Calculation of Unified Thread Limits - Non-standard thread 1 ? 36 UNS-2A



I am wondering if anyone can spot my error in making a calculation for a unified thread limit of size. My company very occasionally needs to use a non-standard thread. I am working with an inherited drawing of a non-standard thread 1 ? 36 UNS-2A with no major or pitch diameter callouts listed.

I used the formulas in ASME B1.1 entered into a spreadsheet to determine the limits of size for entry on the drawing. I compared the results of my calculations against the worked example of tables 18A and 18B in the standard as well as a couple of other standard series threads in Table 2 from the standard.

The calculated results in my spreadsheet match the examples in tables 18A and 18B, but fail to match on the very common 1/4 ? 20 UNC-2A. Table 2 in the standard lists the pitch diameter tolerance on the external thread as being .003731 whereas I calculate that the value should be .003987 using the formula in section 5.8.1(c)(2). Another method of determining the tolerance is to use table 4 in the standard and sum the correct values from the table. Doing this agrees with my calculations, not the listed values in table 2.

It seems to me that I must be making an obvious error, but I just can?t see it. Has anyone else run into this that can steer me in the correct direction?

Thanks in advance for any help.

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