Generally, the countersunk head is used where the surface after connection is required to be smooth and without protrusions, because the countersunk head can be screwed into the part. The round head can also be screwed into the part. The tightening force of the square head can be larger, but the size is large. In addition, in order to meet the needs of locking after installation, there are holes on the head and holes on the rod. These holes can prevent the bolt from loosening when it is vibrated. Some bolts are not threaded smooth rods to be thin, called thin waist bolts. This kind of bolt is conducive to the connection under variable force. There are special high-strength bolts on the steel structure, the head will be larger and the size will also change. In addition, there are special uses: T-slot bolts, the most used on machine tool fixtures, special shapes, and both sides of the head should be cut off. Anchor bolts are used to connect and fix the machine and the ground. There are many shapes. U-shaped bolts, as mentioned above. and many more. There are also special studs for welding. One end is threaded and the other is not. It can be welded to the part, and the nut is directly screwed on the other side.