The nominal size of a thread is the major diameter of the internal thread, or the nut. In other words the tap's major diameter. This is the same for metric and imperial threads.
When cutting an external thread on the lathe, the stock must first be turned to the proper diameter.
When watching machinists on Youtube, even the very experienced ones, they all turn their stock to the nominal size, and they all end up with a tight fit. Not understanding what has gone wrong, they will often call it a "machinist's fit" and ignore the problem.
I have searched and searched, but I have not been able to find a table or formula that tells me the proper size to turn the stock to before cutting external threads on the lathe.
What am I missing?
RoRo
---------------------- If the threads fit there isn't a problem.
According to Machinery's Handbook the Unified tip truncation is 0.125X the vee thread height and the pitch diameter is 0.375X below it. Your answer could be figured from the depths of sharp vee and Unified threads and the pitch diameters, which are in different tables. Then add the tolerance for the class of fit. I'd put the data in a spreadsheet and then compute and print the 29 degree infeed for each pitch on a copy of the threading gearbox chart.
It looks to me like the OD can be the nominal size in the maximum material condition so that's how I cut them, and commercial nuts and tapped threads fit. Custom parts don't have to be universally interchangeable and I think a closer fit with greater root diameter is stronger. The OD of several of my
4-flute 1/4-20 taps averages 0.252" and the tap must be my gauge, not the book value.