Understanding Bend Allowance in Sheet Metal
When sheet metal is bent, the outside surface stretches and the inside surface compresses. The neutral axis — the line that neither stretches nor compresses — determines the true arc length through the bend. Bend allowance is that arc length, and it is the key to accurate flat pattern development.
Bend Allowance (BA)
Bend allowance is the length of the neutral axis arc through the bend. The formula is: BA = (π/180) × θ × (R + K × T), where θ is bend angle in degrees, R is inside radius, K is the K-factor, and T is material thickness.
Bend Deduction (BD)
Bend deduction is what you subtract from the sum of the outside flange lengths to get the flat pattern length. BD = 2 × OSSB − BA. If you dimension your part to the outside mold line, bend deduction gives you the flat pattern directly.
Outside Setback (OSSB)
Outside setback is the distance from the bend tangent point to the apex (where the outside surfaces would meet). For a 90° bend: OSSB = R + T. For other angles: OSSB = tan(θ/2) × (R + T).
Pro tip: When the inside radius equals the material thickness (R/T = 1), the K-factor is most predictable. Going below R/T = 0.5 risks cracking, especially in harder alloys. Always test bend a sample coupon for production runs.
K-Factor Reference Table by Material
| Material | Soft / Annealed | Half Hard | Full Hard |
|---|---|---|---|
| Aluminum (1100, 3003) | 0.33 | 0.35 | 0.38 |
| Aluminum (5052, 6061) | 0.35 | 0.38 | 0.42 |
| Mild Steel / CRS | 0.38 | 0.40 | 0.45 |
| Stainless Steel 304 | 0.42 | 0.45 | 0.50 |
| Copper / Brass | 0.35 | 0.42 | 0.46 |
| Titanium | 0.40 | 0.44 | 0.48 |
| Spring Steel | 0.45 | 0.48 | 0.50 |
K-factors vary with radius-to-thickness ratio, grain direction, and bend method. Values above are typical starting points for air bending with the grain. Always verify with test bends.