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For both of these rules of thumb (85%/90% and major minus pitch), the tap drill size yielded is not necessarily the only possible one, but it is a good one for general use. The 85% and 90% rules works best in the range of 1 ⁄ 4 –1 in (6.4–25.4 mm), the sizes most important on many shop floors. Some sizes outside that range have different ...
The standard designation for a UTS thread is a number indicating the nominal (major) diameter of the thread, followed by the pitch measured in threads per inch.For diameters smaller than 1 / 4 inch, the diameter is indicated by an integer number defined in the standard; for all other diameters, the inch figure is given.
ASME B1.20.7-1991 (R2013) Hose Coupling Screw Threads - Inch: Pipe Thread Form ASME B1.21M-1997 (R2013) Metric Screw Threads - MJ Profile: V Thread Form BS 84: Tables of BS Whitworth, BS Fine and BS Pipe Threads: V Thread Form (55°) 1 ⁄ 4 ″-20 BSW ISO 68-1:1998: ISO general purpose screw threads—Basic profile—Part 1: Metric screw ...
Pitch and TPI describe the same underlying physical property—merely in different terms. When the inch is used as the unit of measurement for pitch, TPI is the reciprocal of pitch and vice versa. For example, a 1 ⁄ 4-20 thread has 20 TPI, which means that its pitch is 1 ⁄ 20 inch (0.050 in or 1.27 mm).
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.
From 1 ⁄ 4 in up to 1 + 1 ⁄ 2 in, thread pitch is the same in both systems except that the thread pitch for the 1 ⁄ 2 in bolt is 12 threads per inch (tpi) in BSW versus 13 tpi in the UNC. Thread form
One aspect of this method of sizing is that the size increment between drill bits becomes larger as bit sizes get smaller: 100% for the step from 1/64 to 1/32, but a much smaller percentage between 1 47/64 and 1 3/4. Drill bit sizes are written as irreducible fractions. So, instead of 78/64 inch, or 1 14/64 inch, the size is noted as 1 7/32 inch.
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.