How to calculate Thread Pitch Diameter for non standard TPI like 5/8" 17.75 NF3


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You can calculate the Pitch diameter using (D-.64952p).
D = major diameter
p = 1/ thds per inch

That should be the max pitch dia for external thds or the min for internal thds. I don't know how to calculate the tolerance for a class 3 thd. We used a program to calculate it for us. I am surprised I remembered that this formula existed.


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how to calculate pitch dia for non standard TPI like 5/8" 17.75 NF3
First note that NF = National Fine. That designation was changed back in 1959 to Unified National Fine. See this discussion here: NF vs. UNF

Second note that the F = the Fine series of threads which is a specific set of threads defined in ANSI/ASME B1.1. Here is the list of UNF sizes.

Third note that since 17.75-TPI (threads-per inch) does not fit any of the UNF defined threads; it is correctly designated UNS (Unified National Special).

Fourth note that in 1959 the thread class changed from 3 to 3A or 3B depending on if the thread is internal or external. For the UNS Class 3A/3B class-of-fit the basic pitch diameter (MMC) is identical for both the internal and the external screw thread. (That statement is not true for the 1A/1B or 2A/2B class-of-fit which have a defined allowance between the male and female screw threads.) The tolerance moves toward the LMC in all cases for screw threads.

Fifth note that there is a piece of software available for you to use for calculating screw thread sizes: ThreadTech. <--This link takes you to a Free 30-Day trial of ThreadTech Screw Thread Engineering software.

Finally; the calculated values for this screw thread pitch diameters are:
3A External Thread Pitch Diameter: 0.5884"/0.5850" (14.945/14.859mm)
3B Internal Thread Pitch Diameter: 0.5884"/0.5928" (14.945/15.057mm)

I hope this information helps.
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