Sheet metal parts look simple: a flat blank, a few bends, some holes. But sheet metal design in CAD is where many first-time product developers get tripped up. A bracket that looks perfect on screen can crack at the bend, need a costly custom tool, or come back from the fabricator a millimeter off because the flat pattern was wrong. At eCadCam in Los Angeles, we model sheet metal enclosures, brackets, chassis, and panels regularly, and the same handful of rules come up on almost every project. Here is what you need to know before sending a sheet metal design out for fabrication.
Why Sheet Metal Design in CAD Is Different From Solid Modeling
A machined or 3D-printed part is modeled as a solid, and the geometry you draw is the geometry you get. Sheet metal is different: every part starts life as a flat blank that gets cut (usually by laser, punch, or waterjet) and then formed on a press brake. That means your CAD model has to “know” how the metal stretches and compresses as it bends.
Dedicated sheet metal tools in SolidWorks, Fusion 360, Inventor, and similar software handle this by building the part from a constant material thickness and calculating a flat pattern automatically. If a sheet metal part is modeled as a regular solid and “shelled” to look thin, the fabricator usually cannot unfold it cleanly, and someone has to rebuild it. When we take on product design work that includes sheet metal, we model it with native sheet metal features from the start.
Key Sheet Metal Design Rules: Bend Radius, K-Factor, and Relief
Inside bend radius
Metal cannot fold to a perfectly sharp inside corner. A common starting rule is an inside bend radius roughly equal to the material thickness for mild steel and many aluminum alloys, with harder materials (like some tempered aluminum or stainless grades) needing more. Too tight a radius can cause cracking on the outside of the bend. Just as important: use the same radius throughout the part whenever possible. Each new radius can mean a press brake tooling change and added cost.
K-factor and bend allowance
When metal bends, the outside stretches and the inside compresses. Somewhere in between is a “neutral axis” that keeps its length. The K-factor describes where that axis sits as a fraction of the thickness, and CAD software uses it to compute the bend allowance and the flat blank size. Typical values fall somewhere around 0.3 to 0.5, but the right number depends on material, thickness, tooling, and the shop’s process. The best practice: ask your fabricator for their K-factor or bend table and plug it into the model. That one step prevents most “the flanges came out too long” complaints.
Bend relief and corner relief
Where a bend runs into an unbent edge, the metal needs somewhere to go, or it will tear. Small rectangular or rounded relief cuts at the end of the bend solve this. Corners where two flanges meet also need relief to avoid overlap and bulging. CAD tools add these automatically, but size them deliberately rather than trusting defaults.
Feature Placement Tips for Manufacturable Sheet Metal Parts
- Keep holes away from bends. Holes and slots placed too close to a bend line will stretch into ovals. A common guideline is to keep them at least a couple of material thicknesses plus the bend radius away from the bend.
- Respect minimum flange length. A flange that is too short cannot be held against the press brake die. Your fabricator’s tooling determines the minimum, but very short flanges are a frequent reason parts get redesigned.
- Mind hole size versus thickness. Punched holes smaller than the material thickness are hard to produce cleanly. Laser cutting is more forgiving, but tiny features in thick plate still distort.
- Plan for hardware. If the part needs PEM-style self-clinching nuts, studs, or standoffs, model them early. They have their own minimum edge distances and hole sizes.
What to Send Your Fabricator: Sheet Metal CAD Deliverables
A complete sheet metal package usually includes:
- 3D model in STEP format (plus native files if the shop uses the same software).
- Flat pattern DXF for laser or punch programming, with bend lines on a separate layer.
- 2D drawing with material, thickness, finish (powder coat, anodize, plating), critical dimensions, bend directions, and hardware callouts.
Many shops recalculate blanks with their own bend deductions, which is normal. Dimension the drawing to the features that matter, such as mounting holes and the overall envelope.
Prototype Before You Commit
For enclosures and assemblies, a quick fit check can save a round of sheet metal revisions. We often 3D print a look-alike of a bracket or housing so clients can confirm mounting points, connector access, and clearances before paying for cut and formed metal. For flat decorative or signage parts, laser cutting and engraving can handle short runs directly.
Get Help With Your Sheet Metal Design
Whether you have a napkin sketch, an old drawing, or a model that will not unfold, eCadCam can build a fabrication-ready sheet metal design with clean flat patterns and drawings your shop can quote on. With 22+ years of CAD and fabrication experience, our Glendale studio serves clients across Los Angeles and beyond. Request a product design quote, call us at 213.489.1173, or visit us at 4212 San Fernando Rd, Glendale, CA 91204.