CAD for CNC Machining: A Beginner’s Guide
CNC machining cuts a part out of solid metal or plastic, and it has different rules than 3D printing. The good news: a few simple habits get you a part a machine shop can actually cut. Here is what a beginner needs to know.
Send a STEP file, not an STL
This is the first thing shops ask for. A machine shop needs precise, editable solid geometry — a STEP file (.step / .stp). An STL is a triangle mesh meant for 3D printing; it throws away the exact dimensions a CNC programmer needs. We cover the full difference in STL vs STEP. For CNC, export STEP.
Design for the tool
A CNC cutter is round, so it cannot make a perfectly sharp internal corner. A few habits keep your part machinable and the quote affordable:
Round internal corners
Add a fillet to inside vertical corners — match a standard tool radius like 3 mm. Outside corners can stay sharp.
Limit deep pockets
Very deep, narrow pockets need long thin tools that flex. Keep depth under ~4× the pocket width where you can.
Give tool access
Every feature has to be reachable from outside. Fully enclosed cavities cannot be milled.
Avoid tiny text and detail
Engraved features smaller than the tool tip cannot be cut. Keep detail coarse, or call it out.
Call out tolerances only where they matter
Machining is precise, but tighter tolerances cost more. Use a normal fit everywhere and only tighten the few dimensions that need it (a bearing seat, a mating bore). For a deeper look at fits and clearances, see our tolerances guide — the fit concepts carry over even though the numbers are tighter for CNC.
Design a machinable part
With PartWork.ai you describe the part in plain English, keep the internal corners rounded, and export a clean STEP file for the shop. Already have a CAD file? You can import STEP, IGES, or DXF and edit from there, then export when it is ready.
Start with 2 free AI generations — no card required
Describe the part, get editable 3D geometry, export a STEP file for CNC. Open the studio. Need more after your free generations? 100 credits for $4.99 (about 5¢ each).