Non-standard thread?
 

Hi,

I need to make an adapter for this rotary tool but the thread has me stumped:
http://hiwaay.net/~bzwilson/prius/turbo_300.jpg
The scale is 1/32 and my lying eyes say the pitch is ~31.5 threads/inch. Converting to metric, 1.279 threads/mm. The diameter is 5/16 inch, 7.9375 mm.

The diameter reads SAE but I can't find any standard thread that matches this threaded shaft. The problem is my adapter needs to screw on the shaft but this does not look like any standard tap would work. I need a tap to make the part.

Right now, it looks like I'll have to turn my own tap (I have access to a lathe at the autoshop.) The reason is finding a tool bit to cut the interior is not a trivial problem given the I.D. is going to be 5/16 inch, 0.3125" (7.9375 mm.) Fortunately, the adapter part will be aluminum so I can make a tap using steel stock; cut the threads; grind gaps for shavings, and; heat treat to harden.

The shaft is hollow but the I.D. is 0.1945" (4.9403 mm) and I need a shaft of 0.250" (6.35 mm.)

Thanks,
Bob Wilson

Would that put it in tolerances for 8mmx1.25? I've run into some threads that need to be "finessed" to fit in standard thread pitches. Manufacturing standards aren't what they used to be.

I think you've nailed it . . . I had put together a spreadsheet but it had an error that kept me from seeing it. Once the error was fixed:

• 1.4% - diameter error, small
• 3.8% - pitch error, long

Thanks!
Bob Wilson

Could it be a Whitworth thread? Mostly found in British stuff prior to the 1960s: British Standard Whitworth - Wikipedia, the free encyclopedia

It's not clear to me what that male thread is attached to, but if you can, why not just take in to the harware store with you? Go over to the bins of nuts. Try your best guesses until something threads on easily by hand. Most stores carry all the standard SAE & metric sizes. That's usually my solution to this kind of thing.