1、 Information from scrap
Scrap is essentially the reverse image of the formed hole. That is, the same part in opposite position. By checking the scrap, you can judge whether the clearance between the upper and lower dies is correct. If the gap is too large, the waste will have a rough, undulating fracture surface and a narrow bright zone area. The larger the gap, the greater the angle between the fracture surface and the bright zone area. If the gap is too small, the waste will show a small angle fracture surface and a wide bright zone area.
Excessive clearance forms holes with large crimping and edge tearing, which makes the profile slightly have a thin edge protruding. Too small a gap forms a band that is slightly curled and torn at a large angle, causing the profile to be more or less perpendicular to the material surface.
An ideal waste material should have reasonable collapse angle and uniform bright zone. In this way, the minimum punching force can be maintained and a clean round hole with few burrs can be formed. From this point of view, extending the die life by increasing the gap is at the expense of the quality of finished holes.
2、 Selection of die clearance
The clearance of the die is related to the type and thickness of the material being punched. Unreasonable clearances can cause the following problems:
(1) If the clearance is too large, the burr of the stamping workpiece is relatively large, and the stamping quality is poor. If the clearance is too small, although the punching quality is good, the wear of the die is relatively serious, which greatly reduces the service life of the die and is easy to cause the break of the punch.
(2) Too large or too small clearance is easy to produce adhesion on the punch material, thus causing the material to be carried during stamping. If the clearance is too small, it is easy to form a vacuum between the bottom of the punch and the sheet metal, which will cause the scrap to rebound.
(3) Reasonable clearance can extend the life of the die, discharge effectively, reduce burr and flanging, keep the plate clean, keep the hole diameter consistent and will not scratch the plate, reduce the number of grinding, keep the plate straight, and punch the hole accurately.
Please refer to the following table to select the die clearance (the data in the table is a percentage)
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3、 How to improve the service life of dies
For users, improving the service life of the die can greatly reduce the stamping cost. The factors affecting the service life of the mold are as follows:
1. Type and thickness of materials;
2. Whether reasonable lower die clearance is selected;
3. The structure of the mold;
4. Whether the materials are well lubricated during stamping;
5. Whether the mold has undergone special surface treatment;
6. Such as titanium plating, carbon titanium nitride;
7. Neutrality of upper and lower turrets;
8. Reasonable use of adjusting shims;
9. Whether the die with inclined cutting edge is properly used;
10. Whether the mold base of the machine tool has been worn;
4、 Problems needing attention in punching holes with special dimensions
(1) Minimum hole diameter: punch φ 0.8—— φ 1.6 Special punch shall be used for punching within the range.
(2) When punching thick plates, please use a larger die relative to the processing hole diameter. Note: At this time, if the normal size die is used, the punch thread will be damaged.
Example 1. For the processing conditions in the following table, although the processing hole diameter corresponds to the mold at station A, please use the mold at station B.
Example 2. For the processing conditions in the following table, although the processing hole diameter corresponds to the die at station B, please use the die at station C.
(3) The ratio of the minimum width to the length of the punch cutting edge should generally not be less than 1:10.
Example 3: When the cutting edge length of rectangular punch is 80mm, the cutting edge width ≥ 8mm is the most appropriate.
(4) The relationship between the minimum size of the punch cutting edge and the plate thickness. It is recommended that the minimum size of the cutting edge of the punch should be 2 times of the plate thickness.
Extended reading:
1. [Process control] All you want about stamping dies are here (II)
2. [Process control] All you want about stamping dies are here (III)
3. [Process control] All you want about stamping dies are here (IV)
