Sheet metal bend

Bend allowance and bend deduction for a sheet metal flat pattern.

Inputs

Material thickness

Inside bend radius

Bend angle

Up to 179°. A hem (180°) has no outside setback, so use the bend allowance only.

K-factor

Where the neutral axis sits, 0.3–0.5. Often 0.33–0.45 for aluminum; measure a test bend.

Result

Bend deduction

0.1564

Bend allowance: 0.2036
Outside setback: 0.18

How it’s calculated

Formula

BA = \theta \, (R + K t)

OSSB = \tan(\theta/2) \, (R + t), \qquad BD = 2 \, OSSB - BA

With your numbers (converted to SI units)

BA = 1.571\,\text{rad} \times (0.002286 + 0.44 \times 0.002286) = 0.005171\,\text{m}

OSSB = \tan(0.7854\,\text{rad}) \times (0.002286 + 0.002286) = 0.004572\,\text{m}

BD = 2 \times 0.004572 - 0.005171 = 0.003973\,\text{m}

What it means

Bending stretches the outside of the sheet and compresses the inside; the neutral axis in between keeps its length. Bend allowance is that arc length. To lay out a flat pattern from outside flange dimensions, add the flanges and subtract the bend deduction for each bend.

Assumptions

θ is the bend angle, not the included angle: a 135° included angle is a 45° bend.
K-factor depends on material and tooling; check a test bend.

Learn the concepts behind this on resources.simbotics.org · All formulas