Sheet metal design rules: bend radius, K-factor and reliefs
Good sheet metal design starts with the shop's real tooling: use a bend radius the press brake can form in that material and thickness, a K-factor measured on test bends, bend reliefs where a bend meets an edge, holes at least about two to three thicknesses away from bends and flanges long enough to sit on the die. Then the flat pattern cut on the laser matches the bent part.
Guilherme Rodrigues ItinoseUpdated 3 min read
Key takeaways
- Ask the shop for its punch radii, die openings and K-factor before modeling.
- Inside bend radius roughly equal to material thickness is a safe starting point for mild steel.
- Add bend reliefs wherever a bend line runs into an edge or cut-out.
- Keep holes and slots away from bend lines or they will stretch into ovals.
- Validate the K-factor with a test strip and update the CAD template.
A flat pattern is only as good as the numbers behind it. When the CAD model uses a bend radius or K-factor the press brake cannot reproduce, the laser cuts a perfect blank that becomes a wrong part after bending. These rules keep the model, the DXF and the shop floor in agreement.
Start from the shop’s tooling
Before drawing the first flange, collect from your fabricator:
- punch radii and die openings available for each thickness;
- the K-factor or bend deduction table they actually use;
- the materials and thicknesses they stock;
- maximum bend length and tonnage of the press brake.
Put those values in your CAD sheet metal template. Every part designed afterwards will inherit them.
Bend radius and K-factor
The inside bend radius should be one the tooling can form. For mild steel, a radius close to the material thickness is a common, safe starting point; harder materials such as stainless and some aluminum alloys usually need more.
The K-factor is the position of the neutral axis inside the bend, as a fraction of thickness. CAD uses it to calculate how long the flat blank must be. A wrong K-factor does not change the 3D model at all, but every flange comes out longer or shorter after bending.
| Situation | What to do |
|---|---|
| New material or thickness | Bend a test strip and measure the result |
| Parts from two suppliers differ | Compare their tooling and tables, not the drawing |
| Tight flange tolerance | Dimension the flange that matters and let the other float |
Reliefs, holes and flanges
- Bend reliefs: where a bend line meets an edge, add a small relief cut so the material does not tear.
- Holes near bends: keep them at roughly two to three thicknesses plus the radius away from the bend line, or they distort into ovals.
- Minimum flange length: the flange must rest on the die while bending; very short flanges slip or cannot be formed.
- Hems and joggles: check with the shop whether they have the tooling before specifying them.
The drawing still matters
The DXF tells the laser what to cut. The bend drawing tells the operator what to do next: bend direction (up or down), angles, sequence when it matters, and which dimensions are critical. Dimension functional features from the same datum the operator will measure from.
How we work on sheet metal parts
At Downway we model sheet metal parts in SolidWorks with the supplier’s own bend table, export clean DXF files for cutting and deliver bend drawings with critical dimensions highlighted. See our CAD engineering service or send a drawing for a fixed-price review.
When to outsource this
- You send flat patterns to several suppliers and parts come back with wrong flange lengths
- You need DXF and bend drawings for a family of enclosures or brackets
When not to
- One simple bracket a week that your bender already knows how to make
FAQ
What K-factor should I use?
Values between about 0.3 and 0.5 are common, but the right number depends on material, thickness, radius and tooling. Bend a test strip, measure it and calculate the K-factor that makes your CAD flat pattern match.
Why do my flange lengths come out wrong?
Usually the CAD bend allowance does not match the press brake: different radius, different die opening or a generic K-factor. Align the CAD template with the real tooling.
Can I bend stainless with the same radius as mild steel?
Stainless springs back more and often needs a larger radius and over-bending. Ask the shop for its stainless table instead of reusing mild steel values.
Do I need a bend drawing if I send the DXF?
Yes. The DXF tells the laser what to cut; the bend drawing tells the operator bend direction, sequence, angles and critical dimensions.
Send us one drawing
We return a review with issues found, suggested fixes and a fixed-price scope.
