CNC Machining: Milling vs Turning
How 3/4/5-axis milling, turning and wire EDM differ, and which to choose
CNC covers three dominant material-removal routes: milling (rotating tool, stationary part), turning (rotating part, stationary tool) and wire EDM (electric discharge through a wire). Milling wins for prismatic, multi-feature parts; turning for axi-symmetric parts at high speed and low cost; EDM for hardened steel, tight internal corners and thin slots that cutting tools cannot reach.
1. Milling: 3, 4 and 5 axes
3-axis milling positions the part on a fixed table and moves the spindle in X/Y/Z — the workhorse for plates, housings and brackets. 4-axis adds a rotary axis (usually around X) for features on the side of a part. 5-axis tilts the spindle or table, allowing undercut-free machining of sculpted faces, deep cavities and turbine-type geometry in one setup.
Choosing more axes does not always mean better parts: 3-axis is the most rigid and cheapest per hour; 5-axis reduces setups and improves surface quality on complex forms but costs more. For most production parts, 3-axis with occasional 4-axis features is the economic optimum.
2. Turning: CNC lathes
Turning spins the workpiece while a fixed tool cuts the profile: shafts, pins, bushings, pulleys and fittings are turned in seconds with excellent concentricity and finish. Live tooling adds milling/drilling of cross features in one operation (mill-turn centers), collapsing what used to be two machines into one part.
Turning tolerances are tight (±0.05 mm routine, ±0.025 mm with care) and surface finishes are fine (Ra 0.8–1.6 µm). Bar-fed lathes make high-volume parts economically; Swiss-type lathes hold ±0.01 mm on long slender parts for medical and electronics.
3. Wire EDM
Wire EDM cuts by spark erosion: a thin wire (0.1–0.3 mm) erodes the workpiece along a programmed path, with the part immersed in deionized water. It machines any conductive material regardless of hardness — hardened tool steel, carbide, exotic alloys — with no cutting force, so delicate features survive.
EDM achieves ±0.005–0.01 mm accuracy and Ra 0.2–0.8 µm surfaces, with minimum corner radius equal to wire radius. It is slow and costly per hour, so reserve it for hardened materials, burr-free fine slots, and features no end mill can reach.
4. Choosing between them
Prismatic part with holes/features → milling; axi-symmetric (shaft, bushing, fitting) → turning; hardened or burr-critical detail → wire EDM; hybrid parts → mill-turn. The quoting engine uses your part envelope and features to suggest the process and its tolerance/finish multipliers.
All three share the same design rules: avoid deep narrow cavities, thin walls and sharp internal corners unless EDM is justified, and keep tolerances on the features that actually mate.
Quick reference
| Process | Motion | Best geometry | Tolerance | Surface Ra | Cost index |
|---|---|---|---|---|---|
| 3-axis milling | Spindle moves X/Y/Z | Prismatic, plates, housings | ±0.127 mm | 1.6–3.2 µm | 1.0 |
| 4/5-axis milling | + rotary/tilting | Complex faces, 1-setup parts | ±0.05–0.127 mm | 0.8–1.6 µm | 1.3–1.6 |
| Turning | Part rotates | Shafts, fittings, pulleys | ±0.05 mm | 0.8–1.6 µm | 0.8–1.0 |
| Mill-turn | Both | Hybrid one-setup parts | ±0.05 mm | 0.8–1.6 µm | 1.1–1.4 |
| Wire EDM | Wire erodes path | Hardened, fine slots, corners | ±0.01 mm | 0.2–0.8 µm | 2.0–3.0 |
Sources & references
- ISO 230 — Machine tool test codes
- ASME B5.57 — CNC axis performance
- Xometry Learning Center — CNC Overview (public engineering reference)