Air bending vs bottoming vs coining on a press brake
Tonnage charts assume a bend method, and the method changes the answer by multiples, not percentages. Air bending, bottoming and coining are three different processes on the same machine — here is what each does to force, radius and your machine spec.
The three methods side by side
| Air bending | Bottoming | Coining | |
|---|---|---|---|
| Sheet contact | Punch tip + die shoulders only | Full contact with punch and die | Punch penetrates the sheet |
| Relative force | 1x (baseline) | Several times air bending | Highest of the three |
| Angle control | Stroke depth + springback correction | Mechanical, springback largely absorbed | Mechanical, very consistent |
| Inside radius | Set by punch tip, predictable | Set by tooling contact | Tight, material thinned at bend |
| Tooling wear | Lowest | Moderate to high | High |
| Best for | Most production work | Thick plate, tolerance-critical parts | Very tight radii, small flanges |
Why the method decides the machine you buy
Every tonnage calculator — includingours — asks which method you are using, because the answer moves the force requirement by multiples. The practical sequence:
- List your part families and decide the method for each. Most shops are 80-90% air bending with bottoming for thick plate.
- Find the worst case across all families: thickest material, longest bend, smallest V-opening, hardest alloy, and the heaviest method.
- Size the machine to that worst case with margin — not to the average part and not to the cheapest method.
Buying a machine that air-bends your average part but cannot bottom your worst one is the classic under-specification. The tonnage guide works through the full calculation.
Springback: the reason the method matters at all
Every material springs back after the punch releases, and the amount depends on the alloy and its temper, not on your machine. Air bending leaves the springback in the part, so the machine or the operator has to over-bend to compensate. Bottoming physically works the material past its elastic limit at the bend, which absorbs most of the springback. Stainless springs back noticeably more than mild steel — the stainless bending guide covers the compensation in detail.
Minimum flange: the constraint people forget
Each method imposes a minimum flange length, driven by the V-opening and the punch geometry. Too short a flange and the sheet falls into the die instead of forming. The V-opening you need for your thickness therefore sets a floor under your part geometry — which is why the die choice comes before the tonnage question, not after it.
Hemming and multi-bend work
Hemming — folding a flange back on itself — needs a hemming die and considerably more force than a single bend, and it is the one operation where air bending is not an option. If hemming is part of your product mix, it belongs in the worst-case calculation even if it is a small fraction of your bends.
A worked way to decide
- Thin to medium sheet, most flanges: air bend. Lowest force, lowest tooling wear, most radius choice.
- Thick plate where angle tolerance matters: bottom. Accept the force and tooling cost.
- Very tight radius or tiny flanges: coin, and budget the tonnage honestly.
- Mixed shop: size for bottoming your worst plate part; you will air-bend everything else on the same machine with margin to spare.
Once you know the method, the V-die selector andtonnage calculator turn it into numbers you can buy against.
Still not sure what your power will support?
Tell us your supply voltage, available breaker size and the thickest material you need to bend. We will tell you straight whether single phase will do it.