Sheet Metal Fabrication — design guidelines
Sheet metal design is governed by the press brake tooling, not by the CAD model. Every bend needs room for the punch and die, and the flat pattern has to account for material stretching around the bend — which is why a folded part is never the sum of its faces.
Constraints specific to this process
These are the limits that come from this process in particular. The general rules on wall thickness, corner radii and hole depth apply on top of them.
| Parameter | Design to thisNo surcharge, no discussion needed | We can reachAchievable, but affects price or lead time | Where the limit comes from |
|---|---|---|---|
| Inside bend radius | = material thickness | 0.5 × thickness | A radius equal to the thickness bends without cracking in any of our stock materials. Tighter risks cracking on the outside face, especially on 5000-series aluminium and hardened steels. |
| Hole-to-bend distance | ≥ 3 × thickness | ≥ 2 × thickness | A hole closer than this deforms into an oval as the material stretches around the bend. If it has to be close, we punch it after bending — that is an extra operation. |
| Minimum flange height | ≥ 4 × thickness + radius | ≥ 2.5 × thickness | A short flange has nothing for the die to grip and slips during forming. Below the limit we have to form it oversize and trim, which shows as a witness line. |
| Bend-to-bend tolerance | ±0.4 mm | ±0.2 mm | Springback varies with each coil of material, so accumulated bend positions drift. Tighter than ±0.2 mm across four or more bends needs a check fixture. |
What we see go wrong most often
Recurring issues on the drawings that reach us. None of them are hard to avoid once you know to look for them.
- Modelling the part as a solid instead of a sheet metal body. We need the bend radii and the flat pattern; a solid with sharp internal edges has to be re-modelled before it can be quoted.
- Four bends that enclose a box with no access for the punch. Bend order matters — one of the flanges usually has to be shortened or welded on instead.
General machining rules
The numbers that apply across every cutting process — wall thickness, radii, holes, threads, tolerances and cost drivers.
- Wall thickness
- Internal corner radii
- Holes and threads
- Pockets, cavities and undercuts
- Tolerances, text and surface callouts
- What actually drives the price
Every quotation includes a manufacturability review against these rules. If something in your model falls outside them, we tell you before you commit, not after the first article fails.
Not sure whether your part clears these limits?
Send the STEP file. A manufacturing engineer marks up anything that will be difficult and returns it with the quotation — no charge, no obligation.