Abstract: CNC machine tools are highly integrated products combining mechanical, hydraulic, electrical, and computer technologies. Most of their failures are a comprehensive reflection of these mechanical, hydraulic, and electrical issues. This paper analyzes the causes of crawling and vibration phenomena in the feed system of CNC machine tools, elaborates on fault diagnosis and repair, and illustrates diagnostic and repair techniques through examples. Keywords: CNC machine tools; crawling; vibration; maintenance Abstract: CNC machine tools are high-integration production, and their malfunctions are mostly a positive reflection of the machinery, hydraulic pressure, and electricity. This paper analyzes the reasons for crawling and vibration phenomena in the feed drive system of CNC machine tools, expounds on malfunction measurement and servicing, and illuminates the technical methods for handling some examples. Key words: CNC Machine Tools, Crawl, Vibration, Servicing 1 CNC Machine Tool Feed System Crawling and Vibration Phenomena and Their Causes During the low-speed operation of the driven moving parts, the CNC machine tool feed system may exhibit a phenomenon where the moving parts initially fail to start, then suddenly accelerate after starting, then stop, and then accelerate again. This cyclical, intermittent, and fluctuating movement of the moving parts is called crawling. When the system operates at high speed, the moving parts will exhibit significant vibration. The cause of crawling in the feed system of CNC machine tools is generally considered to be poor lubrication between moving parts, leading to increased static friction resistance when the machine tool table moves. When the motor drives the table, it cannot move forward, causing the ball screw to undergo elastic deformation, storing the motor's energy in the deformation. When the motor continues to drive, the elastic force generated by the stored energy exceeds the static friction force, causing the machine tool table to creep forward. This cycle repeats, resulting in the crawling phenomenon. However, this is only one possible cause. Other causes could include faults in the mechanical feed transmission chain, problems with the electrical components of the feed system, improper system parameter settings, or a combination of mechanical and electrical faults. 2. Diagnosis and Troubleshooting of Creeping and Vibration Faults For crawling and vibration faults in CNC machine tools, one should not rush to conclusions. Instead, based on the likelihood of the fault, a list of possible contributing factors should be compiled. These factors should then be checked, analyzed, located, and eliminated one by one. Once a problem is identified, it should be analyzed to determine if it is the primary cause of the malfunction, continuing until every possible contributing factor is identified. Finally, a comprehensive solution should be developed to eliminate the fault. The specific methods for troubleshooting crawling and vibration faults in CNC machine tool feed systems are as follows: 2.1 Analyzing the Location of the Fault: Crawling and vibration faults typically require investigating the mechanical components and the feed servo system. This is because crawling at low speeds in CNC machine tool feed systems often depends on the characteristics of the mechanical transmission components, while vibration at high speeds is usually related to the preload of the kinematic pairs in the feed transmission chain. Furthermore, crawling and vibration problems are closely related to the feed speed; therefore, the speed loop and system parameters of the feed servo system must also be analyzed. 2.2 Inspecting and Eliminating Mechanical Component Faults: If the cause of crawling and vibration lies in the mechanical components, the guide rails should be checked first. This is because the frictional resistance experienced by moving parts mainly comes from the guide rails. If the dynamic and static friction coefficients of the guide rails are large, and the difference between them is also large, crawling is likely to occur. Although CNC machine tools widely use rolling guides, hydrostatic guides, or plastic guides, improper adjustment can still cause crawling or vibration. For hydrostatic guides, the focus should be on checking whether the hydrostatic pressure has been established; for plastic guides, check for impurities or foreign objects obstructing the movement of the guide pair; and for rolling guides, check whether the preload is adequate. Poor lubrication of the guide pair can also cause crawling problems; sometimes, crawling is simply due to poor lubrication. In this case, using guide lubricating oil with anti-crawling properties is a very effective measure. This type of guide lubricating oil contains polar additives that can form a non-breakable oil film on the guide surface, thereby improving the friction characteristics of the guide. Secondly, the feed drive chain should be checked. In the feed system, the servo drive unit and the moving parts must pass through a drive chain composed of gears, lead screw nut pairs, or other transmission pairs. Effectively improving the torsional and tensile/compressive stiffness of this drive chain is very beneficial for improving motion accuracy and eliminating crawling. One of the causes of crawling in moving parts is often due to inadequate preload or pretension of bearings, lead screw nut pairs, and the lead screw itself. Excessively long transmission chains, undersized transmission shaft diameters, and insufficient rigidity of supports and bearing seats are also significant factors contributing to crawling; therefore, these aspects should be considered during inspection. Additionally, poor mechanical system connections, such as damaged couplings, can also cause machine tool vibration and crawling. 