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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. People frequently use a chart such ...
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.
A few sizes are close enough to interchange for most purposes, such as 19 mm (close to 3 ⁄ 4 inch (19.05 mm)), 8 mm (close to 5 ⁄ 16 inch (7.94 mm)) and 4 mm (close to 5 ⁄ 32 inch (3.97 mm)). In reality, a wrench with a width across the flats of exactly 15 mm would fit too tightly to use on a bolt with a width across the flats of 15 mm.
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 ...
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.
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Standard Name Type Example ASME B1.1-2003 (R2018) Unified Inch Screw Threads (UN and UNR Thread Form) V Thread Form: 1 ⁄ 4 ″-20 UNC #4-40 UNC ASME B1.5-1997 (R2014) Acme Screw Threads: Trapezoidal Thread Form: Tr 60×9 ANSI/ASME B1.9-1973 (R2017) Buttress Inch Screw Threads: Breech-Lock Thread Form: ASME B1.10M-2004 (R2014) Unified ...
Blue lines are standard metric drill sizes used for metric taps of the size indicated in the Tap Size column. The clearance sizes need to be left as drill nomenclature (3/64, 6.5 mm, #37) rather than converting to decimal dimensions since having that dimension you have to chase down the table to find the actual drill that it represents.