How this calculator works
Bend allowance is the neutral-axis arc length in the curved region. K is the neutral-axis distance from the inside surface divided by thickness. Use a trial bend for the actual material, tooling and process rather than treating K as a universal material constant.
Formula
BA = theta(Ri + KT), where theta is in radians; K = (BA/theta - Ri)/T; neutral-axis offset = KT.
Worked example
For T = 2 mm, Ri = 3 mm, K = 0.4 and a 90 degree rotation, BA = (pi/2)(3+0.8) = 5.96903 mm.
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Frequently asked questions
Do I enter the finished inside angle?
Enter the rotation from flat. A 90 degree corner has a 90 degree rotation; a 135 degree inside corner has a 45 degree rotation.
Is bend deduction the same as allowance?
No. Deduction also involves the outside setbacks and the way the flanges are dimensioned.
Why reject a K-factor outside zero to one?
It would place the neutral axis outside the sheet in this simple model. Check the measured bend allowance and angle convention.