Abstract: CNC machine tools are integrated electromechanical automated equipment widely used in today's manufacturing industry. Their high production efficiency has led to their practical use across many industries. However, these technologically advanced automated machines integrate mechanical and electrical systems, have relatively complex system configurations and diverse mechanical structures. A failure in any part can adversely affect CNC machine operation. With this in mind, the author examines typical current CNC machine faults and, drawing on personal maintenance experience, proposes corresponding repair strategies for reference.
Keywords: CNC machine tools; typical faults; fault repair
1.Overview of CNC machine tools
A CNC machine tool is an automated machine tool equipped with program control. It can logically process programs specified by control codes or other symbolic instructions and control the machine tool system by inputting them into a numerical control device. As a typical mechatronic product, it has the following characteristics: First, strong machining adaptability, suitable for one-off production of mold products; second, high machining precision and assured machining quality; third, coordinated multi-axis movement capable of machining complex parts; fourth, a high degree of machining automation that helps reduce labor intensity; and fifth, relatively high requirements for operators and maintenance personnel.
Generally, it consists of processing program carrier, CNC device, servo and measurement feedback system, machine tool body and CNC machine tool auxiliary equipment. The complex structure constitutes diversified operating modes, making the maintenance and fault repair work of CNC machine tools more complex and diversified. This requires maintenance personnel to adopt corresponding CNC machine tool fault repair strategies according to different situations to ensure that CNC machine tools are in the best operating condition.
2. Typical faults and maintenance strategies of CNC machine tools
Compared with traditional machine tools, CNC machine tools based on computer control technology are less likely to fail, but this does not mean that they will not fail. Generally speaking, failures in CNC machine tools can be divided into the following aspects:
2.1 Feed transmission chain failure
The feed transmission chain is an internal transmission chain. The two ends are the spindle and the tool holder. Its function is to realize the longitudinal or transverse movement of the tool. Under normal circumstances, the guide rails of CNC machine tools are carried out in the form of rolling friction pairs. That is, the failure of the feed transmission chain is mainly due to the decline in motion quality. For example, the mechanical parts do not move to the designated position, the operation is suddenly interrupted, the positioning is inaccurate, the bearing noise becomes louder, etc. At this time, the feed transmission chain failure will occur.
Repairs for this type of fault should address the following aspects: First, improve transmission accuracy by adjusting the preload of each kinematic pair, correcting loose connections, and eliminating transmission backlash. Second, improve transmission stiffness by adjusting and improving the preload of the screw-nut pair and supporting components, as well as the dimensions of the screw itself, to obtain the desired stiffness. Third, focus on motion accuracy adjustment. Reduce the mass of moving components while maintaining the required strength and accuracy of motion, thereby reducing the diameter and mass of rotating parts.
2.2 Spindle component failure
Failures in the spindle drive system of a CNC machine tool result from abnormal operating conditions in the spindle assembly. They generally appear in the following forms. First, spindle overheating may be caused by bearing damage, excessive preload, poor lubricating oil quality, or too much or too little lubricating oil. The appropriate actions are to repair or replace the bearings, adjust the preload, clean the headstock, replace the lubricating oil and check lubrication conditions regularly. Second, excessive spindle noise may result from poor dynamic balance of the spindle assembly, gear wear, damaged bearings, inadequate lubrication, a bent workpiece or unsuitable tooling and fixtures. Each specific cause should be addressed accordingly: rebalance the assembly, repair or replace gears, replace bearings, adjust or replace the drive belt, adjust the lubricating oil quantity, straighten the workpiece and adjust ball screw clearances.
2.3 Automatic tool changer failure
Extensive practical experience shows that more than 50% of faults in machining centers equipped with automatic tool changers are associated with the tool-changing device. The main problems include tool magazine movement faults, excessive positioning errors, unstable gripping of toolholders by the manipulator, and excessive manipulator motion errors. These faults should be addressed as follows. First, when configuring an automatic tool changer, select highly reliable components wherever possible. Sensors, pneumatic components, and hydraulic components should all be selected appropriately. Any component that operates abnormally should be replaced promptly to prevent the fault from spreading. Second, keep excessively heavy or long tools out of the tool magazine to prevent interference between the manipulator and other components during tool changes. Strengthen routine management, define operating procedures, and promote compliance and safety awareness. Third, inspect the installed tools to ensure that the installation arrangement is reliable. Use sequential tool changing and arrange the tool sequence appropriately to prevent inconsistencies between the two.
2.4 Limit switch pressing fault at each axis operating position
Numerous limit switches are often used to detect motion positions and ensure reliable automated operation of CNC machine tools. After prolonged operation, however, the motion characteristics of moving parts gradually change, as do the reliability of the limit-switch actuating mechanisms and the quality characteristics of the switches themselves. These changes can trigger various faults. Maintenance of these faults should focus on the following. First, regularly inspect the actuation of the limit switches. If a potential actuation fault is found, promptly improve or adjust the mechanism, or replace the relevant component if necessary, to prevent deterioration. Second, manage and control the operating environment of the limit-switch actuating mechanisms. During routine CNC machine operation, prevent practices that impair switch actuation so that the mechanisms operate under suitable conditions and related faults are avoided.
3.Conclusion
As complex mechatronic products, CNC machine tools involve extensive and demanding fault repair work that generally requires specialized technical maintenance personnel. Drawing on the experience of numerous CNC maintenance technicians, the following diagnostic approaches can be used during repairs. First, direct observation: use the hands, eyes, ears, nose, and other senses to identify and analyze CNC machine tool faults. Second, self-diagnostic troubleshooting: use the self-diagnostic functions of the CNC control system and analyze its operating status through indicator alarms and software messages to identify a starting point for diagnosis. Third, functional program testing: combine commonly used CNC system functions with special programming to create functional test programs that check each system function. Fourth, spare-part substitution: replace potentially faulty components one by one to determine where the fault occurs, progressively narrow its scope, and ultimately locate its source.
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