1. Home
  2. ⏣ General Metalworking
  3. Pitch diameter formulas for external vs internal threads

Pitch diameter formulas for external vs internal threads

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?

Reply to
Richard J Kinch
Loading thread data ...

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

Reply to
Ned Simmons

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.

Reply to
Richard J Kinch

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.