If your part is round, it probably needs turning. If it isn't, it probably needs milling. That one-line rule gets you 80% of the way there — but the remaining 20% is where quoting mistakes and delays come from.
What CNC turning actually does
A lathe spins the workpiece against a stationary cutting tool. That makes turning the right process for anything that's rotationally symmetric: shafts, bushings, spindles, pins, sleeves, and threaded fittings. The part rotates; the tool moves along and across it.
Turning is fast and efficient for round stock because material removal happens continuously as the part spins — there's no repositioning between cuts the way there often is in milling. On a modern CNC lathe, achievable tolerances typically fall in the IT7–IT9 range (roughly ±0.02–0.05 mm), depending on diameter, material, and how many operations the part needs.
What CNC milling actually does
A rotating cutting tool moves against a stationary (or indexed) workpiece. Milling is the right call for anything with flat faces, pockets, slots, angled surfaces, or holes that aren't on the part's rotational axis: brackets, housings, plates, and machined faces on otherwise-welded assemblies.
Multi-axis milling centers can also produce features that look "round" — a bolt circle, a bore — but the part itself doesn't need to spin to make them. That distinction matters for quoting: a bracket with a few precision bores is a milling job, not a turning job, even though it has round features.
Where the two overlap
Some parts need both. A shaft with a milled keyway or a flat is turned first, then moved to a milling center for the flat. A turned bushing that needs a cross-drilled hole is the same story. This is normal, and it's one of the reasons full-cycle shops that run both processes in-house — rather than splitting the job across two vendors — tend to quote faster and hold tolerance better: the part doesn't lose its reference datum crossing a shop boundary.
A quick decision table
| If the part is mostly... | Use | Typical achievable tolerance |
|---|---|---|
| Rotationally symmetric (shafts, bushings, spindles) | Turning | IT7–IT9 (≈ ±0.02–0.05 mm) |
| Flat faces, pockets, brackets, housings | Milling | ±0.02–0.05 mm |
| Both (turned body + milled feature) | Turning then milling | Set by the tighter of the two operations |
What this means for your RFQ
When you send a drawing, note which features are functional (mate with another part, carry a bearing, seal against something) versus cosmetic. That's what actually drives which tolerances get held tightly and which get relaxed — and it's the single biggest lever for controlling machining cost on a mixed turned/milled part.
We run five HAAS turning centers and five HAAS milling centers in the same shop, specifically so a part that needs both doesn't have to travel. See our CNC turning and CNC milling pages for material-specific tolerance notes, or request a quote directly with your drawing.