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The major minus pitch also works for inch-based threads, but you must first determine the pitch by looking at the number of treads per inch (TPI; for example, 1 ⁄ 20 = 0.050 and 1 ⁄ 13 ≈ 0.077), and your result will only land near a tap drill size (not directly on one).
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.
The Unified thread angle is 60° and has flattened crests (Whitworth crests are rounded). From 1 ⁄ 4 in up to 1 + 1 ⁄ 2 in, thread pitch is the same in both systems except that the thread pitch for the 1 ⁄ 2 in bolt is 12 threads per inch (tpi) in BSW versus 13 tpi in the UNC.
One aspect of this method of sizing is that the size increment between drill bits becomes larger as bit sizes get smaller: 100% for the step from 1/64 to 1/32, but a much smaller percentage between 1 47/64 and 1 3/4. Drill bit sizes are written as irreducible fractions. So, instead of 78/64 inch, or 1 14/64 inch, the size is noted as 1 7/32 inch.
The standard designation for a UTS thread is a number indicating the nominal (major) diameter of the thread, followed by the pitch measured in threads per inch.For diameters smaller than 1 / 4 inch, the diameter is indicated by an integer number defined in the standard; for all other diameters, the inch figure is given.
The metric screw tap and clearance sizes are largely missing, and those clearance hole sizes that are present are more often incorrect; e.g. M5 x 0.8 has 5.1 mm as close fit, but this would be an interference fit, all sources have 5.3 mm. (0.1 mm is below the sum of surface roughness and tolerance of the screw and clearance drill diameter ...