The basic task of manufacturing is to provide workpieces with specified quality characteristics in the demanded quantity in the most time- and cost-efficient way possible. Every manufacturing process is affected by variable disturbances, which can be both external and internal (occurring within the process itself). For this reason, the functionally determinative properties of the components are provided with tolerances. If a characteristic value lies outside of the permissible tolerance, it is defective. Thus important functional characteristics must be tested either already during manufacturing or at the end of manufacturing. Important disturbance factors which must be taken into consideration as possible causes of defects are: disturbances caused by static forces, such as deflections effected by the workpiece weight or clamping errors; disturbances caused by dynamic forces which lead to either self-starting or forced oscillations; disturbances caused by thermal influences, such as process heat or internal sources of heat in the manufacturing machine; and disturbances caused by tool wear (Fig. 2.1). This group also includes disturbances resulting from the engagement kinematics between the tool and the workpiece, such as generated cut deviations occurring during hobbing or diffraction effects on optical surfaces caused by systematically applied tool marks.