If your sheet metal part has bends, send the 3D model of the finished part and let the shop make the flat pattern. Send a flat pattern on its own only when it was built with that shop's bend data, or when the part has already been made and proven.
The reason is simple: a flat pattern is a prediction of how much metal each bend will use, and that prediction depends on the shop's tooling, radius, and material. A 3D model describes what you actually want. Here's what each file is, where flat patterns go wrong, when they're perfectly fine, and what to include with either.
What's the difference?
A 3D model is the folded part as it should look when finished, usually saved as a STEP file. It shows every flange, angle, hole, and radius in place.
A flat pattern is the same part unfolded into a single flat shape, usually a DXF. It's what the laser actually cuts. Bend lines are marked on it (or should be), and the press brake folds it into the final shape.
Every bent part needs a flat pattern at some point. The question is only who makes it.
Why the shop should usually make it
To unfold a bent part, CAD software has to know how much each bend stretches the metal. It does that with a bend radius and a K-factor or bend deduction. Change either, and the flat pattern changes.
Your CAD default might assume a 1 mm radius and a K-factor of 0.5. The press brake might actually produce a 2.5 mm radius with an effective K-factor of 0.42. Each bend comes out a fraction of a millimetre off. On a simple bracket, nobody notices. On a box with eight bends and a lid that has to fit, it shows.
A shop that works from your 3D model builds the flat pattern with the radius and bend data from its own machines. If it's wrong, it's their problem to fix, not yours. Our explainer on the K-factor and bend allowance walks through the maths with a worked example.
Flat pattern vs 3D model at a glance
| 3D model (STEP) | Flat pattern (DXF) | |
|---|---|---|
| Shows the finished shape | Yes | No, only the blank |
| Bend data used | The shop's | Whoever drew it |
| Bend direction and angle | Built in | Must be marked separately |
| Risk of flange length errors | Low | Higher, unless built with the shop's data |
| Good for proven, repeat parts | Yes | Yes |
| Good for parts sent for cutting only | Works, but more than needed | Ideal |
| Easy to check before cutting | Yes | Harder, without a drawing |
When a flat pattern is the right thing to send
Flat patterns aren't bad. They're just easy to get slightly wrong. They make sense when:
- The part has been made before and the flat pattern is proven. Repeat orders should use the same file.
- You've built it with the shop's numbers. Ask for the K-factor or bend deduction for your material and thickness, set your CAD to match, then export.
- You only want it cut, not bent, because you're bending it yourself.
- The bends are non-critical, like a simple guard where a millimetre either way genuinely doesn't matter.
If you send a flat pattern, include a PDF or a 3D view of the folded part as well. That way the estimator can check the result makes sense, and the operator knows which way each bend goes.
Common flat pattern mistakes
These are the ones we see most:
- Bend lines on the cutting layer. The laser cuts them, and the part falls into pieces. Keep bend lines on a separate, clearly named layer.
- No bend direction. A U-channel and a Z-section can have identical flat patterns. Only the bend directions tell them apart.
- Missing relief notches. Where a bend stops partway across the sheet, the flat pattern needs a small relief, or the corner tears. See our guide to bend relief in sheet metal.
- Wrong radius in the CAD settings. The flat pattern was built around a sharp corner or a default radius that doesn't match the tooling.
- Overlapping flanges. On boxes, two flanges that look fine in the flat pattern can collide when folded. A 3D model shows this immediately.
- Text and dimensions left on. Same rule as any cutting file: the DXF should contain only cut geometry and bend lines.
What to include with each file
With a 3D model:
- STEP format is the safest choice for sharing between CAD programs.
- Model it at the real material thickness.
- Use your preferred inside radius, or note "standard radius" if you don't mind.
- State material, grade, quantity, and finish in the quote notes.
With a flat pattern:
- A DXF at 1:1 in millimetres, with closed outlines and no stray lines.
- Bend lines on a separate layer, each with direction and angle noted.
- The K-factor or bend deduction and radius you used.
- A PDF or image of the folded part for reference.
Not sure about formats? Our guide to laser cutting file formats covers DXF, DWG, STEP, and PDF and what each is good for.
Sending it to us
For bent parts, a STEP file is our preference, and our CNC bending service will build the flat pattern from it. If you have a proven flat pattern from a previous run, send that with a reference drawing. The quote form takes DXF, DWG, STEP, PDF, or a photo up to 10 MB; bigger files can go to ctrlcutsa@gmail.com.
Start a quote for CNC bending and include cutting, welding, or powder coating on the same order.