The screw processing technology is divided into two categories, one is pressure forming and the other is turning forming.
The former is generally suitable for high-strength screws, which is the processing technology used by large-scale enterprises. High-strength screws are often manufactured by cold heading. Some large-size high-strength bolts are even pressed by hot forging process, so that the fiber distribution of the metal is not damaged, the mechanical properties are good, and the threaded part is made of silk or silk. The process also does not destroy the continuity of the metal fiber, and the arc at the root of the screw can avoid the problem of stress concentration. Pressure forming screw manufacturing equipment is relatively expensive.
The turning forming process is relatively inexpensive and generally suitable for occasions where the strength requirements are not high. Many small and micro enterprises can't afford high-priced equipment, and often use a manual instrument lathe to turn the screws for a few hundred yuan. At this time, the fiber distribution of the metal is destroyed, the mechanical properties (strength) are poor, and the screws for turning are manufactured. The cutter has no rounded corners, and the root of the screw forms a sharp corner. The stress concentration phenomenon is serious, and it is easy to break when tightened; and the higher the strength of the material, the more likely the stress concentration is.
The strength grade of the screw is divided into more than 10 grades of 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9. If the engineer selects the product when the strength level is selected incorrectly, the possibility of screw failure will increase dramatically.