How to Send a File to a CNC Machine

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  Key takeaways: building a clean CNC machine file
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  What a CNC machine file actually contains
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  Preparing the model before you send the CNC machine file
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  Tolerances that travel with the file
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  How the file reaches the machine after upload
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  7 steps to send a file to a CNC machine
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  Choosing how to send the CNC machine file

A CNC machine never opens your CAD file. It runs G-code, a list of coordinates, feed rates and spindle speeds. Your job when you send a file to a CNC machine is to hand over a model that a CAM programmer can convert into that G-code without guessing. The cleaner the model, the fewer questions come back and the faster the quote.

The machine file the shop wants is usually STEP (.step or .stp). It stores exact NURBS surfaces, so a hole modeled at Ø8.00 mm stays Ø8.00 mm. STL is a mesh of triangles; a 0.05 mm chord tolerance on a Ø8 mm bore can shift the diameter by 0.04 mm before any tool touches metal.

IGES works for older systems but can drop trimmed surfaces and split a single face into patches. Send IGES only when the shop asks for it. Native SolidWorks, NX or Creo files are useful for DFM review, but the STEP export is what goes to the machine.

2D formats have their place. DXF drives laser cutting and waterjet for sheet parts, and a PDF drawing carries tolerances, finishes and notes that a solid model cannot express. For turned parts, a drawing with GD&T is still the controlling document.

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  STEP AP214 or AP242Best default for 3-axis, 4-axis and 5-axis milling.
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  STL only for rough checksAcceptable for a visual quote, not for cutting.
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  DXF for flat profilesPairs with a PDF drawing for sheet metal.
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  PDF drawing stays authoritativeWhen model and drawing disagree, the drawing wins.

Model at final size, in millimeters or inches, and state which. A part modeled at 0.98× scale to allow for shrinkage makes sense for die casting, not for machining. If you scale a model, say so in the upload notes.

Set the coordinate origin on a real datum: the primary locating face, a dowel hole, or a corner that can be probed. A model floating in space forces the programmer to invent a datum, and that is where ±0.05 mm of stack-up quietly appears.

Check the solid is watertight. Open shells, duplicate faces and zero-thickness slivers cause CAM software to fail or, worse, to patch the gap silently. Run a geometry check in your CAD tool before export. Most shops reject a model with more than a handful of surface errors.

Model the threads, chamfers and fillets you actually want. A cosmetic 0.2 mm fillet on an internal corner may be unmachinable with a Ø6 mm end mill. Leave sharp internal corners if a fillet is not functional, and let the shop decide the tool radius.

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  One solid per part numberDelete fixtures, clamps and reference geometry.
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  Units in the file nameBracket-PN4471-RevC-mm.step removes all doubt.
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  Suppress cosmetic threadsKeep the pilot diameter, note the thread on the drawing.

A bare STEP file has no tolerances. If you send only geometry, the shop assumes a general tolerance, often ±0.1 mm on metal. That is fine for a bracket and wrong for a bearing bore. Put critical dimensions on a drawing or in the upload notes.

GreatLight machines to ±0.005 mm (±0.0002 in) when the feature needs it. The cost sits in inspection and cycle time, not in the cut. Apply tight tolerances to the three or four dimensions that mate with something, and leave the rest at general tolerance.

Surface finish is a separate call. As-machined faces run Ra 1.6–3.2 μm, a fine finish reaches Ra 0.2–0.8 μm. A sealing face or a sliding bore may need the fine range; a mounting face rarely does. State it per surface, not for the whole part.

If the part has to match a certification route, say so at upload. ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022 coverage changes how inspection records and documents are handled.

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  Mark critical dimensionsA boxed tolerance or a GD&T callout is enough.
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  Name the datumA-B-C on the drawing, matching the model origin.
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  Specify finish per faceRa 0.8 μm on the bore, as-machined elsewhere.

Once the CNC machine file lands, a CAM programmer builds the setup and toolpaths. For 3-axis work the part may need two or three setups. For a 5-axis center, a single setup can reach five faces and remove most of the repositioning error.

The program is then posted and moved to the control. Some shops run a DNC network straight to the machine; others copy to a USB stick at the console. Either way, the operator proves the program on the first article and checks critical dimensions against the drawing.

This is where a clean file pays off. If the model datum matches the drawing datum, the operator probes once and cuts. If they disagree, the setup is rebuilt and the first article is scrapped. At GreatLight, 100% inspection before shipment covers raw material checks, in-process monitoring and final inspection, with reports on request.

Large parts are not a barrier. The largest travel is 4,000 × 400 × 150 mm, with 16 simultaneous 5-axis machining centers and a Ø400 mm rotary table for round work. If your part fits those envelopes, the transfer method does not change.

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  First article before the run
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  One revision at a time