Metric or International Standard Thread
The metric thread was first adopted in central France from 1898 to 1908, and then widely promoted among countries. Metric thread code m is applicable to the connection of workpieces with a diameter of 0.25 mm to 300 mm. The thread angle is 60 °, the tooth top is flat, and easy to turn, and the tooth bottom is a circular arc, which can increase the thread strength. The metric thread has a coarse thread and fine thread.
British Standard Whitworth (BSW) Thread
Originated in Britain, the thread was invented by British citizen Joseph Whitworth in 1841, hence the name. The thread angle of this thread is 55 °. The crest and the bottom are arc-shaped, also known as BSW thread.
British Standard Fine (BSF) Thread
In shape, this thread is similar to the BSW thread, and the thread angle is also 55 °, but the number of threads per inch is more, that is, the thinner the thread thickness makes the grip stronger. It is used for parts that need high-strength roots or need to bear large vibrations.
United States Standard (USS) Thread
The top and bottom of American Standard thread are flat, with good strength and stress resistance. The thread angle is 60 °, and the specification is expressed in several teeth per inch, which is divided into three grades: coarse tooth (NC), fine tooth (NF), and super fine tooth (NEF).
Unified Thread Standard (UTS) Thread
It is jointly developed by the United States, Britain, and Canada and is the commonly used British thread. Compared with the American Standard thread, the thread angle is 60 °, and the specification is also expressed by the number of teeth per inch. It is also divided into three grades: coarse tooth (UNC), fine tooth (UNF), and ultra-fine tooth (UNEF).
DIN Standard Thread
It is the standard thread specified by din in Germany. The thread is round and suitable for the connection of light bulbs and rubber pipes.