In short: released after bending, sheet opens back 1 to 3° in mild steel, 2 to 5° in stainless, up to 8° in some heat-treated aluminium alloys. Springback is compensated by overbending, by coining in the bottom of the die, or by correcting the programmed angle. It depends on the material, the thickness and the V width, never on one setting valid everywhere.
What happens inside the sheet
As the punch descends, the material undergoes two deformations at once. A plastic deformation, permanent, which makes the bend. And an elastic deformation, temporary, exactly like a spring being compressed.
When the press lifts, the plastic part stays, the elastic part releases. The bend opens slightly. It is not a machine fault: it is a property of the material, and no press anywhere escapes it.
How much, concretely
| Material | Typical springback on a 90° bend |
|---|---|
| Mild steel S235 | 1 to 2° |
| S355 steel | 2 to 3° |
| 304 stainless | 2 to 5° |
| 5754 aluminium | 2 to 4° |
| 6060 T6 aluminium | 5 to 8° |
Three factors move those values, and you need to know them to avoid trial and error.
Yield strength. The stronger the material, the more energy it returns on release. That is why stainless and heat-treated alloys open more than mild steel: not because they bend badly, but because they are more elastic.
The radius-to-thickness ratio. A tight bend plastically deforms a large share of the section, so it opens little. A very open bend stays largely in the elastic range, so it opens a lot. At equal material, an 8 mm radius on 2 mm sheet opens more than a 2 mm radius.
The V width. It drives the radius produced, and therefore springback indirectly. Changing die changes springback, even at an identical programmed angle. It is the most frequent cause of a run that starts right and ends wrong after a tool change.
The three ways to compensate
Overbend
The simplest: you program an angle tighter than the target and the sheet opens back to the right value. For a 90° in stainless that springs 3°, you program 87°.
The advantage is that it needs no special tooling. The drawback is that it assumes you know the springback in advance: on a new material or a different batch, you go by test bend.
Coin in the bottom of the die
The punch crushes the sheet into the bottom of the V, plastically deforming the whole section at the bend. Springback becomes near zero and repeatability is excellent.
The price is force: coining needs three to five times the tonnage of air bending. On thick sheet and a long part, the press does not always follow. Our bend radius chart gives the air bending tonnage: multiply by four to estimate a coining operation before promising a lead time.
Correct in the control
Recent presses accept a correction table by material and thickness, sometimes fed by real-time angle measurement. It is the cleanest solution, provided somebody keeps the table up to date.
In practice that table often lives in a notebook beside the machine, and it disappears with the setter who wrote it. If that is your situation, the first gain is not technical: it is writing it up somewhere it survives.
Why it ends in a dispute
Springback does not show on the first part, it shows at assembly. A box whose four bends each open by 2° does not close. A coping whose end returns are too open does not fit its housing.
And at that point the discussion turns on the angular tolerance on the quote. If it is not there, the workshop pays for the rework.
Write the angular tolerance on the quote. ± 1° is a common shop value in air bending; ± 0.5° demands coining or automatic correction, and therefore a different price. A customer who signed a ± 1° cannot demand 90.0°.
The effect on the flat length
This is the point most often forgotten. If you overbend to 87° to obtain 90°, the flat length is no longer quite that of a 90° bend: the neutral fibre length follows the angle actually formed during bending.
The difference is small on one bend, but it accumulates. On a part with six overbent bends it becomes visible on the overall dimension.
The practical rule: calculate the flat length on the final angle, the one on the finished part, and treat overbending as a machine setting rather than a dimension. Our bend allowance calculator works on the final angle, which is the right default.
What a configurator should do about it
Nothing automatic, and that is worth saying. Springback depends on your tooling and your material batch: no software can guess it for you.
What it can do is apply your correction table, and refuse an angular tolerance your workshop cannot hold. That is what Koventor does: the tolerance announced to the customer is the one you declared, not a generic value that commits you without your knowing.