absurdtools

Press Brake Tonnage & Setup

Whether your brake and tooling can make the part — tonnage, tooling limits, off-centre derating, and the minimum flange that stops most jobs first.

The part

to the outside of the bend — blank to skip the check

The machine and tooling

—
Force needed—tonnes per metre
Total on this bend—
Machine used—
Tooling used—

The flange you cannot go below

A flange has to bridge the die opening and still sit on both shoulders. Since the die is chosen from the thickness, every thickness carries a floor — and it is much higher than most people expect on plate.

What actually stops a job

The flange is usually the limit, not the machine. A flange must bridge the die opening and still have material resting on both shoulders. Below about 0.65 of the vee width it drops into the die instead of bending, and no amount of press capacity changes that. Because the die opening is chosen from the thickness, every thickness carries a minimum flange: about 39 mm on 6 mm plate, about 94 mm on 12 mm. Designers draw 20 mm flanges on 6 mm plate constantly, and the part cannot be made on a brake however big the brake is.

And the tooling gives out before the machine does. A press brake is rated in tonnes; tooling is rated in tonnes per metre, so a short bend concentrates the load into a short length of punch. Coining 3 mm steel needs around 180 t/m, comfortably past the 100 t/m of ordinary precision tooling — while a 150-tonne machine doing a 300 mm bend sees a fraction of its own rating and reports nothing wrong. The punch is what fails, by brinelling or cracking, and it does not warn you first.

  • Bend length matters as much as thickness. Halving the length more than doubles the thickness a machine will take — force per metre works out roughly linear in thickness once the die opening scales with it, and the conventional die opens up further on thick material. A small brake handles surprisingly thick plate in short lengths.
  • Off-centre loading derates the machine. The rating assumes the load is centred between the cylinders. Put it to one side and the near cylinder takes more than half, so you can overload one side while the total sits comfortably inside the plate rating. The symptom is a bend angle that varies along its own length.
  • A tighter die is the usual escape from a short flange — it lowers the minimum flange and tightens the radius, at the cost of more tonnage. Which is the trade this page exists to show you, because those three move together and you cannot change one alone.
  • Coining and bottoming cost four to nine times the force. That multiplier is what buys the repeatability and the punch-controlled radius, and it is also the fastest way to exceed a tooling rating on a part the machine could easily air bend.
  • Leave headroom. These figures assume a sharp punch, a die in good condition, and material at its nominal strength — and real sheet runs above its specified minimum more often than below. A machine at its rating all day is a machine wearing out.

How to use

  1. Enter the material, thickness and bend length.
  2. Enter the shortest flange on the part — this is what usually blocks a job.
  3. Set your machine capacity and tooling rating.
  4. Check which limit you hit first, since the fix is different for each.

Frequently asked questions

Why can I not bend a short flange on thick plate?

Because the flange has to bridge the die opening and still rest on both die shoulders. Below roughly 0.65 of the vee width it drops into the die instead of forming, and no amount of press capacity changes that. Since the die opening is chosen from the thickness — around eight times it on thin material and more on thick — every thickness carries a minimum flange. On 6 mm plate that is about 39 mm and on 12 mm about 94 mm, which is far above what people routinely draw.

What can I do if my flange is too short?

Three things, in rough order of preference. Use a smaller die opening, which lowers the minimum flange but raises the tonnage and tightens the formed radius. Bend it as part of a longer flange and trim it afterward, which costs a second operation but keeps the tooling standard. Or change the process — a folder, a press tool or a welded corner does not have this constraint at all. Buying a bigger press brake does nothing whatsoever, which is the point worth internalising.

Why does my tooling break when the machine says it is fine?

Because they are rated in different units. A press brake is rated in tonnes; tooling is rated in tonnes per metre of punch length. A short bend concentrates the whole load into a short section of punch, so you can be at 180 per cent of a tooling rating while the machine sits at 30 per cent of its own and reports nothing. Coining 3 mm steel is a good example: it needs around 180 tonnes per metre against the 100 that ordinary precision tooling is built for.

How does bend length affect what my machine can do?

Enormously, and more than most people expect. Halving the bend length more than doubles the thickness a machine will take. The reason is that force per metre works out roughly linear in thickness rather than squared, because the conventional die opening is itself a multiple of thickness and the squared term largely cancels. On top of that the die rule opens up further on thick material, making it disproportionately cheaper. A small brake handles surprisingly thick plate in short lengths.

Does it matter where along the bed I bend?

Yes. A machine rating assumes the load is centred between its two cylinders. Bend off to one side and the near cylinder carries more than half the load, so you can overload one side while the total sits comfortably inside the plate rating. The usual symptom is a bend angle that varies along its own length, which people then chase as a crowning or backgauge problem. Centre the work where you can, and take the specific limit from the machine manual.

What is the difference between air bending, bottoming and coining?

How far the punch goes. Air bending stops with the sheet spanning the die shoulders, so the die opening sets the radius and one set of tooling covers many angles. Bottoming presses the sheet into the vee, and coining forces it against the punch face so the punch sets the radius. The gain is repeatability and a tighter radius; the cost is roughly four times the force for bottoming and nine for coining, which is the fastest way to exceed a tooling rating.

How accurate are these tonnage figures?

Close enough to plan with and not close enough to design to. They assume a sharp punch, a die in good condition and material at its nominal strength, and real sheet runs above its specified minimum more often than below. Tooling wear raises the force needed, and a machine run at its rating all day is a machine wearing out. Treat the numbers as a starting point, leave headroom, and check anything marginal against the machine and tooling documentation.

Is this the same as a bend allowance calculator?

No, they answer different questions and you generally need both. A bend allowance calculator tells you what size to cut the flat blank, using the K-factor and the bend deduction. This one tells you whether the machine and tooling can form the part at all. A part can have a perfectly correct flat pattern and still be impossible to make because a flange is shorter than the die opening allows, which is why the flange check here is the first thing it looks at.

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