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For both of these rules of thumb (85%/90% and major minus pitch), the tap drill size yielded is not necessarily the only possible one, but it is a good one for general use. The 85% and 90% rules works best in the range of 1 ⁄ 4 –1 in (6.4–25.4 mm), the sizes most important on many shop floors. Some sizes outside that range have different ...
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.
where is the tap drill size, is the major diameter of the tap (e.g., 3 ⁄ 8 in for a 3 ⁄ 8-16 tap), and / is the thread pitch (1 ⁄ 16 inch in the case of a 3 ⁄ 8-16 tap). For a 3 ⁄ 8 -16 tap, the above formula would produce 5 ⁄ 16 , which is the correct tap drill diameter.
A minimum root-crest clearance of 0.01 in (0.25 mm) (diametral) between opposing threads with 10 tpi (threads-per-inch) or fewer, and 0.005 in (0.13 mm) for finer pitches. (This is also true for the minor diameters of the Centralizing Acme thread, though not its major diameters, where the allowance is made less than the PD allowance.)
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A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the maximum thread diameter) and the pitch P, both expressed in millimetres and separated by a dash or sometimes the multiplication sign, × (e.g. M8-1.25 or M8×1.25). If the pitch is the normally used "coarse" pitch listed in ISO 261 or ...