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A good tap drill is 85% (± 2 pp) of major diameter for coarse threads, and 90% (± 2 pp) of major diameter for fine threads. [ 1 ] For metric V threads , the concept of major minus pitch (i.e., the major or widest diameter of the intended screw in millimeters minus the pitch of the threads of that screw in millimeters per thread) yields a good ...
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., 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 major diameter may be slightly different from the shank diameter, which is the diameter of the unthreaded part of the screw. The diameters are sometimes given approximately in fractions of an inch (e.g. the major diameter of a #6 screw is 0.1380 in, approximately 9 ⁄ 64 in = 0.140625 in.
1 ⁄ 4 ″-20 BSW ISO 68-1:1998: ISO general purpose screw threads—Basic profile—Part 1: Metric screw threads: V Thread Form ISO 68-2:1998: ISO general-purpose screw threads—Basic profile—Part 2: Inch screw threads: V Thread Form ISO 261:1998: ISO general purpose metric screw threads—General plan: V Thread Form: M6x1 ISO 262:1998
The maximum diameter (within tolerance) at the crest of the external threads (called the max. major diameter of external thread) is that of the basic thread form and equals the "nominal diameter", D, stated in the screw's designation.