Pinions.
Well made as to truth of centring, of division, of form of leaves, and polish, are, as the trade well knows, of vital importance to the value of the time-piece.
The making and finishing is one of the most troublesome, as well as most expensive of all the processes in watch work. The nature of the material renders it difficult as it approaches so nearly in hardness to the tools used in cutting. In the ordinary Yankee clock, the lantern pinion has entirely superseded the solid leaf, which substitution was the greatest element of success in their cheap construction. The lantern pinion is really a nearer approximation to the required anti-frictional form than a majority of cut pinions in ordinary clocks. In the process of manufacture of the cut variety, the first consideration is the quality of the steel to be used. For this purpose it should be carefully selected by trial, thus ascertaining its fineness, uniformity, softness when annealed, together with its capacity for taking a good temper, with the least amount of springing during the hardening process. Very few pinions are cut from the solid piece—the drawn pinion wire being quite good enough, when milled and finished, for the ordinary run of watch work.
The steel wire having been selected, the first process is to cut it up in lengths a trifle larger than the required pinion. The separated pieces are then centred with care, and having been placed in a lathe, the staff and pivot are turned up to nearly the required gauge, leaving a portion of the whole piece the full size for the leaves. They are now taken to the milling tool to have the proper form given to the leaves. As this form is of the highest importance, it may be as well to give here the reasons. Supposing a wheel of 60 teeth, depthing into a pinion of 8 leaves, it can readily be seen that the arc of the motion of the wheel tooth is of greater radius than that of the leaf of the pinion, and it follows that if the teeth and the leaves are made in taper form with straight sections, there must occur a sliding motion on the surfaces of both—the power thus absorbed being totally wasted; but if we curve the surfaces we may approach a form so nearly perfect that the wheel teeth, being motors, really roll on the leaves, avoiding almost entirely the friction caused by sliding; the necessity for this curvature becoming greater the more the wheel exceeds the pinion in diameter. This curve, which has been demonstrated by very profound mathematical researches, is the “epicycloidal;” theoretically it should give no more sliding motion than the surfaces of two plain wheels revolving on each other. To obtain this perfect form, very great pains have been taken and expenses incurred, especially by the makers of the best time-keepers.
In the American factories the cutters are very elaborately made, the section being an object of great solicitude—it being an exact counterpart of the space between any two leaves, and also of one-half the top of the leaf from the curvature to the point, so that in milling, the space made by the cutter is its shape, leaving the leaf of the proper form. Generally the pinion passes under two cutters; the first to strike down the rough stock, the other to dress it to size and shape, with a light cut. The care and skill required to make these is certainly very great, and it is a proof of the wonderful ingenuity of man that they are made so perfect as to shape and cutting power.
A very ingenious device is used for dividing the leaves under the cutter, which revolves at a moderate speed over a slide, carrying a pair of centres, between which the turned up piece of pinion wire is placed. The slide is now pushed up to and under the cutter, and in its passage as much of a cut is taken as is desirable; in drawing back the slide the fresh cut space passes under a flat piece of thin steel, screwed on the frame, and set at a slight angle to the axis of the centres. On moving the slide towards the cutter for a fresh cut, the steel plate takes the last cut, and in passing by it the pinion is turned just as much as the angularity of the plate, which must be just one leaf. By this very clever device the division is effected without an index plate. This process, however, is not good enough for work intended to be very accurate—the pinion wire not being always, or indeed rarely correctly divided, the original error will be perpetuated in all the subsequent processes. These are all milled, with oil or soda water for a lubricator, and it follows that the speed of the cutter is regulated to get the greatest cut without dulling the tool. When dull, however, the mill is sharpened on the face of the cutting tooth by means of small grinders of iron, using Arkansas oil-stone dust for the first grinding, and giving the necessary delicacy of the edge by means of crocus, or sharp, followed, when fine work is needed, by rouge.
It is necessary that this care should be taken, for if the edge is left coarse it will become speedily dulled, and leave a very unequal and rough surface on the cut of the pinion, which in the subsequent grinding gives rise to error in shape and size. The pinions, thus cut to gauge, are dried in sawdust, hardened, and tempered; the staff and pivots are now turned up to size, and then pass to the polishers. In the factory they are finished by means of what are called Wig-Wags, which it may be interesting to the reader to have a general description of.
Two Vs are arranged as centres, the pinion is placed between them, the circular parts resting in each V, but free to turn on its own axis. Immediately above the Vs is a frame on which a slide, carrying the polisher, may traverse—generally about two inches. This slide is movable vertically so as to accommodate itself to the pinion; attached to the slide is a connection which leads to a vertical lever, which is put in motion from a crank on the counter shaft. The grinding is effected by bringing the grinder, charged with oil-stone dust in oil, in one of the spaces of the pinion, which, of course, is so arranged as to bring it parallel and central with the grinder. The power being applied, the slide takes a very rapid reciprocatory motion, and the face of the grinder, so charged, rapidly reduces the uneven surface left by the cutter to what is called the gray.
The form of this grinder must be as perfect as the cutters, and the care taken to get the requisite parallelism is in equal proportion, and in all the best polishers is planed up while in its position. The grinder is composed of tin and lead, with sometimes a slight admixture of antimony, rolled to an even thickness, cut off in suitable lengths, and then mounted in the carrier of the Wig-Wag to be planed up to shape. There are too many minute adjustments in the machine to render a full description in this article admissible. It is large compared with the work it has to perform, but it is very admirably made, as indeed all the tools are, in the American factories.