A compensation table that works on S235 is applied to S700, and the first parts come out several degrees off. The control is behaving exactly as programmed. What changed is the ratio between elastic recovery and plastic deformation, and that ratio moves with every coil, every die opening and every change in temper.
1. The practical problem
The failure is rarely a single bad part. It appears as a batch that drifts: the first sheet is close, later sheets from the same stack are under-bent, and the operator adds correction until the last parts are over-bent. Alternatively, parts from one coil are correct and parts from the next are not, with no change to the programme. A third form is a part that is correct along most of the bend but opens at one end, which usually indicates a thickness or hardness variation rather than a compensation error.
2. Failure mechanism
Springback is the elastic recovery of the bend after the punch is withdrawn. In air bending it scales approximately with yield strength over elastic modulus, modified by the ratio of inside radius to thickness. Because high-strength steels have a much higher yield strength, the elastic fraction of the total deformation is larger, and the recovery angle grows accordingly. The compensation model assumes a material state; when the actual yield strength is higher than the value in the table, the model under-predicts recovery. The second mechanism is geometric: the die opening sets the inside radius, and the inside radius sets the recovery, so a change of V opening changes springback even when the material has not changed at all.
3. Key parameters
Five parameters control the outcome: yield strength of the actual sheet, elongation, thickness and thickness tolerance, the V opening, and the punch radius. Mill certificates give a minimum or typical yield, not the value of the sheet in the machine, and batch-to-batch scatter on dual-phase and thermomechanically rolled grades is a normal part of the supply, not a defect. Thickness tolerance matters more than operators expect, because springback is sensitive to the radius-to-thickness ratio.
| Material group | Yield, indicative | Springback trend, air bend | Practical approach |
|---|
| Mild steel S235 | 235 MPa | Low | Library value, small correction |
| Structural S355 | 355 MPa | Moderate | Library value plus trial bend |
| Dual phase / S700 | Approx. 380–700 MPa depending on grade | High, scatter between coils | Trial bend per batch mandatory |
| Wear-resistant plate | Nominal values above 1000 MPa | Very high | Radius control, consider bottom bending within rated tonnage |
| Austenitic 304 | Approx. 215–275 MPa plus work hardening | Moderate but unstable | Trial bend, account for work hardening |
4. Diagnosis and calculation method
- Bend two pieces from every new coil above roughly 450 MPa yield and record the measured angle; use the deviation as a batch offset, not as a change to the master table.
- Check which variable the controller corrects, Y position or angle, because radius change and angle change are not proportional on hard grades.
- Verify the die opening actually fitted against the programme value; a substituted die is a common and invisible cause of a step change.
- Measure thickness at three points on the sheet and compare with the nominal used in the calculation.
- Confirm the Y axis resolution is fine enough for the correction being applied; a coarse axis cannot deliver a small angular adjustment reliably.
- Inspect the punch tip radius and die shoulders before blaming the material, since wear mimics a springback change.
5. Prevention
Treat springback on high-strength steel as a per-batch calibration, not a fixed constant. Store offsets against heat number or coil identity, keep the die opening chosen for the required radius rather than for tonnage convenience, and confirm the tonnage required at that opening stays inside the machine rating. On the machine side, fine Y resolution, stable hydraulics and per-material offsets in the control do most of the work. Our 160 tonne CNC hydraulic press brake is used for general high-strength work in this class, while thicker wear plate and heavy sections are better served by our 400 tonne servo-controlled press brake, which keeps tonnage reserve at the correct die opening.
6. FAQ
Is a bigger V opening better for hard materials?
It reduces tonnage but increases the inside radius and therefore the springback. Choose the opening from the required radius first, then check the tonnage against the machine rating; reversing that order is how machines get overloaded.
Can I just increase tonnage to remove springback?
Only up to the point of coining or bottoming, and only within the rated load. Beyond that the correct answer is a radius or material change, and pressing harder risks tooling damage and frame overload without improving the angle.
Why do two identical sheets behave differently?
Usually thickness tolerance, yield scatter within the delivery, or rolling direction relative to the bend line. All three are measurable, and checking them is faster than adjusting the programme by trial.
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