Originally Posted by Geof  That is not exactly correct is it?
For instance your example using 3/8-16 gives 0.3125" which is the drill size to use for 75% full form thread so it is actually large than the minor diameter.
It is a good approximation for tapping but for cutting a thread I think it will leave a wide flat crest. |
I'm going on memory here, so correct me if I'm wrong: the basic UN spec calls for a 1/6 flat at the crest and 1/24 flat at the root of a female thread. 1-(1/6)-(1/24) = 0.791 = around 79%. TDS is not 75% of a full-form, or normal thread, it's 75% (actually 73.08%) of a full, sharp V, which is really a negligible difference for most practical purposes. 75% is used in tapping rather than 79% (or 73.08%) because:
A) It's easy for sleepy machinists to remember.
B) The thinnest part of the thread contributes the least to it's strength.
C) The thinnest part of the thread contributes the most to cutting forces.
D) It's easy for sleepy machinists to remember.
Usually when I single-point a UN thread I just bore to tap drill size, for all of the reasons stated above.
I would not do this if a drawing specified a 1B thread, rather than the usual 2B, or if I was making a gage.