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. |
My drill chart shows a 1-1/2"-6 thead having a 1-11/32" tap drill ending up with 72% thread.
A 75% thread requires 20% less power to cut, and is only 5% weaker than a full thread.
That's only for "open tolerance threads". If you have a tolerance specified, better look it up and gauge it.