STEP to DXF flat pattern

What "unfolding" actually means

A sheet-metal part starts life flat. It gets its shape from bends, and a bend does not just fold the material. It stretches the outside and compresses the inside. So the flat blank you need to cut is not the sum of the finished part's face dimensions. It is shorter, by an amount that depends on the material, its thickness, the bend radius and the angle.

Unfolding is the calculation that runs that backwards: given the finished 3D part, work out the flat outline that will become it. Get it wrong and the part comes off the brake the wrong size, after you have already cut and bent it.

The K-factor, briefly

Inside every bend there is a line that neither stretches nor compresses: the neutral axis. The K-factor is simply where that line sits through the thickness, expressed as a fraction: 0.5 would be dead centre, and in practice it sits nearer the inside of the bend.

It is not a universal constant. It shifts with material, thickness, and how you form the part. Which is why SheetNest lets you set it per part rather than baking one number into the program: you use the value your own shop has proven on your own brake, and if a job is in a different material you change it for that job.

1. Import the STEP or IGES part

Add the 3D file the way you would add a drawing. The unfolding engine is bundled inside SheetNest, so there is nothing else to install, nothing to configure, and no upload. It runs on your machine, so a part covered by an NDA never leaves it, and it works with the network unplugged.

2. Set the K-factor

Set the bend allowance for that part before unfolding. If every part in the job is the same material and thickness, this is one value you set once; if the job is mixed, each part carries its own.

3. Unfold to a flat pattern

The part is flattened to a two-dimensional outline with its holes and cutouts in place. From this point it behaves exactly like a part you imported as a DXF in the first place: it goes into the nest with everything else.

4. Check the dimensions

Before you commit material, measure the flat pattern with the built-in Measure tool and confirm the developed length is what you expect. This takes ten seconds and it is the step that catches a wrong K-factor while it is still free to fix.

5. Nest it, or export it as DXF

Nest the unfolded part along with the rest of the job, where it is packed by its real outline like anything else, then export one clean, parts-only DXF per sheet layout for your CAM software. If all you wanted was the flat pattern, the DXF export gives you that on its own.

What unfolds, and what does not

Being straight about the limits saves you a wasted download:

• It expects a sheet-metal part. A model of constant wall thickness whose shape comes from real bends is what the engine is built for.
• A general 3D solid is not a sheet-metal part. A machined block or a casting has no bends to unfold, and no flat pattern exists for it. That is geometry, not a software limitation.
• Rolled and stretch-formed shapes are a different problem. A true cone or a stamped dome is not a series of bends, and unfolding it is a different calculation.
• Modelling matters. A part drawn as a solid that merely looks bent (mitred corners, overlapping material at the joints) may not unfold cleanly, because the bends are not really there.

If a part refuses to unfold, the fastest route to a fix is an issue on GitHub with the file attached.

Why do this inside the nesting program?

Because otherwise it is a round trip. Open a CAD package, unfold, export a DXF, save it somewhere, open the nester, import it back. Every one of those steps is a chance for the wrong revision to end up on the sheet. And the CAD package is often the expensive part of the toolchain, sitting on one machine, in use by somebody else.

Doing it in one place means the 3D file goes in and a nested, cut-ready DXF comes out, with the K-factor recorded against the part instead of living in somebody's head.