The short answer
If the part has bends and the shop accepts 3D files, send the STEP. If the part is flat, or the shop only cuts and does not fold, send the DXF. When in doubt, send both.
What each file actually contains
A DXF is two-dimensional geometry: lines, arcs and
polylines on named layers. It describes the flat blank. Bends can only be implied — a line
sitting on a layer someone decided to call BEND-UP. Nothing in the format
says that line is a fold, what angle it folds to, or what radius it uses. That information
travels in a separate bend table, an email, or not at all.
A STEP file is a boundary-representation solid of the finished part. The bends are genuine cylindrical faces with real radii, connected to real planar faces. The angle is not written down anywhere because it does not need to be — it is implied by the geometry, and any CAD kernel can measure it.
Why portals prefer the solid
Instant quoting works by feature recognition. The software loads your part, finds the cylindrical faces, identifies each as a bend, counts them, measures the flange lengths, and prices the cutting and the brake time. All of it comes from the geometry, with nothing to interpret.
Then it does something more valuable: it unfolds the part itself, using its own K-factor and the die tooling it will actually run. The flat pattern it derives is more accurate than yours, because yours was calculated for assumed tooling and theirs is calculated for the machine standing on their floor.
Hand the same system a DXF and it has to guess which layer means what. Layer naming is convention, not standard — BEND, FOLD, BEND-UP, BEND_UP and half a dozen others are all in use. Guess wrong and your fold lines get quoted as cuts, or ignored entirely and quoted as a flat part.
What a flat DXF still does well
Plenty. Send a DXF when:
- The part has no bends — a gasket, a bracket plate, a panel.
- The supplier is a cutting house rather than a fabricator, and folding is not their job.
- You have already agreed the flat pattern and want that exact blank cut, not a re-derived one.
- The shop asks for one, which some still do out of habit and tooling.
If you do send a flat DXF for a part that will be bent, get the layers and contours right and state the bend angles, directions and radii explicitly in the email. Do not assume the layer names carry it.
The failure modes, concretely
- Bends quoted as cuts. The portal reads your bend layer as cut geometry and prices a part sliced into strips. Obvious in the quote if you check the preview, easy to miss if you do not.
- Bends silently ignored. The portal recognises nothing on that layer, quotes a flat blank, and you receive flat parts. This one is expensive because the price looks plausible.
- Flat pattern mismatch. You calculated with K = 0.4 and a 2 mm radius; their die produces 2.6 mm. Every flange comes out short. Sending the solid avoids this entirely, because they unfold it themselves.
- Wrong units. DXF units are a hint the reader may ignore. STEP records units explicitly.
What neither format carries
Material, thickness, quantity, finish, tolerance and grain direction are not in a DXF, and only thickness is inferable from a STEP. Every quote needs them stated separately regardless of which file you attach. A three-line email covers it: material and thickness, quantity, finish.
Coobic exports both formats from the same drawing — a DXF flat pattern with bends split onto BEND-UP and BEND-DOWN layers, and a STEP solid of the folded part built with a real CAD kernel so the bends survive as true cylindrical faces.
Frequently asked questions
Should I send a DXF or a STEP file for a sheet-metal quote?
Send a STEP file of the folded part if the shop or online portal accepts 3D. It carries the bends explicitly, so the quoting system detects them and unfolds the part with its own tooling numbers. Send a flat DXF when the part has no bends, or when the shop only cuts.
Why do quoting portals prefer STEP over DXF?
A STEP file describes the folded solid, so bends appear as real cylindrical faces the software can recognise, count and price. A DXF is flat geometry where bend lines are only lines on a layer whose name the portal has to guess.
Does sending a STEP file mean the shop ignores my flat pattern?
Usually yes, and that is the point. The shop unfolds your solid using their own K-factor and die tooling, which produces a more accurate flat pattern than yours because it matches the machine that will actually bend it.
Can I send both a DXF and a STEP file?
Yes, and it is often the safest option. The STEP defines the part, and the DXF shows the flat pattern you expect. If they disagree, the shop will ask — which is exactly the conversation you want before anything is cut.