CAD File Formats for CNC Machining: A Guide for Customers

In modern manufacturing, CNC machines have replaced manual machines for most applications. Computer-controlled machinery can cut parts with a high level of precision, saving machinists time and ensuring accuracy.

But as CNC machines have replaced traditional machines, CAD files have taken the place of technical drawings as the primary source of machining instructions. Technical drawings are still useful for human machinists, but CAD files contain the digital information that will be transferred to the machine.

Our CNC machining centers can do a lot, but they can’t read your handwriting.

This article serves as a basic guide to CAD files for CNC machining, helping you prepare a suitable CAD file that can be uploaded to our online platform, ready for machining.

What is a CAD file?

A computer-aided design (CAD) file is a digital document containing information about a 2D or 3D model.

It mainly specifies geometric dimensions, but it may also contain material properties and manufacturing information.

CAD files are prepared using CAD software, a type of computer application used by engineers, designers, and other professionals to create 2D and 3D models. CAD software has many advantages over manual drafting, such as precision, transferability, and even simulation.

However, CAD files must be exported into the right format before they can be used by CNC machines and other manufacturing technologies.

A typical workflow goes like this:

1. The engineer/designer(you) designs the part using CAD software and exports a neutral CAD file.
2. The machinist(3ERP) imports the CAD file into computer-aided manufacturing (CAM) software to define the ideal cutting instructions for the part, then uses a CAM post-processor to export a G-code file that can be read by the specific CNC machine.
3. The CNC machine(3ERP) reads the G-code file and cuts the physical part.

Common CAD File Types for CNC Machining

A few different CAD file formats exist, just like different image formats exist (JPEG, PNG, GIF, etc.). These formats were developed at different times for different purposes.

First, it is important to understand the difference between neutral and native CAD formats.

• Neutral or universal CAD formats like STEP and IGES are standardized formats that can be read by most CAD or CAM software. They contain the core geometric information about the model.
• Native CAD formats like PRT and SLDPRT contain more information than neutral file formats, including the model’s full “history tree,” which documents individual changes made by the designer. However, these files can only be opened in the software in which they were made.

A common mistake made by first-time customers is trying to upload native 3D CAD files that aren’t compatible with our CAM software and CNC machines.

Fortunately, most CAD software provides the option to export files in a neutral format, even if the default file format is native.

The table below compares some of the main CAD file formats for CNC machining.

Format Extension Native Geometry Compatibility 3ERP Upload STEP .stp, .step No Full solid All CAM systems ✅ IGES .igs, .iges No Surface Most CAM systems ✅ X_T .x_t, .x_b No Full solid Most CAM systems SAT .sat No Full solid Most CAM systems DXF .dxf No 2D vector Most CAM systems ✅ DWG .dwg Yes 2D vector Autodesk AutoCAD PRT .prt Yes Full solid + metadata PTC Creo, Siemens NX SLDPRT .sldprt Yes Full solid + metadata Dassault Solidworks F3D .f3d Yes Full solid + metadata Autodesk Fusion

STEP is the most useful and widely used format and fully preserves geometry. IGES is older but is still widely used for surface and wireframe models. The Parasolid X_T format offers high precision with translation-free interoperability between programs, while the ACIS SAT format is useful for geometry exchange and eliminating translation errors.

DXF and DWG are vector formats for 2D CAD. Other listed formats are native and non-universal.

Recommended CAD Files: STEP and IGES

So why are STEP and IGES the best CAD file formats for CNC machining?

In short, we can use these files on our in-house software and equipment without encountering errors or requiring further information from the customer.

If deciding between IGES vs STEP, then STEP is the preferable format because it is more up-to-date and preserves full solid geometry. However, IGES is good for surface models and wireframe models.

Special Case: CAD Files for CNC Routers

It is worth noting that 2D vector files are often sufficient for jobs that require a CNC router, laser cutter, or plasma cutter. These machines are often used for simple cutting, engraving, or 2.5D operations on soft or sheet materials.

Vector CAD file formats include AutoCAD’s DWG format and the universal DXF format.

2D Technical Drawings in CNC Machining

While it is true that CAD files now perform most of the functions that 2D technical drawings used to, 3ERP still recommends submitting a technical drawing.

This is because you can use your technical drawing to provide manufacturing details that are not included (or not fully explained) in the CAD file. These may include tolerances, hole positions, and surface finishing instructions.

In short, a technical drawing allows a designer to explain exactly what they want to the machinist, rather than just providing them with a shape.

How to Prepare CAD Files for CNC Machining​

CAD design for CNC machine operations requires adherence to design for manufacturing (DfM) principles. For tips on how to design your CNC parts, download our CNC machining design guide.

Preparing your finished CAD file for CNC machining means exporting the project in a CNC file format that can be read by CAM software—typically a STEP or IGES file.

This process is usually the same regardless of the CAD software:

1. Click File in the top left corner of the screen.
2. Select Export from the menu.
3. Navigate to and click on STEP or IGES.
4. Name the file.
5. Click Export.

If exporting is not possible, the part’s geometry may be invalid. For example, unstitched surfaces without proper solid volume cannot be exported as a STEP file.

Another possible issue is that some free versions of CAD software do not allow exporting to a universal format.

If you are unsure about your CAD file format or are encountering issues, contact 3ERP.

Ready to upload your model? Explore our CNC machining services or get a quote today.

Frequently Asked Questions (FAQ)

Which CAD file formats preserve geometry best for milling?

For CNC milling, STEP is the industry standard for 3D geometry, keeping smooth curves and surfaces intact.

Which CAD file format supports complex surfaces for CNC?

IGES was specifically designed for transferring surface data and wireframes, making it good for complex surfaces. However, STEP remains the industry standard among CNC file types and preserves the most detail.

What is a STEP file?

A STEP file is a neutral 3D CAD file that uses exact mathematical data (NURBS) to preserve curved surfaces.

The acronym stands for Standard for the Exchange of Product model data.

Why is STEP considered a standard format for CNC machining?

Most engineers choose a STEP file for CNC machining because it is a neutral file format that fully preserves solid geometry with mathematical descriptions. All CAM software can read STEP files.

Is STL suitable for high-precision CNC machining tasks?

STL is a triangular mesh format commonly used in 3D printing. It is not suitable for CNC machining because it replaces true arcs and circles with flat polygons. CNC requires exact mathematical descriptions to ensure manufacturing accuracy.

Other 3D printing formats like OBJ should also be avoided.

Does DXF work well for laser cutting and CNC routing?

Yes, DXF files work well for laser cutting, CNC routing, and other manufacturing technologies, principally used for 2D/2.5D profile cutting of sheet material. They store vector geometry with exact measurements.

DXF files are neutral (non-native) and can be read by most CAM systems.

What is the best CAD file format for outsourcing CNC?

When sending your 3D model to a digital manufacturer or CNC service bureau like 3ERP, you need to use a neutral format like STEP or IGES.

Your CAD software might save files in a native format like PRT or SLDPRT by default, but you should export your model in a neutral format when sending it to a third party.

Is PDF ever an acceptable CAD file format for CNC?

PDFs can be used as a file format for your technical drawing, which contains blueprints, tolerances, material specifications, surface finishes, and other information.

However, the technical drawing is for human eyes only; you also need to provide a STEP or IGES file that can be read by our CAM software.