What File Format Do CNC Machines Use?

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  The machine runs G-code, not your CAD file
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  The CAD file you design in stays at home
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  STEP is the default cnc machines file format for milling
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  IGES, Parasolid and STL each have a narrow job
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  From STEP to G-code: what happens after you upload
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  When the format stops being the problem
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  How to prepare files before you request a quote
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  Which cnc machines file format to send

Ask what file format do CNC machines use and the literal answer is G-code, sometimes called NC code: a plain text list of coordinates, feed rates, spindle speeds and tool changes. A Fanuc or Siemens control reads that list line by line and moves the axes. Nothing else reaches the machine directly.

But you never hand-write G-code for a real part. It is generated by CAM software from a 3D model or a 2D drawing, and the quality of that source model decides how clean the toolpaths are. A sloppy source costs you air cuts, gouges and scrapped stock.

So there are three format layers in play: the CAD format you design in, the exchange format you send, and the G-code the control executes. Most confusion about cnc machines file format comes from mixing those layers up.

The practical takeaway: send us a solid model in a kernel-based format, and we handle the CAM side. Everything after that is our problem, not yours.

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  Design layerNative CAD: SolidWorks, Inventor, NX, Creo, CATIA.
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  Exchange layerSTEP, IGES, Parasolid, STL, DXF.
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  Machine layerG-code plus setup sheets and tool lists.

Native formats such as SLDPRT, IPT, PRT and CATPart carry full design intelligence: feature history, sketches, constraints, assembly mates and material properties. That is useful to the designer and nearly useless to a machine shop, because the shop may not run the same software version.

A native file can also be fragile across versions. A part built in SolidWorks 2023 may open with broken features in an older release. If you send native files, tell us the exact version and include a STEP export as a fallback.

We do accept native CAD when a customer prefers it, and we can read the major packages. But it adds a translation step that nobody needs, and translation is where dimensional surprises appear.

Keep native files as your internal master. Export a neutral solid for anyone outside your team. That habit removes most back-and-forth on a first order.

STEP (AP203 or AP214) stores exact analytic geometry: planes, cylinders, cones, NURBS surfaces and B-rep topology. A tapped hole is still a cylinder with a defined diameter, not a pile of triangles. CAM software can therefore pick edges, offset faces and verify a Ø6.5 mm hole against the drawing.

That exactness matters when you hold ±0.005 mm. Triangle meshes cannot represent a true cylinder, so a nominally round bore becomes a polygon and the CAM toolpath drifts at the flats.

AP214 carries colour and assembly structure; AP203 is geometry-focused. Either works for machining. If your exporter offers AP242, that is also fine and keeps tolerances as annotations.

For a part with tight bores, thin walls or blended surfaces, STEP is the format we ask for first. It is the safest single file you can send.

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  Best forPrismatic and contoured milled parts, turned parts, assemblies.
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  KeepsExact surfaces, edge topology, assembly tree (AP214).
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  Watch outExport in mm, not inches, unless the drawing says otherwise.

IGES is the older neutral format. It handles trimmed surfaces and wireframe, and it still appears in aerospace and tooling supply chains. The trouble is loose tolerance settings at export: two adjacent surfaces can leave a gap, and the CAM kernel then fails to stitch a solid. If you must send IGES, set the export tolerance to 0.001 mm and check that it reads as a closed solid.

Parasolid (X_T) is a kernel format. It is exact and compact, and it round-trips cleanly between SolidWorks, NX and Solid Edge. When STEP exports look faceted or won't heal, Parasolid is a good second try.

STL is a triangle mesh with no units, no features and no exact surfaces. It sizes the model but cannot be dimensioned. Use it for a visual check or a quick form study, not for a part that must fit a mating bore.

DXF earns its place for 2D work: laser-cut plates, waterjet blanks, bend profiles and turned profiles defined in a section view. Send DXF for the flat pattern, STEP for anything with 3D features.

A manufacturing engineer imports your STEP into CAM, checks that it heals into a watertight solid, then sets the machining strategy. On a 3-axis job that means selecting tool diameters, stepovers and depths of cut. On a 5-axis job it also means defining tool axis tilt to keep the tool shank clear of the walls.

Feeds and speeds come from the material and the tool. A 6061-T6 aluminum block cuts fast with high spindle speed; 17-4PH stainless needs lower surface speed and more passes. Those choices never appear in your CAD file, which is why sending only a model is usually enough.

The CAM output is posted to a machine-specific G-code dialect. A Fanuc post will not run unchanged on a Heidenhain control. Post-processors are shop-side assets, so you do not need to supply them.

What you should supply is the drawing or a 3D PDF with tolerances, plus a note on critical faces. G-code cannot tell us which bore is a bearing seat and which is a clearance hole.

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  Import checkSolid heals watertight; units confirmed as mm.
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  StrategyTool selection, stepover, depth of cut, workholding.
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  PostG-code in the control dialect, plus setup sheet.

A clean STEP file does not make an unmachinable part machinable. A pocket 8 mm deep and 2 mm wide cannot be reached by a tool stiff enough to hold tolerance, whatever the file says. Tool reach and aspect ratio set that limit, not the format.

Sharp internal corners are another case. A cutter has a radius, so a square internal corner becomes a radius unless you allow a relief. If the print calls for a sharp corner, expect an EDM or a corner-relief note.

Thin floors deflect under cutting force. On a 0.8 mm aluminum floor we often reduce depth of cut and add support, or flip the part and machine it in two setups. None of this is visible in the file.

So the file is necessary but not sufficient. It gives us exact geometry. The drawing, the material callout and a short note about function give us the rest.