The CNC (computer numerical control) system of a machine tool includes the control unit itself, as well as less visible parts of the motion-control system such as the servomotors, drives and axis positioning devices. These components are part of any CNC machine tool, but they might be bought separately and retrofitted later on older machines that are upgraded or refurbished. Also part of the control system are sensors that may allow the control to make certain real-time decisions during unattended or lightly attended machining processes. These can include probes for measuring the position of the part or machined features, as well as monitoring systems for detecting the presence of the tool or the force that is being exerted in the cut.