25 tpi is used on many old wood working tools to make the screws proprietary. The biggest offender by far was Stanley Tool Works. I made a living for a few months making replacement screws for the venerable old Stanley #45 and #55 combination planes. I suspect that this thread may be oddball for similar reasons.


I had to do the math on my change gears in order to turn 25 tpi. Almost all lathes with screwcutting ability can be used to make oddball threads that are not listed on the change gear charts. It is a matter of fractions. If you have a 16 tpi leadscrew then the fraction is 16/25 or .64 turns of the leadscrew for each turn of the spindle. Not many 16 tooth change gears out there, so multiply the fraction by two to get 32/50. Those gears I have. Engage a 32 tooth gear with the bull gear on the spindle and slap a 50 on the end of the leadscrew. Use an idler gear of any size to hook them together and you have 25tpi. You can get some really odd pitches this way. Just remember that gear ratios are merely fractions, maintain the relationship between the numerator and denominator and you will be fine. This also works with compound gearing which is where you can get some incredibly odd pitches. How about 114.6 tpi? Hmmm??? Multiply the possibilities immensely if you have a quick change gearbox on the leadscrew.

For an excellent dissertation on figuring out your own change gear combos, I recommend Using the Small Lathe by L.C. Mason. It is published by Tee Publications in England