CNC turning

CNC turning is a machining method in which the workpiece rotates in the spindle while a tool (a turning tool, usually with an indexable insert) removes material by feeding along and across the axis of rotation. Unlike milling, the workpiece rotates rather than the tool, so the process mainly produces surfaces of revolution: cylinders, cones, faces, grooves, threads and holes on the axis.

A basic CNC lathe controls two axes: X (diameter) and Z (along the workpiece axis). The workpiece is held in a chuck, or in a collet for bar stock; longer shafts are supported by a tailstock center and very slender ones also by a steady rest. Typical operations are OD turning and facing, boring (internal turning), grooving and parting off, drilling and reaming on center, and single-point threading.

Turning centers add live tooling and a C axis (controlled spindle positioning), often also a Y axis and a sub-spindle. A part can then get milled flats, slots, hexes or off-axis holes in the same setup and be machined from both ends. Small, slender parts in high volumes are made on sliding-headstock (Swiss-type) automatic lathes with bar feeders.

Finish turning routinely holds diameters to tolerance grades IT7 to IT8 (a tolerance zone of 21 to 33 µm on a 20 mm diameter) with a surface roughness of Ra 0.8 to 3.2 µm, which depends mainly on the feed rate and the tool nose radius. Surfaces machined in one setup are highly concentric. More precise surfaces, such as bearing seats on hardened shafts, are ground after turning.

Choose CNC turning for parts whose main surfaces are round: shafts, pins, bushings, spacers, flanges, pulleys, nozzles or custom screws. A round part is usually faster and cheaper to make on a lathe than on a mill, because the cut runs continuously around the circumference and bar stock is easy to hold. Common materials are steels, stainless steels, aluminum, brass, titanium and plastics such as POM, PA or PEEK.

If the part also needs milled features (wrench flats, keyways, cross holes), ask for a turning center with live tooling. You save a second setup on a mill and improve the relative position of the features. Turning covers everything from a single prototype to large runs on automatic lathes and is also common for replacement parts made from a sample.

The tool has a nose radius (typically 0.2 to 1.6 mm), so a shoulder cannot have a perfectly sharp internal corner. If a bearing sits against the shoulder, the fillet radius must be smaller than the bearing chamfer (listed in the bearing catalog), or an undercut is used. Threads need a runout or a relief groove, and deep small-diameter holes and deep internal grooves need special tools. Longer shafts (roughly from a length-to-diameter ratio of 3 to 4) must be supported by the tailstock; very slender parts deflect under the cutting force, and tight tolerances on them are expensive.

Choose the largest diameter with standard bar sizes in mind: a part that exceeds a standard size by a few tenths of a millimeter has to be turned from much larger stock. On the technical drawing, state the material, ISO 286 fits only on functional diameters, the roughness of bearing seats and sealing surfaces, runout to a clearly defined datum axis, and general tolerances. Ask whether the supplier will machine the part in one setup or flip it for a second operation, and how they will measure the critical diameters.