As the reliability of modern numerical control (NC) systems continues to improve, the frequency of internal system faults is gradually decreasing. However, most of the problems encountered are actually caused by external factors rather than the NC system itself. External faults typically arise from issues with detection switches, hydraulic components, pneumatic elements, electrical actuators, and mechanical devices. These types of failures can be further classified 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 still adjusting to the system. Hard faults, on the other hand, stem from physical damage to external hardware. One of the more common failure points in CNC systems is the servo unit. Each axis movement is controlled by the servo unit, which drives the ball screw through a servo motor. A rotary encoder provides speed feedback, while a grating scale offers position feedback. Common failure areas include the rotary encoder and the servo driver module. Additionally, power-related issues can 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 service, it is essential to return to the reference point after a power cycle. If the axes are manually moved outside the non-interference zone, they must be returned to the reference point before restarting. Otherwise, collisions may occur. To avoid this, it’s best to move the machine tool axes to a safe position at the end of each day, ensuring smooth operation after shutdown or restart. External hardware-related faults are among the most frequent issues in CNC repairs. These are often caused by problems with sensors, hydraulic systems, pneumatic systems, electrical actuators, or mechanical components. Many of these faults can be identified using alarm information. Most CNC systems have built-in diagnostic tools or fault alarms that help maintenance personnel narrow down the issue. However, not all alarms clearly indicate the root cause, requiring further analysis based on both the alarm message and observed symptoms. Example 2: In one instance, a turning unit equipped with the SINUMERIK 840C system suddenly shut down during operation, displaying a spindle temperature alarm. After checking, the issue was traced to a faulty temperature meter. Adjusting the peripheral circuit resolved the problem, and replacing the meter 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, even though the robot had successfully picked it up. By reviewing the PLC diagram and inspecting the manipulator, it was found that the sensor switch hadn’t been fully activated due to an incomplete stroke. Adjusting the clamping force of the robot resolved the issue. Example 4: A vertical machining center using the FANUC-OM control system triggered alarms 414# and 410# during X-axis rapid movement. The alarms indicated a speed control failure and an X-axis servo drive anomaly. Although the issue could be temporarily resolved by restarting, it recurred each time the X-axis moved quickly. Further investigation revealed a phase-to-phase short circuit in the servo motor power cord due to arc creep. Trimming the damaged wiring eliminated the problem. Example 5: Improper operator behavior can also lead to system failures. For example, a CNC lathe using the 840C system functioned normally on the first day but failed to start on the second. When switched to automatic mode, it displayed the alarm “EMPTYING SELECTED MOOE SELECTOR.” Despite no visible faults, the issue was traced to a misaligned mode switch under automatic conditions. This highlights the importance of checking switch positions in different working modes when unexpected alarms occur. Some faults do not trigger any alarm messages but prevent certain actions from completing. In such cases, troubleshooting requires experience, knowledge of the machine's working principles, and an understanding of the PLC status. Effective diagnosis of external CNC faults often involves analyzing the machine’s operation sequence and using PLC ladder diagrams or system status displays for guidance. In summary, identifying and resolving external CNC faults requires a combination of technical knowledge, systematic analysis, and attention to detail. While the diagnostic process can sometimes be complex, once the root cause is identified, the solution is usually straightforward. Two key principles should guide the diagnosis: first, understanding the machine’s operational logic and sequence; second, utilizing PLC diagrams and NC system status monitoring tools. Following these principles ensures timely resolution of most common CNC system issues.

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