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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). If the pitch is the normally used "coarse" pitch listed in ISO 261 or ...
They then rounded to 2 significant figures in metric and then converting to inches and rounding to the thousandth of an inch. This anticipated worldwide metrication by about a century. The design was first proposed by the British Association in 1884 [ 1 ] [ 2 ] with a thread angle and depth based on the Swiss Thury thread , [ 3 ] it was adopted ...
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.
The size is imprinted on the spanners in millimeter values or inch sizes with intermediate sizes in fractions (older British and current US spanners). The two systems are in general not compatible, which can result in rounding of nuts and bolts (i.e. using a 13 mm (0.51 in) spanner in place of a 1 ⁄ 2 inch (12.70 mm)).
Metric drill set, 1.0–6.0 mm by 0.1 mm, jobber length. The case that holds them in an indexed order (by size), via a graduated series of holes, is called a drill index. Metric drill bit sizes define the diameter of the bit in terms of standard metric lengths. Standards organizations define sets of sizes that are conventionally manufactured ...
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.
In the modern standard metric version, it is simply a size number, where listed diameter size is the major outer diameter of the external thread. For a taper thread, it is the diameter at the "gauge length" (plus/minus one thread pitch) from the small end of the thread.
To calculate the major diameter of "aught" size screws count the number of extra zeroes and multiply this number by 0.013 in and subtract from 0.060 in. For example, the major diameter of a #0000 screw is 0.060 in − (3 × 0.013 in) = 0.060 in − 0.039 in = 0.021 in.