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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 ≈ 0.541P to 0.52P.
Below is a comprehensive drill and tap size chart for all drills and taps, 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 half-angle (between the taper surface and the axis of the pipe) is arctan(1 ⁄ 32) ≈ 1.7899° ≈ 1° 47′. The standard for Nominal Pipe Size (often abbreviated NPS, which should not be confused with the abbreviation NPS for the straight thread form standard) is loosely related to the inside diameter of Schedule 40 series of sizes.
A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the maximum thread diameter) and the pitch P, both expressed in millimetres and separated by a dash or sometimes the multiplication sign, × (e.g. M8-1.25 or M8×1.25).
For example, compare the 6–32, 8–32, 10–24, and 10–32 options in this table with the UTS versions of those sizes, which are not identical but are so close that interchange would work. Survey results on the use of SAE standards (including screw size standards), reported in the journal Horseless Age, 1916
Sizes that may interchange, depending on the precision needed, include 2 mm (close to 5 ⁄ 64 inch (1.98 mm)), 4 mm (close to 5 ⁄ 32 inch (3.97 mm)) and 8 mm (close to 5 ⁄ 16 inch (7.94 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.
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To simplify matters, the term hexagon is used in this section to denote either bolt head or nut. Two spanners, both nominal size 5 ⁄ 8 in, with a diagram superimposed to show the logic that allows them both to be nominal size 5 ⁄ 8 in when their actual sizes are clearly different (across-flats distance vs screw diameter). The across-flats ...