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Number drill bit gauge sizes range from size 80 (the smallest) to size 1 (the largest) followed by letter gauge size A (the smallest) to size Z (the largest). Although the ASME B94.11M twist drill standard, for example, lists sizes as small as size 97, sizes smaller than 80 are rarely encountered in practice.
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%.
The twist drill bit is the type produced in largest quantity today. It comprises a cutting point at the tip of a cylindrical shaft with helical flutes; the flutes act as an Archimedean screw and lift swarf out of the hole. The modern-style twist drill bit was invented by Sir Joseph Whitworth in 1860.
One particular type of reduced-shank drill bits are Silver & Deming (S&D) bits, whose sets run from 9 ⁄ 16-inch (14 mm) to 1 + 1 ⁄ 2-inch (38 mm) drill body diameter with a standard 1 ⁄ 2-inch (13 mm) reduced shank for all. This allows drill presses with 1 ⁄ 2-inch (13 mm) chucks to run the larger drills. S&D bits are 6 inches (150 mm ...
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 pilot drill may be a standard twist drill, another type of drill bit appropriate for the material, or, when the primary purpose is precisely locating a hole, may be made with a short, stiff center drill. The pilot hole also reduces the power needed to turn a large drill bit, and reduces the large bit breakage risk.