## 2 Analysis
### Fault Analysis of Feed and Spindle Servo System  
The fault types of CNC machine tools are diverse, including feed system, spindle servo system and auxiliary mechanism, and other parts, and any link problems will affect the normal operation of CNC machine tools. In practical applications, the servo system of CNC machine tools has a high probability of failure. Some faults can be displayed through the CRT or the operation panel alarm, some can be displayed using the hardware display on the servo unit, and some only show that the feed movement is abnormal, but there is no warning information, so such faults are more difficult to judge and bring great difficulty for subsequent maintenance and management work. In such a complex situation, it is of great significance to predict, diagnose, and eliminate various faults quickly and accurately to improve the production efficiency and machining accuracy of the machine tool.  
To determine and eliminate the system fault timely and accurately, a comprehensive overview of the common fault types of the existing servo drive system is made, and its fault mechanism is deeply analyzed, as shown in Figure 4. The types of fault that occur in the servo system can include the servo shaft moving, the spindle speed becoming unstable, being unable to reach the highest speed, acceleration and deceleration failure, speed deviation being too large, etc. Some of the reasons for these failures are analyzed as follows:  
Servo shaft moving: feed transmission chain has reverse clearance, servo drive gain is too large.  
Servo shaft crawling: servo system gain is too low, poor lubrication of the feed transmission chain, etc.  
Servo shaft vibration: the bearing of the motor is poorly lubricated, the fastening screw inside the motor is loose, etc.  
The spindle speed is unstable: the tachometer generator installed at the tail of the spindle fails, the speed command voltage is poor or wrong, etc.  
Cannot reach the highest speed: motor excitation current adjustment is too large, the excitation control loop is bad.  
Acceleration and deceleration failure: current feedback loop setting, mechanical transmission system is poor.  
Overload: excessive load, poor lubrication of the feed transmission chain, etc.  
Spindle does not turn: machine load is too large, mechanical connection falls off, etc.  
Excessive speed deviation: improper adjustment of setting of speed regulator or speed measurement feedback loop, etc.  
Figure 3: Statistical histogram of CNC system failure analysis.  
The root cause of servo drive failure is explored, and it is found that it is related to the transmission device, drive system, and detection components of the machine tool. When the transmission device fails, the power of the motor and other devices cannot be transferred to the actuator, and the failure mostly occurs in the coupling, lead screw, bearing, machine tool guide, and other parts. The drive system refers to the servo motor and other driving devices, and the failure of the drive system mainly includes the failure of the drive control unit and the servo motor. The detection parts mainly include the encoder, grating ruler, and other detection parts, and the fault of the detection part is mainly manifested as the feedback data error being too large or no feedback.  
The faults of the servo drive system mentioned in the above analysis can be classified into three categories according to the fault nature: feed speed fault, spindle speed fault, and spindle load fault. In the case of complex fault causes, the accurate identification of the fault category can provide help for the subsequent determination of the fault location and the formulation of maintenance plans, which is of great significance to ensure the production reliability and efficiency of CNC machine tools.