I. Information Revealed by Scrap
Scrap is essentially the reverse image of the formed hole, meaning the same area appears in the opposite position. By inspecting the scrap, you can determine whether the clearance between the upper and lower dies is correct. If the clearance is too large, the scrap will exhibit a rough, undulating fracture surface and a narrow bright band area. The larger the clearance, the greater the angle between the fracture surface and the bright band area. If the clearance is too small, the scrap will show a small-angle fracture surface and a wide bright band area.
Excessive clearance produces holes with large rollover and edge tearing, causing the contour to protrude slightly and the edges to be thinner. Insufficient clearance causes slight rollover and large-angle tearing of the strip, resulting in a cross-section that is more or less perpendicular to the material surface.
Ideal scrap should have a reasonable burnish angle and a uniform bright band. This maintains low punching force and produces a clean, round hole without burrs. From this perspective, increasing die life by increasing clearance comes at the expense of finished hole quality.
II. Selection of Die Clearance
Die clearance is related to the type and thickness of the material being stamped. Unreasonable clearance can lead to the following issues:
1. If the clearance is too large, the stamped workpiece will have significant burrs and poor stamping quality. If the clearance is too small, although the blanking quality is good, the die wears severely, greatly reducing die life and easily causing punch breakage.
2. Either too large or too small clearance can easily cause material adhesion on the punch, leading to taper in the material during stamping. If the clearance is too small, a vacuum can form between the punch face and the sheet, causing scrap rebound.
3. Reasonable clearance extends die life, improves stripping performance, reduces burrs and flanging, keeps the sheet clean, ensures consistent hole diameters without scratching the sheet surface, reduces the frequency of sharpening, keeps the sheet flat, and ensures accurate punching positioning.
III. How to Improve Die Service Life
For users, extending the service life of the die can significantly reduce stamping costs. The factors affecting die service life are as follows:
1. The type and thickness of the material.
2. Whether a reasonable die clearance is selected.
3. The structural design of the die.
4. Whether the material is properly lubricated during stamping.
5. Whether the die has undergone special surface treatment.
6. Such as titanium coating or titanium carbonitride coating.
7. The alignment of the upper and lower turrets.
8. The proper use of adjustment shims.
9. Whether the use of beveled-edge dies is appropriate.
10. Whether the die holder on the machine is worn.
4. Precautions for punching holes of special sizes
1. Use a dedicated punch for small-hole punching in the φ0.8–φ1.6 mm range.
2. When punching thick plates, use a die one size larger than the hole diameter. Note: If a standard-size die is used in this case, the punch threads may be damaged.
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: For a rectangular punch with a cutting edge length of 80 mm, a cutting edge width of ≥8 mm is appropriate.
5. Die sharpening
1. Importance of die sharpening
Regular die sharpening is key to ensuring consistent punch quality. Sharpening dies on a regular basis not only extends die service life but also improves machine service life. However, the correct sharpening timing must be properly determined.
2. Specific characteristics of dies that require sharpening
There is no strict number of strokes that determines when die sharpening is required. It mainly depends on the sharpness of the cutting edge, which is determined by the following three factors:
1. Check the radius of the cutting edge. If the corner radius reaches R0.1mm (the maximum R value should not exceed 0.25mm), sharpening is required.
2. Check the punching quality—are there any large burrs?
3. Determine whether sharpening is needed based on the noise of the stamping machine. If the same die produces abnormal noise during stamping, it indicates that the punch has become dull and requires sharpening.
Note: If the cutting edge becomes rounded or the rear of the cutting edge is rough, sharpening should also be considered.
3. Sharpening Methods
There are many methods for sharpening dies, which can be achieved through a dedicated sharpening machine or a surface grinder. The sharpening frequency ratio for punches to lower dies is generally 4:1. After sharpening, please adjust the die height.
1. Hazards of Incorrect Sharpening Methods: Incorrect sharpening can accelerate the rapid destruction of the cutting edge, greatly reducing the number of strokes between each sharpening.
2. Benefits of Correct Sharpening Methods: Regularly sharpening the die maintains consistent stamping quality and precision. The die cutting edge wears more slowly, resulting in a longer service life.
4. Sharpening Rules
The following factors should be considered when sharpening:
(1) The edge radius, when in the range of R0.1-0.25mm, depends on the sharpness of the cutting edge.
(2) The surface of the grinding wheel should be kept clean.
(3) It is recommended to use a soft, open-structured, coarse-grit grinding wheel, such as WA46KV.
(4) The grinding depth (depth of cut) per pass must not exceed 0.013mm. Excessive grinding depth can cause overheating of the die surface, which is equivalent to annealing, softening the die and significantly reducing its service life.
(5) Sufficient coolant must be applied during sharpening.
(6) During grinding, the punch and die should be securely fixed, and dedicated fixtures should be used.
(7) There is a specified sharpening limit for the die. Once this limit is reached, the punch must be scrapped. Continuing to use it can easily cause damage to the die and the machine, which is not worth the loss.
(8) After sharpening, the edges should be treated with an oil stone to remove excessively sharp burrs or lines.
(9) After sharpening, the die should be cleaned, demagnetized, and oiled.
Note: The amount of die sharpening mainly depends on the thickness of the stamped sheet metal.
VI. Precautions Before Using Punches
1. Storage
(1) Wipe the inside and outside of the upper die sleeve with a clean cloth.
(2) When storing, be careful to avoid scratches or dents.
(3) Apply oil to prevent rust.
2. Preparation Before Use
(1) Thoroughly clean the upper die sleeve before use.
(2) Check the surface for scratches and dents. If any are found, remove them with an oilstone.
(3) Apply oil to both the inside and outside.
3. Precautions when installing the punch into the upper die set
(1) Clean the punch and apply oil to its long shank.
(2) Regarding large-station dies