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STEP vs IGES vs STL: which CAD file to send

Sending the right file gets you an accurate quote and a correct part. Sending the wrong one gets you delays, follow-up emails, or a part that's subtly wrong. Here's the two-minute version.

The short version: send a STEP file (.step or .stp). It's the universal, accurate, machine-ready format every shop can read. Only reach for IGES if STEP genuinely isn't available, and never quote a real machined part from an STL.

STEP (.step / .stp) — send this

STEP is the international standard (ISO 10303) for exchanging solid CAD models. It carries the exact geometry — real surfaces, edges and solids — so a machinist or a quoting engine sees precisely the part you designed. Every serious CAD package exports it and every CAM package imports it. If you export one thing, export STEP. The newer AP242 flavour can even carry tolerances and annotations (PMI), though most parts still travel as plain geometry plus a drawing.

IGES (.igs / .iges) — the old fallback

IGES is an older exchange format from the 1980s. It still works, but it describes models as a collection of surfaces rather than a watertight solid, which sometimes leads to tiny gaps that need repairing on import. If your system can produce STEP, prefer it. Use IGES only when you're dealing with legacy data or a package that won't give you anything better.

STL (.stl) — do not machine from this

STL describes a surface as thousands of tiny triangles — a faceted approximation, not true geometry. It's perfect for 3D printing, and useless for precision machining: curved faces become visibly chunky, exact dimensions are lost, and there's no way to hold a real tolerance. If all you have is an STL, we can look at it, but you'll get a far better quote and part from a STEP export of the original model.

Native files (SolidWorks, Fusion, Inventor…)

Native CAD files (.sldprt, .f3d, .ipt and so on) hold everything, but only open in the software that made them. Rather than assume your shop runs the same package and version, export a neutral STEP — it's the safest way to guarantee they see exactly what you see.

Format	Use it for	Verdict for CNC
STEP (.step/.stp)	Machining, quoting, everything	✅ First choice
IGES (.igs)	Legacy data only	🟠 OK if STEP unavailable
STL (.stl)	3D printing, visualisation	❌ Not for machining
Native (.sldprt etc.)	Working in the same CAD	🟠 Export STEP instead

Should I send a drawing too?

For anything with critical dimensions, tapped threads, tolerances or a specific finish — yes. The 3D model defines the shape; a 2D drawing (PDF) defines the requirements. Our quote page lets you upload a PDF drawing alongside your model, and you can flag tapped threads and tolerance bands directly.

Got a STEP file? Try it now.

Drop your model in and get an instant, geometry-based price. You can add a PDF drawing and thread callouts in the same step.

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