FINISHING THE METAL-WORK.
The batch of parts, having passed through their various stages of manufac will not. Thus no two screws will jam, out of any possible combination that can be made. Hundreds of such gauges are used for the many dimensions required in lens settings, all being periodically inspected for signs of wear. To overcome the diffi culty of expansion by friction, compressed air is blown upon the work to keep it cool. Even with this and other precau tions, it is found difficult to shape metals, by means of cutting tools alone, within limits of error much smaller than -a„-„ in. And although such variations are really small compared with those com monly to be found in good instrument work, there are some parts of lens ture, are carefully inspected in detail, any which do not fulfil the prescribed con ditions being at once rejected. Tho passed pieces are then received into stock, and presently, with other parts, are finished and assembled to form complete lens settings. In finishing, the object is to retain the accuracy already given to the pieces, and at the same time to give them the good appearance of finished work. Fig. 543 shows a rack of inter changeable holders or chucks, by means of which the prepared parts are held true in various machines. The polishing of brasswork calls for no particular remark. The fluting of the rims of the cells, a marking similar to that seen on the peri phery of silver and gold coins, is pro duced in the lathe by rotating the piece and impressing it with a steel roller having the requisite pattern cut in its rim. The lacquer or varnish, used to
preserve the metal-work from tarnishing, is composed of shellac dissolved in alcohol. As shown in Fig. 541, the lacquer for focussing, and all other inscriptions usually found on a lens, are engraved upon the mount by the machine shown in Fig. 515, of which mention has already been made. This machine is, in principle, a de velopment of the well-known pantagraph. A copy of the inscription to be engraved is used, several times larger than required, and the operator, by moving a style along the lines of this copy, works a graving tool about the lines to be engraved. The is applied with a brush, the work having previously been heated sufficiently to evaporate the alco hol as the lacquer is applied. The shellac is thus distributed evenly over the surface, and be comes hard as the metal cools. It will be seen that mounts should not afterwards be un necessarily warmed, or the lac quer may thus be softened and damaged ; n Or should they be washed with alcohol, as is some times advised, for that removes the shellac.