The Best File Format to Send a Machine Shop
When you ask a CNC shop to make a part, the first thing they need is a file they can quote and program from. Send the wrong format and you get a slower quote, a back-and-forth email, or a request to “please send the solid model.” The short answer: send a STEP file. Here is why, what to include alongside it, and what to avoid.
The short answer: STEP
A machine shop programs toolpaths from a real 3D solid model. The STEP format (.step or .stp) is the universal, neutral way to give them exactly that — a watertight solid that opens cleanly in any CAM software. It is the format almost every shop asks for, and sending it up front skips a whole round of emails.
Why STEP and not the shop's native format
You do not need to match whatever CAD the shop runs. STEP is vendor-neutral, so it imports cleanly regardless of their software. That is the entire point of a neutral format — one file works everywhere.
STEP and IGES are both neutral formats, but STEP is the modern default and preserves solids more reliably. If a shop specifically asks for IGES, see STEP vs IGES for when each one fits.
What NOT to send (and why)
Not an STL
An STL is a mesh of triangles for 3D printing. It approximates curves with flat facets and carries no true edges, so a machinist cannot reliably program from it. Great for a printer, wrong for a CNC quote.
Not only a PDF or photo
A drawing or sketch helps as a supplement, but a shop cannot machine from a picture. They need the 3D solid to generate toolpaths.
Not a locked native file
A proprietary CAD file may not open in the shop's software at all. STEP avoids the compatibility lottery entirely.
Watch your units
Export in the units you designed in and state them (mm or inch). A units mix-up is the most common cause of a part coming back the wrong size.
What to include with the STEP file
The STEP solid is the core deliverable, but a few extras make for a faster, more accurate quote — and fewer surprises:
- Material — aluminum 6061, mild steel, Delrin, etc. It drives both price and how the part is cut.
- Quantity — one prototype prices very differently from fifty.
- Critical tolerances — note any dimension that must be tight; leave the rest at standard tolerance so you are not overpaying.
- Finish — as-machined, bead-blasted, anodized, and so on.
Designing the part to be machinable in the first place matters too — tool access, avoiding sharp internal corners, and sensible tolerances. Our beginner's guide to CAD for CNC machining walks through designing parts a shop can actually cut.
Export a quote-ready STEP in your browser
You do not need a CAD license to produce a proper STEP file. Describe the part to PartWork.ai, get editable solid geometry, and export STEP directly — ready to attach to a quote request. Need to bring in a supplier's model first? See importing files for the formats you can load.
Start with 2 free AI generations — no card required
Open the studio, describe your part, and export a clean STEP solid to send your machine shop. More credits: 100 for $4.99 (~5¢ each).