As modern cnc increasing the reliability of the system, the NC fault system itself increasingly low, and most of the failures occurred, Non-system itself causes. Outside the main fault systems that due to detection switch, hydraulic components, pneumatic components, the implementation of electrical components, mechanical devices and other problems caused. NC external equipment failures can be divided into soft failures and external hardware damage caused by hardware failures. Soft failure is due to operation, adjustment caused by improper handling of such failures occurred in the early use of equipment or equipment used staff adjustment period. The cnc system, another easy to fault the local unit for the servo. Since the axis of movement is on servo control unit servo motor driven ball screw to achieve. With a speed of rotation encoder feedback, with grating device for position feedback. General easy for the failure of the local Rotary encoder and servo unit drive module. A few of the reasons is because the power system caused the confusion. Especially those with computer hard drives store data systems. For example, Germany's Siemens system 840 C. A CNC lathe just put into use when the power system after the restart, must return to the reference point. That is, when used manually will be moved to the axis of non-interference areas, and then to return to the axis reference point. Otherwise, possible crashes. Therefore, a day after processing, machine tools to the best of the axis moved to a safe place. At this point again after the operation or electricity will not be a problem. Arising from the operation of external hardware failure is common in the NC repair faults. Are generally due to detection switch, hydraulic systems, pneumatic systems, the implementation of electrical components, mechanical problems caused the device. Some of these failures can cause alarm information to find fault. The general cnc fault diagnosis system has features in terms of information or warning.