Milling and turning are the two dominant CNC processes, and most parts lean clearly toward one or the other based on geometry. Cylindrical parts turn; prismatic (block-like) parts mill. But modern mill-turn machines blur the line, and the wrong choice can double your cost. This guide covers how to identify which process fits your part.
Milling: the part is fixed, the cutting tool rotates. Material is removed by moving the tool along X, Y, Z axes (and optionally rotation). Best for prismatic parts (blocks, brackets, plates) with features on multiple faces.
Turning (lathe): the part rotates, the cutting tool is stationary (but moves along X and Z). Material is removed concentrically around the rotating axis. Best for cylindrical parts (shafts, pins, rings, flanges).
These differ fundamentally in:
Standard lathe operation. Cycle times 1-5 minutes for typical parts. Concentricity held to ±0.005 mm easily.
Circular parts with optional off-center features (holes, slots) machined in secondary operation or on mill-turn.
Threads cut on lathe are the industry standard. Fast, accurate, cost-effective at any volume.
For production runs above 100 units of a cylindrical part, CNC turning is 2-5× faster than milling equivalent. Use mill-turn or dedicated lathes.
Parts under 25 mm diameter with tight tolerances. Swiss lathes are purpose-built for this, holding ±0.005 mm while producing at ~30 parts per minute.
Prismatic parts with holes, pockets, slots on multiple faces. 3-axis milling with single setup is faster than trying to turn a brick-shaped part.
Curved pockets, angled surfaces, complex contours. Milling is the only CNC process that handles these natively.
Mounting plates with holes aligned across multiple setups. Milling with precision fixtures holds the geometry better than turning + secondary operations.
Large brackets from thick plate. Milling handles the bulk efficiently. Turning from large round stock wastes material.
Under 100 units, milling setup for prismatic parts is more economical than dedicated turning tooling.
Mill-turn machines combine both operations on one machine:
Used for:
Cost: higher per hour ($180-300 US, $65-100 China) but often faster overall for parts requiring both operations. Eliminates multiple setups and their associated tolerance stack-up.
| Process | US hourly rate | China hourly rate | Best for |
|---|---|---|---|
| 3-axis milling | $65-95 | $20-35 | Prismatic parts, general CNC |
| Standard turning | $55-85 | $18-30 | Cylindrical parts |
| Swiss turning | $80-130 | $30-50 | Small precision cylindrical |
| Mill-turn | $180-300 | $65-100 | Hybrid geometry |
| 5-axis milling | $110-160 | $40-60 | Complex 3D surfaces |
Rule of thumb for mixed geometry: split into separate parts if possible. A shaft with a bracket attached is typically cheaper as two parts joined by a dowel pin, rather than one complex mill-turn part.
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