This section is from the book "Applied Science For Metal Workers", by William H. Dooley. Also available from Amazon: Applied Science For Metal Workers.
A simple method of finding a tap drill for a V thread, or a United States standard thread tap, is provided by the following formulas (see
(a) Taper Tap
(b) Plug Tap
(c) Bottoming Tap
FIG. 119. - Taps.
Copyrighted by Miller Falls Co.
Fig. 120.
Fig. 120), where S is the desired size, T the diameter of the tap or screw, and N the number of threads per inch.
For V thread:
S = T - 1.733 / N
For U. S. standard thread:
S = T - 1.3 / N
Example. - What is the tap size drill for a 3/4 in. diameter 10 thread per inch V-thread tap?
S = T - 1.733 / N = 3 / 4 - 1.733 / 10 = .75 - .1733 = .5767 in.
 
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