Various Conditions That Occur When The Die Clearance Is Too Large Or Too Small

Various conditions that occur when the die clearance is too large or too small

When the clearance between the punch and die is too small (significantly less than the material thickness or recommended standard), the material is subjected to excessive compressive and shear forces

1. Severe Adhesion and Wear (Galling): Minimal clearance creates excessive friction and high lateral pressure on the punch and die walls. This causes the workpiece material (especially soft steel, stainless steel, and aluminum) to adhere or cold-weld to the tool surfaces, leading to rapid material transfer and damage .

2. Excessive Shavings and Reduced Tool Life: The small clearance turns the process into a combination of shearing and extrusion, generating fine, unwanted shavings. This also subjects the tools to extreme side loading and wear, drastically shortening the life of the punch and die.

3. Slow and Erratic Stripping Force: Insufficient clearance causes the material to be severely compressed and tightly grip the punch. The stripper needs to apply a much greater force to remove the part from the punch, resulting in slow, inconsistent stripping and potential tool deflection.

4. Poor Hole Quality (Non-Perpendicular Walls): Excessive compression and lateral squeezing can cause the hole walls to be non-perpendicular or tapered (conical). The hole size may also be undersized.

5. High Heat Generation: The excessive shearing, friction, and compressive forces convert significant mechanical energy into thermal energy, leading to abnormal temperature increases in the tools and material. High heat accelerates galling and tool degradation.

6. Sheet Metal Warping/Distortion: High, concentrated pressure, friction, and large residual stresses around the pierced hole can cause the sheet to exhibit local deformation, dishing, or warping.

7. Due to the small gap, the material is predominantly sheared and extruded rather than allowed to fracture. This results in a much larger burnish zone and a smaller, less noticeable initial burr.

8. Larger, Thicker Running Burr (Accelerated Growth): While the initial burr is small, the rapid tool wear, galling buildup, and intense compressive stresses cause the burr to grow extremely fast, leading to very large and thick burrs after a short period of production.

9. Punching Noise: The insufficient clearance causes the material to be extruded/sheared over a longer period, resulting in a smoother, less abrupt energy release. The sound of the punch event is typically quieter (but this is achieved at the expense of tool life).

10. Reduced Rollover (Radius): Small clearance restricts the material's plastic flow and depression before fracture, resulting in a very small, defined radius (rollover) at the edge of the hole.

11. Reduced Break-away Area (Fracture Zone): Most of the material thickness is formed by shearing (burnish), meaning the fracture zone proportion on the hole wall is significantly reduced.

12. Less Slug Pulling: The smaller clearance creates a larger burnish zone and a straighter slug wall profile, making the slug less likely to snap back and stick to the punch face on the return stroke.

13. Excessive shear and compression cause the material at the burr location to undergo severe plastic deformation, making it extremely hard and difficult to remove in subsequent deburring processes.

When the clearance between the punch and die is too large (significantly more than the material thickness or recommended standard), the material is primarily bent and stretched before it fractures.

1. Increased Slug Pulling/Snapping: Large clearance causes the slug to undergo excessive bending and tensile stress before breaking. The slug's side wall often forms an hourglass shape, and the stored elastic energy causes the slug to snap back (pull), making it much more likely to adhere or be pulled back onto the punch face.

2. Creation of Rough Shavings/Tearing: Due to the large gap, the material is stretched excessively before clean fracture occurs. This causes the material to tear rather than shear cleanly at the end of the process, producing rough, stringy shavings.

3. Poor Hole Quality (Dimensional and Edge): Excessive clearance leads to over-stretching and bending of the material. This results in holes that may be oversized and have irregular, rough, and poor-quality edges.

4. Much more Workpiece Distortion: The material is severely stretched and bent rather than cleanly cut. This generates large plastic deformation and residual stress around the punched area, increasing the likelihood of the sheet dishing or buckling.

5. Increased and Rougher Burr: Since the material mostly tears under tension and bending rather than shears cleanly, the fracture zone is large. This results in a taller, sharper, and much rougher burr.

6. Increased Rollover (Radius): The large clearance allows the material excessive space for plastic flow. This creates a very large and prominent radius or depression (rollover) at the edge of the hole before fracture.

7. Increased Break-away Area (Fracture Zone): Since the shearing forces are not concentrated effectively, the material tears and fractures through most of its thickness. The fracture zone proportion on the hole wall is very large, while the clean burnish zone is minimal.

8. Rounded/Deformed Slug Edges: The slug undergoes extreme bending and deformation as it is pushed out. Its edges may become rounded or irregularly deformed rather than maintaining clean, sharp sheared corners.

9. Work Hardened Burrs: Due to the excessive stretching and tearing in the large fracture zone, the material at the burr edge can still experience work hardening, though potentially less severe than the extreme compression of insufficient clearance.