A lathe does something quietly counterintuitive: the workpiece spins and the cutting tool stands almost still. Every other machining process moves the tool through stationary material. Turning inverts that relationship, and almost everything distinctive about the process follows from it. Because the part rotates, cutting speed is not a machine setting but a function of diameter. Surface speed is pi times diameter times spindle speed, so a bar being faced from 100 mm down to the centre passes through every speed from full rate to zero. Carbide inserts have a narrow window in which they perform, and drifting below it causes built-up edge, smearing and torn finishes rather than clean shearing. This is why constant surface speed exists as a control mode: the spindle accelerates as the tool travels inward, holding the cutting zone at a fixed velocity until the chuck limit is reached. The geometry of the cut is governed by three parameters: - Depth of cut, which sets chip thickness and th...