Turning is a machining process that involves cutting a rotating workpiece with a lathe tool and belongs to lathe machining. This process utilizes the combination of workpiece rotation and the linear or curved motion of the tool. By replacing tools with drills, reamers, countersinks, etc., operations such as drilling and boring can also be performed. It can machine various rotary surfaces, including internal and external cylindrical surfaces, end faces, conical surfaces, and threads, making it suitable for manufacturing parts like shafts, discs, and sleeves. Based on machining accuracy, it is divided into rough turning (achieving IT11 grade tolerance) and finish turning (achieving IT10-IT7 grade tolerance). Among these, mirror finishing can achieve a surface roughness of Ra 0.04-0.01 micrometers, while rainbow turning can create decorative stripes. The process characteristics include advantages such as ensuring positional accuracy of the workpiece, stable cutting process, suitability for precision machining of non-ferrous metals, and simple tool structure.

The development of the lathe began with ancient human-powered devices. In 1797, the Englishman Henry Maudslay invented the leadscrew-driven tool holder, laying the foundation for the modern lathe. In 1845, the American John Fitch developed the turret lathe, enhancing the level of automation. At the beginning of the 20th century, motor-driven lathes became widespread. Program control technology emerged in the 1950s, and CNC lathes developed rapidly in the 1970s, driving continuous improvement in machining accuracy and efficiency.

When turning external and internal cylindrical surfaces, the tool moves parallel to the axis of rotation of the workpiece. When facing an end or cutting off a workpiece, the tool moves horizontally perpendicular to the axis of rotation. If the tool path is at an angle to the workpiece axis, a conical surface can be machined. Formed rotary surfaces can be machined using form tools or by controlling the tool path. During turning, the workpiece is rotated by the machine spindle as the primary motion, and the tool, held in the tool post, performs the feed motion. The cutting speed (v) is the linear speed (meters/minute) at the point of contact between the rotating workpiece surface and the tool. The depth of cut is the perpendicular distance (mm) between the workpiece surface prior to cutting and the surface after cutting for each cutting pass; however, during parting and form turning, it refers to the contact length (mm) between the tool and workpiece perpendicular to the feed direction. Feed rate is the displacement of the tool in the feed direction per revolution of the workpiece (mm/rev), or it can be expressed as the tool's feed per minute (mm/min). When turning ordinary steel with a high-speed steel tool, the cutting speed is generally 25-60 m/min, while with a carbide tool it can reach 80-200 m/min. Using coated carbide tools, the maximum cutting speed can exceed 300 m/min.

Turning is generally categorized into rough turning and finish turning (including semi-finish turning). Rough turning aims to maximize efficiency by using large depths of cut and feed rates without reducing cutting speed, but the achievable accuracy is only around IT11, with a surface roughness of Ra 20-10 micrometers. Semi-finish and finish turning employ high speeds with smaller feed rates and depths of cut, achieving accuracies of IT10-IT7 and surface roughness values of Ra 10-0.16 micrometers. On high-precision lathes, high-speed fine turning of non-ferrous parts using a finely honed diamond tool can achieve accuracies of IT7-IT5 and a surface roughness of Ra 0.04-0.01 micrometers; this process is called "mirror finishing". If the cutting edge of the diamond tool is honed to create concave and convex shapes of 0.1-0.2 micrometers, the turned surface will exhibit extremely fine, regularly arranged stripes. Due to light diffraction, this surface displays a satin-like luster and can be used as a decorative finish; this process is called "rainbow turning".

During turning, if the tool rotates in the same direction as the workpiece (typically at a rotational speed several times that of the workpiece) while the workpiece rotates, the relative motion path between the tool and workpiece is altered, allowing the machining of workpieces with polygonal cross-sections (such as triangular, square, rhombic, and hexagonal). If a periodic radial reciprocating motion is imparted to the tool post relative to each revolution of the workpiece while the tool moves longitudinally, surfaces like cams or other non-circular cross-sections can be machined. On relief grinding lathes, a similar principle is used to machine the flank surfaces of the teeth of certain multi-toothed cutting tools (such as form cutters and gear hobs), a process known as "backing off" or "relief grinding".