2.3 Inspection and Troubleshooting of Feed Servo System Faults If the cause of crawling and vibration lies in the feed servo system, each relevant component needs to be inspected separately. This includes checking the speed regulator, servo motor or tachogenerator, system interpolation accuracy, system gain, whether system parameters related to position control are set correctly, whether the shorting bar on the speed control unit is set correctly, whether the gain potentiometer adjustment is off-center, and whether the speed control unit's wiring is in good condition. Each component should be checked and categorized for troubleshooting. 2.3.1 Speed ​​Regulator Testing For speed regulator faults, the main checks are for problems with the given signal, feedback signal, and the speed regulator itself. The given signal can be detected by measuring the analog signal VCMD output from the position deviation counter via D/A conversion to the speed regulator. Whether this signal has a vibration component can be observed using an oscilloscope on the pins of the servo board. If there is only a one-cycle vibration signal, then the machine tool vibration is undoubtedly correct; the speed regulator itself is not the problem, but rather the upstream stage. Then, check the D/A converter or deviation counter for the problem. If our measurement results show no periodic waveform of vibration, then the problem is definitely with the feedback signal or the speed regulator. 2.3.2 Detection of the Tachometer Motor Feedback Signal The feedback signal and the given signal are exactly the same for the regulator. Therefore, fluctuations in the feedback signal will inevitably cause the speed regulator to adjust in the opposite direction, thus causing machine tool vibration. Since the machine tool is vibrating, it means that the machine tool speed is oscillating violently, and naturally, the waveform fed back by the tachometer generator will also be volatile. If, at this time, there is a precise ratio between the machine tool vibration frequency and the motor rotation speed, for example, the vibration frequency is four times the motor speed, then we need to consider a fault in the motor or the tachometer generator. 2.3.3 Motor Inspection When the machine tool vibration frequency is proportional to the motor speed, the first step is to check for motor malfunctions. Inspect the carbon brushes, commutator surface condition, and lubrication of the ball bearings. Additionally, a faulty motor armature coil can also cause system vibration. This can be confirmed by measuring the motor's no-load current. If the no-load current increases proportionally with the speed, it indicates a short circuit inside the motor. In cases of this fault, the commutator should generally be cleaned first, and the brushes checked before measurement confirmation. If the fault persists, there may be a short circuit between coil turns, requiring motor repair. If no problems are found, the tachogenerator should be checked. 2.3.4 Pulse Encoder or Tachogenerator Testing For faulty pulse encoders or tachogenerators, the following measurement and inspection methods can be used. First, disconnect the position loop and speed loop, manually rotate the motor, and observe the voltage of the F/V converter on the speed control unit's printed circuit board. If a sudden voltage drop waveform appears, it indicates a faulty feedback component. A common problem in tachogenerators is that carbon dust worn off by the carbon brushes accumulates in the slots between the commutator segments, causing short circuits between the commutator segments. This leads to vibration. 2.3.5 System Parameter Adjustment A closed-loop system can also experience oscillations due to improper parameter settings. The best way to eliminate oscillations is to reduce the amplification factor. In a FUNAC system, adjusting RV1 counterclockwise will immediately improve the situation. However, because the RV1 adjustment potentiometer has a relatively small range, it may not be able to be adjusted sufficiently. In such cases, the short-circuit bar must be changed, i.e., the feedback resistor value is cut off, reducing the overall amplification factor of the regulator. 2.3.6 Handling External Interference For constant interference, check the waveforms of the F/V converter, current detection terminals, and synchronization terminals to check for interference and take appropriate measures. For occasional interference, effective shielding and reliable grounding should be used to avoid it as much as possible. If these methods fail to completely eliminate vibration, or are ineffective, consider replacing the speed regulator board or thoroughly checking the waveforms after replacement. 