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.
Air bending, bottoming and coining compared
| Method | Tool contact and angle control | Force and springback | Practical use |
|---|---|---|---|
| Air bending | Three-point contact; punch depth sets the angle. | Lowest force; compensate for elastic recovery. | Flexible angles from one compatible tool set. |
| Bottoming | Sheet seats against die faces; tooling angle matters. | Higher force; generally less springback. | Repeat angles with matched tools and verified capacity. |
| Coining | High pressure plastically forms the tool profile into the bend. | Highest force; least springback, not necessarily zero. | Dedicated tooling where the specified geometry justifies the load. |
Force multipliers are estimates, not tooling ratings. Check concentrated load limits and the supplier’s required force for each method.
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
What is the difference between air bending and bottoming?
Air bending supports the sheet at three contact points and controls angle through punch depth. Bottoming seats the sheet against the die faces, uses matched tooling and needs more force.
Does coining eliminate springback?
Coining generally produces the least springback because it plastically forms the bend under high pressure. Do not assume zero recovery; confirm the unloaded angle on a trial part.
Can I switch from air bending to bottoming with the same tools?
Only if the tooling and machine supplier approve the geometry and load. Bottoming needs compatible punch and die angles and substantially more force; an air-bending setup is not automatically suitable.