CNC File Format Overview: What Each Format Carries and Where It Fails

• 1
  What a CNC file format actually stores
• 2
  How STEP, IGES, Parasolid, STL and DXF differ
• 3
  Import errors that change the part
• 4
  Which format fits which machining job
• 5
  Export settings and file hygiene
• 6
  CNC file formats at a glance

A CAD file is a set of instructions for building a surface, not a picture of a part. Two families exist. Boundary representation formats store exact analytic geometry: a cylinder is defined by its axis, radius and trim curves, so the surface stays mathematically exact at any zoom level. Mesh formats store only triangles. Tolerance is fixed the moment the file is written.

That single difference drives most of what happens on the shop floor. B-rep files pass cleanly into CAM and generate true arcs and helical moves. Mesh files force the CAM system to approximate, so a hole that should be round comes in faceted. The toolpath then chatters through the facet edges.

A third family handles 2D only. DXF and DWG carry wireframe geometry, layers and dimensions. Useful for profiles, bend lines and plate layouts. Useless for anything with a contoured 3D surface. Picking the wrong family is the most common reason a quote comes back with questions.

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  B-repExact surfaces, arcs and trim data. Best for milled and turned parts.
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  MeshTriangles only. Facet size sets the achievable accuracy.
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  2D wireframeProfiles and layers. Good for sheet metal, not for 3D solids.

STEP (AP203, AP214, AP242) is the default exchange format. It carries solid bodies, exact curves, assemblies, materials and, in AP242, PMI tolerances. A STEP file opens in most CAM systems without repair. For a part with freeform surfaces and a ±0.005 mm tolerance band, it is the safest single file to send.

Parasolid (X_T) is the native kernel behind SolidWorks, NX and Solid Edge. Because those systems share the kernel, X_T often imports with fewer healing operations than STEP. When a design has many intersecting fillets and lofts, X_T tends to survive the trip better. Not every CAM package reads it, so send STEP as a companion.

IGES is the older standard. It works, but it stores surfaces rather than solids more often than not. Imported IGES bodies frequently arrive as open shells with gaps at the seams. Those gaps stop CAM from generating a watertight toolpath. Send IGES only when a partner explicitly asks.

STL stores faceted triangles. Its accuracy is set by the chord tolerance chosen at export. Export at 0.01 mm and the file is large but usable for visualization. Export at 0.5 mm and a Ø6 mm bore becomes a visible polygon. STL has no units, no color, no feature history.

DXF carries 2D geometry. It is the right format for laser-cut blanks, waterjet profiles and sheet metal flat patterns. It is the wrong format for a bracket with a drafted rib.

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  STEPBest general-purpose B-rep format. Send this by default.
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  ParasolidTightest import fidelity for SolidWorks and NX users.
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  IGESSurface-only legacy. Expect gap repair and extra setup.
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  STLMesh. Watch chord tolerance before exporting.

A translated model is not the model you drew. Healing a gap of 0.02 mm sounds harmless, but on a sealing face that gap becomes a leak path. On a bearing bore it becomes a 0.02 mm change in fit. Always ask what was repaired and how much material moved.

Unit mismatches are the quiet killer. A file authored in inches and imported as millimeters comes in 25.4 times too large. A file authored in millimeters and read as inches comes in undersized and looks plausible on screen. Confirm units in writing with your supplier before cutting metal.

Missing surface normals cause CAM to machine the wrong side of a wall. Faceted tessellation on a curved sealing surface creates a scalloped profile that leaks. Threads modeled as simple cylinders lose pitch, so the CAM system bores a plain hole. Send a drawing or note thread callouts in the file.

Native CAD files avoid most translation entirely. SolidWorks, Creo and NX files preserve feature trees, configurations and design intent. The tradeoff is that the supplier must own a matching seat and version. Older file versions open more reliably than new ones.

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  UnitsState mm or inch explicitly. Never assume the importer guesses right.
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  HealingAsk for the repair log. Any gap change affects fit.
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  ThreadsModeled cylinders lose pitch. Add callouts in text.

For 5-axis contoured work, send STEP or Parasolid. The exact surfaces let CAM generate smooth lead-in and lead-out moves without gouging. Complex impellers, medical housings and aerospace brackets all fall into this group. STL will produce a visibly faceted surface on those same parts.

For turned parts, STEP or a native file with a defined axis works best. Add a 2D DXF of the profile if the shop runs a lathe with a profile tracer. Thread callouts belong on the drawing, not only in the model.

For sheet metal, DXF is the practical choice. Send the flat pattern with bend lines on a separate layer. STEP is still useful as a reference for formed geometry. Sending only the folded STEP forces the shop to unfold it and guess the K-factor.

For prototypes and simple prismatic parts, almost any format with correct units will do. A 3-axis job with flat faces and drilled holes does not need a perfect B-rep. It needs correct hole positions and depths.

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  5-axis contoursSTEP or Parasolid. Exact surfaces matter here.
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  Turned partsSTEP plus a drawing with thread callouts.
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  Sheet metalDXF flat pattern with bend lines on their own layer.

Export STEP as AP242 when your CAD supports it. That version carries assembly structure and PMI tolerances, which cuts review time on both sides. If the model has assemblies, decide whether to send them merged or separate. Separate bodies let the shop quote each part cleanly.

Set STL chord tolerance to 0.01–0.05 mm for inspection meshes. Anything coarser is fine for a quick visual check but not for quoting a tolerance-critical feature. Check the exported file size and triangle count before sending.

Strip construction surfaces, sketches and reference planes from the deliverable. A clean solid body imports faster and reduces the chance of a stray surface confusing the CAM system. Keep one authoritative version of the file. Version confusion causes more scrap than format choice.

Name files with part number and revision. A revision note in the drawing is useless if the STEP file carries no revision in its name.

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  STEP versionAP242 when available. Carries PMI and assembly data.
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  STL tolerance0.01–0.05 mm for inspection-grade meshes.
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  File namingPart number plus revision. One authoritative file.