Sheet metal is cheap, light and fast — but only if you design for how it's actually made: cut flat, then folded on a press brake. The number-one thing to understand is that bending stretches the material, so your flat blank is not simply the sum of the flange lengths.
Bend allowance and the K-factor
When you bend a sheet, the outside stretches and the inside compresses; somewhere between them is a neutral axis that neither stretches nor compresses. Its position, as a fraction of the material thickness, is the K-factor (typically 0.3–0.45). The arc length of the neutral axis through the bend is the bend allowance, and that's what you add to the flat pattern. Work out the flat length on the Sheet-Metal Bend tool and keep the formulas on the cheat sheets.
Inside radius and minimum flange
Don't bend to a sharp inside corner — it cracks. A good default is an inside radius at least equal to the material thickness. Flanges need a minimum length to sit on the die: roughly 4× thickness plus the bend radius, or the brake can't form them. And keep holes and slots back from the bend — about 2.5× thickness from the bend line — or they distort into ovals.
Design for one setup
Every bend that needs the part flipped or re-clamped costs money and stacks tolerance. Try to keep bends on consistent orientations, uniform radii (one tool), and avoid bends that trap the tooling. Consistent radii also let the shop use a single punch — cheaper and more repeatable.
Tolerancing reality
Sheet metal is not a milled block. Expect bend-angle tolerance around ±1°, and don't chain tight tolerances across multiple bends — the errors add. Use ISO 2768 general tolerances and reserve tight callouts for the one or two features that truly need them.