In some engines the edge of the dividing-plate, or rather a rim just within the edge on the under side of the plate, has a screw cut upon it, into which a tangent-screw is made to act ; and the head of this screw consists of a small wheel, or dividing-plate, removeable at pleasure, in order to substitute enother of a different number. By such an arrange ment as this just described. not only may any number (even prime numbers) be cut, but the plate itself may be divided. The cutting of wheels. or rather shifting the plate by the screw, would take too much time for ordinary wheel-cutting; hence the numbers most in use are drilled in the plate as above described. A great many ingenious methods are resorted to in wheel-cutting for the purpose of cutting odd numbers, for moving the plate over any number of divisions rapidly without the possibility of making a mistake, the details of which our limits will not allow us to go into : what we have said above wo deem sufficient to illustrate the principle. It is necessary here to observe that the cutter, previous to commencing to operate, should be adjusted so that a plane passing through the ccutre of its cutting-edge, and parallel with its either, should pass also through the centre of the arbor on which the wheel is fixed ; otherwise the teeth will not be cut in straight to the centre, and will have the appearance of being bent on oue side. It is also necessary that the downward motion of the slide containing the cutter-arbor should be perfectly at right angles with the plane of the wheel. We have hitherto spoken of the wheels as being screwed to the arbor c, but we have also said the arbor c is hollow, for the purpose of receiving the axes of the wheels to be cut. This hollow arbor is, hi fact, capable of receiving end-pieces, which are firmly fixed therein ; and it is ou these latter pieces that the wheels which have no axes of their own, but simply a hole through them, are screwed. But when the wheel to be cut is already fixed on an axis, the hollow arbor is used with an apparatus shown in tho figure, which we shall now describe :-1, 1, part of a very firm bar attached to the lower frame, and extending horizontally across the centre of the engine plate, the extremities of which bar terminato in two pins or pivots ; 2, 2, one of which is not seen, extending a short distance beyond the edge of the dividing-plate ; on to these pins are hooked two pieces, 3,3, which, at their upper ends, pass through slits in the piece of iron, 4,4, which has a screw, 5, passing through its centre. (In all this apparatus, except the screw, 5, there must be no nice fitting, but perfect freedom.) The screw 5 Ilea a centre, either male or female, by which it presses down the hollow cone y; but the foregoing apparatus, called the gal lows, will be beat understood by describing the mode of putting on a wheel having an arbor In it :—Ilemove the gallows; on to the end of axis c drop a flauge r, a little less in diameter than the circle which corresponds with the bottom of the teeth to be cut : through the centre of this flange drop the longer part of the arbor of the wheel rr, to be cut, so that the wheel rests on the flange x; then over the wheel drop the flange or collar x' (of the same diameter as r); over the shorter part of the axis of wheel re place the hollow cone y : bring the gallows over the cone ; screw down screw 5 just sufficiently tight to hold the gallows in its position, but not to set the wheel re fruit ; remove the pin k from the plate P r, and cause the latter to revolve rapidly by the hand or otherwise; the wheel so will in all probability be far from concentric with the plate, as will also the centre of cone y. With the hand slip the wheel ve or cone y, or both, as may be required, together with the upper flange x', as nearly concentric with the plate r r as the hand and eye will enable you in one or two seconds to do; next slide the piece 0 so as to bring the crew 8 to a convenient distance from the wheel ,r, and set fast by screw 7 ; then by revolving the plate rapidly, and gradually bringing the end of 8 to the edge of wheel w, it will be got perfectly concentne ; next screw down 5 sufficiently tight to prevent the wheel from moving, and set fast screw 5 by tightening nut 9 : the cutter being then adjusted so as to cut the teeth a proper depth, the cutting proceeds as before described. This operation of setting the wheel perfectly concentric will not occupy an experienced hand more than fifteen seconds.
