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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.
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.
For example, for a 1/4-28 thread, both 5.5 mm and 5.6 mm drills make good tap drills for virtually any purposes, even though 7/32" is the principal tap drill. The idea is explained more at List of drill and tap sizes § Rules of thumb, and then worked examples are given at List of drill and tap sizes § Examples. The only thing that would stop ...
The diameter-to-length ratio of the drill bit is usually between 1:1 and 1:10. Much higher ratios are possible (e.g., "aircraft-length" twist bits, pressured-oil gun drill bits , etc.), but the higher the ratio, the greater the technical challenge of producing good work.
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.