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The standard thread pitch is approximately 1 / 4 of the major diameter. The thread form is slightly modified to increase the minor diameter, and thus the strength of screws and taps. The major diameter still extends to within 1 / 8 H of the theoretical sharp V, but the total depth of the thread is reduced 4% from 5 / 8 H ...
The major minus pitch technique also works for inch-based threads, but you must first calculate the pitch by converting the fraction of threads-per-inch (TPI) into a decimal. For example, a screw with a pitch of 1/20 in (20 threads per inch) has a pitch of 0.050 in and a 1 ⁄ 13 in pitch (13 threads per inch) has a pitch of 0.077 in.
Unified Inch Screw Threads (UN and UNR Thread Form) V Thread Form: 1 ⁄ 4 ″-20 UNC #4-40 UNC ASME B1.5-1997 (R2014) Acme Screw Threads: Trapezoidal Thread Form: Tr 60×9 ANSI/ASME B1.9-1973 (R2017) Buttress Inch Screw Threads: Breech-Lock Thread Form: ASME B1.10M-2004 (R2014) Unified Miniature Screw Threads: V Thread Form: 0.60 UNM ANSI/ASME ...
where is the tap drill size, is the major diameter of the tap (e.g., 10 mm for a M10×1.5 tap), and pitch is the pitch of the thread (1.5 mm in the case of a standard M10 tap) and so the correct drill size is 8.5 mm. This works for both fine and coarse pitches, and also produces an approximate 75% thread.
Because only 5 ⁄ 8 of this height is cut, the difference between major and minor diameters is 5 ⁄ 4 × 0.8660 × P = 1.0825 × P, so the tap drill size can be approximated by subtracting the thread pitch from the major diameter. In an external (male) thread (e.g. on a bolt), the major diameter D maj and the minor diameter D min define ...
The nominal diameter of Metric (e.g. M8) and Unified (e.g. 5 ⁄ 16 in) threads is the theoretical major diameter of the male thread, which is truncated (diametrically) by 0.866 ⁄ 4 of the pitch from the dimension over the tips of the "fundamental" (sharp