Classification of common nut cold heading processes
Hexagonal nuts are also a widely used fastener. There are many production methods for them. Nuts with specifications below M24 are generally produced by cold heading (pressing). Commonly used nut cold heading processes are as follows:
1. Cold heading nuts with smaller diameter wire
This is the most commonly used production method in cold heading nuts. The wire diameter do=0.60s~0.70s, s is the opposite side size of the nut. The workstations (processes) of cutting, shaping, ball upsetting, hexagonal pressing, and punching are used. They are produced on three-station and four-station automatic cold heading machines, and can also be produced in sequence on a press. Shaping can be omitted in production on a three-station cold heading machine, but for nuts with specifications larger than M12, the end face quality and uniformity of the bald corners are difficult to control without shaping.

2. Cold heading of larger diameter wire to produce nuts
This process uses wire diameter do≈0.9s, and is made by cutting, shaping, initial heading, preforming, fine forming, and punching. It is generally produced on a five-station automatic cold heading machine with a clamp with a flip mechanism.
3. Hexagonal steel forming process
This process method is rarely used and is generally used for the production of large-size nuts above M20. It is completed by sequential cold pressing on a press. The process flow is produced according to cutting, initial pressing, fine pressing, and punching.

Process analysis and process parameters of cold heading (pressing) of nuts
1. Cutting
In the multi-station production of automatic cold heading machines or the sequential production on presses, cutting is the first process and the most critical process. Because the flatness of the cut surface and the size of the horseshoe print formed by the cutting plate have a direct impact on the shaping and ball upsetting in the next order.
2. Shaping
Shaping is to flatten the end face of the material column and upset (press) a 1~2×45° chamfer at the lower end. The purpose is to repair the defects of the cut material and ensure the quality of the next ball pressing process.
3. Ball upsetting
Ball upsetting is to upset (press) the shaped material column into a drum-shaped ball, as shown in Figure 4. Its quality affects the clarity and quality of the end face, bald corners, and edges of the nut. When determining the geometric dimensions of the drum ball, according to experience, when the chamfer is determined at 40°, the dimensions dM and h should be as small as possible. In this way, when pressing the hexagon, the friction force of the corresponding part should be small, and the metal has good fluidity under the action of the pressing force, and it is easy to fill the hexagon. If dM and h are too large, it is not easy to fill the hexagon when pressing the hexagon. If the pressing force is increased in order to fill the hexagon, flash will be generated on the end face of the nut.
4. Pressing
Pressing, that is, upsetting the hexagon of the nut to meet the requirements of the hexagonal nut shape size. Whether the deformation size is reasonable directly affects the quality of the product and the life of the mold.
5. Punching
The size and quality of the punching are to meet the requirements of the next tapping. The inner diameter of the nut is generally determined by the maximum size of the minor diameter. Considering that the hardness of the steel affects the quality of the punching, the hole diameter can be set between the minimum and maximum sizes of the nut minor diameter, and the operator can flexibly control it within its tolerance range. In fact, considering the factors of tapping, the tolerance of the punching size should be smaller than the tolerance of the minor diameter.