We have now arrived at an important station in staff making, a junction, we may term it, where many lines branch off from the main road. At this particular spot is where authorities differ. I have no hesitation in saying that at this particular point the split chuck should be removed from the lathe head and carefully placed in the chuck box and the cement chuck put in its place. I believe that all of the remaining work upon a staff should be executed while it is held in a cement chuck. On the other hand I have seen good workmen who turned and finished all the lower part of a staff while in a split chuck, cut it off and turned and finished the upper part in a cement chuck. All I have got to say is that they had more confidence in the truth of their chucks than I have in mine. I have even read of watchmakers who made the entire staff in a split chuck, but I must confess I am somewhat curious to examine a staff made in that way, and must have the privilege of examining it before I will admit that a true staff can be so made.
We will suppose that the workman has a moderately true chuck, and that he prefers to turn and finish all the lower portions in this way. Of course the directions for using a cement chuck on the upper part of a staff are equally applicable to the lower. Before going further I think it advisable to consider the requirements of a pivot, but will reserve this for another chapter.
CHAPTER II.
The chief requirements of a pivot are that it shall be round and well polished. Avoid the burnish file at all hazards; it will not leave the pivot round, for the pressure is unequal at various points in the revolution. A pivot that was not perfectly round might act fairly well in a jewel hole that was round, but unfortunately the greater proportion of jewel holes are not as they should be, and we must therefore take every precaution to guard against untrue pivots. Let us examine just what the effect will be if an imperfect pivot is fitted into an unround hole jewel, and to demonstrate its action more clearly let us exaggerate the defects. Suppose we pick a perfectly round jewel and insert into the opening a three-cornered piece of steel wire, in shape somewhat resembling the taper of a triangular file. We find that this triangular piece of steel will turn in the jewel with the same ease that the most perfect cylindrical pivot will. Now suppose we change the jewel for one that is out of round and repeat the experiment. We now find that the triangular steel soon finds the hollow spots in the jewel hole and comes to a stand-still as it is inserted in the hole. The action of a pivot that is not true, when in contact with a jewel whose hole is out of round, is very similar, though in a less marked degree. If the pivot inclines toward the elliptical and the jewel hole has a like failing, which is often the case, it is very evident that this want of truth in both the pivot and hole is very detrimental to the good going of a watch.
Fig. 4.
Fig. 5.