These are common starting points for air bending, not guarantees. Exact values depend on material grade, tooling and bend angle, and are checked on every drawing.
Inside bend radius
When sheet is bent, the inside of the bend is compressed and the outside is stretched. Bending too tightly can crack the outer surface, especially on harder materials. For mild steel, an inside radius roughly equal to the thickness is a practical default. Stainless steel and harder aluminium grades generally need a larger radius.
In air bending, the radius is set mainly by the width of the V-die opening. A common rule is a die opening of six to ten times the thickness, with eight times typical for mild steel. Giving a sensible inside radius on the drawing lets matching tooling be chosen.
Minimum flange length
The flange is the leg of metal on either side of the bend. If it is too short, it falls into the die opening and cannot be formed. A common guideline is a minimum flange of about 0.7 times the die opening. With a die of eight times the thickness, that is roughly five and a half times the thickness, so six times or more is a safe target.
Holes near bends
Holes too close to a bend line stretch into ovals and can tear. Keep holes at least 2.5 times the thickness plus the inside radius from the bend line. If a hole has to sit closer, it can be cut after bending, at extra cost.
Bend relief
Where a bend runs into a flat edge, the metal can tear at the corner. A small relief cut fixes this: at least as wide as the thickness and slightly deeper than the bend radius. When laser cutting and bending happen together, reliefs are added to the flat pattern as part of the job.
Material behaviour
- Springback: every bend opens slightly after the punch lifts. Stainless steel springs back more than mild steel, corrected by over-bending.
- Grain direction: bending across the rolling direction is less likely to crack, which matters most for tight radii.
- Aluminium grade: softer grades bend well, while hard tempers can crack on tight bends. State the grade on the drawing.
Worked example: a 3 mm mild steel bracket
| Feature | Rule of thumb | Value for 3 mm MS |
|---|---|---|
| Inside bend radius | about 1 × thickness | about 3 mm |
| V-die opening | about 8 × thickness | about 24 mm |
| Minimum flange length | about 0.7 × die opening | about 17 to 18 mm |
| Hole distance from bend line | 2.5 × thickness + radius | about 10.5 mm |
| Bend relief width | at least 1 × thickness | 3 mm or more |
Most brackets, channels and enclosure panels designed to these values bend cleanly first time. See the sheet metal bending service or send a drawing from the contact page.