Ensure optimal die clearance
Die clearance is related to the thickness and material of the sheet, as well as the stamping process. Choosing the appropriate die clearance ensures good punching quality, reduces burrs and collapse, keeps the sheet flat, effectively prevents strip feeding, and extends the die life.
By examining the waste from the stamping process, one can determine if the die clearance is suitable. If the clearance is too large, the waste will have a rough and uneven fracture surface and a smaller bright surface.
The larger the clearance, the larger the angle formed between the fracture surface and the bright surface, leading to edge rolling and fracturing during punching, and even the formation of a thin edge protrusion.
Conversely, if the clearance is too small, the waste will have a small angle fracture surface and a larger bright surface.
When performing slotting, step punching, shearing, and other local stamping operations, lateral forces will cause the punch to deflect, resulting in a single-sided, excessively small clearance. Sometimes, excessive blade offset can scratch the lower die, causing rapid wear of the upper and lower dies.
When the die punches with the optimal clearance, the fracture surface and bright surface of the waste have the same angle and overlap each other, minimizing the shearing force and producing small burrs.
Timely sharpening can effectively extend the life of the die
If the workpiece has excessive burrs or abnormal noise during stamping, it may be due to die dulling. Inspect the punch and lower die, and when the blade edge wear produces a radius of about 0.10mm, it is time to sharpen.
Practice shows that regular, small-scale sharpening rather than waiting until it is absolutely necessary not only maintains good workpiece quality and reduces shearing force, but also extends the die life by more than double.
In addition to knowing when to sharpen the die, mastering the correct sharpening method is especially important. Die sharpening procedures are as follows:
- During sharpening, hold the punch vertically in the V-groove or fixture of the magnetic chuck on a surface grinder, with a grinding depth of 0.03-0.05mm per pass. Repeat grinding until the punch is sharp, with a maximum grinding depth of 0.1-0.3mm.
- Use sintered alumina grinding wheels with a hardness of D-J and grit size of 46-60, preferably wheels suitable for high-speed steel grinding.
- When the grinding force is large or the die is close to the grinding wheel, add cooling fluid to prevent the die from overheating and cracking or annealing. Choose a high-quality, multi-purpose cooling fluid according to the manufacturer’s requirements.
- For downward grinding depth, use 0.03-0.08mm; for lateral grinding depth, use 0.13-0.25mm; and for lateral feed rate, use 2.5-3.8m/min.
- After sharpening, polish the cutting edge with an oilstone to remove burrs and create a 0.03-0.05mm radius round corner to prevent edge chipping.
- Demagnetize the punch and spray it with lubricating oil to prevent rust.
Methods to eliminate and reduce adhesive materials
During stamping, pressure and heat can cause small particles of the sheet material to adhere to the punch surface, leading to poor hole quality. To remove adhesive materials, use a fine oilstone for grinding, and ensure the grinding direction is the same as the punch’s movement direction.
This will prevent further adhesion. Do not use coarse cloth for grinding, as this may cause the punch surface to become rougher and more prone to adhesion.
A reasonable die clearance, good stamping process, and necessary sheet lubrication will all reduce adhesive material formation. To prevent overheating, lubrication is generally used, which reduces friction. If lubrication is not possible or waste material rebounds, consider the following methods:
Alternate the use of multiple punches of the same size in rotation for stamping, allowing for a longer cooling time before being reused. Rest overheated dies by pausing their use. Change dies using programmatic control, interrupting their long-term repetitive work, or reducing their stamping frequency.
Measures to prevent sheet deformation when punching many holes
If many holes are punched on a single sheet, the sheet cannot remain flat due to the cumulative cutting stress. With each punch, the material around the hole will deform downward, causing tensile stress on the upper surface of the sheet and compressive stress on the lower surface.
For a small number of holes, the impact is not significant, but as the number of holes increases, the tensile and compressive stresses accumulate in some areas until the material deforms.
One method to eliminate this type of deformation is to punch every other hole first, then go back and punch the remaining holes. Although this method still generates stress, it alleviates the stress accumulation that occurs when punching sequentially in the same direction.
Additionally, it allows the stresses from the two groups of holes to counteract each other, thereby preventing sheet deformation.
Try to avoid punching excessively narrow strips
When the die is used for punching sheets with a width smaller than the sheet thickness, the punch may bend and deform due to lateral forces, causing one side of the clearance to be too small or experience increased wear. In severe cases, the lower die may be scratched, damaging both the upper and lower dies simultaneously.
It is recommended not to punch strips narrower than 2.5 times the sheet thickness. When cutting excessively narrow strips, the sheet tends to bend into the lower die opening rather than being completely cut, or it may even wedge into the side of the punch die. If the above situations cannot be avoided, consider using a full-guided die with support from a backing plate for the punch.
Punch surface hardening and its application range
Although heat treatment and surface coatings can improve the punch surface properties, they are not general solutions for solving stamping problems and extending die life.
Generally, coatings increase the punch surface hardness and improve the lubricity of the side surfaces, but these advantages disappear after about 1,000 punches when dealing with high-tonnage and hard materials.
Surface-hardened punches can be used in the following situations:
- Punching soft or sticky materials (such as aluminum).
- Punching thin abrasive materials (such as glass epoxy sheets).
- Punching thin hard materials (such as stainless steel).
- Frequent step-punching.
- Non-normal lubrication situations.
Surface hardening usually involves methods such as titanium plating and nitriding, which create a molecular structure layer with a thickness of 12-60μm. It is part of the punch base rather than just a coating.
Surface-hardened dies can be sharpened in the usual way. Surface hardening reduces the wear of the die when punching stainless steel sheets but does not extend its service life. Proper lubrication, timely sharpening, and following standard operating procedures are effective methods.
Maintenance when the punch press die alignment is poor
If the alignment of the punch press die is poor, causing rapid punch dulling and poor workpiece processing quality, consider the following maintenance points:
- Check the machine’s leveling condition and readjust if necessary.
- Inspect and lubricate the die holes and guide keys on the turret; repair any damage promptly.
- Clean the lower die seat of the turret to ensure accurate installation and inspect the wear of its keys or keyways, replacing if necessary.
- Use a specialized core rod to calibrate the die position, adjusting promptly if deviations are detected.
The above content is applicable to general situations. However, considering that punch press and die types and specifications may vary, users should also combine their practical experience and understanding to fully utilize the optimal performance of the dies.
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