There are four common process structures of fastener threads, namely: chamfer, screw tail, undercut and drill angle.

1. Chamfer

In order to facilitate the assembly of internal and external threads and prevent end bumps, the thread ends are generally chamfered. The chamfer is generally 45°, and its size is specified by the standard.

2. Snail tail

When cutting threads, due to the impact of tool withdrawal or the structure of the thread tool, the last few teeth are incomplete and cannot be used. This incomplete thread is called the screw tail (l).

3. Undercut

Sometimes, in order to avoid the thread tail and facilitate assembly and processing, an undercut groove is pre-cut at the end of the thread.

4. Drill bit angle

For small diameter threaded holes that are not drilled (also called blind threaded holes), the general manufacturing process uses a drill to drill holes, and then uses a tap to tap the inner wall of the hole. Since the bit angle of the drill is 118°, it is simplified to 120°

when drawing the picture. When tapping the thread, since the bottom of the hole cannot be tapped with a tap, it is impossible to make the depth of the hole full thread.