Ten years ago a quote request came with a DXF. Now it comes with a STEP file: a 3D model of the finished bracket, folded, with the hardware modeled in and maybe the whole weldment in one file. The laser cuts flat blanks. Somebody has to get from one to the other, and if that somebody is you, every quote waits on CAD work before the first number exists. Here’s what unfolding actually does, why two people unfolding the same part can get two different blanks, and how to handle a STEP that turns out to be twelve parts.
What a STEP File Is (and Isn’t)
STEP is the neutral 3D format every CAD system can write: SolidWorks, Inventor, Fusion, Onshape, NX. It describes solid bodies by their faces, edges, and surfaces, which is why it survives the trip between programs where a native file wouldn’t. What it does not contain, in the usual export, is a flat pattern. The customer’s CAD may have had one on screen, but the STEP carries the folded solid, so the flat pattern has to be rebuilt on your side from the geometry.
What Unfolding Does
An unfolder looks at a solid and asks whether it is sheet metal: a uniform thickness, planar faces, joined by cylindrical bend faces of consistent radius. If it is, it picks a base face, lays out every flat face in the plane, and replaces each bend with a strip of material equal to the bend allowance. The result is a closed 2D outline, the blank, with the holes and cutouts in their unfolded positions and, ideally, the bend lines marked so the brake operator knows where to fold.
That outline is what the rest of the shop runs on. The nester needs it to place the part on a sheet. The quote needs its area for material, its perimeter for cut length, and its hole count for pierces. The brake needs the bend lines. None of that exists until the part is flat.
Why Two Unfolds Don’t Match
Metal stretches on the outside of a bend and compresses on the inside, and somewhere in between is a neutral axis that keeps its length. Where that axis sits, expressed as a fraction of the thickness, is the K-factor, and it decides how much flat material each bend consumes:
Bend allowance = angle (radians) × (inside radius + K × thickness)
K runs from roughly 0.3 to 0.5 depending on material, tooling, and how tight the bend is. Take a 90° bend in 1/8" steel with a 1/8" inside radius. At K = 0.33 the allowance is about 0.261". At K = 0.50 it’s about 0.295". That’s 0.034" per bend, and a four-bend enclosure is off by more than an eighth of an inch overall if the unfold used a different K than the brake actually produces. Holes near bends land in the wrong place; flanges come up short.
For quoting, this barely matters: the blank is within a fraction of a percent either way, and material cost doesn’t move. For cutting, it matters completely, which is why the K-factor should come from your brake and your tooling, not from a default in somebody else’s CAD. If the customer sends their own flat pattern as well as the STEP and the two disagree, ask which one governs before you cut anything.
When the STEP Is an Assembly
The other surprise inside a STEP file is that it’s often not one part. A weldment exports as one file with every body in it: four sheet-metal panels, two plate gussets, a piece of tube, and eight bolts. To quote it you need each body as its own part, with its own quantity (three identical panels are one part, quantity three), the sheet-metal ones unfolded, the plate ones taken flat as they are, and the hardware ignored.
Doing that by hand means opening the assembly, saving out bodies one at a time, unfolding each, and exporting each blank: twenty minutes of CAD before you can price a job that might take ten minutes to cut. An assembly separator does the split for you, names the parts, counts the duplicates, and hands the sheet-metal bodies to the unfolder.
The Things That Go Wrong
- Sharp bends. The model has zero-radius corners because the designer never set a bend radius. There’s no bend to unfold; the unfolder needs a radius supplied.
- Non-uniform thickness. A machined step, a chamfer, a weld bead modeled in: the body stops being sheet metal and the unfolder rightly refuses. Strip the feature or quote it as a machined part.
- Formed features. Louvers, embosses, and drawn dimples don’t flatten into a cut outline; the blank is an approximation and the tooling is a separate operation.
- Hems and jogs. Legitimate sheet-metal features that some unfolders handle and some don’t. Check the blank against the model, especially closed hems.
- Units. A STEP written in millimeters and read in inches gives you a part 25 times too big. Confirm one known dimension before nesting, every time.
- The customer’s flat pattern. If they send one, it was unfolded with their K-factor. Treat it as a reference, not gospel.
Why This Belongs in the Quoting Program
The cost of a STEP file isn’t the unfolding. It’s the round trip: out of the quote, into CAD, split the assembly, unfold, export, back into the quote, discover the units were wrong, repeat. The fix is having the unfolder and the assembly separator inside the program that does the quoting. In PolygonLogix Standard, that’s Draw: import the STEP, separate the assembly into individual parts with quantities, unfold the sheet-metal bodies into blanks, and drop them straight into the nest and the quote, bend counts feeding your forming operations. The 3D file the customer sent becomes a priced job without leaving the window.
Lite, the browser version, doesn’t unfold; it works from DXF and DWG. If your customers send STEP, that’s the clearest sign the desktop program is the one you need.