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Schematic of a machine tap. Below is a comprehensive drill and tap size chart for all drills and taps: Inch, imperial, and metric, up to 36.5 millimetres (1.44 in) in diameter. In manufactured parts, holes with female screw threads are often needed; they accept male screws to facilitate the building and fastening of a finished assembly.
The thread form is slightly modified to increase the minor diameter, and thus the strength of screws and taps. The major diameter still extends to within 1 / 8 H of the theoretical sharp V , but the total depth of the thread is reduced 4% from 5 / 8 H = 5 / 8 cos(30°) P ≈ 0.541 P to 0.52 P . [ 5 ]
US tap and drill bit size and reference chart [7] Machine screw size Number of threads per inch (TPI) Major diameter Minor diameter Tap drills Clearance drill 75% thread for aluminum, brass, & plastics 50% thread for steel, stainless, & iron Close fit Free fit Drill size Decimal equiv. Drill size Decimal equiv. Drill size Decimal equiv. Drill size
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 ...
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.
The minor diameter is the lower extreme diameter of the thread. Major diameter minus minor diameter, divided by two, equals the height of the thread. The minor diameter of a nut is its inside diameter. The minor diameter of a bolt can be measured with go/no-go gauges or, directly, with an optical comparator.
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The major diameter is given by 6p 1.2, [4]: 12 rounded to two significant figures in mm and the hex head size (across the flats) is 1.75 times the major diameter. BA sizes are specified by the following British Standards: BS 57:1951 — B.A. screws, bolts and nuts