Sheet Metal Bending Design Guidelines
Inside bend radius, minimum flange length, hole-to-bend distance, bend relief and springback: the core rules for manufacturable sheet metal parts.
Table of contents 7 sections
Sheet metal parts can look simple on paper, yet ignoring a few basic bending rules can make them impossible to produce or lead to unexpected costs.
In short: keep the inside bend radius at least equal to the sheet thickness, avoid very short flanges, keep holes away from bend lines, add bend relief where needed and use the same radius on all bends.
Inside bend radius
When sheet is bent, the outer surface stretches and the inner surface compresses. If the radius is too small, the outer surface can crack. As a general rule, keep the inside bend radius at least equal to the sheet thickness (1t). Some aluminum tempers and high-strength steels need a larger radius.
Using the same inside radius on every bend avoids tool changes on the press brake and reduces cost.
Minimum flange length
The bent edge (flange) must sit securely on the V-die. If it is too short, it drops into the die and cannot be bent accurately. Minimum flange length depends on the V-die opening; in practice, avoiding flanges shorter than about 3–4 times the sheet thickness is a good starting point. Confirm the exact value against the manufacturer’s tooling.
Hole-to-bend distance
Holes too close to a bend line stretch into an oval during bending. Keep the hole edge at least 2–3 sheet thicknesses (plus the bend radius) away from the bend line. If the hole will be made after bending this rule does not apply, but that is an extra operation.
Bend relief
If a bend does not run all the way to the edge of the part, material can tear or deform where the bend line ends. A small relief cut at the end of the bend prevents this. The relief should be at least as wide as the sheet thickness and deeper than the bend radius plus the sheet thickness.
Common mistake: no relief where two bends meet at a corner. Material overlaps at the corner and the part cannot be bent cleanly.
Springback
When the bending force is released, the material opens up slightly. The effect is stronger in stainless steel and high-strength materials. Experienced manufacturers compensate by adjusting the tool angle, but a reasonable tolerance such as ±1° on bend angles is still necessary.
Flat pattern and K-factor
The flat length of a bent part is calculated with the K-factor, which describes where the neutral axis lies. It typically ranges from 0.3 to 0.5 depending on material, thickness and radius. If you prepare the flat pattern yourself, ask which value the manufacturer uses; the safest option is to send the 3D model of the bent part and let the manufacturer create the flat pattern.
Checklist
- Inside bend radius at least the sheet thickness and the same on all bends
- Flanges above the minimum length
- Holes far enough from bend lines
- Bend relief where bends do not reach the edge
- Reasonable tolerance on bend angles
- Drawing states whether dimensions are inside or outside
Upload your technical file and collect comparable quotes from verified manufacturers.