A metric thread is measured in millimeters, and it has a cusp angle of 60 degrees. US and Imperial threads are in inches. The cusp angle of the American thread is also 60 degrees, while the imperial thread is 55 degrees.
Screws are a common fastener widely used in machinery, electrical appliances and buildings. The general material is metal or plastic, it is cylindrical, and the grooves engraved on the surface are called threads.
Due to the different units of measurement, description of various threads are also different. For example, M16-2X60 means metric thread. The nominal diameter of the screw is 16MM, the pitch is 2MM, and the length is 60MM. Take another example: 1/4—20X3/4 means inch thread, which means that the nominal diameter of the screw is 1/4 inch. There are 20 teeth on the inch and the length is 3/4 inch. In addition, American screws generally add UNC and UNF to the inch screws to distinguish between American coarse teeth or American fine teeth.
The main types of screws include ordinary screws, machine screws, self-tapping screws and expansion screws. According to different purposes, they are divided into many categories. For example, machine screws can be divided into longitudinal fastening screws and transverse expansion screws. According to the thread, it can be divided into triangular thread, triangular thread for pipe, trapezoidal thread, square thread, etc.