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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.
where is the tap drill size, is the major diameter of the tap (e.g., 3 ⁄ 8 in for a 3 ⁄ 8-16 tap), and / is the thread pitch (1 ⁄ 16 inch in the case of a 3 ⁄ 8-16 tap). For a 3 ⁄ 8 -16 tap, the above formula would produce 5 ⁄ 16 , which is the correct tap drill diameter.
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 ...
Because only 5 ⁄ 8 of this height is cut, the difference between major and minor diameters is 5 ⁄ 4 × 0.8660 × P = 1.0825 × P, so the tap drill size can be approximated by subtracting the thread pitch from the major diameter.
pipe size Thread density Thread pitch P Hand-tight engagement [6] Effective thread [6] Overall length L 4 [6] Actual outside diameter D Tap drill; Length L 1 Turns Diameter E 1 Length L 2 Turns Diameter E 2; inch inch −1 inch mm inch inch inch inch inch inch mm inch mm 1 ⁄ 16: 27 0.03703704 0.9407 0.1600: 4.32: 0.28118 0.2611: 7.05: 0.2875 ...
Thus, if the thread pitch is p, the height of the fundamental triangle is H = p/(2tanΘ) = 0.96049106p. However, the top and bottom 1 ⁄ 6 of each of these triangles is cut off, so the actual depth of thread (the difference between major and minor diameters) is 2 ⁄ 3 of that value, or h = p/(3tanΘ) = 0.64032738p.
In the case of M6-1.0, the rule of thumb that you mentioned, major minus pitch, yields a 5mm tap drill at ~75% thread. However, the tap drill that the chart mentions, 5.2mm, will give a ~60% thread, which is also acceptable, provided that a 60% thread is what the user wants.
] BSF threads use the 55 degree Whitworth thread form. It was introduced by the British Engineering Standards Association in 1908. [2] The table provides BSF sizes, the threads per inch and spanner jaw sizes. The BSC column indicates where BSF and BSC threads match. The table shows suitable tapping drill sizes. Uncommon sizes are shown in italics.