Bending Guide
Air Bending vs Bottoming vs Coining
Three ways to form the same bend, three very different tonnage requirements. Here's how they compare and when each one earns its keep.
All three methods use the same basic tooling — a punch pressing sheet metal into a V-die — but how far the punch goes changes everything downstream: tonnage, accuracy, springback and tooling wear. Choosing the right one is mostly a question of how tight and repeatable the finished angle needs to be, and how much tonnage you're willing to spend to get it.
| Method | Tonnage vs air bend | Springback control | Typical use |
|---|---|---|---|
| Air bending | 1× | Needs compensation | General fabrication, most jobs |
| Bottoming | ≈5× | Better suppression | Tighter tolerance, less angle variation |
| Coining | ≈5-10×+ | Strongest control | Sharp, precise corners; minimal springback |
Estimate air bending force first
Most jobs run as air bends — check the baseline tonnage before deciding whether bottoming or coining is worth the extra force.
Air bending
The punch forms the sheet against the V-die without pressing it all the way to the bottom — the final angle is set by how deep the punch travels, not by the die geometry itself. That's what makes air bending so flexible: one die set and one punch can produce a whole range of angles just by changing stroke depth. It needs the least tonnage of the three methods (see the full formula and worked example), which is why it's the default for most general fabrication work.
The trade-off is springback — the material relaxes back slightly after the punch retracts, so the punch angle has to overshoot the target to land on spec. See springback explained for compensation methods.
Bottoming
The punch presses the sheet fully against the die at the bottom of the stroke, which locks in the angle more precisely and suppresses springback more than air bending does. It typically needs around 5 times the tonnage of an equivalent air bend, because the machine is now working against the full resistance of the die rather than just forming against the V opening. Worth it when air bending's springback variation is too much for the tolerance on the drawing.
Coining
Coining goes a step further and permanently deforms the material to match the exact die radius under very high pressure — the material effectively "flows" into the die shape. It gives the tightest, most repeatable corners and the least springback of the three, but at a steep tonnage cost: 5-10 times air bending force, sometimes more for very small inside radii. It's usually reserved for parts where corner sharpness or angle repeatability genuinely can't be achieved any other way, since the tonnage (and tooling wear) cost is significant.
Choosing between them
- Default to air bending unless there's a specific reason not to — it's cheaper on tonnage, tooling wear and cycle flexibility.
- Step up to bottoming when air bending's springback variation is causing parts to fail angle tolerance.
- Reserve coining for parts where the drawing genuinely calls for a sharp, minimal-radius corner with tight repeatability — and confirm the machine has the tonnage to spare before committing.
Check your machine has the tonnage
Enter your bending method's multiplier alongside material and geometry to see where you land against your press brake's rating.
Frequently asked questions
How much more tonnage does bottoming need than air bending?
Roughly 5 times more, as a rule of thumb, because bottoming presses the sheet fully against the die rather than just forming it against the V opening.
How much more tonnage does coining need than bottoming?
Coining can need 5-10 times the tonnage of air bending, or more for very tight radii, since it permanently forms the exact die geometry into the material under high pressure.
Which method gives the least springback?
Coining, followed by bottoming. Both suppress springback far more effectively than air bending because the material is pressed into the die shape rather than relaxing back after the punch retracts.
Why is air bending used more than bottoming or coining?
It needs far less tonnage, one die set covers a range of angles just by adjusting punch depth, and it's flexible enough for most general fabrication work where extreme precision isn't required.