How I Turn a Staff
Lacking a suitable set of micrometers to measure the length of the various seats on the staff, I have found the following very useful and quite accurate.
I have an ordinary micrometer of the one inch outside diameter type, but this cannot get in the close quarters to measure all the seats 'on a staff while it is in a lathe. I use the following system:
After removing the old staffs, I measure diameter. (a) and reduce a piece of wire to this diameter. This can be done with the mike.
Then I use the balance wheel itself as a guage to turn the balance seat to its exact diameter. Next, I lay the old staff down along side the new staff in the lathe so the end of the balance seat just touches the large flange of the new staff. Now the flange on the old staff marks the exact spot to make the undercut for the balance seat of the new staff.
After cutting this and turning the new hairspring seat to the approximate size. I measure the old staff hairspring seat (measurement c) and turn the new hairspring seat to exact size. The diameter of (d) is not critical but it should be reduced below the diameter of (c) and polished; the exact length of (c) and (d) may be determined by the same type of measurement as is shown in drawing (b) by laying the end of each so as to butt against the next larger diameter and the end of the new hairspring seat; for example will be indicated by the position of the undercut part of the balance seat of the old staff. The shape of the pivots should, be duplicates of the original staff and maybe fitted so as to just enter the hole jewel in the balance cock (with endstone removed) without freedom, then polished, which will give them the necessary freedom. The jewel should tip about 15 degrees to each side after polishing and be free in the jewel with a little of the pivot projecting through; next turn the back side of the flange and the approximate shape of the roller seat; then cut off and, reverse in a new chunk. Turn the seat to the required taper and turn down so the roller goes on till the space between the top of the roller and the bottom of the flange is about equal to the thickness of the jewel pin roller table. This gives about the right tension to the roller after staking on. Finally turn and grind the lower pivot to the same jewel fitting as the top pivot and polish carefully. By using the old staff in the same manner as is shown in, illustration (b), the new roller seat may be gotten to the exact length of the old. I usually leave the pivots a trifle long and then cut to exact fit after riveting on the balance wheel. I find it best to cut the top pivot to the exact "length and leave the bottom pivot a trifle long as roller seat is difficult to chuck in the ordinary chuck.