Where F(b) = Blanking force (kN), P = Perimeter of blank (mm), S(s) = Shear strength (MPa), c = a constant for impact loadings.
Grinding a shear angle on a punch minimises the shock load on a press and gives a quieter punch operation. For zero shear, when s = 0, c = 1.25. For part shear, when s = 0.3t, c = 1.0. For full shear, when s = t, c = 0.75. (NB: on a small diameter, multi-punch set-up, staggering the length of punches gives a similar effect on shock loads.) e.g. 150mm diameter blank in 3mm mild steel with part shear on punch gives: