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Square head cap screws up to and including 3 ⁄ 4 inch (19.05 mm) have a head 1 ⁄ 8 inch (3.175 mm) larger than the shank; screws larger than 3 ⁄ 4 inch (19.05 mm) have a head 1 ⁄ 4 inch (6.35 mm) larger than the shank. [9] In 1919, Dyke defined them as screws that are threaded all the way to the head. [10] socket screw
A machine screw or bolt is usually a smaller fastener (less than 1 ⁄ 4 inch (6.35 mm) in diameter) threaded the entire length of its shank that usually has a recessed drive type (slotted, Phillips, etc.), usually intended to screw into a pre-formed thread, either a nut or a threaded (tapped) hole, unlike a wood or self-tapping screw. Machine ...
Self-tapping screws can be divided into two classes: [3] those that displace material (especially plastic and thin metal sheets) without removing it, known as "thread-forming" self-tapping screws, and self-tappers with sharp cutting surfaces that remove the material as they are inserted, termed "thread-cutting" self-tapping screws.
where is the tap drill size, is the major diameter of the tap (e.g., 3 ⁄ 8 in for a 3 ⁄ 8-16 tap), and / is the thread pitch (1 ⁄ 16 inch in the case of a 3 ⁄ 8-16 tap). For a 3 ⁄ 8-16 tap, the above formula would produce 5 ⁄ 16, which is the correct tap drill diameter. The above formula ultimately results in an approximate 75% thread.
The preload achieved by torquing a bolt is caused by the part of the torque that is effective. Friction in the threads and under the nut or bolt head uses up some fraction of the applied torque. Much of the torque applied is lost overcoming friction under the torqued bolt head or nut (50%) and in the threads (40%).
Example (inch, fine): For 7 ⁄ 16-20 (same diameter as the previous example, but this time with 20 threads per inch, which is considered fine), 0.437 in × 0.90 = 0.393 in (i.e., if the threads are to be fine, then a slightly larger diameter drill bit should be used before tapping the hole for the screw).
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