Several common mistakes in stamping production

Various problems will occur more or less in the process of stamping, and a large part of these problems are caused by very low-level mistakes. It reflects that our basic knowledge in stamping is not enough. The following summarizes some common basic mistakes in the stamping process.

1. When stamping, the depth of the upper die entering the lower die is too large.
When stamping, the depth of the upper die entering the lower die should not be too large, and it is generally advisable to just punch the sheet metal, and this depth is 0.5-1 mm. If the depth of the upper die entering the lower die is too large, it will aggravate the wear of the upper die and the lower die. Under the condition of poor precision of die guidance and punch movement, the cutting edge of the die will be gnawed, especially when punching thick materials, small holes and high-speed stamping, the depth of the upper die entering the lower die should not be too deep. In order to prevent the upper die from entering the lower die too deeply, a limit column can be installed on both sides of the die to limit the depth of the upper die entering the lower die. When grinding the upper die, the limiting sleeve is also ground to the same grinding amount.

2. There is eccentricity between the punching pressure center and the punching press pressure center.
The point of action of the resultant force of punching pressure is called the punching pressure center. Taking a small punch as an example, if the punching pressure center and the punch pressure center (generally located on the axis of the die handle hole) are not on the same axis, the punch slider will bear eccentric load, which will lead to abnormal wear of the slider guide rail and the die guide part, damage the movement accuracy of the punch, reduce the die life and even damage the die. Therefore, determining the stamping pressure center is an important work in die design. For the workpiece with simple and symmetrical shape, the action point of punching resultant force is at its geometric center, and the pressure center need not be calculated. For workpieces with complex shapes and multi-process continuous stamping dies, the method of finding the resultant force point of parallel force system should be adopted to determine the stamping pressure center.

3. The punching force exceeds the nominal pressure of the punching machine.
The selection of punching machine is mainly based on punching force. The principle is that the punching force cannot exceed the nominal pressure of the punch. The main factors affecting punching force are material thickness and mechanical properties, peripheral length of stamping parts, die gap and sharpness of cutting edge. When stamping high-strength materials or workpieces with large thickness and long stamping contour (such as thick plate stamping), the required punching force often approaches or exceeds the nominal pressure of the punching machine, and the available punching machines in the factory are limited, so we should consider trying to reduce the punching force from the die structure. The main methods to reduce the punching force are: inclined edge stamping, stepped upper die stamping, parts stamping step by step, heating stamping and so on. Oblique-edge stamping method is to make the cutting edge of the upper die (when punching) or the lower die (when blanking) into an angle with its axis, which is less than 150 degrees, generally 80~100 degrees, which is similar to oblique-edge shearing. The whole cutting edge does not contact at the same time, but gradually punches the material, so the punching force is significantly reduced, and the vibration and writing noise during stamping can be alleviated. Hot stamping is the stamping (or red stamping) of materials under heating. Because the shear strength of metal materials generally decreases obviously under heating, the stamping force can be effectively reduced. However, the disadvantage of this method is that the oxide scale is produced after the material is heated, which affects the surface quality of the parts, so it is generally used for stamping thick plates or parts with low requirements on size and surface quality. In addition, the blunt, chipped or not sharp cutting edge of the die will obviously increase the punching force. Therefore, keeping the sharp cutting edge is one of the conditions for the normal work of the stamping die. In order to keep the cutting edge of the die sharp, the cutting edge should be polished after the die is stamped for a period of time.

4. When fixing the upper die, there is a big hole in the bottom of the die seat of the fine die.
The structure types of fine dies can be divided into fixed upper die and movable upper die. Different die structures require that the structure of the punch table should be matched accordingly. For the movable upper-mode fine stamping die, it is required that the workbench of the punch press is fixed as the center with a floating hydraulic workbench composed of an annular oil cylinder and a plunger around it. For the fixed upper die precision stamping die, it is required that the middle of the workbench of the punching machine has a plunger cylinder. The die structure is characterized in that the upper and lower dies are fixed on the lower die holder, and the blank holder keeps relative movement with the upper and lower dies through the dowel bar and the die holder. It is not suitable for a fine die with a fixed upper die to have a large cavity under the die base, because when the upper die is pressed down, the hydraulic cylinder moves down under the action of the dowel bar, so a large cavity appears under the die base, and all the punching force acts on the top of the cavity, which makes the upper and lower dies bend, which is very unfavorable. However, under the action of increasing punching force, the lower parts of the upper and lower dies will be in danger of cracking due to bending. In order to avoid this situation, when the punching force is large, it is necessary to use a special joint ring to improve the supporting conditions of the lower die holder and avoid bending the upper and lower dies due to a big loss. Due to the development of fine blanking technology to large-scale and composite technology, it is necessary to punch the porous or large internal contour, and the punching pressure is very large, which requires large blank holder force and counter pressure. Therefore, the middle of the workbench of the punch is required.

5. The movable upper die is used to punch porous parts or parts with large internal contour.
The upper and lower dies of the movable upper die fine stamping die are directly fixed in the center of the workbench, and the supporting conditions are good. This die structure is characterized in that the upper and lower dies are movable relative to the die holder, and the upper and lower dies are guided by the inner holes of the die holder and the blank holder. The lower die and the blank holder are fixed on the upper and lower die bases respectively, and the upper and lower dies are kept in relative positions through the blank holder, so the gap between the upper and lower dies is required to be smaller, and the alignment can be guaranteed only if the upper and lower dies are guided for a long time and positioned correctly. Therefore, the movable upper die can’t punch the parts with holes or large internal contour, because it is difficult to center the die assembly and ensure the gap, so it is mainly suitable for fine blanking of medium and small parts.

6. The heat treatment hardness of the upper die and the lower die of the stamping die is lower than 55HRC.
The upper die and the lower die of the stamping die are in contact with the stamping material, which is stressed and worn quickly. Therefore, the upper die and the lower die of the stamping die must be heat-treated, and the hardness should not be lower than 55HRC, because the higher the hardness, the higher the strength of the die and the more wear-resistant. Different die steel materials have different heat treatment processes and hardness. Cold-working die steel Cr12MoV and high-speed steel W18Cr4V2 have high heat treatment hardness, good hardenability, small quenching deformation and no cracking, which are suitable for stamping parts with complex shapes, while T8A hardening capacity is good, but its hardenability is poor, and it is easy to crack due to large quenching deformation, so it is often used to punch parts with simple and soft shapes. Because the processing of the lower die is more difficult than that of the upper die, the hardness of the lower die is higher than that of the upper die, generally 2-3 Rockwell hardness, that is, the heat treatment hardness of the upper die is generally 58-60~62HRC, and the heat treatment hardness of the lower die is 60-62 HRC.

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