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Fault Diagnosis and Maintenance of CNC Machine Tools in 4 Cases
As the reliability of modern CNC systems continues to improve, the frequency of internal system faults is gradually decreasing. However, most of the problems encountered are not caused by the system itself but by external factors. These external issues typically involve problems with sensors, hydraulic components, pneumatic elements, electrical actuators, and mechanical devices.
External faults in CNC equipment can be categorized into soft faults and hard faults. Soft faults usually result from improper handling or adjustments, often occurring during the early stages of equipment use or when users are adjusting settings. Hard faults, on the other hand, are caused by physical damage to external hardware.
Another common failure point in CNC systems is the servo unit. Each axis movement is controlled by a servo motor driven by the servo unit, which uses a rotary encoder for speed feedback and a grating scale for position feedback. Common failure areas include the rotary encoder and the servo driver module. In some cases, power supply issues may also lead to system confusion, especially in systems that store data on computer hard drives, such as the Siemens 840C system.
For example, when a CNC lathe is first put into operation, it must return to the reference point after a power cycle. If the axes are manually moved outside the safe zone and not returned properly, a collision could occur. Therefore, it's recommended to move the machine tool’s axes to a safe position at the end of each day to avoid issues upon restart or power loss.
Faults caused by external hardware are among the most common in CNC repairs. They often stem from faulty detection switches, malfunctions in the hydraulic or pneumatic systems, electrical actuators, or mechanical components.
Example 1: A turning center using the SINUMERIK 840C system suddenly shut down during operation, displaying a spindle temperature alarm. After checking, the issue was found in the temperature meter. Adjusting the peripheral circuit resolved the problem, and replacing the faulty instrument restored normal operation.
Example 2: On a different turning center, the CRT displayed alarm code 9160: “NO PART WITH GRIPPER 1 CLOSED – VERIFY V14-5.†This indicated that the robot had not picked up the workpiece. However, the robot had actually grabbed it. Upon checking the PLC diagram, the fault was traced to a sensor switch. The manipulator’s stroke was not fully engaged, so the switch wasn’t activated. Adjusting the robot’s clamping force resolved the issue.
Example 3: A vertical machining center using the FANUC-OM control system triggered alarms 414# and 410# during an automatic X-axis rapid move. These alarms indicate a speed control failure and a servo drive anomaly. Restarting the machine temporarily cleared the error, but it reoccurred every time the X-axis moved rapidly. Further investigation revealed that the servo motor’s power cable had shorted due to arcing.
Example 4: Improper operator actions can also cause failures. For instance, a CNC lathe using the 840C system worked perfectly on the first day but failed to start on the second. When switching to automatic mode, it displayed the alarm “EMPTYING SELECTED MOOE SELECTOR.†Even though no faults were found in the machine parts, the issue was traced to a misaligned mode switch under automatic conditions. This highlights the importance of checking switch positions in different operating modes when encountering unknown alarms.
Some faults do not trigger any alarm messages but prevent the machine from completing its intended action. In such cases, troubleshooting requires analyzing the machine’s working principle, reviewing maintenance experience, and checking the PLC status.
When repairing CNC machines, identifying the root cause is crucial—especially for external faults. Although the diagnostic process can be complex, once the issue is located, the solution is usually straightforward. To effectively diagnose external faults, two key principles should be followed: first, understanding the machine’s working principle and sequence of operations; second, using the PLC ladder diagram, NC system status display, or an off-board programmer to monitor the PLC’s operation. By following these guidelines and being cautious, most CNC faults can be resolved efficiently.