Clarification on Threading Infeed

I just wanted to clarify the infeed for threading on both sharp v threads and also UN threads. I think I understand for sharp v threads the depth of cut is equal to the pitch of the thread. So for a 2 1/4-8 sharp v thread with the compound set at 29 degrees the calcualtion is .125*cos 29=.1094"

For a UN thread the infeed is 7/8ths of the pitch or .875*.125/.875=.125 So the infeed at 29 degrees would be .125"

This seems logical since when cutting UN threads you are truncating the triangle by 7/8 ths. Therefore .125*7/8= .1094

The UN thread is 7/8ths of the sharp v thread.

Is this correct?

Thanks, Steve

Reply to
Steve
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Steve,

You need to account for a flat on both the crest and the root. If you use a sharp V tool, you don't get a flat on the root of the thread, but the flat on the crest is still produced by limiting the infeed.

For now, assume the compound set at 30 degrees (because the math is easier), external thread, a sharp V tool where you won't have a flat on the root, and you want a proper 1/8 flat on the crest, you'd advance the compound by 7/8 of the pitch. So, for example, a 8 TPI thread should be advanced 1/8 * 7/8 or .1094. But if the tool is formed for a 1/8 flat at the root, you might advance

3/4 of the pitch (or .0938 for 8 TPI).

To be more precise, with the compound set at X degrees, you'd multiply the above calculation by Cos(30) / Cos(X). For a 29 degree compound, that value is about .9902. So, the V formed thread is infeed .1083 and the UN formed thread infeed is .0928.

Regards, Bruno

Reply to
Bruno

Reply to
Robert Swinney

Agreed. But it still helps to correctly calculate the infeed so you know when you're close. Also, if you're doing threading on non-standard sizes and/or small pitched threads, you may not have a test nut or other means to accurately test while cutting.

-Bruno

"Robert Sw>Not to worry re. the sharp V at the root. Grind a tiny radius on the V

Reply to
Bruno

Also agreed! Having a thread gauge close by is good for verifying infeed. Technically, you will need 2 of these "fishtails" - 1 for Sharp V and 1 for UN. As Bruno says, for non-standard sizes (when you are nutless) there is no substitute for doing it right.

Bob Swinney

Reply to
Robert Swinney

1/8 pitch? Shouldn't the valleys be truncated by 1/4 P? 1/8 P is the truncation of the peaks? That's how I understand _Machinery's Handbook_.
Reply to
Richard J Kinch

Reply to
Steve Austin

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