Thread, is tooth type cross section through the axis of the cylindrical or conical, and along the surface of the spiral movement form continuous convex part. That is in a cylinder or cone matrix surface produced spiral line shape, with a particular section of continuous convex part.

In mechanical processing, thread is in a cylindrical shaft (or inner hole surface) with a cutting tool or grinding wheel, work piece turn a turn at this time, the cutting tool along the work piece axis movement fixed distance, cutting tool cut trail out of work piece is thread. Of course, in addition to threaded work piece, bolt and nut have thread, so that they are firmly connected.

Thread according to matrix shape can be divided into cylindrical thread and conical thread. According to its location in the matrix can be divided into external thread and internal thread, that is to say, excircle surface shapes thread called external thread; inner bore surface shapes thread called internal thread. According to its cross section shape can be divided into triangle thread, rectangle thread, trapezoid thread, sawtooth thread and other special shapes thread. In other words, what shape is the section of thread, is called what thread. Such as the cross section is circular arc is called circular arc thread.