Bend allowance & flat pattern
Sheet metal bend allowance, outside setback, bend deduction and flat pattern length from thickness, inside radius, K-factor and bend angle. Inches or millimeters.
Bend allowance & flat pattern
Same math I use for laser-cut and press brake parts. K-factor 0.44 is a good start for air-bent 5052 and mild steel with a bottom V of about 8x material thickness.
BA = (π/180) · angle · (R + K·T) is the arc length of the neutral axis.
OSSB = tan(angle/2) · (R + T) is the outside setback from the bend line to the mold line.
BD = 2·OSSB − BA and Flat = A + B − BD for two flanges dimensioned to the outside.
K is where the neutral axis sits as a fraction of thickness. 0.33 is a tight bend, 0.50 is pure stretch with no shift. Air bending on a V about 8x thickness lands near 0.44.
Real K-factor depends on tooling, V-die width and material. Bend a test coupon and back-calculate before you commit a production flat pattern.
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