The main purpose is to make the industrial products into a fixed unit. In use, it often occurs that the teeth and the teeth cannot be tightly sealed, if the screws are too tightly locked, the screw heads are broken, or the dents are not tightly locked, etc., and the quality is Precision problem. Screws are "quantity products", not hand-made artworks. In mass production, they are supplied to consumers with high-precision and stable quality and popular prices. The accuracy of the screw is usually 6g (2, American standard "IFI" is 2A), and the coarse screw for construction is 8g (3, "IFI" is 1A). The value of the screw is very important. There are examples of the bankruptcy of the car factory due to poor screw quality in the world. There are also examples of aircraft crash and vehicle subversion caused by poor screw quality.