Seven methods for diagnosing CNC machine tool faults

CNC machine tools are highly complex systems involving multiple applied disciplines. Furthermore, the variety of CNC systems and machine tools themselves, with their diverse functions, makes it impossible to find a single universal diagnostic method suitable for all CNC machine tools and all types of faults. Here, we only introduce some commonly used general methods. These methods are interconnected and should be used comprehensively in actual fault diagnosis.

1. Self-Diagnostic Function Method: Although modern CNC systems have not yet reached a high level of intelligence, they possess strong self-diagnostic capabilities. They can monitor the working status of the CNC system's hardware and software at any time. Once an abnormality is detected, alarm information is immediately displayed on the CRT or LEDs indicate the approximate cause of the fault. Using the self-diagnostic function, the status of the interface signals between the system and the host machine can also be displayed, thus determining whether the fault occurs in the mechanical part or the CNC system part. This method is one of the most effective methods in current maintenance work.

2. Functional Program Testing Method: This method involves manually or automatically programming common and special functions of the CNC system, such as linear positioning, circular interpolation, helical cutting, fixed cycles, and user macro programs, into a functional test program. This program is then input into the CNC system, and the system is started to run it. This checks the accuracy and reliability of the machine tool in executing these functions, thus identifying the possible causes of faults. This method is particularly useful for first-time startups of CNC machine tools that have been idle for a long time, and for situations where machine processing results in scrap but no alarms are triggered, making it difficult to immediately determine whether the cause is a programming error, an operational error, or a machine tool malfunction.

3. Isolation Method: The isolation method involves disconnecting certain control loops to narrow down the fault location. Example: A machining center feeds smoothly in JOG mode, but malfunctions in automatic mode. First, determine whether the fault is in the NC or the servo system. Disconnect the servo speed input signal and use battery voltage as the signal; if the fault persists, the NC system is not the problem. Further investigation reveals a fault in the Y-axis clamping device.

4. Local Heating Method: After long-term operation, components in a CNC system will age and their performance will deteriorate. When they are not completely damaged, the faults may appear intermittently. In this case, a hot air blower or soldering iron can be used to locally heat the suspected components to accelerate their aging and thoroughly expose the faulty parts. Of course, when using this method, it is essential to pay attention to the temperature parameters of the components to avoid damaging previously good components.

5. Tapping Method: When the faults in a CNC system are intermittent, the tapping method can often be used to locate the fault. This is because a CNC system consists of multiple printed circuit boards, each with many solder joints, and the boards or modules are connected by connectors and wires. Therefore, any cold solder joint or poor contact can cause a fault. When you gently tap the suspected area with a cold solder joint or poor contact with an insulated object, the fault will definitely reappear. If it does, the tapped area is likely the location of the fault.

6.Comparison Method: For ease of adjustment and maintenance, CNC system manufacturers design multiple test terminals on the printed circuit boards. Users can also use these terminals to compare and measure the differences between normal and faulty printed circuit boards (PCBs). The voltage and waveform of these measurement terminals can be detected to analyze the cause and location of the fault. Sometimes, even normal PCBs can be artificially "faulted," such as by disconnecting wires, short-circuiting, or removing components, to determine the true cause of the fault. Therefore, maintenance personnel should regularly collect the correct waveforms and voltage values of critical or fault-prone parts of the PCB under normal conditions, as CNC system manufacturers often do not provide this information.

7. Diagnostic Using Alarm Numbers Using alarm numbers for fault diagnosis is one of the main methods for diagnosing CNC machine tool faults. If a machine tool malfunctions and an alarm number is displayed on the CRT, the first step is to analyze and diagnose based on the alarm number's content. Maintenance personnel can then analyze the phenomena indicated by the alarm number, narrow down the scope of the inspection, and perform targeted checks on specific aspects. Alarm numbers (error codes) generally include the following types of fault (or error) information: (1) Programming error or operational error; (2) Memory malfunction; (3) Servo system failure; (4) Programmable controller failure; (5) Connection failure; (6) Abnormal temperature, pressure, liquid level, etc.; (7) Incorrect status of limit switch (or proximity switch). In addition to the commonly used fault inspection and testing methods mentioned above, there are also the plate pulling method, voltage pull-off method, open-loop testing method, etc. Including the diagnostic methods mentioned above, all these inspection methods have their own characteristics. According to different fault phenomena, several methods can be selected simultaneously and applied flexibly to comprehensively analyze the fault, so as to gradually narrow down the fault range and eliminate the fault more quickly. Once the fault location is found, but there are no replacement spare parts available, the transfer and borrowing method can be used as an emergency measure to solve the problem.