What is the minimum bend radius?
The minimum bend radius is the tightest inside radius the material can take before the outer surface cracks. It depends on the material's elongation, its temper and the direction of the bend relative to the sheet's rolling direction.
What minimum bend radius should I allow by material?
As an industry reference, soft mild steel and annealed stainless steel tolerate an inside radius of about half the thickness, while hard tempers, high-strength steel and some aluminium alloys need two to four times the thickness.
| Material | Indicative minimum inside radius |
|---|---|
| Mild steel, soft temper | ≈ 0.5 × t |
| Mild steel, hard temper | ≈ 1.0 × t |
| Stainless steel, annealed | ≈ 0.5 × t |
| Stainless steel, work hardened | 1.0 – 2.0 × t |
| Aluminium, soft (O temper) | ≈ 0.5 × t |
| Aluminium 5052-H32 | ≈ 1.0 × t |
| Aluminium 6061-T6 | 2.5 – 4.0 × t |
| Copper, soft | ≈ 0.5 × t |
| High-strength steel | 1.5 – 3.0 × t |
Why does the rolling direction matter?
Sheet has higher ductility across the rolling direction than along it. Bending so the bend line runs perpendicular to the rolling direction puts the stretch across the grain, which tolerates a tighter radius than bending along it.
- Best: bend line perpendicular to the rolling direction.
- Worst: bend line parallel to the rolling direction, where cracking starts first.
- Nesting: plan the layout so tight-radius bends cross the grain.
- Where the part shape forces a bad orientation, increase the radius or change the material.
Does the air bending radius from the die respect the minimum?
Not automatically. Air bending produces an inside radius of roughly 0.16 times the die opening, whatever the material, so a die chosen purely for tonnage can produce a radius below the material's cracking limit.
| Step | Value | Comment |
|---|---|---|
| Eight times rule opening | V = 24 mm | For general air bending |
| Resulting inside radius | ≈ 0.16 × 24 = 3.8 mm | What the die produces |
| Material minimum | 2.5 – 4.0 × 3 = 7.5 – 12 mm | Cracking limit |
| Conclusion | Radius too tight | Widen the die or change the temper |
What is sharp bending?
Sharp bending is when the punch tip radius is small enough that it digs a crease into the inside surface instead of forming a smooth arc. The crease weakens the bend and makes the angle less predictable.
A common rule of thumb is that the punch tip radius should be at least about 63 percent of the material thickness for a normal bend, and larger when the material is thick or high strength.
Related technical pages
V-die opening selection chart
The opening that sets the radius in air bending.
Bend allowance guide
How the resulting radius feeds into the flat pattern.
Radius dies
Dies for large-radius bends, where cracking is not the constraint.
Acute punches
Narrow-tip punches and the loads they impose.
Next step
Send the material grade, thickness and the radius on the drawing and we will confirm whether the radius is achievable and which die produces it.
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