Aluminium bend allowance calculator
Bend geometry
Flange lengths
Bend allowance is the arc length along the neutral axis. Add it to both flange lengths to get the blank you cut.
BA = π (R + K × t) × θ / 180°Flat blank = f1 + f2 + BA
| Inside radius / thickness | Typical K-factor |
|---|---|
| R < 2t (tight) | 0.33 |
| R = 2t – 4t | 0.40 |
| R > 4t (generous) | 0.44 |
| Annealed (O) temper | 0.50 |
- Keep inside radius at or above the plate thickness to avoid surface cracking on the outside of the bend.
- Bend lines should run across the rolling direction where possible; bending parallel to rolling direction cracks more easily.
- For pressure-boundary parts, check the class rule for minimum bend radius before you set the tooling.
People also ask
What is the K-factor measuring?
How far the neutral axis sits from the inside face, as a fraction of thickness. It shifts with the radius-to-thickness ratio and with temper.
Why does my part come out short?
Usually springback: the press relaxes and the bend opens slightly, so the part consumes more material than the ideal angle suggests. Over-bend by 2–4°.
Can 5083 H116 be bent at all?
Yes, with care. The practical minimum inside radius is about 1.5–2 × thickness for transverse bends. Tight radii on H116 crack at the outer fibre.
What is the difference between bend allowance and bend deduction?
Allowance adds material to the neutral-axis length; deduction subtracts the material that overlaps at the corner. Same geometry, two bookkeeping conventions.
Should I anneal before bending?
Only for very tight radii. Annealing a finished hull part removes the H116/H321 temper that gives it stress-corrosion resistance, so it is not done on classified plate.
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