CNC turning: how it works, costs and design rules

The part spins while a stationary tool cuts it. Turning makes round parts quickly and accurately: shafts, pins, bushings, spacers, fittings and knobs. Lathes with live tooling can also drill and mill flats.

Typical tolerance

±0.05 mm general, ±0.01 mm on diameters

Quantities

1 to 100,000+

Lead time

3–15 working days

Materials

Steel, stainless, brass, aluminium, plastics, titanium

How it works

Bar stock is clamped in a chuck or collet and rotated. Tools cut outside diameters, faces, grooves, bores and threads. Swiss-type lathes guide long slender bar through a bushing, which keeps small parts straight and allows very high volumes.

Benefits

• Fast and cheap for round parts, especially from bar stock.
• Very good roundness and concentricity.
• Bar-fed lathes run unattended, so large batches become cheap.
• Fine surface finishes on diameters for bearings and seals.

Limitations

• Only features around the turning axis are cheap; flats and cross holes need live tooling or a second operation.
• Long slender parts (length more than about 8× diameter) deflect.
• Internal grooves and deep small bores need special tools.

What drives the cost

• Bar diameter: the largest diameter decides the stock.
• Number of operations (second side, cross holes, milled flats).
• Tight diameters and fits that need grinding.

Design rules

• Design the largest diameter just under a standard bar size.
• Add a small undercut or relief where a shoulder meets a thread or a fit.
• Use standard groove widths and thread sizes.
• Avoid sharp internal corners on shoulders; a 0.2–0.5 mm radius is natural for a turning insert.

Preparing your STEP file

• Model the axis on a main coordinate axis (Z is common).
• Keep threads cosmetic in the model and specify them on the drawing.
• Show knurls as a note, not as modelled geometry.

See also the full STEP export checklist and the comparison of all processes.