As the reliability of modern numerical control (NC) systems continues to improve, the frequency of internal system faults has significantly decreased. However, most of the issues encountered are actually caused by external factors rather than the NC system itself. These external faults typically arise from problems related to 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 are usually the result of improper handling, incorrect adjustments, or user errors. They often occur during the initial use of the machine or when operators are still familiarizing themselves with the system. Hard faults, on the other hand, stem from physical damage or malfunctions in external hardware. A common weak point in the CNC system is the servo unit, as it controls the movement of each axis through a servo motor that drives the ball screw. This system relies on a rotary encoder for speed feedback and a grating scale for position feedback. Common failure points include the rotary encoder and the servo drive modules. In some cases, power-related issues can also cause system confusion, especially in systems that store data on computer hard drives, such as the Siemens 840C system.
Example 1: 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 may occur. To prevent this, it's best practice to move the machine’s axes to a safe location at the end of each day, ensuring smooth operation upon restart.
External hardware-related faults are among the most common issues in CNC repairs. These can stem from problems with switches, hydraulic systems, pneumatic systems, electrical actuators, or mechanical components. Many of these faults can be identified using alarm messages generated by the system. Most modern CNC systems have built-in diagnostic tools or alarm functions that help maintenance personnel narrow down the issue. However, not all alarms clearly indicate the true root cause, so troubleshooting often requires combining alarm information with visual inspection and analysis of the fault symptoms.
Example 2: In our factory, a turning unit equipped with a SINUMERIK 840C system experienced an unexpected shutdown during operation, displaying a spindle temperature alarm. After checking the system, we found that the issue was with the temperature meter. Adjusting the peripheral circuit resolved the problem, and replacing the faulty instrument restored normal operation.
Example 3: Another turning center displayed alarm code 9160: “NO PART WITH GRIPPER 1 CLOSED – VERIFY V14-5.†This indicated that the workpiece wasn’t detected by the gripper. However, the robot had indeed picked up the part. Upon checking the PLC diagram, we discovered that the sensor switch wasn’t triggered due to an incomplete stroke of the manipulator. Adjusting the clamping force of the robot fixed the issue.
Example 4: A vertical machining center using a FANUC-OM control system frequently triggered alarms 414# and 410# during X-axis rapid movement. The alarms indicated a speed control failure and a servo drive anomaly. Although restarting the system temporarily resolved the issue, the alarms reappeared every time the X-axis moved rapidly. Further investigation revealed a phase-to-phase short circuit in the servo motor power cable due to arcing. After trimming the damaged wires, the fault was permanently resolved.
Example 5: Improper operator behavior can also lead to system failures. For instance, a CNC lathe using the 840C system operated normally on the first day but failed to start on the second. When switching to automatic mode, it displayed the alarm “EMPTYING SELECTED MOOE SELECTOR.†Despite no visible faults in the machine, we found that a mode switch was incorrectly positioned under automatic working conditions. This highlights the importance of checking switch positions in different operating modes when encountering unexplained alarms.
Some faults do not trigger any alarm messages but prevent the machine from completing its intended actions. In such cases, troubleshooting requires a combination of experience, understanding of the machine’s working principles, and analysis of the PLC status.
In CNC machine tool repair, identifying the root cause is crucial—especially for external faults. While the diagnosis process can be complex, once the issue is pinpointed, the solution is often straightforward. Two key principles should guide external fault diagnosis: first, thoroughly understand the machine’s working principle and operational sequence; second, utilize PLC ladder diagrams, the NC system’s status display, or an off-board programmer to monitor the PLC’s operation. By following these guidelines and exercising caution, most common NC faults can be quickly resolved.
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