Springback in Bending: Prediction and Compensation

Every bent sheet metal part opens up slightly when the tool releases it. This is springback. If the die is built to the drawing angle, the part comes out wrong by that amount. With mild steel the error is a degree or two and easy to absorb. With stainless steel, hard aluminum tempers and high-strength steels it can be several degrees and vary from coil to coil. This article explains what controls springback, how to estimate it, and how to remove it in the die.

What happens in the bend

When sheet is bent, the outer fibres are stretched and the inner fibres compressed. Most of that strain is permanent (plastic), but a part of it is elastic. A band around the middle of the thickness is never stressed beyond the elastic limit at all. When the tool releases the part, the elastic strain recovers: the bend angle decreases and the bend radius becomes slightly larger.

The factors

The two ratios that matter most are yield strength divided by elastic modulus (the material) and R/t (the geometry).

Estimating springback

A commonly used first estimate relates the radius before release (Ri) to the radius after release (Rf):

• Ri ÷ Rf = 4·x³ − 3·x + 1, where x = (Ri × Y) ÷ (E × t)

Y is the yield strength, E the elastic modulus and t the thickness. The angle after release is then:

• Angle after = angle under load × (Ri + t/2) ÷ (Rf + t/2)

Example. Austenitic stainless steel 304, t = 1.0 mm, bent to 90° over Ri = 4.0 mm. E = 193,000 N/mm². The initial yield strength is about 300 N/mm², but the material work-hardens strongly, so the flow stress in the bend is taken as about 600 N/mm².

With the initial yield strength of 300 N/mm², the same formula gives only 1.5°, which is too low. The formula ignores work hardening, so always use the flow stress at the bend strain for materials that harden strongly. Treat the outcome as a starting value for the die, to be confirmed at try-out.

Typical values

Indicative springback for 90° bends in a V-die without bottoming:

These ranges are wide because strength varies within a grade. That variation is the real difficulty: a die can be adjusted to any constant springback, but not to one that changes with each coil.

Compensation in the die

For progressive and transfer dies running varying material, build in adjustment: shims under form blocks, adjustable cams, or a restrike station whose depth can be set. A die with fixed compensation is correct for only one coil.

Design choices that reduce the problem

1. Use the smallest bend radius the material tolerates. A smaller R/t leaves less elastic core. Check the minimum bend radius for the material and grain direction to avoid cracking.
2. Bend across the grain where the layout allows; the minimum radius is smaller in that direction.
3. Specify the material tightly when angles are critical: a narrower yield strength range from the supplier is often cheaper than scrap.
4. Tolerance the angle realistically. ±0.5° on a mild steel bracket is achievable in a good die; the same tolerance on a high-strength channel needs calibration features or a restrike.
5. Keep flanges long enough to be gripped and overbent. As a guide, the flange should be at least four times the thickness plus the bend radius.

Try-out procedure

Sources of variation in production

Common mistakes