Springback in sheet metal: predicting and correcting it

What springback does to a stamped bend

When sheet is bent, part of the strain is plastic and stays, and part is elastic and recovers. The tool releases, the bend opens, and the finished angle is shallower than the angle the punch commanded. The inside radius also grows, because the elastic recovery reduces the curvature. A 90 degree form can finish at 88 degrees, and a 2.0 mm inside radius can finish nearer 2.1 mm.

On a single bend the error looks small. On a channel with four bends it accumulates, and the last flange can finish outside its position even though each bend is within its own angle tolerance. That is why springback is a stack-up problem as much as a forming problem. It also explains why a part that measures correctly on the bench can move after it is clamped into an assembly.

The drivers that matter on the drawing

The first driver is yield strength. A higher yield means the material resists plastic flow and recovers more of the applied strain, so high-strength steel and hard tempers spring back more than mild steel. The second is the elastic modulus. Steel sits around 200 to 210 GPa, stainless near 193 GPa, aluminium near 70 GPa and copper near 120 GPa. For the same strength, a lower modulus means less recovery.

The third driver is the radius to thickness ratio. A large radius bends a longer arc, and the elastic recovery acts over more length, so the angle loss grows. A tight radius localises the strain and reduces the recovery as a share of the total. The fourth is thickness: a thicker sheet is stiffer in bending, so the same tool force produces less curvature and a smaller recovery.

Tooling sits on top of those four. A punch that deflects, a die cushion that gives, or a gap that is too large all add to the apparent springback. Check the tool and the press before blaming the coil.

Estimating the recovery before the tool is cut

A full prediction needs the stress strain curve and the tool geometry, which is rarely available at the quotation stage. A useful planning estimate treats the elastic recovery as proportional to the yield strength and the bend radius, and inversely proportional to the modulus and the thickness. The relationship is enough to rank materials and to see which bends are sensitive.

For illustration, a 90 degree bend in 1.5 mm mild steel at an inside radius near one thickness might recover 1 to 2 degrees. The same geometry in a 1.5 mm stainless or a high-strength grade might recover 3 to 5 degrees. These are illustrative ranges, not design values, but they show why a tool built for mild steel cannot simply be used on a harder grade.

Use the estimate to set the starting overbend and to flag the bends that need a trial. Do not use it to sign off a drawing, because the real recovery depends on the tool, the press and the coil lot.

The correction methods and when each one fits

The simplest correction is overbend: the punch is set past the required angle so the part recovers onto the drawing. It costs nothing in tooling and suits a part with one or two bends at a stable radius. Its weakness is that the overbend is a setting, so it drifts when the material lot changes and it needs a disciplined setup record.

Bottoming forces the sheet against the die at the end of the stroke, which reduces the recovery by plastically deforming the bend zone. Coining goes further and deliberately thins the bend. Both give a more stable angle but need more press force and a stronger tool. A wipe die or a re-strike station can also correct the angle after the main form, at the cost of an extra station.

For a part that must hold a tight angle over a long run, the most reliable answer is often a feature rather than a force. A stiffening rib, a gusset or a small stake near the bend changes how the part behaves in service and can reduce the visible movement.

Material and thickness effects

Mild steel is the forgiving case. Its yield strength is modest, its modulus is high, and the recovery is small enough that a simple overbend holds across a lot. Stainless steel is less forgiving: it work-hardens as it forms, so the yield strength at the bend rises during the stroke and the recovery with it. Tool wear is also higher, which changes the radius over the life of the tool.

Aluminium has a lower modulus, so it springs back less than steel of the same strength, but its yield strength varies more between tempers and suppliers. Copper and brass are soft and recover little in the annealed condition, but a half-hard temper behaves quite differently. The temper callout on the drawing is therefore part of the springback specification, not a purchasing detail.

Thickness tolerance has a direct effect. A coil at the thin end of the tolerance bends to a larger radius for the same tool and recovers differently from a coil at the thick end. If the part is angle-sensitive, ask for the mill tolerance and check the first article from each lot, not just the first lot.

The correction loop on the first trial

Start with a trial blank and the estimated overbend. Form one part, release it fully, and measure the angle at the same points the drawing uses. Let the part settle for a few minutes before the final measurement, because some alloys continue to move after the tool opens. Record the angle, the radius and the coil lot.

Compare the measured angle with the target and calculate the gap in degrees. Add that gap to the commanded angle, re-set the tool and run three more parts. If the three parts agree, the setting is stable enough to record. If they scatter, the problem is not the average springback but the variation, and the cause is usually the material, the tool deflection or the strip location.

Lock the setting in the setup sheet with the coil lot and the date. Then put the bend on the in-process check plan at a frequency that matches the run length.

What belongs on the drawing

Give the bend angle with a realistic tolerance, the inside radius and the datum from which the angle is measured. Do not tighten the angle to compensate for springback. A tight angle tolerance on a bend that the tool cannot hold simply transfers the problem to the gauge, and the supplier will either add a re-strike station or quote a more expensive tool.

If the part is critical, state the functional requirement instead of a stack of angles. A flange that must sit flat against a mating face can be controlled by a gap or a flatness call at assembly, which is closer to the function than the angle at the bend.

• Bend angle, inside radius and the measurement datum
• Material grade and temper, including the thickness tolerance
• Which bends are functional and which are clearance
• Any feature that must sit flat after forming
• The inspection method for the critical angle