1.There are two types of threads: thread and external thread;

2. According to the tooth shape, it can be divided into:
1) Triangular thread
2) Trapezoidal thread
3) Rectangular thread
4) Serrated thread;

3. According to the number of threads, there are single thread and multi thread;

4. According to the screw-in direction, there are two kinds of left-handed threads and right-handed threads. The right-handed thread is not marked, and the left-handed one is added with LH, such as M24×1.5LH;

5. According to different uses, there are: metric common thread, pipe thread sealed with thread, non-threaded pipe thread, 60° tapered pipe thread, metric tapered thread, etc.




Metric usual thread
1. Metric threads are usually indicated by a capital M, and the tooth angle is 2α=60° (α indicates the half angle of the tooth);

2. According to the pitch, the metric normal thread is divided into two types: coarse thread normal thread and fine thread normal thread;
2.1. Generally, the thread symbol for coarse thread usually does not indicate the pitch. For example, M20 indicates the coarse thread; the symbol for the fine thread must indicate the pitch. For example, M30×1.5 indicates the fine thread, and the pitch between them is 1.5.
2.2. Threads are usually used for connection and fastening between mechanical parts. Generally, coarse threads are used for thread connection. The strength of fine threads is slightly higher than that of coarse threads of the same nominal diameter, and the self-locking function is better.

3. Symbols of metric common thread: M20-6H, M20×1.5LH-6g-40, among which M indicates metric common thread, 20 indicates the nominal diameter of the thread is 20mm, 1.5 indicates the pitch, LH indicates left-handed, 6H, 6g indicate the thread Accuracy class, the uppercase accuracy class code indicates the internal thread, the lowercase accuracy class code indicates the outer thread, and 40 indicates the screwing length.