Manufacturing Process
CNC Turning
CNC 车削
Rotating a workpiece against a cutting tool to machine cylindrical features, diameters, grooves and threads.
Process Steps
- Bar loaded
- Rough turning
- Finish turning
- Threading/grooving
- Inspection
What is CNC turning?
Direct answer: CNC turning rotates the workpiece against a single-point tool, so the part comes out round. Turning produces diameters, faces, bores, grooves, chamfers and threads — anything that is a surface of revolution.
It is the most economical way to make a round metal part with tight concentricity, and it is the process that ironing in a deep-drawn part is measured against when a buyer asks whether machining can be removed from a route.
What a modern lathe does in one setup
| Operation | What it produces |
|---|---|
| Turning | External diameters to tolerance, with controlled surface finish |
| Facing | Square ends and datum faces |
| Boring | Internal diameters and bore-to-bore concentricity |
| Grooving | O-ring grooves, undercuts, relief grooves |
| Threading | Metric and imperial, internal or external, single or multi-start |
| Parting and chamfering | Finished part off the bar with a deburred edge |
| Live tooling / mill-turn | Flats, cross-holes and slots without a second machine |
What turning holds
| Property | Typical |
|---|---|
| Diameter tolerance | ±0.005–0.02 mm |
| Concentricity between turned features | 0.01–0.03 mm, better when both are cut in one setup |
| Roundness / cylindricity | 0.005–0.015 mm |
| Surface finish | Ra 0.4–1.6 µm as turned; better after a finishing pass |
| Thread accuracy | Class 6H / 6g standard, tighter by gauging |
Where the money goes
- Cycle time. Turning is fast, so the piece price is mostly seconds per part. Removing a chamfer or opening a tolerance that required a second pass changes the price more than a cheaper bar does.
- Bar size and material. Bar stock is priced by the kilogram and by the millimetre of diameter — machining a Ø30 mm part from Ø40 mm bar pays for a lot of chips.
- Setup count. Work that can be finished off one bar with a bar feeder runs almost unattended. Work that needs a second op on a mill costs the second setup.
- Concentricity that needs one setup. If two diameters have to be coaxial within a few microns, they must be cut without re-chucking — which constrains the process, not the tolerance.
Turning, or forming?
Turning and deep drawing overlap more than most buyers expect. A cylindrical housing can be turned from bar, cold formed and machined, or deep drawn — and a deep-drawn wall that is then ironed can hold a diameter and wall thickness well enough to remove a turning operation entirely. The crossover is volume and geometry: turning suits small volumes and thick sections; drawing plus ironing wins on thin-walled parts at volume. See ironing tolerances and surface finish.
Frequently asked
What is the difference between CNC turning and CNC milling?
Turning rotates the workpiece against a stationary tool, so it makes round parts. Milling rotates the tool, so it makes flats, pockets and contours. Mill-turn machines do both in one setup.
How accurate is CNC turning?
Typically ±0.005–0.02 mm on diameters and 0.01–0.03 mm concentricity, with a surface finish of Ra 0.4–1.6 µm as turned. Tighter means grinding after turning.
Can turning be replaced by a forming process?
Sometimes. If the part is a thin-walled cup or sleeve, deep drawing plus ironing can hold the inner and outer diameter tightly enough to delete the turning operation — and that removes both the machining cost and the chip waste. It only works where the geometry suits a die.
What is the smallest part you turn?
Small enough that bar feeding and part handling, not the cutting, become the constraint. Below that size the economics of a die usually take over.
Related: CNC turning capability · CNC milling · ironing tolerances · metal machining services · deep drawing