Your advice is earnestly asked in the following problem.
My present method of restoring rough or damaged balance pivots is to remove the hairspring and roller, mount the balance wheel in the split chuck and go to work on the pivots with oil-stone powder, diamontine, iron and bell metal slips. This method is satisfactory in producing results but is slow, particularly in adjusting, where it is sometimes necessary to grind the cone of the pivot back.
The balloon type of chuck is unsatisfactory except for working on the pivot end. I have concluded that the American type pivot polisher as made for the American lathe would be slower than my above method as used in general repair work.
The Jacot lathe seems to be the answer but while I have studied your articles on this tool which appeared in Horology, also the article in "Practical Benchwork," I still do not understand its use thoroughly. It is clear that no grinding or polishing compounds can be used in connection with this tool as the rests and the lanterns would soon be ruined.
It is stated that pivot files are to be used. But how? Are they drawn to and fro over the rests while the staff is rotated with the bow? It seems to me that this would soon ruin both file and rest. Also I find that watchmakers' files will not cut all staffs. Before burnishing a pivot with steel it has been my belief that it was first necessary to prepare it for burnishing by polishing with diamontine. Assuming that I have the pivot filed to the desired size how am I to proceed to polish it on the Jacot lathe? Am I to understand that a proper polish can be had by burnishing only?
Answer: Most American balance staffs are too hard to be filed in the Jacot lathe. Neither is there any reason why American staffs should require filing.
Any make of balance staff may be burnished on the Jacot lathe with a burnisher to which oil is applied. Dry burnishing should never be practiced. The burnisher will also reduce the pivot in diameter and if a burnisher with a cone file is used, the pivot may thus be lengthened. The degree of cutting which a burnisher will do depends on the grade of emery paper on which it is dressed.
If much material is to be removed from a pivot a burnisher dressed 011 coarse paper should be used. Finer polishing is done with a burnisher dressed on fine emery paper. Diamontine or cutting stones should never be used on the Jacot lathe except for touching up a pivot when supported by the lantern spindle. The pivot files, if properly used, will not damage the tool.