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Regarding the proportion of tap drill to thread major diameter, for standard V threads (ISO V thread and UTS V thread), there are several rules of thumb with strong predictive power: A good tap drill is 85% (± 2 pp ) of major diameter for coarse threads, and 90% (± 2 pp ) of major diameter for fine threads.
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.
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.
Machine screws with different thread diameters Hex key set with metric sizes from 2 mm to 10 mm according to sizes for use with ISO 262 screw sizes (except 3.5 mm for M6 internal set screw) For each size bolt or screw and type of head, there is a corresponding size driver prescribed by various ISO standards, including:
Pipe Thread Form ANSI B1.20.3-1976 (R2013) Dryseal Pipe Threads (Inch) Pipe Thread Form 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 ...
For a taper thread, it is the diameter at the "gauge length" (plus/minus one thread pitch) from the small end of the thread. The taper is 1:16, meaning that for each 16 units of measurement increase in the distance from the end, the diameter increases by 1 unit of measurement.
My own view is that DN is preferable because this refers to the pipe size, and any given pipe size can only be threaded to the same thread size, whereas mm is an exact mm dimension but one which has no direct relationship to the thread size, eg, 1/2" is 12.7mm not 15mm, and 3/4" is 19.1mm not 20mm, etc. —Preceding unsigned comment added by ...
[1] [25] 7.5 threads per inch were selected not just as a compromise between the 6, 7, and 8 threads per inch then in use, but because tools and methods had been developed to facilitate the conversion of existing 7 tpi and 8 tpi threads to 7.5 tpi threads, for couplings with an outside diameter ranging from 3 inches to 3 1/8 inches to the new ...