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Many thread sizes have several possible tap drills, because they yield threads of varying thread depth between 50% and 100%. Usually thread depths of 60% to 75% are desired. People frequently use a chart such as this to determine the proper tap drill for a certain thread size or the proper tap for an existing hole.
Acme Screw Threads: Trapezoidal Thread Form: Tr 60×9 ANSI/ASME B1.9-1973 (R2017) Buttress Inch Screw Threads: Breech-Lock Thread Form: ASME B1.10M-2004 (R2014) Unified Miniature Screw Threads: V Thread Form: 0.60 UNM ANSI/ASME B1.11-1958 (R2016) Microscope Objective Thread: V Thread Form ASME B1.13M-2005 (R2015) Metric Screw Threads: M Profile ...
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.
The metric standard used the same thread geometry as the USS standard but differed in that the dimensions and pitch were based on metric units. In 1906, the A.L.A.M developed what would be the SAE thread standard for threaded fasteners based on the USS standard but with a finer thread pitch. [6] [7]
The thread form follows the British Standard Whitworth standard: Symmetrical V-thread in which the angle between the flanks is 55° (measured in an axial plane) One-sixth of this sharp V is truncated at the top and the bottom; The threads are rounded equally at crests and roots by circular arcs ending tangentially with the flanks where r ≈ 0 ...
The metric screw tap and clearance sizes are largely missing, and those clearance hole sizes that are present are more often incorrect; e.g. M5 x 0.8 has 5.1 mm as close fit, but this would be an interference fit, all sources have 5.3 mm. (0.1 mm is below the sum of surface roughness and tolerance of the screw and clearance drill diameter ...
ISO metric threads consist of a symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other. The thread depth is 0.54125 × pitch. The outermost 1 ⁄ 8 and the innermost 1 ⁄ 4 of the height H of the V-shape are cut off from the profile.
So a 1 / 2 in (12.7 mm) diameter drill will be able to drill a hole 4 + 1 / 2 in (114.3 mm) deep, since it is 9 times the diameter in length. A 1 / 8 in (3.175 mm) diameter drill can drill a hole 1 + 5 / 8 in (41.275 mm) deep, since it is 13 times the diameter in flute length. [3]