How to Prepare CAD Files for CNC Machining (Complete Guide)

You've designed something awesome in CAD, now it needs to become a real-world part. But before your CNC machine can cut that geometry, your design needs to be carefully prepped and exported. Let's walk through exactly how to go from CAD to CNC, what formats machines accept, and how to avoid expensive mistakes along the way.

This guide will walk you through the entire process, from choosing the right CAD format to generating CNC-ready files, so you can avoid costly mistakes and keep your production running smoothly.

At JLCCNC, we work with designers, engineers, and makers at all levels, which means we see the same avoidable CAD preparation issues come up again and again. That's why our online upload system accepts multiple file types, checks them for compatibility, and flags potential issues before they reach the machining stage.

Why Proper CAD File Preparation Matters

Whether you're making a one-off prototype or a large production run, accuracy starts at the file level. CNC machines follow instructions down to the fraction of a millimeter, so if your CAD data is incomplete, in the wrong format, or overly complex, you can end up with:

• Wasted material
• Longer machining times
• Rework and extra costs
• Parts that don't fit or function correctly

In short: your CNC is only as good as the file you give it.

Not sure if your CAD file is ready for machining? Upload it here, and our engineers will review it for free. You can get a free quote too!

1. Choosing the Best File Formats for CNC

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Before your CAD design can be machined, you need to export it into a format your CNC shop (or CAM software) understands.

Best formats for CNC machining:

• STEP (.stp/.step) – A widely used industry format that maintains precise geometry and keeps curves as smooth, true arcs.
• IGES (.igs/.iges) – Great for surface models, widely compatible
• Parasolid (.x_t/.x_b) – Common for Solid Edge and NX workflows
• Native CAD files (SolidWorks .sldprt, Inventor .ipt). Only if your CNC vendor can open them

Avoid mesh-based formats like STL or OBJ for machining, they work for 3D printing but not for CNC because they break smooth curves into tiny triangles.

If you're working with JLCCNC, you can upload STEP, IGES, Parasolid, or even native CAD files directly through our portal, our system automatically converts and optimizes them for machining while preserving accuracy.

2. CAD to CNC: The Complete Workflow

Here's how the process typically flows:

CAD Design → Export to CNC-friendly format (STEP/IGES) → Import into CAM software → Create toolpaths → Post-process to G-code → Load into CNC machine → Machining begins

Let's break that down step-by-step.

Step 1: Design in CAD

Use CAD software that supports CNC workflows:

• Fusion 360: Affordable, integrated CAD/CAM
• SolidWorks: Industry powerhouse with advanced modeling tools
• AutoCAD: Strong for 2D profiles and drafting, less for complex solids

Step 2: Export to a CNC-Friendly Format

• Always ask your machine shop which format they prefer (99% will say STEP)
• Double-check units, inches vs millimeters mismatches cause disasters
• Include any 2D DXF files for profiles, flat patterns, or engraving

Step 3: Import into CAM Software

CAM (Computer-Aided Manufacturing) is where your part is translated into toolpaths, the movements your CNC cutting tool will make.
Popular CAM tools:

• Fusion 360 CAM
• Mastercam
• SolidCAM
• Autodesk HSM

Your CAD file directly impacts the quality of your CNC toolpaths. Learn more in our CNC toolpath guide.

Step 4: Post-Processing to G-Code

Your CAM software will use a post-processor to output machine-specific G-code.
For example:

• Haas machines use different G/M codes than Fanuc
• A Siemens controller might require different syntax than Mazak
• Want to understand the code your CAD file becomes? Check out our CNC programming guide for a beginner-friendly intro.

Step 5: Load into CNC & Machine the Part

Now your CNC machine has all the instructions it needs to cut metal, plastic, or composite exactly as you designed it.

3. Best Practices for Preparing CNC Files

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We've found that following these best practices not only speeds up machining but also reduces costly revisions. Our engineers routinely optimize client files to meet these guidelines before production begins.

Follow these to ensure smooth production:

Common Mistakes to Avoid

• Sending mesh files (STL/OBJ) for CNC work
• Forgetting to define hole sizes or thread specs
• Not checking units before exporting
• Using overly complex spline curves instead of arcs

FAQ:

Q: What files do CNC machines use?
A: CNC machines run on G-code, which is generated from CAD files via CAM software.

Q: Can I send an AutoCAD file directly to a CNC machine?
A: No. You'll need to export to a compatible format (usually STEP) and run it through CAM first.

Q: Which CAD software is best for CNC work?
A: Fusion 360, SolidWorks, and Inventor are the most widely used for CNC workflows.

All in All

We understand how hard it can be to machine all on your own, getting the right type of file and tweaking the g-code commands takes a lot of time and effort. The best part is: You can relax, JLCCNC will take care of everything from start to finish. Simply upload your CAD file through our portal, and we'll send you a free quote.