Punching force calculator
Punching force is the length of the cut times the sheet thickness times the shear strength: F = L·t·τ. A 20 mm square in 3 mm mild steel cuts 80 mm of outline and takes about 83 kN (8.4 t). Choose the shape, enter its size, the thickness and the material, and the calculator gives the force, the cut length and the area sheared, and checks it against your press.
- Punching force F
- In tonnes-force
- Cut length L
- Area sheared L·t
- Shear strength τ
- 345N/mm²
How it’s calculated
F = L·t·τ, with the cut length L and thickness t in mm and the shear strength τ in N/mm², gives newtons (÷ 1000 for kN; 1 t = 9.80665 kN). L is the outline: π·d round, 4·a square, 2·(a + b) rectangle, 2·(a − b) + π·b slot, the length for a straight cut on a flat blade; punches that hit together add. Where only the tensile strength Rm is known, τ ≈ 0.8·Rm. The force is for a sharp, flat-faced punch: a dull one takes more.
Force for this shape by thickness (kN)
| t (mm) | Aluminium | Brass | Mild steel | Stainless |
|---|---|---|---|---|
| 0.8 | 8.7 | 12.1 | 17.3 | 26.0 |
| 1 | 10.8 | 15.2 | 21.7 | 32.5 |
| 1.2 | 13.0 | 18.2 | 26.0 | 39.0 |
| 1.5 | 16.2 | 22.7 | 32.5 | 48.7 |
| 2 | 21.7 | 30.3 | 43.3 | 65.0 |
| 2.5 | 27.1 | 37.9 | 54.1 | 81.2 |
| 3 | 32.5 | 45.5 | 65.0 | 97.5 |
| 4 | 43.3 | 60.6 | 86.6 | 129.9 |
| 5 | 54.2 | 75.8 | 108.3 | 162.4 |
| 6 | 65.0 | 91.0 | 129.9 | 194.9 |
| 8 | 86.7 | 121.3 | 173.3 | 259.9 |
| 10 | 108.3 | 151.6 | 216.6 | 324.8 |
Why does the punch need margin on the press?
The force is for a sharp, flat-faced punch. As the cutting edge wears, the force rises, so tool makers keep a job to about 80 % of the press. A punch face ground with shear (roof-top or angled) cuts a little at a time and needs less, but the saving shrinks as it dulls, so it is not a way to fit a job onto a press that is too small. Very narrow punches in thick or hard sheet buckle or break: in stainless, keep the punch at least twice as wide as the sheet is thick.