Pass Along MW
September 08

Cutting Odd-Count Threads
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.

Please do us a tremendous favor and forward this e-mail on to your machining friends!

Are you not a subscriber to MACHINIST'S WORKSHOP ? Visit us at and request a no-obligation issue, or call and request it at 1-800-447-7367. Tell them you received an e-mail from a friend!

Your friend thinks you’ll enjoy MW, or you wouldn’t have this e-mail!

We hope you enjoyed this e-mail tip, brought to you by your friends at Machinist's Workshop, dedicated to precision metalworking.

To unsubscribe to these pass along tips just send this email message.

Go to the Pass Along Archive