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Thumb screws are one-piece screws that have a large flattened head, with or without a shoulder, and external machine screw threads. They are designed to be turned using finger pressure—no special driver or wrench is needed. Like wing nuts, use where frequent adjustment or disassembly is required and when the amount of torque produced by finger pressure is sufficient to tighten and loosen. There are two types: Type A, also called Type S (presumably for Shoulder), has a shoulder under its head the diameter of which is a little less than the head width. Type B does not have a shoulder and is also referred to as Type P (possibly for Plain). Thumb screws are also known as:
spade head screws.
Type A screws (with shoulder) are found in applications where the shoulder contacts a mating surface. Type B (without shoulder) is often used as a compression fastener where the tip contacts the mating surface, like a set screw. The point consists of a plain sheared end.
Threads, specified as the number of threads per inch, are standard right-hand and Unified inch coarse series (UNC, Unified National Coarse). One size, #10, may also be available in Unified inch fine (UNF, Unified National Fine). Before plating or coating thumb screws, threads are Class 2A. After plating or coating the threads can be Class 3A maximum and Class 2A minimum. They are fully threaded.
Typically, thumb screw sizes—listed as whole numbers and fractional inches—range from #6 to 3/8" in diameter; 1/2" may also be available. Lengths start at 1/4" and go up to about 6". Only larger sizes are available in long lengths. Measure Type A length from under the shoulder to the tip. Type B length is measured from the point where the head meets the shank to the tip.
Head dimensions are standard but the contour of the head can vary by manufacturer. Although regular and heavy patterns heads are both said to exist, regular pattern is the style normally available.
Steel and stainless steel are common materials. Steel is usually zinc plated.
Although thumb screws can be used with nuts, they normally mate with internally threaded (tapped) holes.
Refer to American Society of Mechanical Engineers Standard ASME B18.6.8 Thumb Screws and Wing Screws (Inch Series), for specifications relating to thumb screws.
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