I need to single point some ID 3/8 NPT threads but for the life of me can't figure out the minor diameter at the beginning of the bore. I found the thread height in the machinist's handbook to be .04444 but nothing that leads me to a minor/major diameter. I understand the taper amount (1/16 per foot) and how to program it with a G76 but just need a starting diameter to work from. Any help would be much appreciated, thanks.
The Machinist's Handbook gives the pitch diameter at the front of the OD thread. This is shown on the drawing to be at a defined distance from the front of the ID thread.....L3 if I remember correctly. No Handbook at home. For minor diameter, offset half the thread height, and use a little trig to figure out the bore diameter at the front of the thread.
Or simply multiply 1.78333 times L3, times 2, plus the given pitch diameter, minus .0444 to be close enough for government work.
Or do as I do....lay it out in Mastercam?
The major diameter can be figured the same way. Naturally this diameter will depend on how long the thread is.