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Such an appropriately sized drill is called a tap drill for that size of thread, because it is a correct drill to be followed by the tap. Many thread sizes have several possible tap drills, because they yield threads of varying thread depth between 50% and 100%. Usually thread depths of 60% to 75% are desired.
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.
Unified Coarse/Fine diameters and tap drill sizes (U.S. units) Unified Coarse/Fine tap drill sizes (U.S. units) Imperial Metric fastening size conversion charts; International Thread Standards; Conversion chart Whitworth/BSF/AF and metric Archived 2 May 2010 at the Wayback Machine; Spanner Jaw Sizes Additional information and spanner jaw size ...
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.
This would be a simple table where the topic of percentage of thread was not even broached and which simply told the target user subaudience (i.e., those doing hobbyist, DIY, or non-critical garden-variety maintenance work) that, "hey, if you want a 3/8 coarse thread, drill a 5/16 hole, end of story", or, "if you want a 1/2 coarse thread, drill ...
This additional truncation is achieved by using a slightly larger tap drill in the case of female threads, or by slightly reducing the diameter of the threaded area of workpiece in the case of male threads, the latter effectively reducing the thread's major diameter. In the case of female threads, tap drill charts typically specify sizes that ...