Learn about minimum bend radii for different materials and thicknesses. Understand how to prevent cracking, design for manufacturability, and select appropriate radii.
Introduction
The minimum bend radius is critical in sheet metal design. Bending below the tolerable radius causes cracking. At Fulei Metal, we help clients specify appropriate bend radii.
What Determines Minimum Bend Radius?
Material ductility, thickness, temper, grain direction, and edge condition all determine the minimum radius. Ductile materials tolerate tighter radii. Thicker materials need proportionally larger radii. Annealed tempers allow smaller radii. Bending across grain is safer.
Minimum Bend Radius by Material
Mild Steel
Rule: 0.8 times thickness across grain. For 1 mm: 0.8 mm. For 2 mm: 1.5 mm. For 3 mm: 2.4 mm. For 5 mm: 4.0 mm. For 10 mm: 8.0 mm.
Stainless Steel
Rule: 1.0 times thickness. Excellent ductility (40-60% elongation). For 1 mm: 1.0 mm. For 3 mm: 3.0 mm. For 5 mm: 5.0 mm.
Aluminum
Varies by alloy. Soft alloys (5005-O): 0.5-1.0 times thickness. Medium (5052-H32): 1.0-1.5 times. Hard (6061-T6): 1.5-3.0 times.
Galvanized Steel
Base metal: 0.8-1.0 times. To prevent coating cracking: 1.0-1.2 times.
Consequences of Bending Below Minimum
Cracking on the outside of the bend. Reduced strength from micro-cracks. Coating damage. Aesthetic defects.
Designing for Appropriate Bend Radii
Use standard radii (0.5, 0.8, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 6.0, 8.0, 10.0 mm). Design with material in mind. Consider grain direction. Add bend relief. Avoid multiple tight bends in sequence.
Testing Bend Feasibility
Always perform test bends for new materials, unusual thicknesses, or tight radii. Cut a sample, bend, inspect for cracking. Test both grain directions if unknown.
Common Mistakes
Specifying radius equals zero (impossible). Underestimating aluminum requirements. Ignoring grain direction. Not testing new materials.
Conclusion
Understanding and specifying appropriate minimum bend radii prevents cracking. At Fulei Metal, our experience enables us to advise on appropriate radii and produce bends meeting structural and aesthetic requirements.
Minimum Radius and the Die That Produces It
Below these values the outer fibre of the bend starts to crack. The relationship that makes this actionable is that air bending produces an inside radius of roughly 0.156 times the die opening, so a radius callout on a drawing is also a die choice.
| Material | Recommended minimum inside radius (90° bend) | Practical note |
|---|---|---|
| SPCC / mild steel | 0.5 × t up to 2 mm, 1 × t above | the usual shop default |
| SECC galvanized | 1 × t | a tighter radius can fracture the zinc layer at the outer fibre |
| SUS304 2B | 1 × t | 0.5 × t is possible but raises cracking risk when bending with the grain |
| Aluminium 5052-H32 | 1 × t | acceptable down to about 0.8 × t across the grain |
| Aluminium 6061-T6 | 2 to 3 × t | below this it usually cracks; consider forming in a softer temper |
| Aluminium 1100-O | 0 to 0.5 × t | can often be folded flat |
Converting that to tooling: to obtain an inside radius of 1 × t you need a die opening near 6.4 × t, and for 2 × t the opening moves to about 12.8 × t. That second figure is why a “generous” radius on thin material can become awkward — the wide opening needed produces a long flange requirement and lower forming force, which in turn makes the angle harder to hold consistently.
Frequently Asked Questions
The bend cracked on our prototype. Was the material wrong?
Usually the radius was below the minimum for that grade, or the bend ran parallel to the rolling direction. Both are drawing-stage fixes: increase the radius or rotate the nest so critical bends run across the grain.
Does a small radius make the part stronger?
It concentrates strain at the outer fibre, which is exactly where cracks start. Unless space forces the issue, a radius of about 1 × t is both easier to produce and more reliable in service.
Can you test whether our combination of grade and radius will work?
Yes. We form sample coupons from the actual material batch and inspect for cracking before committing to a production program — far cheaper than discovering it in a batch.
Questions about a specific part are usually faster to answer against the drawing — send it through the review route below.
Practical next steps. Start with bending capability and tooling for the drawing review, read choosing the die that sets the radius for process context, and if you are choosing between routes, how material choice changes bending and DFM review set out the alternatives side by side.
Once the radius is settled, the next number to fix is the flat pattern. Bend allowance and bend deduction works through the formulas, the K-factor behind them and a chart of 90 degree values, and includes a calculator for checking a tooling assumption before a program is released.