Turning is a machining process in which a cutting tool is held nearly stationary to cut a rotating workpiece. The cutting tool can be slowly moved back-and-forth, and in-and-out to cut cylindrical shapes, and flat surfaces on the workpiece. Turning is usually done with a lathe.
Usually the term "turning" is used for cutting external surfaces, and "boring" for internal surfaces, or holes. Thus the phrase "turning and boring" categorizes the larger family of processes known as lathing. Additionally, "facing" is cutting the ends of the workpiece, to create flat faces.
Turning is typically done with either a manual lathe, or a computer numerical control (CNC) lathe. With a manual lathe, an operator turns cranks to move the cutting tool. On a CNC lathe, the cutting tool is moved by a computer, controlling electric motors to follow a pre-programmed path. Early manual lathes could be used to produce complex geometric figures, even the platonic solids; though this is now usually done with CNC machines.
Different turning processes are typically carried out on a lathe, such as straight turning, taper turning, profiling or external grooving. Those types of turning processes can produce various shapes of materials such as straight, conical, curved, or grooved workpieces. In general, turning uses simple single-point cutting tools.
The waste metal cut off of the workpiece from turning operations is known as chips in North America, or swarf in Britain. In some areas they may be known as turnings.
A component that is made by turning is often called a turned part.
Turning is usually performed on a lathe, which range in size from tabletop machines suitable for jewelry, to building-sized machines for manufacturing ship propeller shafts.
Turning tooling usually cuts with a single point, as opposed to a drill bit or end mill which cut with several sharp edges. The angles around the sharp point are important, such as rake angle , side rake angle, cutting-edge angle, and relief angle. Additionally, the cutting point may have a nose radius which creates radii when cutting corners in the workpiece and extends the life of the tool tip, but increases machining forces.
The tool bit is held with a rigid tool holder during operation. CNC machines are typically able to switch tools automatically.
Significant forces are required for turning operation, and must be accounted for in the design and selection of the machine tools, the workpiece holding, and any additional supports. A setup that is not strong enough may allow the workpiece to break free during operation, ruining the part being made, and endangering the operator. A setup that is not rigid enough will deform during operation, and create chatter when cutting, creating an undesirable surface finish, and inaccurate dimensions.
There are three principal forces in turning:
Speeds and feeds refer to the rotational speed that the workpiece is rotated at, and the feed speed that the tool is moved at. They are chosen based on the workpiece material, cutter material, setup rigidity, machine tool rigidity, spindle power, and whether coolant is used.
The rotational speed is measured in revolutions per minute (rpm).
The feed rate is the distance that the tool is fed into the workpiece per workpiece revolution. It is measured either as millimeters per revolution (mm/rev), or as inches per revolution (in/rev).