STEP, IGES, DWG, DXF: which CAD file to send for a CNC quote

The file you attach decides how much of your part the shop has to guess at. Here is what each format carries, in the order you should prefer them for EDC work.

STEP (.step, .stp) — send this

A neutral solid model. It carries the full 3D geometry, which means the programmer can take dimensions directly off the model rather than interpreting a drawing. Every CAM system reads it.

STEP does not carry tolerances, surface finish callouts, material or threads-as-specifications. It carries the shape. That is why you send a drawing with it.

Use AP214 or AP242 if your CAD asks. AP242 can carry annotation; most shops will not use it, but it costs nothing to export.

IGES (.iges, .igs) — acceptable, older

Surfaces rather than a watertight solid. Usable, but imports sometimes arrive with gaps that the programmer has to heal, which is unpaid time that quietly shows up in your price. Send STEP if you have the choice.

DWG and DXF — 2D only

Native and neutral 2D. Genuinely useful for flat parts: knife scales with no 3D contour, liners, spacers, pocket clip blanks before forming. For anything with a machined contour, a 2D file alone means the shop is reconstructing your part from views.

DXF is the one to send for laser or waterjet blanks and for wire EDM profiles.

PDF — the drawing, not the part

A dimensioned PDF is where tolerance, finish, material, thread callouts, datums and inspection requirements live. Nobody can machine from it alone at EDC complexity, but no quote is complete without it.

Make sure it is a vector PDF exported from CAD, not a photo of a printout. Scaled screenshots have caused more scrap than any other file problem we see.

Parasolid, SLDPRT, F3D — native formats

If the shop runs the same CAD you do, native is the richest option. If it does not, native is a file nobody can open. Send STEP as well, always.

STL — almost never

Triangulated mesh, built for 3D printing. Curves become facets, so a cylindrical bore arrives as a many-sided polygon. Quoting from STL means either the shop rebuilds your model or you accept faceted surfaces. Do not send STL for CNC.

The combination to send

For a typical EDC part: one STEP file plus one dimensioned PDF. Add a DXF if there is a flat profile that will be cut separately.

Name the files with the part name and revision — scale-left-rev-c.step, not Part1 (2) final FINAL.step. Revision confusion is a real source of scrap on repeat orders.

One more thing

Put the units in the drawing title block. Millimetres and inches look identical in a STEP file until someone machines a scale that is 25.4 times too big. It happens, it is expensive, and a title block prevents it.

Ready to send? Upload STEP and PDF together and you will get a quote without a round of questions first. See also: how to write an RFQ a shop can quote.

What a drawing must carry that a model cannot

A STEP file is geometry. Everything below has to come from the drawing, and a quote produced without it is a guess:

• Material and grade, with an acceptable equivalent if you have one
• Tolerances, and which dimensions are critical
• Datum scheme — which face or feature everything is measured from
• Surface finish callouts, and whether dimensions are before or after finishing
• Thread specifications: form, pitch, class, depth of full thread
• Edge condition — break all sharp edges, or a specified chamfer
• Marking: what, where, how big, and vector artwork attached
• Inspection requirements and what documentation you want back
• Quantity and revision

That list is the difference between a model and a specification. See how to write an RFQ a shop can quote for how to package it.

Exporting STEP properly

Three settings worth checking in your CAD:

Solid, not surface. Export as a solid body. A surface export arrives as a collection of faces that the programmer has to stitch, and the stitch is not always watertight.

One body per file, or an assembly that opens cleanly. Sending an assembly is fine for context but send each part as its own file too, named for the part.

Units. Set them explicitly and state them in the drawing title block. The millimetre-versus-inch error is rare and catastrophic.

Do not simplify or defeature the model before sending unless you intend those features gone. A shop will machine what it receives.

Assemblies and context

For anything that fits something else — a scale into a frame, a head onto a body — send the mating part as a STEP too, even though you are not ordering it.

Cost: nothing. Benefit: the programmer can see what your part has to fit, and will flag an interference before it exists in metal. This is one of the highest-value, lowest-effort things you can do in an EDC RFQ and almost nobody does it.

File size and transfer

STEP files for EDC parts are small, usually well under 10 MB. If yours is enormous, it probably contains an entire assembly or a very high tessellation setting.

Zip the package — model, drawing, artwork — into one archive named with the part and revision. It keeps the set together and prevents the classic failure where the drawing arrives and the model does not.

Forms with file upload can struggle on mobile. If the upload fails, email the archive rather than abandoning the enquiry.

The files are half of the brief; the other half and the rest of the route are in our guide to custom EDC manufacturing.

FAQ

Can a shop quote from a PDF alone? For a simple flat part, sometimes. For anything contoured, no — it will either be declined or quoted with a margin for uncertainty, which you pay for.

Is STEP AP214 or AP242 better? AP242 can carry annotation and is the more modern standard. In practice most shops read the geometry and use your PDF for everything else, so either works. Export AP242 if your CAD offers it.

What about Fusion 360 or Onshape links? Useful as a supplement, not as the deliverable. Cloud links expire, permissions change, and version history creates ambiguity about which revision was quoted. Export a STEP.

Should I send STL as well? No reason to. It adds a file that could be machined from by mistake and gives you faceted geometry if it is.

Do I need to send a drawing for a very simple part? Yes. Even a bead needs material, finish, hole size and edge condition stated somewhere, and “somewhere” should be a drawing rather than an email thread that nobody can find in six months.

Who owns the CAM program derived from my model? A separate question worth settling before the first order — see NDA, IP and tooling ownership.