F = (1.42 × K × L × S²) / (1000 × V)
K is usually taken as 41 – 46 daN/mm² for mild steelThe formula works. But one thing is rarely spelled out: K is a fixed coefficient you choose yourself, while the actual bending force depends on the real tensile strength of the sheet in front of you.
How the die maker does it
's standard die catalogue (2021) does not use a fixed coefficient. It publishes a force table based on carbon steel at Rm ≈ 450 N/mm², then converts to other materials with:
k = Rm / 450In other words: read the force off the mild-steel table, then multiply by actual tensile strength divided by 450.
Part of 's 90° air-bending table, in kN/m:
2 mm sheet V12: 220 V14: 190 V16: 170 V18: 150 V20: 130 V25: 110
3 mm sheet V18: 340 V20: 300 V25: 240 V32: 190 V40: 150
4 mm sheet V25: 430 V32: 340 V40: 270 V50: 210 V63: 170How far apart the two methods land
Take an example circulating on Vietnamese industry sites: 5 mm sheet, 3000 mm long, V50 die.
Thietbi365, K = 41 → 87.33 ton
VNTECH, K = 42 → 89.46 ton(Both are local Vietnamese equipment-information sites.)
Two articles, two numbers, about 2 tons apart — purely because K differed by one. Neither is wrong. The point is that K is a number you pick.
If you know the steel grade and hold a material certificate, converting by Rm tracks the real material more closely.
On the shop floor
- Just need a quick figure to size a machine: either formula is fine. A few percent will not change the machine class.
- Bending near the machine's limit: work from actual Rm, and run one test bend first.
- Not sure of the grade: don't guess. Ask your steel supplier, or request the material certificate.
We put both methods side by side in the tonnage calculator so you can compare them yourself.