely that it will rub against it, and the point will be out of cut. If it meets it below, there will be a tendency for the point to catch in, and the work to roll up upon the face of the tool, which, in fact, it very often does with careless workmen, and then there comes a smash of some kind—lathe centres snapped off, the tool broken, the bar bent beyond remedy, and possibly the operator’s toes made unpleasantly tender.
The most common slide-rest tool for outside work is that given at H². It is made straight, as shown, or bent sideways to right or left to cut shoulders on the work, or enter hollows, or creep sneakingly round corners, or any other of those crooked ways in which man delights; but whether straight or not, these tools have all most commonly the cranked form shown here. This gives the tools a slight degree of elasticity—not very much, because that would only injure the perfection of the work; therefore they are not very considerably cranked. The angles are ground as directed in the table of tool-angles, and if the point is too low, slips of iron are placed below the shank upon the tool-plate of the slide-rest; if too high, the grindstone must be resorted to; and the advantage of these cranked tools is, that they can be ground down several times without being brought too low to be packed up with iron slips to the right level. Thus a cranked tool is by far more advantageous for the slide-rest than one made straight like those used for hand-turning. For inside work, however, or “holing,” the crank form is not possible, unless the hole is of large size, and so, for this purpose, straight side-tools are used, like K.
If the tool is well placed, as well as correctly made, nothing can be more easy and delightful than slide-rest work. You merely advance the tool to take the required cut (beginning generally at the right-hand end of the bar), and then gently turning the other handle, you will see it traverse along, as if work was a pleasure to it, as it ought to be to all young mechanics. Not infrequently, however, instead of this even, steady work, the tool jumps and catches, or rubs and shrieks: it is out of temper, I suppose; at any rate, in some one or more particulars it needs correction.
Although with the slide-rest you can generally venture upon taking a deeper cut than you could with hand-tools, it is by no means well to hurry the work. At first, especially before it has become cylindrical, the tool will only cut partly round its surface, and the work is done in an uncomfortable, jerking, dissatisfied sort of way, and the deeper you drive the tool the worse it is; but as soon as the outer skin is off, and the work has become cylindrical, a long, clear, bright shaving curls off pleasantly from end to end, and the surface ought, if the tool is wetted, to become at once of a finished appearance.
You should always, with a slide-rest, take the whole run of the piece from end to end to a certain depth, and then, commencing again at the end, repeat the same process, and so on until the required size is almost attained. When it is, take out the tool with the pointed end which has been in use, and insert one freshly sharpened with a broad point, getting it so placed as to cut the shaving both from the surface below, and from the shoulder to which it is attached at the side, as I explained to you in the chapter on grinding and setting tools, and which must be well understood before you can hope to make good work with tools rigidly fixed in a slide-rest. With this tool, kept wet with soap and water (or soda water, which is better for this than for your stomach), take a very light shaving from end to end, taking especial care to turn the handle which gives the traverse slowly and evenly. If you stop, or almost stop, the tool will be sure to draw a little deeper into cut, which will make a scratch upon the work, or, it may be, plough a groove, and so far spoil the appearance of it.
Whenever you finish turning any bar that has been centred at each end, be careful to leave the centre marks just as they were when the work was in the lathe. The ends will have been otherwise trimmed off at the very commencement, and it may happen that at some future day it may be desired to re-mount the piece for repair, when, if these marks are gone, and new centres have to be drilled, the whole will run so much out of truth that it will have to be entirely re-turned from the commencement. Do not, therefore, fancy that these centre marks are unsightly, and forthwith file them out, but be content to leave them.
Slide-rest tools, made in the ordinary way, are so far troublesome in use that if they get broken you must have them re-forged, and few country smiths know anything about such matters. I have a tool now lying by me made by a smith (true, it was a Welsh smith), and although I stood by and explained how it should be done, and cut one out of a piece of wood, it never arrived at a proper shape, and was never even placed upon the rest. I keep it as old Izaak Walton kept the Londoner’s artificial fly, viz., “to laugh at,” and as a caution to all concerned, never to go to a country blacksmith for slide-rest tools. The following plan answers very well for many kinds of outside work, and is on the whole a plan that may be satisfactorily followed by the young mechanic.
Instead of having the tools constructed from a large bar of steel half an inch or so in the square, they are made of short pieces about an inch long, fitted into a peculiar holder.
The advantage of this arrangement consists in the ease with which you can make your own tools out of broken round, triangular, or square pillar files, small chisels and such like. These can be shaped by the grindstone alone, and the blacksmith will not have to be called into requisition. I shall give you two forms of tool-holders, more or less simple, because I may suppose my young mechanic to be fast growing into an old hand, and able to appreciate differences in these arrangements.
Fig. 55, A, B, represents two of such holders, one for round, the other for flat steel cutters. You can see at once that when these are upon the bed of the rest, they form a tool with cranked end, as previously described, and can therefore be used in precisely the same manner. I shall give no directions for making these tool-holders, which are, nevertheless, very simple affairs, and can be readily understood from the drawings here given.
Fig. 55.
Another form is shown at C. The part d e is a clamp, which is separately drawn at f. This, like the last, enables one to use all sorts of odds and ends for tools. There are several other patterns of tool-holders, arranged either to use the little pieces of square, round, or triangular steel bars, so that one side, at least, of these may remain without grinding, and others in which two entirely new faces must be given to the tool by the grindstone. The latter are, perhaps, generally the best, because you can then, with the aid of the table of tool-angles, shape your cutters very accurately to the work required of them.
Although such tool-holders and cutters are generally used for metals, there are others intended for wood; and constructed to hold miniature gouges and chisels, which perform their work admirably. A capital tool for outside work, Fig. 55, E, which was used extensively at Portsmouth dockyard for brass turning, is made simply by filing off at an angle of about 45° a round short bar of steel. This angle, however, is unusually small for brass and gun-metal, 80° being better. For iron it will answer better, because though filed, or rather ground at 45°, the cutting edge, a little way from what may be called the point of the tool, is nearer 60°.
Similar to these last are the tube gouges, short bits of steel tube ground off and sharpened. These fixed in a holder answer beautifully for soft wood, and do not “catch in.” If the holder is bent so as to bring the tool into proper position, inside work can be rapidly effected by these, such as hollowing out large bowls and similar heavy work. All this can, of course, be done rapidly with the slide-rest, so far as regards the removal of the greater part of the wood. But in the case of a bowl, in which a curve predominates over a straight line, hand-tools must be used to finish it (generally the inside hook-tool). This last is, in fact, almost identical with the tube gouge; for the slide-rest, and that which makes it so difficult a tool to use, is that, being a hand-tool, and subject to slight unintentional changes of position upon the part of the workman, it catches in, and is either wrenched out of the hand, or a piece is chopped off the wood. Rigidly held in the slide-rest, the exact angle, once found, is of course maintained.