3. Fault Diagnosis and Repair Examples 〖Example 1〗 A machining center equipped with a FUNAC 11ME system vibrated during forward X-axis movement after long-term use. Fault Analysis and Handling: The main reasons for vibration and crawling in the servo feed system are as follows: (1) Improper installation or adjustment of mechanical parts; (2) Malfunction of servo motor or speed/position detection components; (3) Improper setting and adjustment of the driver; (4) External interference, poor grounding, poor shielding, etc. In order to identify the fault location, considering that the machine tool servo system is a semi-closed-loop structure, the machine was restarted after disconnecting the motor from the lead screw. It was found that the fault still existed. Therefore, it was initially determined that the fault was caused by the electrical part of the servo drive system. In order to further identify the fault cause, the servo motors of the X and Y axes were replaced during the repair test. It was found that the fault was transferred to the Y axis. Therefore, it was determined that the fault was caused by the malfunction of the X-axis motor. The signal of the encoder built into the servo motor was measured with an oscilloscope. It was finally found that the fault was caused by the malfunction of the encoder. After replacing the encoder, the machine tool returned to normal operation. Example 2: A machining center equipped with a FUNAC 6ME system experienced unstable X-axis speed during movement. During the process from movement to a stop, significant oscillations occurred at the stopping position, sometimes preventing positioning and requiring a shutdown before restarting. Fault Analysis and Handling: Careful observation of the machine tool's vibration revealed a low X-axis oscillation frequency with no abnormal noise. Based on the oscillation phenomenon, the fault was related to the closed-loop system parameter settings, such as an excessively high system gain or an excessively large integral time constant. Checking the system parameter settings, servo driver gain, and integral time potentiometer adjustment, all were within appropriate ranges and consistent with the adjustments made before the fault. Therefore, it could be preliminarily determined that the X-axis oscillation was unrelated to parameter settings and adjustments. To further verify this, the parameters were readjusted and tested while recording the original adjustment values. The fault persisted, confirming the initial assessment. Based on the above handling, the parameters and adjustment values ​​were returned to their original settings, and the servo motor and measurement system were inspected. First, the commutator surfaces of the tachogenerator and servo motor were cleaned, and the tachogenerator windings were checked using a digital multimeter. Inspection revealed a loose connection between the tachogenerator rotor and the motor shaft of the servo motor; the bonded parts had detached. After reconnection, the machine was tested and the fault disappeared, and the machine returned to normal operation. 〖Example 3〗 When machining a workpiece with an arc, tool travel marks appeared after arc interpolation, resulting in substandard machining quality. Fault Analysis and Handling: Inspection revealed X-axis crawling. Adjustments to the speed and position loops were ineffective. Inspection of the mechanical mechanism revealed that the worktable did not float from the hydrostatic guide rail. Further inspection of the hydraulic system revealed a leak in the worktable branch. After adjusting the leak, the worktable floated, the X-axis crawling disappeared, and the machining quality was satisfactory. 〖Example 4〗 A vertical milling machine equipped with a FUNAC 6ME CNC system exhibited crawling during automatic machining of a curved part, resulting in high surface roughness. Fault Analysis and Handling: During test program execution, no crawling occurred during linear and circular interpolation, indicating the cause was in the programming. After careful inspection of the machining program, it was found that the machining curve was composed of numerous small arc segments. The correct positioning check instruction G61 was used during programming. Removing G61 from the program and replacing it with G64 eliminated the crawling phenomenon. 4. Conclusion A CNC machine tool is a complete organic whole; the mechanical, electrical, and hydraulic controls are interconnected and mutually influential. Therefore, when analyzing and solving crawling and vibration faults, a holistic concept and experience are necessary to effectively solve practical problems. If the fault is caused by both mechanical components and the feed servo system, and it is difficult to identify the main cause, multi-faceted testing, patient and meticulous analysis and diagnosis are required until the root cause is found. If the root cause of the fault is due to a combination of factors, only a comprehensive troubleshooting approach can resolve the problem. References [1] Wang Chunhai, Zhang Zengliang. Pitch error detection and compensation of CNC machine tools. Microcomputer Information [J], 2006 (1): 228-229 [2] Xu Heng. Fault diagnosis of CNC machine tools [M]. Beijing: Chemical Industry Press, 2005 [3] Gong Zhonghua. CNC machine tool maintenance technology and typical examples [M]. Beijing: Posts & Telecom Press, 2005 [4] Ren Jianping. Fault diagnosis and maintenance of modern CNC machine tools [M]. Beijing: National Defense Industry Press, 2005 [5] Yu Zhongyu. CNC machine tool maintenance [M]. Beijing: Machinery Industry Press, 2005