Of the Callers, and the mode of reakisg and wring them.—Tho cutters vary with the nature of the work to be done, and much depends upon having good ones and well adapted for the purpose. The first kind wo shall describe is used for the commonest work, that is, merely for cut ting spaces whose sides are parallel through their whole length (in which case the teeth have to be rounded up afterwards), and the cutter is merely a circular disc of steel with teeth on its edge, similar to a circular saw, the two sides being slightly undercut, as seen in the sec tion No. 1 : this allows the cutter to pass freely through the metal
without clogging, which it would do were the sides parallel. These teeth in the cutter are sometimes made with a file by hand, but they are much better inado iu the engine. Cutters of this description are generally tempered so low, after having been hardened, as to allow of the teeth being renovated with a 3-equare file when they become dull, and answer the purpose well enough for common pluposes. A better 'Sort of cutter, and more lasting, is made with from six to sixteen or eighteen teeth, as in No. 2, which gives a sido-view and section : these are left very hard, being tempered only to a light straw-colour, and are sharpened with emery and oil on a metal mill or lap ; these take much longer making than those first described. When these consist of not more than six teeth, each tooth is geuerally relieved in direction shown in section No. 3, but not on the aides, by which means the cutter frees itself better, and cuts on its sides to the very root of its tooth, which can be kept sharper and in much better working condition than the foregoing.
Cutters are also made having but one tooth, others with two, three, and four ; but these are more frequently used when the teeth are to be cut and rounded up at one operation, in which case the side view and section No. 3 will represent the form in which they require to be made to produce the necessary curve for the wheel-teeth ; and great nicety is required to produce two curves ou each side of the cutting tooth exactly similar, for unless they are so, the teeth in the wheel cut therewith will have a very unsightly appearance.
It will be necessary to say a few words on the different kinds of work to which the various cutters above described are applicable. No. 1 is toad, when tempered low, for soft metal, such as gold and brass and gun-metal ; if left hard, they should be sharpened with oilstone dust or very fine emery, and they can then be used for harder metals, such as iron and steel. No. 2 may also be used fur the same purposes, but when used for the harder metals should have a greater number of teeth, from 20 to 30 not being too many in a cutter of three-quarters of an inch in diameter. In general, the harder the metal to be cut the greater the number of teeth required in the cutter ; and with some metals, such as steel and bell-metal, it is requisite to keep the cutter well supplied with oil, and in all cases where hard metals are under operation the cutter should have a slow molion. Cutters with from one to six teeth may revolve from 10,000 to 20,000 times in a minute iu cutting the softer metals, and those with 10 to 20 teeth from 5000 to 8000 or 10,000; in fact, cutters with few teeth cannot be used for the harder metals with advantage, and the others should not have more than from GO to 100 revolutions iu a minute. These observations apply to cutters whose diameter is about thrce-quartera of an inch ; if the cutters are larger, they must have a proportionably less number of revolutions. Cutters are sometimes made for emitting steel, and indeed for all the metals, having very fine teeth, from 200 to 300 in the inch ; but the use of them is now almost obsolete : in fact, the process approximates very closely to filing, and such cutters may be regarded as circular files. One great disadvantage is that the keen edges are soon lost, and they cannot be renovated without softening.
Wheel-cutting is one of thoeo operations in which much depends upon the manual dexterity and judgment of the operator ; for it some times happens that steel, one of the hardest metals, may be cut with a more rapid motion, and a loser number of teeth in the_ cutter, than is stated above; but it is only when the metal of a particularly mild quality, or has been prepared especially for the purpose by softening. The following mode of softening the steel to be cut is a very good one, and may be frequently adopted with great advantage:— Envelope the articles in a mass of loam, clay, or lime, with sufficient moisture to make it adhesive, taking care that the articles to be softened have a considerable thickness of the material all round them. Heat the mass gradually to a blood-red heat; keep it at this heat as long as convenient, and let it remain in the fire to coo] gradually as the fire goes out, after which the articles may be removed, and will be found more uniform in texture and easier to be cut than before being subjected to the operation. In making cutters, as well indeed as any other edge-tools, the susceptibility of receiving and retaining a keen edge will be considerably increased by subjecting the steel to great condensation, by cold hammering, previous to the process of hardening, and more especially if the scale left after forging be first removed with a file. Harden in water, but do not heat your steel too hot ; for every degree that steel is heated above its hardening point deteriorates its quality.