The short answer: for air bending, required force is P = (1.42 × TS × S² × L) / V — tensile strength times thickness squared, times bend length, divided by the die opening. For mild steel over the standard die opening of 8× material thickness, that works out to roughly 8 t per metre at 1 mm, 17 t/m at 2 mm and 110 t/m at 6 mm. Run the number for your thickest, longest, hardest part, then add 20–30% margin and round up to the next standard frame size.
Key takeaways
- Air-bending force follows P = (1.42 × TS × S² × L) / V — it rises with the square of thickness, so 4 mm plate needs about four times the force of 2 mm, not twice.
- With the classic V = 8 × S die opening, mild steel needs ≈ 8 t/m at 1 mm, 17 t/m at 2 mm and 110 t/m at 6 mm.
- Multiply mild steel figures by ≈ 1.5 for 304 stainless, ≈ 0.65 for 5052 aluminum, and 1.6 or more for high-strength steel.
- A 2 mm stainless part 2.5 m long over a V16 die needs about 64 t — the right machine is 100–125 t, not 70 t.
- The formula covers air bending only; coining the same part can demand 3–5× the force.
Why tonnage matters more than any other spec
Tonnage is the one press brake specification you cannot work around. Too little and jobs simply don't finish: angles open up short of target, operators start "cheating" with narrower dies that mark the sheet, and eventually someone overloads the machine trying to force a part through. Too much and you have paid for frame, hydraulics and floor loading you will never use — a 200-ton machine bending 1.5 mm enclosure panels all day is capital sitting idle.
Because force scales with the square of thickness, small changes in your product mix have outsized effects. Moving from 3 mm to 4 mm mild steel raises the per-metre requirement from about 33 t to about 48 t. That is why sizing starts with arithmetic, not with a brochure.
The air-bending tonnage formula
Every tonnage chart in every manufacturer catalogue is a tabulation of the same rule of thumb:
- P — required bending force in kN. Divide by roughly 10 to get metric tons.
- TS — tensile strength of the material in N/mm² (mild steel ≈ 400–470, 304 stainless ≈ 600–700, 5052 aluminum ≈ 230–260).
- S — material thickness in mm.
- L — bend length in metres.
- V — width of the V-die opening in mm.
The 1.42 coefficient bundles up die friction and geometry; some references use 1.33, and real charts vary a few percent either way. Treat every result as an estimate to be rounded up, never a value to run at exactly.
Choosing V: the 8 × thickness rule
The die opening V is the lever in the formula — double it and required force halves. The classic starting point is V = 8 × S: a V16 die for 2 mm sheet, V24 for 3 mm, V48 for 6 mm plate. Go narrower (6 × S) when you need a tighter inside radius or a shorter minimum flange; go wider (10–12 × S) on thick plate to keep force and cracking under control.
Two side effects come with the choice: the inside bend radius lands at roughly V/6, and the minimum flange you can bend is about 70–80% of V. This is also why double-sided dies earn their keep — a V16V24 lower die covers both 2 mm and 3 mm work in one $100 tool, no die change needed beyond flipping it.
Tonnage chart: mild steel, air bending, V = 8 × S
The figures below are indicative values in metric tons per metre of bend, for mild steel around 420–470 N/mm² tensile strength. They track typical manufacturer charts, which run slightly above the bare formula at heavier gauges.
