Here is Hex Cap Screws Manufacturer talking about Introduction To Screw Classification.
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Screw standard specification:
Standards are norms, and each country and department has its own standards. At present, the most commonly used standards in our usual business are the following:
GB-China National Standard (GB) ANSI-US National Standard (American Standard)
DIN-German National Standard (German Standard) ASME-American Society of Mechanical Engineers Standard
JIS-Japan National Standard (Japanese Standard) BSW-UK National Standard
The GB-national standard is one of many standards in China, in addition to industry standards, professional standards, and departmental standards. National standards are divided into GB (mandatory standards) and GB/T (recommended standards) and GBn (national internal standards). We usually see GB30, GB5783 and so on are mandatory standards.
In addition to some basic dimensions such as head-to-edge, head thickness, etc., the most important ones are the difference in threaded parts. Threads of GB, DIN, JIS, etc. are all in MM (mm), collectively referred to as metric threads. Another thread like ANSI, ASME, etc. is called the American standard thread in inches. In addition to metric and American threads, there is also a BSW-inch standard, the thread is also in inches, commonly known as Wyeth thread.
Metric threads are in MM (mm) and have a cusp angle of 60 degrees. Both US and inch threads are in inches. The cusp angle of the US thread is also 60 degrees, while the cusp angle of the inch thread is 55 degrees. Due to the different units of measurement, the representation of various threads is not the same. For example, the M16-2X60 represents a metric thread. His specific meaning is that the nominal diameter of the screw is 16MM, the tooth pitch is 2MM, the length is 60MM, and another example is: 1/4-20X3/4 is the inch thread, his specific meaning is the nominal diameter of the screw. It is 1/4 inch (one inch = 25.4 mm) with 20 teeth on one inch and 3/4 inches in length. In addition, if you want to indicate the US-made screws, you will usually add UNC and UNF to the back of the inch screws to distinguish between American-made coarse teeth and American-made fine teeth.