Before You Upload That File: What Format Actually Works for CNC? 

We’ve seen it too many times. A customer uploads a file, thinking the hardest part is over. They’ve modeled their part, saved it, and sent it off for a CNC quote. Then the waiting begins. Sometimes we write back right away: “Could you please send this in STEP format instead?” Other times, we try to make do—but honestly, it doesn’t always end well.

So what’s the problem? It’s not your part. It’s the format you used to send it.

File format isn’t just a tech detail. It decides whether the machine sees a clear instruction—or a fuzzy guess. And if you're investing in custom CNC machining, you don’t want anyone guessing about your tolerances, thread types, or surface finishes.

We get a lot of STL files. That makes sense—many 3D design tools export STL by default. For 3D printing, it’s fine. But CNC is a different game. STL files are made up of triangles. Just surfaces. No real solids. They don’t contain information about wall thickness, hole types, thread specs, or what kind of finish you expect.

We can sometimes reverse-engineer from an STL, but it's like trying to machine a part based on a photo of it—it introduces risk. And you don’t want risk where a micron-level fit matters.

💡 Need help converting STL to machinable formats? Use our free support form →

Now compare that to a STEP file. When we receive a proper .step or .stp file, it’s like someone handed us a clean blueprint—with real geometry, real edges, and real machining intent. We can analyze features, detect blind holes, generate toolpaths, simulate CAM—all without guesswork.

We often recommend attaching a 2D technical drawing in PDF as well. Not because STEP isn’t enough, but because drawings show intent: threads, tolerances, depth callouts. One client’s STEP file had a simple countersink; their PDF clarified it was an M4 threaded hole to 10mm depth. Saved both sides from a costly error.

✍️ Learn more: How to Prepare a CNC Drawing →

What about DXF? It works well—for flat profiles. If you’re cutting a plate, panel, or letters, DXF is fast and easy. But if you're sending DXF expecting us to machine a 3D part... you’ll need to supplement it.

We’ve received DXF files with no indication of thickness, no material callout, no tolerance notes. If you're using DXF, let us know:

What’s the thickness?

What material?

Are any edges chamfered or deburred?

Or better—include a STEP model alongside the DXF.

Want to test 2D files instantly? Try our CNC Quote Tool →

Customers also ask about SolidWorks, Fusion 360, or native CAD formats. They work—if the receiver uses the same system. Otherwise, you risk version mismatch or missing features. That’s why we recommend converting to neutral formats like STEP for 3D, DXF for 2D, and PDF for annotation.

We’ve seen native files (.sldprt, .ipt) that open fine for the sender—but show up empty or broken on our end.

If all you have is STL, that’s okay—but understand its limits. We can quote it, but likely ask: “Do you have a STEP file too?” You might lose detail or get surface artifacts if STL resolution is too low. We’ve had customers export STLs at 0.5mm tolerance and wonder why their holes came out oval.

👀 [Compare file quality in our upcoming visual guide – Coming Soon]

The better the input, the better the output. Use a file format that speaks clearly. A clean STEP file says, “This is exactly what I want.” An STL file says, “Here's a rough idea.” A DXF file says, “This is a flat sketch—guess the rest.” Your file is your voice. Make it precise. 

What We Recommend Before You Upload:

✔ A .STEP file with full geometry

✔ A .PDF drawing for key tolerances or threads

✔ A clear filename like Bracket_M6_Al6061.step

✔ Optional .DXF for 2D profiles (with notes on thickness/material)

Ready to quote?
📩 Upload your file for instant CNC review →

Frequently Asked Questions (FAQ)

What is the best file format to send for CNC machining?
The best file format is STEP (.step or .stp). It contains solid geometry and feature data, allowing precise toolpath generation and clear manufacturing intent.

Can I use STL files for CNC machining?
Technically yes, but STL files are surface-based and lack critical info like tolerances and threads. They're better suited for 3D printing.

Is DXF enough for machining?
DXF is ideal for 2D cutting tasks (e.g., laser cutting), but not suitable for complex 3D milling unless combined with other file types.

Do you accept native files like .sldprt or .ipt?
We can review them but recommend exporting STEP instead to avoid compatibility issues or missing features.

What if I don’t know which file to send?
No problem. Contact us with what you have—we’ll walk you through the best way to prepare your files.

Author's Note

This article is written by the EKINSUN CNC Team, based on real-world experience reviewing thousands of part files every year. We specialize in custom CNC machining, offering global delivery, low MOQ production, and personal engineering support.

If you're unsure what file to use or how to structure your design data, reach out. We believe good machining starts with good communication—and it starts with your file.