There are some very specialised methods for CNC machining, such as electric discharge machining (EDM), additive machining and 3D laser printing. Wire machining, for example, uses conductive materials – typically metals – and electric discharge machining to wear the workpiece into complex shapes.

However, here the focus is on milling and turning processes – two subtractive methods that are widely available and often used in precision CNC machining.

Milling vs. turning

Milling is a machining process that uses a rotary, cylindrical cutting tool to remove material and create shapes. Milling equipment, known as a mill or machining centre, produces a universe of complex part geometries on some of the largest machined metal objects.

An important feature of milling is that the workpiece remains stationary while the cutting tool rotates. In other words, the rotating cutting tool in a milling cutter moves around the workpiece, which remains stationary on the substrate.

Turning is the cutting or shaping of a workpiece on what is called a lathe. Typically, the lathe rotates the workpiece about a vertical or horizontal axis while a stationary cutting tool (which may or may not rotate) moves along the programmed axis.

The tool cannot move around the workpiece. The material rotates so that the tool can perform the programmed operations. (There is a subset of lathes where the tools rotate around a spool wire, but that is not discussed here.)

In turning, the workpiece rotates, unlike in milling. The workpiece rotates on the lathe spindle and the cutting tool is brought into contact with the workpiece.

Manual and CNC machining

Manual versions of both milling machines and lathes are available, but for small parts, i.e. applications where large quantities of parts are produced to tight tolerances, CNC machines are better suited due to their scalability and repeatability.

Precision CNC machines include not only simple two-axis machines where the tool moves in the X and Z axes, but also multi-axis machines where the workpiece also moves. Unlike lathes, where the workpiece only rotates and the tool moves to create the shape.

This multi-axis configuration allows for more complex shapes to be produced in a single operation without the need for additional work by the machine operator. This not only facilitates the manufacture of complex parts, but also reduces or eliminates the possibility of operator error.

In addition, the use of high-pressure coolant in precision CNC machining can prevent swarf from entering the workpiece, even on machines with vertical spindles.