File Format Comparison
Not all CAD file formats work the same way. Some carry full solid geometry. Others lose data during export. The table below shows how the most common formats compare.
| Format | Extension | Geometry | CNC Compatibility | Verdict |
|---|---|---|---|---|
| STEP | .stp, .step | Full solid | All CAM systems | Best choice |
| IGES | .igs, .iges | Surface only | Most CAM systems | Good backup |
| Parasolid | .x_t, .x_b | Full solid | Many CAM systems | Good option |
| STL | .stl | Mesh (triangles) | 3D printing only | Not for CNC |
| Native CAD | .sldprt, .f3d, etc. | Full solid | Requires same software | Avoid unless asked |
| DXF/DWG | .dxf, .dwg | 2D only | 2D profiles only | Flat parts only |
Why STEP Is Best
STEP stands for Standard for the Exchange of Product Data. It is an open format. Every CAM system can read it. Here is why it works best for CNC.
- Full solid geometry. STEP keeps faces, edges, and volumes. Nothing is lost.
- Universal support. Every CNC shop can open a STEP file. No license needed.
- Small file size. STEP files are much smaller than STL files for the same part.
- Accurate surfaces. STEP uses math curves, not triangles. Surfaces stay smooth.
- Version stable. STEP AP214 and AP203 work across all major CAD tools.
Use STEP AP214. It carries color and layer data along with geometry. AP203 works too, but it strips color info.
Export Instructions by CAD Tool
Follow these steps for your CAD tool. Each one exports STEP files a bit differently.
SolidWorks
- Open your part file.
- Click File > Save As.
- Set file type to STEP (.stp).
- Click Options. Choose AP214.
- Save the file.
Fusion 360
- Right-click the part body in the browser tree.
- Click Save As Mesh is wrong. Click Export instead.
- Choose STEP as the file type.
- Pick a save location and click Export.
Autodesk Inventor
- Open the part.
- Click File > Save Copy As.
- Set file type to STEP (.stp).
- Click Options. Choose AP214.
- Save the file.
CATIA
- Open the part.
- Click File > Save As.
- Set file type to STEP (.stp).
- Use the default export settings.
- Save the file.
Always open your exported STEP file in a viewer before sending it. Missing faces and broken surfaces are common. Catch them before the shop does.
Common File Mistakes
These errors slow down quotes and cause confusion. Avoid them.
- Sending STL files. STL files are mesh, not solid. CNC machines need solid geometry. STL causes errors in toolpath creation.
- Leaving in assembly data. Export only the part body. Remove assembly references, mates, and constraints before exporting.
- Wrong units. Check your units before export. A part designed in millimeters but exported in inches will be 25.4 times too small.
- Multiple bodies in one file. Each part should be its own STEP file. Multi-body files confuse quoting tools.
- Outdated revisions. Name your files with the revision. Example: bracket-rev-C.stp. Old files cause wrong parts.
- Suppressed features. Un-suppress all features before export. Hidden holes and pockets will not appear in the STEP file.
What to Send with Your Quote Request
A complete submission gets a faster, more accurate quote. Here is the full list.
| Item | Format | Required? | Why |
|---|---|---|---|
| 3D model | STEP (.stp) | Yes | Shows the geometry for programming |
| 2D drawing | Yes | Controls tolerances, finishes, notes | |
| Quantity | Text or email | Yes | Pricing depends on batch size |
| Material | On drawing or email | Yes | Affects price, lead time, and tooling |
| Finish spec | On drawing | Yes | Anodize, passivate, or as-machined |
| Deadline | Text or email | Helpful | Rush orders may need different pricing |
Send a STEP file and a PDF drawing with every quote request. The STEP shows geometry. The drawing controls the specs. You need both.
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Upload CAD for Instant QuoteFrequently Asked Questions
What is the best file format for CNC machining?
STEP (.stp or .step) is the best format. It keeps full solid geometry and works with every major CAM system. Always send a STEP file with your quote request.
Can I send a native CAD file instead of STEP?
Most shops prefer STEP files. Native files (like .sldprt or .f3d) need the same software version to open. If the shop asks for native files, send them. Otherwise, export STEP.
Why is my STEP file not opening correctly?
The most common cause is a failed export. Open the STEP file in a free viewer like eDrawings or CAD Assistant before sending it. Look for missing faces, broken surfaces, or open edges.
Should I include a 2D drawing with my 3D model?
Yes. A 2D drawing defines tolerances, finishes, and notes that a 3D model cannot carry. The STEP file shows geometry. The drawing controls the specs.
Do I need to remove internal features from my STEP file?
No. Keep all features in the model. The shop needs to see every hole, pocket, and thread. Simplified models cause quoting errors and delays.
