Numerical Investigation of Optimal Rooftop Punch Shape for Force Reduction and Dimensional Accuracy Control

The rooftop punch is proposed to reduce the maximum cutting force during the trimming operation for advanced high strength steels (AHSS), by introducing a shearing angle at the tool edge. However, such non-simultaneous shearing mechanism results in the inconsistent deformation around the hole perimeter, and consequently affects the dimensional accuracy of the trimmed hole. A numerical study was conducted to investigate the effects of punch tipping angle and tipping heights on the force reduction and dimensional discrepancies. The 60mm hole punching operation for DP 1180 (1.2mm) material was simulated with finite element analysis. The tipping height was reduced by introducing flat portions to the rooftop punch and it can mitigate the material deformation difference before trimming. The results showed tipping height played a significant role of dimensional accuracy control by adopting small tipping angle and broad flat portions. However, such geometric combination could increase the maximum shearing load at the same time. Additionally, a concave rooftop punch shape was also evaluated to compare with the conventional convex rooftop punch.