| Thickness S | Die opening V (8 × S) | Approx. force (t/m) |
|---|---|---|
| 1 mm | V8 | ≈ 8 |
| 1.5 mm | V12 | ≈ 13 |
| 2 mm | V16 | ≈ 17 |
| 3 mm | V24 | ≈ 33 |
| 4 mm | V32 | ≈ 48 |
| 5 mm | V40 | ≈ 75 |
| 6 mm | V48 | ≈ 110 |
Material factors: steel is the baseline, not the rule
Charts are published for mild steel. For anything else, scale by tensile strength:
| Material | Multiplier vs mild steel | Notes |
|---|---|---|
| Mild steel (Q235 / S235) | 1.0 | Baseline, TS ≈ 400–470 N/mm² |
| Stainless 304 | ≈ 1.5 | Higher TS plus more springback — expect over-bending |
| Aluminum 5052 | ≈ 0.65 | Low force, but hard tempers crack over narrow dies |
| High-strength steel | 1.6 and up | Check the actual datasheet TS — grades vary widely |
Worked example: 2 mm stainless, 2.5 m part
Say your defining job is a 2.5 m long bend in 2 mm SUS304 over a V16 die. From the chart, 2 mm mild steel at V16 needs about 17 t/m. Apply the stainless multiplier and the length:
Could a 70- or 80-ton machine do it? On paper, barely — and that is the problem. Running a machine at 90%+ of rating leaves nothing for harder-than-spec material, a slightly narrower die, or the 3 mm job that walks in next year. The sensible purchase here is a 100–125 t machine with a 3–3.2 m bed: the stainless part runs at a comfortable 50–65% of capacity, and you keep headroom for thicker mild steel work across the full bed.
For day-to-day work at the machine, the 钣金人 app's bend calculators run this same arithmetic — plus unfold length and bend deduction — from your phone at the brake. And once you have a tonnage and bed length pinned down, you can request free quotes from vetted Chinese manufacturers against that exact spec instead of a guess.
Common mistakes when sizing a press brake
- Ignoring crowning. On bends beyond ~2 m the ram and bed deflect, opening the angle mid-part. A machine sized "just enough" with no crowning system produces boat-shaped parts at exactly the loads where you need it most. Budget for crowning on any machine 3 m or longer.
- Sizing to today's thickest job with zero margin. If your worst case computes to 64 t, a 70-ton machine is already at its limit on day one. Materials run hard, dies wear, and work gets thicker over a machine's 15-year life — buy 20–30% above the computed worst case.
- Confusing coining with air bending. All figures in this guide assume air bending. Bottoming and coining press the sheet into the die and can multiply required force by 3–5× or more. If you coin for precision, the whole calculation changes — and so does the machine class.
Where tooling fits into the tonnage question
Tooling is the cheap way to manage force. A wider V-die halves tonnage on thick plate; a die change from V16 to V24 turns a marginal job into an easy one, at the price of a larger radius and flange. Remember that punches and dies carry their own maximum t/m ratings — exceeding them chips tool shoulders long before the machine notices.
Die choice also touches surface quality: standard V-dies at high unit loads leave witness marks on stainless and aluminum, which is where mark-free dies and films come in — our mark-free bending guide compares the options and when each pays off. And if you are specifying a laser to cut the blanks that feed the brake, size that with the same discipline using the fiber laser power guide.
Frequently asked questions
Can I bend 6 mm steel on a 100-ton press brake?
Only for short bends. Air bending 6 mm mild steel over the standard V = 48 mm die opening takes roughly 110 t per metre, so 100 t of force covers only about 0.9 m of bend length. Stepping up to a V60–V63 die drops the requirement to roughly 85–90 t/m — buying you about 1.1 m — at the cost of a larger inside radius and minimum flange. A full-length 6 mm bend across a 2.5–3 m bed is out of reach on a 100-ton machine.
What happens if I exceed rated tonnage?
Modern CNC press brakes limit hydraulic pressure and will typically stall or fault before serious damage, but repeated overloads still stress the frame, ram and cylinders and usually void the warranty. Older machines without overload protection can crack tooling, permanently deflect the ram or bend the bed. Also watch concentrated loads: a short, heavy bend in the middle of the bed can locally overload the ram even when total force is below the machine rating — respect the per-metre limit, not just the headline number.
Is coining tonnage different?
Yes — very. The formula and chart in this guide apply to air bending only, where the punch stops short of the die bottom. Coining forces the material fully into the die and typically demands 3–5 times the air-bending force, sometimes more. A job that air-bends comfortably at 60 t could need 200 t or more to coin, so never size a machine from air-bending numbers if your process is bottoming or coining.