Thread is a shape with uniform helical protrusions on the cross-section of the outer or inner surface of a solid. According to its structural characteristics and uses, it can be divided into three categories: (1) Ordinary thread: the tooth shape is triangular, which is used to connect or fasten parts. Ordinary threads are divided into coarse and fine threads according to the pitch, and the connection strength of fine threads is higher. (2) Transmission thread: The tooth shape includes trapezoidal, rectangular, saw-shaped and triangular. (3) Sealing thread: used for sealing connection, mainly pipe thread, tapered thread and tapered pipe thread.
 
A helical, continuous convex portion with a specific cross-section made on the surface of a cylindrical or conical parent body. The thread is divided into cylindrical thread and conical thread according to the shape of its parent body; it is divided into external thread and internal thread according to its position in the parent body, and it is divided into triangular thread, rectangular thread, trapezoidal thread and zigzag thread according to its cross-sectional shape (tooth shape). And other special-shaped threads, triangular threads are mainly used for connection, rectangular, trapezoidal and zigzag threads are mainly used for transmission; according to the direction of the helix, it is divided into left-handed thread and right-handed thread, and right-handed thread is generally used; according to the number of helixes There are single-thread threads, double-thread threads and multi-thread threads; most of them are single-thread for connection, and double-thread or multi-thread for transmission; they are divided into coarse thread and fine thread according to the size of the teeth, and are different according to the application occasion and function. , can be divided into fastening thread, pipe thread, transmission thread, special thread, etc.