Ensure optimal die clearance
Die clearance is related to the sheet thickness and material, as well as the stamping process. Choosing appropriate die clearance ensures good punching quality, reduces burrs and collapses, keeps the sheet flat, effectively prevents strip feeding, and prolongs the die life.
By examining waste from the stamping process, it can be determined whether the die clearance is adequate. If the gap is too large, the waste will have a rough, uneven fracture surface and a smaller shiny surface.
The greater the gap, the greater the angle formed between the fracture surface and the bright surface, leading to edge rolling and fracture during punching, and even the formation of a thin edge protrusion.
On the other hand, if the gap is too small, the waste will have a small-angle fracture surface and a larger shiny surface.
When performing grooving, step punching, shearing and other local stamping operations, lateral forces will cause the punch to deflect, resulting in an excessively small and one-sided gap. Sometimes excessive blade travel can scratch the lower die, causing rapid wear of the upper and lower dies.
When the die punches with ideal clearance, the fracture surface and the shiny surface of the residue have the same angle and overlap, minimizing shear force and producing small burrs.
Timely sharpening can effectively extend the service life of the die
If the workpiece has excessive burrs or abnormal noise during stamping, it may be due to dullness of the die. Inspect the punch and lower die, and when the blade edge wear produces a radius of about 0.10mm, it's time to sharpen.
Practice shows that regular, small-scale sharpening, rather than waiting until absolutely necessary, not only maintains good part quality and reduces shear force, but also extends the life of the die by more than double .
In addition to knowing when to sharpen the die, it is especially important to master the correct sharpening method. Die sharpening procedures are as follows:
- During sharpening, hold the punch vertically in the V-groove or magnetic chuck attachment on a surface grinder, with a grinding depth of 0.03-0.05 mm per pass. Repeat grinding until the punch is sharp, with a maximum grinding depth of 0.1-0.3 mm.
- Use sintered alumina grinding wheels with DJ hardness and grit size of 46-60, preferably grinding wheels suitable for grinding high-speed steel.
- When the grinding force is large or the die is close to the grinding wheel, add cooling fluid to prevent the die from overheating, cracking or annealing. Choose a high-quality multipurpose coolant according to the manufacturer's requirements.
- For downward grinding depth, use 0.03-0.08mm; for side grinding depth, use 0.13-0.25 mm; and for side feed rate, use 2.5-3.8m/min.
- After sharpening, polish the cutting edge with an oil stone to remove burrs and create a round corner with a radius of 0.03-0.05 mm to avoid chipping on the edge.
- Demagnetize the punch and spray lubricating oil to prevent rust.
Methods for eliminating and reducing adhesive materials
During stamping, pressure and heat can cause small particles of sheet material to adhere to the surface of the punch, resulting in poor quality holes. To remove adhesive materials, use a fine oil stone for sanding and make sure the grinding direction is the same as the direction of movement of the punch.
This will prevent greater adhesion. Do not use a rough cloth for sanding, as this may cause the surface of the punch to become rougher and more likely to stick.
Reasonable die clearance, good stamping process and necessary sheet lubrication will reduce the formation of adhesive material. To prevent overheating, lubrication is usually used, which reduces friction. If lubrication is not possible or residual material ricochets, consider the following methods:
Alternate use of several punches of the same size in rotation for stamping, allowing longer cooling time before reuse. Rest overheated dies by pausing their use. Change dies using programmatic control, stopping your long-term repetitive work or reducing your stamping frequency.
Measures to avoid deformation of the sheet when drilling many holes
If many holes are drilled in a single sheet, the sheet cannot remain flat due to the cumulative shear stress. With each punch, the material around the hole will deform downward, causing tensile stress on the top surface of the sheet and compressive stress on the bottom surface.
For a small number of holes, the impact is not significant, but as the number of holes increases, tensile and compressive stresses build up in some areas until the material deforms.
One method to eliminate this type of warping is to drill all the other holes first and then go back and drill the remaining holes. Although this method still generates tension, it relieves the tension buildup that occurs when punching sequentially in the same direction.
Furthermore, it allows the tensions of the two groups of holes to counteract each other, thus preventing deformation of the sheet.
Try to avoid punching excessively narrow strips
When the die is used to punch sheets with a width smaller than the sheet thickness, the punch may bend and deform due to lateral forces, causing one side of the gap to be too small or to suffer greater wear. In severe cases, the lower die can be scratched, simultaneously damaging the upper and lower dies.
It is recommended not to drill strips narrower than 2.5 times the thickness of the sheet. When cutting excessively narrow strips, the sheet tends to fold into the bottom opening of the die instead of being completely cut, or it may even get stuck on the side of the punch die. If the above situations cannot be avoided, consider using a fully guided die supported by a backing plate for the punch.
Punch surface hardening and its application range
Although heat treatment and surface coatings can improve punch surface properties, they are not general solutions to solve stamping problems and extend die life.
Generally, coatings increase the hardness of the punch surface and improve the lubrication of the side surfaces, but these advantages disappear after about 1,000 punches when dealing with hard, high-tonnage materials.
Punches with a hardened surface can be used in the following situations:
- Drilling soft or sticky materials (such as aluminum).
- Drilling fine abrasive materials (such as glass epoxy sheets).
- Drilling hard and thin materials (such as stainless steel).
- Frequent drilling.
- Abnormal lubrication situations.
Surface hardening generally involves methods such as titanium plating and nitriding, which create a molecular structure layer 12-60μm thick. It is part of the base of the punch and not just a coating.
Dies with a hardened surface can be sharpened in the usual way. Surface hardening reduces die wear when punching stainless steel sheets, but does not extend their service life. Proper lubrication, timely sharpening, and adherence to standard operating procedures are effective methods.
Maintenance when punch press die alignment is poor
If the alignment of the punch die is poor, causing rapid blunting of the punch and poor processing quality of the part, consider the following maintenance points:
- Check the leveling status of the machine and readjust if necessary.
- Inspect and lubricate the die holes and guide keys in the tower; repair any damage immediately.
- Clean the lower tower seat to ensure accurate installation and inspect its keys or keyways for wear, replacing them if necessary.
- Use a specialized core rod to calibrate the position of the die, adjusting immediately if deviations are detected.
Conclusion
The above content is applicable to general situations. However, considering that the types and specifications of punching machines and dies may vary, users must also combine their practical experience and understanding to fully utilize the optimal performance of the dies.
