How to calibrate a K-factor from a trial bend

Record the setup before selecting K

Use a coupon made with the intended material and process. Record blank length L, actual thickness T, formed inside radius R, and outside flange dimensions A and B extended to the virtual sharp. Also record material, rolling direction, tooling, machine, and angle turned from flat θ. This guide covers one right-angle bend in a uniform-thickness sheet.

K is the neutral-axis offset from the inside surface divided by thickness: a parameter of the unfolding model for that setup.

Work backward from deduction to K

1. Use mm throughout. For θ = 90°, θrad = π/2.
2. Calculate BD = A + B − L.
3. Calculate SB = (R + T) × tan(θrad/2), then BA = 2 × SB − BD.
4. Calculate K = (BA/θrad − R)/T. Keep intermediate precision; round the reported value to suit the measurements.

An illustrative coupon

These assumed measurements are not a recommended material setting. Substituting the unrounded K into BA = θrad(R + KT) returns BA = 6.000 mm and L = 96.000 mm. This checks arithmetic, not predictive accuracy on another coupon.

Validate independently, then use the tool

Test a separate coupon made under the same conditions. Compare predicted and measured flat length against the drawing and process requirements. SheetMetal Master includes bend and K-factor calculations to help check the inputs and unfolding approach; the actual parameters still come from validated shop measurements. You can first check the equations with our single-bend calculator.

If K looks implausible, check the dimension convention, angle, formed radius, and measurement errors. Do not simply clamp the result to a familiar value.

Scope and references

Revalidate after changes to material, thickness, tooling, or bending method. This geometric model does not automatically account for springback or local thinning. See Autodesk’s trial-bend K-factor guidance and the definition in Fusion sheet metal rules.