Bend allowance, bend deduction and the flat blank length you need to cut before bending sheet metal.
The K-factor is the position of the neutral axis as a fraction of the sheet thickness: K = t ÷ T, where t is the distance from the inside face to the neutral axis and T is the thickness. It is always between 0 and 0.5 for practical bends, and it moves toward 0.5 as the inside bend radius gets larger relative to the thickness.
| Material (air bending) | R ≤ T | T < R ≤ 3T | R > 3T |
|---|---|---|---|
| Soft: aluminium, copper, brass | 0.33 | 0.40 | 0.50 |
| Medium: mild steel | 0.38 | 0.43 | 0.50 |
| Hard: stainless steel | 0.40 | 0.45 | 0.50 |
Actual K-factors depend on the tooling, the die opening, the grain direction and the exact alloy and temper. For accurate flat patterns, bend a test strip on the same press brake and tooling, measure the flanges, and back-calculate K with the formulas below. Use that value for production parts.
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