The cold heading (pressing) of fastener products is accomplished by presses and automatic cold heading machines. Among the various types of cold heading, such as sequential cold pressing, single-station, and multi-station cold heading, the shape of the semi-finished product from the previous process or station directly affects the forming of the next process or station. Therefore, determining the correct deformation shape based on a reasonable distribution of deformation ratio has a direct impact on subsequent deformations and product quality.
1. Cold heading (pressing) process for rod-shaped fasteners
For the cold heading (pressing) process of rod-shaped fasteners, relevant parameters of each process (station) should be considered. The main parameters include the heading ratio, where Lo and do represent the original length and original diameter of the heading part of the blank, respectively; D and H indicate the diameter and height of the workpiece after heading, respectively.
Lo/do is a measure of the longitudinal stability of the blank upsetting deformation, which refers to the ability of the upsetting part of the blank to resist longitudinal bending during upsetting. The smaller the value of Lo/do, the more favorable it is for the head to undergo upsetting and forging; when the value of Lo/do is too large, longitudinal bending occurs in the upsetting and forging part of the blank. In addition to the value of Lo/do, there are other factors that affect the longitudinal stability of the blank upsetting deformation. Whether it is an automatic cold upsetting machine or a cutting machine, whether it is a blade cutter or a sleeve cutter, the cut surface of the blank cannot be perpendicular to its axis, and should have an inclination of 1° to 5°. This way, during cold upsetting (pressing), the initial impact point of the force on the blank is not at the center, but rather eccentric, causing uneven stress on the blank, resulting in uneven deformation and leading to folding due to longitudinal bending during head forming. For blanks with a small inclination angle of the cut surface, the longitudinal bending caused by deformation is not obvious and does not reach a level that affects the quality of the head. In the cold upsetting (pressing) process, arranging a blank shaping step after cutting is mainly for this purpose.
In addition, the bottom end of the initial punching cavity is the transmission surface for applying the upsetting force to the blank. If there is a center offset, the center of the resultant force will inevitably be offset, which is also a factor affecting the occurrence of longitudinal bending. Adopting a spring-loaded ejector pin in the initial punching can mitigate this effect. Other factors such as the operational accuracy of the machine tool and the operator's level of adjustment for tooling installation also have an impact on the initial punching forming.
In order to enhance the stability of the blank during the initial stamping deformation, especially for steel grades with poor severability such as low carbon steel, a cylindrical cavity with a height of 1.5 to 2mm should be added to the small end working cavity of the initial stamping, in addition to the conical shape.
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