Turning is a CNC machining process in which a cutting tool (usually a non-rotating tool) describes a helical toolpath by moving more or less linearly as the workpiece rotates.

Generally, the term "turning" is reserved for the creation of an outer surface by this cutting action, while when applied to an inner surface (a hole of one kind or another) this same basic cutting action is called "boring". ". Thus, the phrases "turning and boring" classify a wider range of machining processes as turning operations. Whether using a turning tool or a boring tool, the faces that are cut on a workpiece are called "faces" and can be classified as either category as a subset.

Turning can be done manually with a traditional form of lathe, which often requires constant supervision from an operator, or with an automatic lathe where it is not required. Today, the most common type of automation of this type is computer numerical control, also known as CNC. (In addition to turning, CNCs are commonly used in many other types of machining.)

In turning, the workpiece (a relatively rigid piece of material such as wood, metal, plastic, or stone) rotates and the tool moves along 1, 2, or 3 axes of motion to produce precise diameters and depths. Turning can be performed on the outside or inside of a cylinder (also known as boring) to produce tubular components of various geometries. Although very rare now, early lathes could even be used to produce complex geometries, even platonic solids; although the use of non-computerized toolpath control for this purpose has become unusual since the advent of CNCs.

The turning process is usually carried out on lathes, which are considered the oldest machine tools, and can be of different types such as straight line turning, tapered turning, profile turning or external grooving. These types of turning processes can produce various shapes of material such as straight, tapered, curved or fluted workpieces. In general, turning uses simple single-point cutting tools. Each set of workpiece materials has a set of optimal tool angles developed over many years.

Scrap metal fragments from turning operations are known as chips (North America) or chippings (UK). In some regions, they may be called turnings.

The tool's axes of motion may actually be a straight line, or they may follow a set of curves or angles, but they are inherently linear (in a non-mathematical sense).

Components affected by turning operations may be referred to as "turned parts" or "machined components". Turning operations are performed on lathes that can be operated manually or numerically controlled.