Press brake tonnage calculator
Air-bending force grows with the square of the thickness and falls with the V opening: F = 1.42·Rm·t²·L / V. 3 mm mild steel on a 24 mm V takes about 240 kN per metre of bend. Enter the sheet, the die and the bend length, and the calculator gives the force, the V the makers recommend for that thickness, the inside radius it forms and the shortest flange that will bend on it, and checks it against your press.
- Force for 1000 mm
- In tonnes-force
- Force per metre
- V by the makers' rule
- 8·t = 24.0 mm
- Natural inside radius ri
- Minimum flange b
How it’s calculated
F = 1.42·Rm·t²·L / (1000·V) kN, with t, V and L in mm and Rm in N/mm², for a 90° air bend. Mild steel is taken at Rm 450 N/mm², the force charts’ basis; stainless at 1.5 times and soft aluminium at 0.5 times that force. ri ≈ 0.16·V (mild steel), b ≈ 0.77·V; 1 t = 9.80665 kN.
Force by thickness, on the rule’s V
| t (mm) | V (mm) | Rule | F (kN/m) | F (t/m) | ri (mm) | b (mm) |
|---|---|---|---|---|---|---|
| 0.5 | 3.0 | 6·t | 53 | 5.4 | 0.5 | 2.3 |
| 0.8 | 4.8 | 6·t | 85 | 8.7 | 0.8 | 3.7 |
| 1 | 6.0 | 6·t | 107 | 10.9 | 1.0 | 4.6 |
| 1.2 | 7.2 | 6·t | 128 | 13.0 | 1.2 | 5.5 |
| 1.5 | 9.0 | 6·t | 160 | 16.3 | 1.4 | 6.9 |
| 2 | 12.0 | 6·t | 213 | 21.7 | 1.9 | 9.2 |
| 2.5 | 15.0 | 6·t | 266 | 27.1 | 2.4 | 11.6 |
| 3 | 24.0 | 8·t | 240 | 24.4 | 3.8 | 18.5 |
| 4 | 32.0 | 8·t | 320 | 32.6 | 5.1 | 24.6 |
| 5 | 40.0 | 8·t | 399 | 40.7 | 6.4 | 30.8 |
| 6 | 48.0 | 8·t | 479 | 48.9 | 7.7 | 37.0 |
| 8 | 64.0 | 8·t | 639 | 65.2 | 10.2 | 49.3 |
| 10 | 100.0 | 10·t | 639 | 65.2 | 16.0 | 77.0 |
| 15 | 180.0 | 12·t | 799 | 81.4 | 28.8 | 138.6 |
| 20 | 240.0 | 12·t | 1065 | 108.6 | 38.4 | 184.8 |
How do you choose the V-die?
The makers’ rule of thumb is a V of about 6 times the thickness for thin sheet, 8 times from 3 to 8 mm, 10 times around 10 mm and 12 times for plate. A wider V needs less force but forms a larger inside radius (about 0.16·V in mild steel) and needs a longer flange (about 0.77·V) to stay on the shoulders; a narrower one the reverse. Makers’ force charts differ by up to about 15 %, so leave margin on the press.