Help with Pitch Diameter Tolerence!

I’m having a bit of trouble with finding pitch diameter tolerances for odd-ball threads. I’m able to look some common threads up online and in the machinery’s handbook, but when I run into something like .4707-32, or #14-24, I’d like to be able to calculate them on my own.

I’ve been pretty successful in finding reliable formulas to calculate the basic pitch diameter of any thread, but now I’d like to be able to calculate the pitch diameter tolerance as well. From “ASME B1.1-2003,” the pitch diameter for 2B threads is given as:

1.3*(0.0015*(BasicMajorDiameter)^(1/3)+0.0015*(LengthOfEngagement)^(1/2)+0.015*(Pitch)^(2/3))

I’ve been adding what I obtain from that formula to the basic pitch diameter, and comparing the values to this table:

[url]http://www.threadcheck.com/technical-documents/standard-inch-pitch-diameters-pg54.pdf[/url]

I’m unclear as to what the “length of engagement” should be set as. I get closest to this table when I set it to one diameter. When I do this, in some cases, I’m spot on, in many cases, I’m off by one ten thousandth, and in some cases I’m off by as much as a thousandth.

Can anyone help me?

This is just for unified threads. I haven’t even STARTED looking at metric yet…