Pitch diameter formulas for external vs internal threads

Dec 14, 2004 2 Replies
R

I need pitch diameters for unusual external and internal Unified Screw threads that aren't in _Machinery's Handbook_, which glibly says, "Thread data for special combinations ... are also given in the Standard but are not given here." This is for optical instruments that use ultra-fine threads. Examples would be 2"-40 (typical inch size), or M58x0.75 (typical metric size, like a camera filter thread). The handbook is rather poorly written concerning the exact standard used to make the tables, but it appears they mean ANSI/ASME B1.1-1989, "Unified Inch Screw Threads". [These organizations want to monopolize technical information by not publishing it except on costly paper versions.]



Anyway, the formulas for the external vs internal pitch diameter are hidden in "the Standard" which doesn't seem to be available online. All I want is the formulas. Anyone have this information?


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N

There's enough information there to calculate the basic (theoretical with no tolerances and allowances applied) pitch diameter. What's missing are the tolerances and allowances for the various thread classes--allowance being the intentional deviation from the basic dimension that allow for clearance between mating fasteners. The allowance for class 3 threads is zero, and greatest in class 1.

I did once try looking at the pattern in the allowances for PD and major and minor diameters, and while there were obvious trends, I couldn't find evidence of a hard and fast formula being applied to generate the numbers in the tables. I don't blame you for being reluctant to extrapolate as far out as a 2.000-40 if the part is intended to mate with another that you can't bring to the lathe.

Ned Simmons

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R

I suspect it is a formula, but which contains tabular constants, similar to how pitch series (coarse, fine, etc) are arbitrarily tabulated.

Exactly. The mating part is typically in a customer's lab halfway around the world.

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