What Bend Allowance means
Bend allowance is the arc length measured along the neutral axis between the two tangent points of a bend. Because the metal on the inside of a bend compresses and the metal on the outside stretches, the flat blank cannot simply be the sum of the finished leg lengths. Bend allowance is the correction that lets a flat pattern unfold to the right finished size.
Why it matters on the shop floor
On the shop floor this is the number that decides whether a folded part measures 100 mm or 100.6 mm when it comes off the press brake. Get it wrong by a fraction of a millimetre on a part with four bends and the error compounds across the part — which is why the flat pattern is calculated from the tooling and material actually used, not from a generic table.
Formula
BA = 2π × (R + K × T) × (θ / 360)R = inside bend radius · K = K-factor · T = material thickness · θ = bend angle in degrees
What to remember
- Bend allowance grows with both radius and thickness; a 2 mm sheet with a 2 mm inside radius needs roughly twice the allowance of a 1 mm sheet at the same radius.
- The K-factor is the only unknown; it depends on the tooling, the material and the bending method (air bending vs bottoming).
- The complement of bend allowance is bend deduction — the amount subtracted from the sum of the outside dimensions to reach the same flat length.