Line is a figure composed of line types, and there are many kinds. The most intuitive is the spiral protruding part with a specific section made on the surface of the cylinder or cone matrix. Threads are divided into cylindrical threads and conical threads according to their mother shapes; According to its position in the matrix, it can be divided into external thread and internal thread, and according to its cross-sectional shape (tooth type), it can be divided into triangular thread, rectangular thread, trapezoidal thread, serrated thread and other threads with special shapes. Triangular threads are mainly used for connection, while rectangular, trapezoidal and serrated threads are mainly used for transmission. According to the spiral direction, it is divided into left-handed thread and right-handed thread, which is generally used; According to the number of threads, it can be divided into single thread, double thread and multi-thread thread; Most connections are single-wire, and transmission is double-wire or multi-wire; According to the size of the tooth, it can be divided into coarse thread and fine thread, and according to the different use occasions and functions, it can be divided into fastening thread, pipe thread, transmission thread and special thread.