EngineerBenchCalculating sheet-metal bend allowance
A bent sheet's flat length is not simply the sum of its outside flange dimensions. Material follows a curved path through the bend. Bend allowance estimates the length along the neutral axis using inside radius, sheet thickness, bend angle, and K-factor; setback relates that curved region to the outside dimensions.
Use the expected angle convention
Enter the bend angle measured from flat as labeled in the tool. An included angle from a drawing may use a different convention. Use outside flange dimensions for this model and keep the radius and thickness in the selected units.
Calibrate K-factor with a real bend
The reverse calculation can estimate K-factor from a measured blank and finished geometry. It is useful for a specific material and tooling combination, not a universal material constant. Springback, tooling, and process variation still affect the final part; confirm a production blank with a test bend.
Tracing flat length through the bend
The neutral-axis arc length is bend angle in radians multiplied by inside radius plus K-factor times thickness. Degrees must be converted to radians in that relationship. Outside setbacks connect the straight flange dimensions to the curved region. Mixing an inside dimension from one flange with an outside dimension from the other makes the flat-length calculation inconsistent even if every number looks reasonable.
Keep bend allowance and bend deduction distinct. Allowance describes length through the bend; deduction adjusts the sum of the outside flange dimensions to the flat blank. Their values are not interchangeable. For a measured test bend, record blank length, finished flange dimensions, thickness, radius, and the angle convention before solving backward for K-factor. A poor measurement of one of those quantities can produce a misleading fitted factor. Use a result established with the same material, thickness, tooling, and bend process when planning similar parts, and confirm tolerances with the shop rather than relying on an apparently precise calculated blank length.
Formula
BA = θ(R + Kt); setback = (R+t)tan(θ/2); flat = A+B−2×setback+BA.