1.1 Basic concepts of cold heading (extrusion) process
1.1.1 Cold heading and cold pressing
At room temperature, the billet is placed in the mold of an automatic cold heading machine or press, and pressure is applied to the mold. The relative motion of the upper and lower molds is used to deform the billet in the mold cavity, reduce its height, and increase its cross-section. This pressure processing method is called cold heading for automatic cold heading machines and cold pressing for press machines.
In actual production, the cold forming process of fasteners often involves extrusion during the cold heading process. Therefore, the cold heading process for fastener products alone is actually a composite processing method that includes both cold heading and extrusion.
1.1.2 Cold heading (extrusion) deformation method
a. Punching separates a part of the blank from the main body. Such as cutting wires, punching nuts, and trimming the heads of hexagonal bolts.
b. The processing method of upsetting to shorten the height and increase the cross-section of the blank, such as upsetting the nut ball, pre upsetting the bolt head, precision upsetting, etc.
c. During cold heading, when the billet is deformed in the lower mold, the direction of metal flow is consistent with the direction of motion of the upper mold. The reduction of the diameter of the thick rod in cold heading bolts and cylindrical head hexagon screws is a form of positive extrusion.
d. During the deformation of reverse extrusion blanks, the direction of metal flow is opposite to the direction of motion of the upper mold. The forming of the head of a cylindrical hexagonal screw belongs to reverse extrusion.
e. Composite extrusion
The flow direction of the metal during deformation of the blank is partially the same as the movement direction of the upper mold, and partially opposite. There are both positive and negative compressions in deformation. In the deformation process of a cylindrical hexagonal screw at the same workstation, there are both rod shrinkage (positive extrusion) and head forming (negative extrusion).
2.1.3 Cold heading (extrusion) deformation degree
a. Degree of deformation
It refers to the ratio of the compression amount at the end of the forging of the billet to the original height, or the ratio of the increase in the cross-sectional area of the billet at the end of the forging to the original cross-section.
b. Method for representing the degree of deformation
The first method uses the forging ratio (S), as shown in Figure 36-7.
Namely: (Formula 36-5)
In the formula: h0- the original height of the forged part
D0- Original diameter of the forged part
The forging ratio can determine the difficulty or ease of forging
The smaller the size, the smaller the deformation, and the easier it is to deform. Upset forging
The larger the forging ratio, the harder the deformation, and the metal fibers flow
Irregular, some fibers are bent to form a longitudinal pattern
Bending phenomenon. As shown in Figure 36-8.
Figure 36-7 Deformation degree during upsetting of bar material
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