Press Force (Tonnage) Calculator

How is the cutting force for sheet metal calculated?

A punch shears the sheet along the cut perimeter. The peak force needed is the sheared area times the shear strength of the material: F = L × t × τ, where L is the cut length, t the sheet thickness and τ the shear strength. For a round hole the perimeter is π × d, for a rectangle 2 × (a + b).

With dimensions in mm and strength in N/mm² (MPa), the force is in newtons. Divide by 1000 for kN and by 9807 for tonnes.

Example

A 20 mm hole is to be punched in 3 mm sheet with a shear strength of 320 N/mm². The perimeter is π × 20 = 62.8 mm and the sheared area 62.8 × 3 = 188.5 mm². The force is 188.5 × 320 = 60 319 N, about 60 kN or 6.2 tonnes. With a 20 percent margin, choose a press of at least 72 kN.

If the shear strength is unknown

Certificates often give only the tensile strength. For steel sheet the shear strength is commonly taken as about 0.8 times the tensile strength. That is a rough assumption, and the calculator lets you change the ratio.

Bending and hydraulic presses

A widely used empirical relation for V air bending is F = 1.33 × L × R_m × t² / V. A wider die opening lowers the force. On a hydraulic press, force is oil pressure times ram area: a 200 mm ram at 250 bar gives 785 kN, about 80 tonnes.

Limits

The calculation gives force, not the work capacity of the press. On thick sheet a mechanical press may run short of energy. Shear angle on the punch lowers the force and dull tooling raises it. Use manufacturer data to select the press and tooling.