Stem Dies and Crown Taps
By Ronald L. Ives
Some years before the present war started, crown taps and "Swiss" screw plates became scarce, and (with some exceptions). of poor quality. Since the war began, these items are practicaly unobtainable. Crowns still need tapping, and probably more new stems are being made today than ever before.
Published screw tables indicate that the following stem tap sizes are "standard":
Diameter, Inches Threads per inch
Search for suitable threading tools that are obtainable shows that there is no good substitute for the .048/110 tap and plate thread.
A standard machinist's tap of 0-80 size has a nominal diameter of .0600 inches, with 80 threads to the inch. An 0-80 bottoming tap can be used in place of a .061 crown tap with about 95 percent success.
The 0-80 die can be used to thread a stem for this tap, or if a round split die, such as a Greenfield type 382 is used, it can be opened sightly, by use of the adjusting screw, so that threads cut with it will make a fit of any tightness desired in a pretapped crown.
If the thread is started with the front portion of the die, which is relieved, and finished with the rear portion (by reversing the die in the holder) a very clean thread can be cut.
For the .077/72 crown tap a 1-72 machinist's tap can be used, this tap having a nominal diameter of .0730 inches. This substitution will work 9.bout 80 percent of the time without further work. The 1-72 round split die can be used as suggested for the 0-80.
There is no standard machinist's die having a 60 thread count. Both the 2-56 and the 2-64 tap have a diameter close to .0860 inches, which is .005" smaller than the .091 nominal diameter of the 60 TPI crown tap. Both of these sizes will work satisfactorily in stem and crown assemblies, although the 2-64 size, with its greater root diameter, is probably to be preferred. Both of these sizes, it should. be realized, even though they are standard in machine and instrument practice, are "bastards" so far as the average watchmaker is concerned, and their use, even though mechanically sound and justifiable, should be reserved for emergencies.