By Clinton James
I’ve read lots of articles and books and have not seen this idea anyplace. Of course, it’s an adaptation of the long-employed procedure required when using a lathe without quick-change gears.
I have occasion to cut 1/8″ NPT threads on parts I’m making on my lathe. My Taiwan-built lathe has no listed capacity for 27 threads per inch; the nearest is 26 and 28. However, as one of the gear sets provided for metric thread-cutting, the supplier included a 30-tooth gear that will fit on the quick-change box input drive shaft. Since the normal gear on this shaft is a 40-tooth gear, replacing the 40-tooth with the 30-tooth causes all the gears in the quick-change to rotate at 4/3 the indicated speed. This has the effect of causing the threading tool to move at 4/3 the indicated thread-spacing or 3/4 the indicated thread count. So, I set the quick-change at 36 tpi and the lathe cuts 27 threads per inch. Don’t expect the thread-chasing dial to work when you’ve destroyed the synchronization with the spindle gear. The carriage must be engaged at all times, just as when cutting metric threads with an inch-cut lead screw. It’s a bit clumsy having to back the carriage into position each time, but that’s the price for the ability to cut odd-count threads.
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