Sheet Metal Bend Allowance and Flat Length

Bend allowance and flat length

Bend allowance is the arc length along the neutral axis. Add K times thickness to the inside radius, then multiply by the angle in radians. An inside radius of 2 mm, thickness of 2 mm and K of 0.5 give 4.712389 mm allowance for a 90 degree turn.

Dimension convention

Enter only the two straight tangent lengths in the flat length tab. Straight sections of 30 and 40 mm give 74.712389 mm overall. To calibrate K, subtract these straight sections from a measured blank to obtain bend allowance. Verify K for the actual material, tooling and process.

Relate a bend test to the flat length

For an example bend, choose inside radius 3 mm, thickness 2 mm, K factor 0.4 and rotation from flat 60°. The neutral radius is 3 + 0.4 × 2 = 3.8 mm. The angle in radians is π/3, so bend allowance is 3.8π/3 ≈ 3.979351 mm. With straight tangent lengths of 20 and 25 mm, total flat length is 48.979351 mm. The example K factor is not a material recommendation.

Enter the same allowance in the mode for K from a test to recover the original factor. Entering the total measured length without first subtracting the straight portions overstates K. For a very small bend angle, small length measurement errors can cause a substantial change in the inferred factor. Consider measurement precision as well.

Bend allowance and bend deduction based on outside dimensions are different quantities. Convert outside flange dimensions to the proper tangent lengths before using them. For a part with several bends, count each arc and straight section once. The result is not a machine program. Material orientation, tooling and springback require manufacturing validation.

Frequently asked questions

How do you calculate bend allowance?

For inside radius 2 mm, thickness 2 mm, bend angle 90° and example K factor 0.5, bend allowance is 4.712 mm. Use the neutral axis arc length relation, verifying the actual K factor by shop trials for the material and tooling, since springback is excluded.

Calculate with these values