- Dimension to a face or a bend line, not to a theoretical sharp corner.
- Inside radius is set by tooling and material, not by the drawing.
- Send flat DXF plus a folded PDF and the part comes back right first time.
A press brake does not fold metal along a line. It pushes a punch into a die and forms a radius, and the material on the outside of that radius stretches while the inside compresses. The flat blank therefore has to be shorter than the sum of the finished legs. That difference is the bend allowance, and getting it wrong is the single most common reason a trade folding job, sent from a Sydney fabricator or a Brisbane builder, comes back out of tolerance.
The sharp corner does not exist
The most frequent problem with drawings we receive is dimensions taken to the intersection of two theoretical outside faces. That point is not on the part. On 3mm stainless folded in a 30mm die you will have an inside radius of roughly 3mm, and the corner your dimension refers to is several millimetres away from any surface a tape can touch.
- Dimension outside face to outside face, or from a hole centre to a face.
- State the inside radius if it matters, or write 'fabricator's standard' if it does not.
- Nominate which face is the datum, so tolerance stacks in the direction you can live with.
- Say which side is the visible side, so tool marks land on the back.
Springback and why the first part is a test
Stainless work-hardens as it forms and then springs back when the pressure releases, typically one to three degrees on 304, and more on 316. We overbend to compensate, but the exact figure depends on the coil, the thickness and the tooling. On a run of any size we fold one part, check the angle, adjust, and only then run the batch. If a quote promises fifty parts with no first-off check, the first fifteen are the check.
Tolerances worth putting on paper
| Feature | Realistic tolerance | Note |
|---|---|---|
| Cut length, sheared | ±0.5mm | Per cut, not cumulative |
| Bend angle | ±0.5° | After first-off check |
| Leg length, single bend | ±0.3mm | From the datum face |
| Leg length, four bends | ±0.8mm | Tolerance stacks |
| Flatness over 2400mm | ±1.5mm | Before welding |
| Hole to bend distance | ±0.5mm | Keep holes clear of the radius |
Grain direction
Rolled sheet is not isotropic. A tight fold across the rolling direction is more likely to crack at the outside of the radius than the same fold with the grain. On brushed stainless this collides with the aesthetic requirement, because the visible grain also has to run consistently across an assembly. On visible work we nest to the finish and choose tooling to suit; on hidden work we nest for yield.
What to send
A flat DXF of the developed blank plus a PDF of the folded part with dimensions and material called out. If you have a STEP model, send that instead and we will develop the flat ourselves in Moorebank and ship across Australia. What we cannot work from is a folded DXF with no bend lines, because the bend allowance has already been baked into it by somebody else's tooling.
When a client orders fifty brackets, the test is not whether one fits. It is whether the fiftieth goes on without a hammer.