Common basic errors in the stamping process

Common basic errors in the stamping process
During the stamping process, various problems will occur more or less, and a large part of these problems are caused by very low-level errors. The following summarizes some common basic errors in the stamping process:
1. The depth of the upper die entering the lower die during stamping is too large
During stamping, the depth of the upper die entering the lower die should not be too large. Generally, it is appropriate to just punch the sheet metal. This depth can be taken as 0.5-1mm. If the upper die enters the lower die too deeply, it will increase the wear of the upper die and the lower die. In the case of poor die guide and punching machine movement accuracy, it will also gnaw the die edge. Especially when punching thick materials, punching small holes and high-speed stamping, the depth of the upper die entering the lower die should not be too large. In order to prevent the upper die from entering the lower die too deeply, a limit sleeve can be installed on both sides of the upper die to limit the depth of the upper die entering the lower die. When grinding the upper die, the limit sleeve is also ground off by the same grinding amount.
2. The stamping pressure center and the punching machine pressure center are eccentric
The point of action of the combined force of the punching pressure is called the stamping pressure center. If the stamping pressure center and the punching machine pressure center (generally located on the axis of the die handle hole) are not on the same axis, the punching machine slide will bear eccentric loads, which will cause abnormal wear of the slide rail and the die guide part, damage the motion accuracy of the punching machine, reduce the life of the die, and even damage the die. Therefore, determining the stamping pressure center is an important task in die design. For workpieces with simple and symmetrical shapes, the point of action of the stamping force is at its geometric center, and the pressure center does not need to be calculated. For workpieces with complex shapes and continuous stamping dies with multiple processes, the method of finding the point of action of the parallel force system should be used to determine the stamping pressure center.
3. The punching pressure exceeds the nominal pressure of the punching machine
The selection of stamping presses is mainly based on the punching pressure. The principle is that the punching pressure cannot exceed the nominal pressure of the punching machine. The main factors affecting the punching pressure are material thickness and mechanical properties, the peripheral length of the stamping part, the size of the die gap, and the sharpness of the cutting edge. When stamping high-strength materials or workpieces with large thickness and large circumference of the stamping contour (such as thick plate stamping), the required stamping force is often close to or exceeds the nominal pressure of the punch press. When the factory has limited punch presses to choose from, it is necessary to consider reducing the stamping force from the mold structure. The main methods to reduce the stamping force are: oblique blade stamping method, stepped upper die stamping method, part step stamping method, heated stamping method, etc. The oblique blade stamping method is to make the edge of the upper die (when punching) or the lower die (when blanking) into a shape that is inclined at an angle to its axis. This angle is less than 150 degrees, generally 80~100 degrees, which is similar to oblique blade shearing. The entire blade is not in contact at the same time, but gradually punches the material, so the stamping force is significantly reduced, and the vibration and noise during stamping can be reduced. Heated stamping is the stamping of the material in a heated state (or red stamping). Since the shear strength of metal materials generally decreases significantly in a heated state, it can effectively reduce the stamping force. However, the disadvantage of this method is that after the material is heated, oxide scale is produced, which affects the surface quality of the parts. Therefore, it is generally used for stamping thick plates or parts with low requirements for size and surface quality. In addition, if the die edge is blunt, chipped or not sharp, it will also significantly increase the punching force. Therefore, maintaining a sharp edge is one of the conditions for the normal operation of the stamping die. In order to keep the die edge sharp, the die should be sharpened after a period of stamping.
4. There is a large cavity under the die base of the fixed upper mode fine stamping die
The fine stamping die structure type can be divided into fixed upper mode fine stamping die and movable upper mode fine stamping die. Different die structure forms require the punching machine worktable structure to match it accordingly. For the movable upper mode fine stamping die, the punching machine worktable is required to be a floating hydraulic worktable with a fixed center and surrounded by an annular cylinder and a plunger. For the fixed upper mode fine stamping die, the punching machine worktable is required to have a plunger cylinder in the middle, as shown in Figure 3-29. The characteristics of this mold structure are: the upper and lower molds are fixed on the lower mold base, and the blank holder ring maintains relative movement with the upper and lower molds through the force transmission rod and the mold base. It is not suitable for a large cavity to appear under the mold base of the fixed upper mold fine punching die. This is because: when the upper mold is pressed down, the hydraulic cylinder moves downward under the action of the force transmission rod, so a large cavity appears under the mold base, and all the punching pressure acts on the top of the cavity, causing the upper and lower molds to bend, which is very unfavorable. Under the action of the increasing punching pressure, the lower part of the upper and lower molds will bend due to the danger of cracking. In order to avoid this situation, when the punching pressure is large, a special joint ring is required to improve the support conditions of the lower mold base to avoid large swirls and bending of the upper and lower molds. As the fine punching technology develops towards large-scale and composite processes, it is necessary to punch multiple holes or large internal contours, and the punching pressure is very large. The blank holder force and counter pressure are both large. Therefore, it is required to be in the middle of the workbench of the punch press.
5. The movable upper mode fine punching die is used to punch parts with multiple holes or large inner contours
The upper and lower dies of the movable upper mode fine punching die are directly fixed to the center of the workbench, and the support conditions are good. The characteristics of this die structure are: the upper and lower dies are movable relative to the die base, and the upper and lower dies are guided by the inner holes of the die base 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 maintain relative positions with the blank holder and the lower die. Therefore, the gap between the upper and lower dies and the lower die is required to be smaller. Only by making the upper and lower dies have a longer guide and correct positioning can the centering be guaranteed. Therefore, the movable upper mode fine punching die cannot punch parts with multiple holes or large inner contours, because the die assembly is difficult to center and the gap is difficult to guarantee, so it is mainly suitable for fine punching of medium and small parts.
6. The heat treatment hardness of the upper and lower dies of the stamping die is lower than 55HRC
The upper and lower dies of the stamping die are in contact with the stamping material, and are subjected to greater force and wear faster. Therefore, the upper and lower dies of the stamping die must be heat treated, and the hardness cannot be lower than 55HRC, because the higher the hardness, the higher the strength of the die and the more wear-resistant it is. 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. T8A has good hardenability, but poor hardenability, large quenching deformation and easy cracking, and is often used for punching simple and soft parts. Since the processing of the lower die is relatively 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 higher, that is, the heat treatment hardness of the upper die is generally 58~60HRC, and the heat treatment hardness of the lower die is 60~62HRC.