It is mainly used for connecting small parts. It has pan head screw, cylindrical head screw, half countersunk head screw and countersunk head screw. The head strength of pan head screw and cylindrical head screw is higher, and the shell is connected with common parts; the head of half countersunk head screw is arc-shaped, and its top is slightly exposed after installation, and it is beautiful and smooth, which is generally used in instruments or precision machinery; countersunk head screw is used for places where the nail head is not allowed to be exposed.