Turning (lathe)

The part spins; a single-point tool cuts it

Turning makes round parts: a lathe (a machine that spins the workpiece) rotates a bar while a single sharp tool slides along it and shaves off material. A wooden baseball bat, a table leg and the steel shaft inside an electric motor are all turned. It is the standard way to give a part an accurate diameter.

Photo: David English, CC BY-SA 4.0, via Wikimedia Commons

Turning on a lathe · side view, 5 stepsCircled numbers match the steps below; the dashed outline shows the bar's surface before this pass.

Turning (lathe) in a minute, by MecMade

How it works

Clamp the part. The bar is gripped in a chuck (a rotating clamp with jaws) on the headstock, the motor end of the lathe. A tailstock center (a pointed support at the far end) steadies long parts.
Spin it. The spindle turns the part, typically at a few hundred to a few thousand rpm (revolutions per minute). Small diameters and soft metals spin faster.
Set the depth of cut. The tool moves in toward the axis by the depth of cut (how deep it bites), typically 1–4 mm for roughing and 0.1–0.5 mm for the finishing pass.
Feed the tool along. The carriage (the sliding tool holder) moves the tool parallel to the axis at the feed rate, typically 0.05–0.5 mm per revolution.
Peel off the chip. The tool shears the outer layer into a chip and leaves a smaller, perfectly round diameter behind. The same lathe can also face the end flat and cut grooves or threads. It can bore (enlarge a hole from inside) and part off (cut the finished piece free).

Key facts

Materials: Nearly all metals (steel, aluminum, brass, titanium), plus plastics and wood
Typical parts: Shafts, pins, bushings (sleeves), pulleys, threaded fittings, knobs
Part size: From watch parts under 1 mm across to rolls and shafts several meters long
Precision: Typically ±0.01–0.05 mm on diameters; about ±0.005 mm with care
Surface finish: Typically Ra 0.8–3.2 µm (Ra = average roughness; 1 µm is a thousandth of a millimeter), with fine feed marks
Batch size it suits: One-offs to millions; automatic lathes with bar feeders run unattended for high volume
Tooling and machine cost: Inserts (replaceable carbide tool tips) cost a few dollars each; lathes run from a few thousand dollars (manual) to tens or hundreds of thousands (CNC, computer-controlled)
Speed: Seconds to a few minutes per small part; hours for a large shaft

Strengths and limits

Strengths

Very accurate diameters, all on one shared axis
Works on almost any material that can be cut
Cheap, standard tools; no part-specific tooling
Easy to automate with bar feeders and CNC

Limits

Only round shapes, unless the lathe also carries spinning tools
Turns part of the bar into chips (recyclable, but still waste)
Long, thin parts bend and vibrate unless supported
For millions of simple parts, forming (cold heading) is faster and wastes less

Where it's used

AutomotiveShafts, wheel hubs, brake discs, valve parts
AerospaceLanding gear parts, engine shafts, hydraulic fittings
Energy and heavy industryTurbine rotors, pump shafts, steel-mill rolls
MedicalBone screws, implant pins, dental implants
Everyday goodsPen barrels, knobs, furniture legs, baseball bats

How to recognize it

Fine, evenly spaced rings or a spiral running around round surfaces: the feed marks of the tool.
Flat ends with a pattern of circles around the center, sometimes with a tiny nub or swirl left in the middle.
Steps, grooves, chamfers (small bevels on edges) and threads that all line up perfectly on one axis.