Calibrate K from a bend coupon

Measure the same geometry the model uses

Measure the original blank length L before bending. After unloading, measure thickness t, final inside radius Ri, bend rotation θ and outside legs A and B. The legs extend to the virtual sharp intersection of the outside faces, not to the tangent points. θ is the rotation from flat, so the included angle is 180° − θ.

Assumptions: one uniform-thickness, constant-radius bend, straight legs, no trimming after the blank measurement. The following are invented teaching measurements, not production trial data.

Calculate K from the measured blank

1. Outside setback: OSSB = (Ri + t) tan(θ/2) = (2 + 2) tan(45°) = 4.00 mm.
2. Bend allowance: BA = L − A − B + 2 OSSB = 96.40 − 50 − 50 + 8 = 4.40 mm. The two straight portions are each 50 − 4 = 46 mm.
3. Neutral-axis model: BA = θrad (Ri + Kt), hence K = (BA/θrad − Ri)/t.
4. Result: K = [4.40/(π/2) − 2.00]/2.00 = 0.40056… ≈ 0.4006, dimensionless.

Forward check: BD = 2 OSSB − BA = 8.00 − 4.40 = 3.60 mm; L = A + B − BD = 96.40 mm. Using rounded K = 0.4006 gives 96.4001 mm; extra digits do not imply measurement accuracy.

Enter these inputs in the linked bend calculator and set “Measured blank length” to 96.4. Its three-decimal display should show K ≈ 0.401.

Keep the calibration tied to its process

• Record grade, temper, batch, thickness, rolling direction, punch, die opening, bending method and measured geometry with the result.
• Use the unloaded inside radius and angle, not automatically the punch radius or programmed angle. A fitted K is not a springback prediction.
• Repeat the trial and validate a separate part before releasing a flat pattern. Do not reuse this example's K for another material or tool setup.
• If the result falls outside this calculator's 0–0.5 range, check units, leg definitions, angle convention, radius and measurements before changing the model. Do not clip the result into range.