tap formula

Cool tip! Saved and remembered (hopefully.) Thanks, Karl.

that cool tip follows from 6 degree thread pitch - 1/2 base times height

- it's in all the handbooks.

You mean 60 degree, of course. And "... 1/2 base times height" is the area of a triangular thread cross section & has nothing to do with diameters.

The height of the triangle is P/2 * SQRT(3)(where P = pitch), which is the difference between the major & minor _radii_ of a _triangular_ form thread. The difference in diameters is then P*SQRT(3) which is quite a bit larger than P (1.7 times).

But the standard thread form is not a triangle - the crest and root are both truncated. So by a happy cancellation of inaccuracies (the pitch being to small a difference for triangular threads and the standard thread not being triangular), the pitch is a close-enough difference between major & minor diameters.

I suppose that if one went through the geometry of the truncations, the difference would come out being (almost)equal to the pitch. But it certainly does not come from the height of a 60 degree triangle whose base is equal to the pitch.

Bob

Quicker to simply look at a chart than to convert say 1/13" or 1/11" into a decimal format...

I've had most of the common tap drill sizes memorized nearly forever anyways although frequently I'll use a slightly larger size, depending on the actual job

Same here, a larger hole makes hand tapping so much easier and cuts down on tap breakage.

A common internal thread, drilled so that it results in 50% of full thread will break the external thread, before the internal thread will strip. A common internal thread drilled out so that it contains 100% of full thread is only 5% stronger than a 75% height of thread, yet it requires 3 times the power to tap.

Best Regards Tom.

Do you have "CRS" disease? (can't remember shit) Welcome to the

---whatever it is!

If I recall correctly that is true for holes that the engagement length is at least as long as the diameter.

Dan

" snipped-for-privacy@krl.org" fired this volley in news: snipped-for-privacy@googlegroups.com:

Rule of thumb is five full threads of engagment. For common coarse threads, that's _about_ one major diameter. (i.e. 1/4-20... 5 threads equals 1/4")

LS

I distantly remember from college that the minimum engagement length was a good rule of thumb to apply if you didn't have data on the material properties. Later I heard it in auto and aircraft contexts.

A Grade 8 screw in an aluminum casting won't fail the same way as a Grade 2 in a steel nut, and coarse threads are more likely to break at the thread root, fine ones more likely to strip. If we knew the properties we were supposed to calculate how much the male element would stretch and IIRC design so the threads fail by stripping progressively, beginning at the hole surface where bolt stretch is greatest.

Herreshoff designed his rigging turnbuckles such that the barrel and eyebolts stretched identically. When tested to failure all parts deformed before any broke.

The college class was for chemists and perhaps not as extensive in mechanical failure modes as one for MEs, but it went far into alloying, grain structure and corrosion.

jsw

The ROT that I read here, a long time ago*, was 4 threads for steel and

Bob

• - i.e., too long ago to remember the author and so to determine credibility