Sheet Metal Design Guide: Holes, Bend Radii and Flanges
Numeric rules for hole spacing, bend radius and flange length that keep parts manufacturable
Sheet metal parts fail or cost extra when a design ignores three basic rules: how close a hole can sit to a bend line, what bend radius the material can form without cracking, and how short a flange can be before the press brake tooling cannot reach it. Following these numeric limits at the design stage avoids rework and keeps quoted prices predictable.
This guide covers hole-to-bend distance, minimum bend radius by thickness, and minimum flange length, with a worked example for a typical bracket.
Hole distance from a bend line
A hole positioned too close to a bend line distorts during forming, becoming oval instead of round, or the bend line itself deflects around the hole. As a rule, keep the edge of any hole at least 2.5 times the material thickness away from the inside of a bend line, and further for thicker material or tighter bend radii.
For a 2 mm sheet, that means roughly 5 mm minimum clearance; for 4 mm sheet, roughly 10 mm.
| Material thickness | Minimum hole-to-bend distance |
|---|---|
| 1 mm | 2.5 mm |
| 2 mm | 5 mm |
| 3 mm | 7.5 mm |
| 4 mm | 10 mm |
Minimum bend radius
Every sheet material has a minimum inside bend radius below which it cracks or excessively thins at the bend. As a general guide, mild steel S235JR can be bent to roughly 1x material thickness, aluminium 5754 needs roughly 1.5-2x thickness due to lower ductility, and stainless 304 (1.4301) can typically match mild steel at around 1x thickness in the annealed condition.
Specifying a bend radius below the material's minimum forces the supplier to either reject the design or bend it with visible cracking risk at the outer surface.
| Material | Minimum bend radius (approx.) |
|---|---|
| Mild steel S235JR | 1x thickness |
| Stainless 304 (1.4301) | 1x thickness |
| Aluminium 5754 | 1.5-2x thickness |
| Aluminium 6061-T6 | 2-2.5x thickness |
Minimum flange length
A flange (the short leg after a bend) shorter than roughly 4 times the material thickness plus the bend radius cannot be reliably formed, since the press brake tooling needs clearance to grip the flange without marking or slipping. For 3 mm steel with a 3 mm bend radius, this puts the practical minimum flange length at roughly 15-18 mm.
Flanges shorter than this minimum are either rejected at quoting or require a secondary forming operation, adding cost and lead time.
Worked example: an electrical enclosure bracket
A bracket in 2 mm mild steel with a 90-degree bend needs: bend radius at least 2 mm (1x thickness), any mounting hole at least 5 mm from the bend line, and any flange at least 8-10 mm long. A design with a hole only 2 mm from the bend line or a flange of 5 mm would be flagged during quoting and need revision before production, adding a day or more to the quoting cycle.
Design tips
- Keep holes at least 2.5x material thickness away from the inside of any bend line.
- Use a bend radius of at least 1x thickness for mild steel and stainless, 1.5-2x for aluminium.
- Keep flange length at least 4x thickness plus the bend radius to allow press brake tooling clearance.
- Avoid slots or cutouts crossing a bend line; they concentrate stress and can tear.
- Add relief cuts at the ends of bend lines that stop short of a part edge to prevent tearing.
- Where two bends are close together, leave at least 3x thickness of flat material between them.
- Specify material grade and temper (e.g. 5754-H111 vs 6061-T6) since minimum bend radius varies by temper.
Applications
| Industry | Example parts | Typical material |
|---|---|---|
| Electrical | Panel enclosures, brackets | Mild steel, 304 stainless |
| HVAC | Duct brackets, mounting plates | Galvanised steel DX51D |
| Furniture | Bent frames, legs | Mild steel, aluminium |
| Machinery OEM | Guards, covers | S235JR |
| Solar | Mounting brackets | Galvanised steel, aluminium |
| Signage | Bent letter housings | Aluminium 5754 |
Pricing and lead time
Sheet metal part cost is driven by three factors: material grade and thickness, number and complexity of bends, and quantity ordered. Standard laser-cut and bent parts run 3-5 working days from file approval; parts needing design revision for hole spacing or flange length add 1-2 days to the quoting stage before production begins.
Frequently asked questions
As a rule, keep the hole edge at least 2.5 times the material thickness away from the inside of the bend line; closer than that risks the hole becoming oval during forming.
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