Electrical fault diagnosis for CNC machine tools comprises three stages: fault detection, fault identification and isolation, and fault localization. The first stage, fault detection, involves testing the CNC machine tool.

  Electrical fault diagnosis for CNC machine tools comprises three stages: fault detection, fault identification and isolation, and fault localization. In the first stage, fault detection involves testing the CNC machine tool to determine whether a fault is present. The second stage entails characterizing the nature of the fault and isolating the faulty component or module. The third stage focuses on pinpointing the fault to a replaceable module or printed circuit board, thereby minimizing repair time. To promptly detect system faults, quickly identify their location, and resolve them in a timely manner, fault diagnosis should be as simple and minimal as possible, with the diagnostic process taking the shortest possible time. To achieve this, the following diagnostic methods may be employed:

  Intuitive method

  Utilize the senses to observe various phenomena that occur during a fault, such as whether sparks or bright flashes are present, whether there are unusual noises, where abnormal heating is detected, and whether there is a burning smell. Carefully examine the surface condition of each printed circuit board that may be involved, looking for signs of scorching or damage, in order to further narrow down the scope of the inspection—this is one of the most basic and commonly used methods.

  The self-diagnostic function of the CNC system

  Leveraging the CNC system’s rapid data-processing capabilities, multi-channel, high-speed signal acquisition and processing are performed on fault‑prone areas. Subsequently, a diagnostic program conducts logical analysis and evaluation to determine whether a fault exists and to promptly pinpoint its location. The self‑diagnostic functions of modern CNC systems can be categorized into the following two types:

  (1) Power-On Self-Diagnosis Power-on self-diagnosis refers to the automatic execution, from the moment power is applied until the system enters its normal ready‑to‑operate state, of internal diagnostic routines that perform functional tests on modules such as the CPU, memory, bus, and I/O units, as well as on printed circuit boards, CRT displays, optical readers, and floppy disk drives, thereby verifying that the system’s major hardware components are functioning properly.

  (2) Fault Information Alerts: When a malfunction occurs during machine tool operation, the CRT display will show an error code and its corresponding description. Based on this information, consult the relevant maintenance manual to identify the root cause of the fault and the appropriate troubleshooting procedures. In general, the more comprehensive the fault information provided by the CNC machine tool’s diagnostic system, the easier it is to pinpoint the issue. However, it should be noted that for some faults, the fault description and manual reference allow for direct identification of the cause; for others, the actual cause may not match the indicated symptom, or a single fault message may list multiple potential causes. In such cases, maintenance personnel must discern the underlying relationships among these factors to indirectly determine the true cause of the problem.

  Data and Status Check

  The CNC system’s self-diagnostic function not only displays fault alarm messages on the CRT monitor but also provides machine tool parameter and status information in the form of multi-page “diagnostic addresses” and “diagnostic data.” Common data and status checks include parameter verification and interface diagnostics.

  (1) Parameter Verification: The machine tool data of a CNC machine are critical parameters obtained through a series of tests and adjustments, serving as the foundation for its proper operation. These parameters include gain settings, acceleration values, contour‑monitoring tolerances, backlash compensation, and lead‑screw pitch compensation, among others. When subjected to external disturbances, such data may be lost or become corrupted, causing the machine tool to malfunction.

  (2) Interface Inspection: The input/output interface signals between the CNC system and the machine tool include those connecting the CNC system to the PLC, as well as those linking the PLC to the machine tool. The CNC system’s I/O interface diagnostics can display the status of all discrete signals on the CRT monitor, using “1” or “0” to indicate the presence or absence of a signal. By leveraging this status display, it is possible to verify whether the CNC system has transmitted signals to the machine‑tool side and whether discrete signals from the machine‑tool side have been received by the CNC system, thereby enabling fault localization either on the machine‑tool side or within the CNC system.

  The alarm indicator light is signaling a fault.

  Inside the CNC system of modern CNC machine tools, in addition to the aforementioned software‑based alarms such as self‑diagnosis functions and status displays, there are numerous hardware alarm indicators. These are distributed across components like the power supply, servo drives, and input/output units, and their indications can be used to identify the root cause of a fault.

  Board Replacement Method

  Using a spare circuit board to replace a suspected faulty module is a quick and straightforward method for diagnosing the cause of a malfunction; it is commonly employed in CNC system functional modules such as CRT modules and memory modules. It should be noted that, prior to swapping in the spare board, the relevant circuits must be inspected to prevent short circuits from damaging the good board. Additionally, verify that the selector switches and jumpers on the test board match those of the original module; for certain modules, also ensure that the potentiometers are properly adjusted. After replacing a memory board, perform the necessary initialization procedures according to the system’s requirements; otherwise, the system will remain unable to operate normally.

  Exchange method

  In CNC machine tools, modules or units with identical functions are often available. By swapping identical modules or units and observing whether the fault shifts to another location, one can quickly pinpoint the faulty component. This method is commonly employed for diagnosing faults in servo feed drive systems and can also be applied to interchangeability testing among identical modules within the CNC system.

  Tapping method

  CNC systems consist of various circuit boards, each containing numerous solder joints; any cold solder joint or poor contact can lead to malfunctions. When gently tapping suspected faulty circuit boards, connectors, or electrical components with an insulating material, if the fault reappears, it is highly likely that the problem lies at the tapped location.

  Comparison Method of Measurement

  For ease of testing, test terminals are provided on modules or units. By using instruments such as multimeters and oscilloscopes to measure the voltage levels or waveforms at these terminals, and comparing the normal values with those observed under fault conditions, it is possible to identify the root cause of the fault and pinpoint its location. Given the comprehensive and complex nature of CNC machine tools, the factors that can lead to malfunctions are multifaceted. The diagnostic methods described above often require the simultaneous application of several techniques, enabling a holistic analysis that facilitates rapid identification of the faulty component and subsequent troubleshooting. Moreover, some fault symptoms may appear electrical in origin yet be caused by mechanical issues; conversely, mechanical symptoms may stem from electrical problems. In other cases, both electrical and mechanical factors may be involved. Consequently, diagnosing such faults cannot be reduced solely to electrical or mechanical causes; rather, a comprehensive, multidimensional approach is essential.