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Fastener terminology of hex nuts: d: Nominal thread diameter k: Wrench size (width across flats) h: Thickness e: Width across corners = Width across flats is the distance between two parallel surfaces on the head of a screw, bolt or nut. The width across flats will define the size of the spanner or wrench needed.
A hex key (also, hex wrench, Allen key and Allen wrench, Unbrako or Inbus) is a simple driver for bolts or screws that have heads with internal hexagonal recesses ().. Hex keys are formed from a single piece of hard hexagonal steel rod, having blunt ends that fit snugly into similarly shaped screw sockets.
The hex socket screw drive has a hexagonal recess and may be driven by a hex wrench, also known as an Allen wrench, Allen key, hex key, or inbus as well as by a hex screwdriver (also known as a hex driver) or bit. Tamper-resistant versions with a pin in the recess are available.
Confusion can arise because each Whitworth hexagon was originally one size larger than that of the corresponding BSF fastener. This leads to instances where for example, a spanner marked 7 ⁄ 16 BSF is the same size as one marked 3 ⁄ 8 W. In both cases the spanner jaw width of 0.710 in, the width across the hexagon flat, is the same.
Originally named Allen Manufacturing Company, the business produced hexagonal set screws and wrenches to fasten them. The terms "Allen wrench" (American English, though "Allen key" is also common in the US) and "Allen key" (British English) [3] are derived from the Allen brand name and refer to the generic product category "hex keys".
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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
Example (inch, coarse): For size 7 ⁄ 16 (this is the diameter of the intended screw in fraction form)-14 (this is the number of threads per inch; 14 is considered coarse), 0.437 in × 0.85 = 0.371 in. Therefore, a size 7 ⁄ 16 screw (7 ⁄ 16 ≈ 0.437) with 14 threads per inch (coarse) needs a tap drill with a diameter of about 0.371 inches.