HOW TO USE A CEMENT CHUCK.
The shellac is melted in an iron vessel, and the ultramarine added and stirred to incorporate the parts. Care should be observed not to burn the shellac. While warm, the melted mass is poured on to a cold slab of iron or stone, and while plastic made into sticks about 1/2" in diameter.
We show at Fig. 178 a side view of the outer end of a cement chuck with a cylinder in position. We commence to turn the lower pivot of a cylinder, allowing the pivot z to rest at the apex of the hollow cone a, as shown. There is something of a trick in turning such a hollow cone and leaving no "tit" or protuberance in the center, but it is important it should be done. A little practice will soon enable one to master the job. A graver for this purpose should be cut to rather an oblique point, as shown at L, Fig. 179. The slope of the sides to the recess a, Fig. 178, should be to about forty-five degrees, making the angle at a about ninety degrees. The only way to insure perfect accuracy of centering of a cylinder in a cement chuck is center by the shell, which is done by cutting a piece of pegwood to a wedge shape and letting it rest on the T-rest; then hold the edge of the pegwood to the cylinder as the lathe revolves and the cement soft and plastic. A cylinder so centered will be absolutely true. The outline curve at c, Fig. 178, represents the surface of the cement.
The next operation is turning the pivot to the proper size to fit the jewel. This is usually done by trial, that is, trying the pivot into the hole in the jewel. A quicker way is to gage the hole jewel and then turn the pivot to the right size, as measured by micrometer calipers. In some cylinder watches the end stone stands at some distance from the outer surface of the hole jewel; consequently, if the measurement for the length of the pivot is taken by the tool shown at Fig. 175, the pivot will apparently be too short. When the lower end stone is removed we should take note if any allowance is to be made for such extra space. The trouble which would ensue from not providing for such extra end shake would be that the lower edge of the half shell, shown at e, Fig. 171, would strike the projection on which the "stalk" of the tooth is planted. After the lower pivot is turned to fit the jewel the cylinder is to be removed from the cement chuck and the upper part turned. The measurements to be looked to now are, first, the entire length of the cylinder, which is understood to be the entire distance between the inner faces of the two end stones, and corresponds to the distance between the lines v d, Fig. 171. This measurement can be got by removing both end stones and taking the distance with a Boley gage or a douzieme caliper.
A CONVENIENT TOOL FOR LENGTH MEASUREMENT.
A pair of common pinion calipers slightly modified makes as good a pair of calipers for length measurement as one can desire. This instrument is made by inserting a small screw in one of the blades—the head on the inner side, as shown at f, Fig. 180. The idea of the tool is, the screw head f rests in the sink of the cap jewel or end stone, while the other blade rests on the cock over the balance. After the adjusting screw to the caliper is set, the spring of the blades allows of their removal. The top pivot z of the cylinder is next cut to the proper length, as indicated by the space between the screwhead f and the other blade of the pinion caliper. The upper pinion z is held in the jaws of the cutting pliers, as shown in Fig. 177, the same as the lower one was held, until the proper length between the lines d v, Fig. 171, is secured, after which the cylinder is put back into the cement chuck, as shown at Fig. 178, except this time the top portion of the cylinder is allowed to protrude so that we can turn the top pivot and the balance collet D, Fig. 171.
The sizes we have now to look to is to fit the pivot z to the top hole jewel in the cock, also the hairspring seat D and balance seat D'. These are turned to diameters, and are the most readily secured by the use of the micrometer calipers to be had of any large watchmakers' tool and supply house. In addition to the diameters named, we must get the proper height for the balance, which is represented by the dotted line b. The measurement for this can usually be obtained from the old cylinder by simply comparing it with the new one as it rests in the cement chuck. The true tool for such measurements is a height gage. We have made no mention of finishing and polishing the pivots, as these points are generally well understood by the trade.