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

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(a) Taper Tap

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(b) Plug Tap

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(c) Bottoming Tap

FIG. 119. - Taps.

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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.