Seven methods of fault diagnosis for CNC machine tools

CNC machine tools are very complex systems involving multiple application disciplines. In addition, there are many types of CNC systems and machine tools themselves, and their functions are different. It is impossible to find a 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 related to each other. In actual fault diagnosis, these methods should be used in combination.

1. Self-diagnosis function method Although modern CNC systems have not yet reached a high level of intelligence, they already have strong self-diagnosis functions. They can monitor the working conditions of the hardware and software of the CNC system at any time. Once an abnormality is found, the alarm information is immediately displayed on the CRT or the approximate cause of the fault is indicated by the light-emitting diode. Using the self-diagnosis function, the status of the interface signal between the system and the host can also be displayed, so as to determine whether the fault occurs in the mechanical part or the CNC system part. This method is one of the most effective methods for current maintenance work.

2. Functional program test method The so-called functional program test method is to compile the common functions and special functions of the CNC system, such as linear positioning, circular interpolation, spiral cutting, fixed cycle, user macro program, etc., into a functional test program by manual programming or automatic programming, input it into the CNC system, and then start the CNC system to run it, so as to check the accuracy and reliability of the machine tool to execute these functions, and then determine the possible cause of the fault. This method is a good judgment method for the first time inspection of a long-idle CNC machine tool and when the machine tool processing causes scrap but there is no alarm. It is difficult to determine whether it is a programming error or an operating error, or the cause of the machine tool failure.

3. Isolation method The isolation method is to disconnect certain control loops to achieve the purpose of narrowing the fault search area. Example: In a machining center, in JOG mode, the feed is stable, but the automatic is abnormal. First, it is necessary to determine whether it is an NC fault or a servo system fault. First, disconnect the servo speed setting signal and use the battery voltage as a signal. The fault still exists, indicating that there is no problem with the NC system. Further inspection shows that the Y-axis clamping device is faulty.

4. Local heating method After long-term operation, the components of the CNC system will age and their performance will deteriorate. When they are not completely damaged, the faults will become intermittent. At this time, a hot hair dryer or electric soldering iron can be used to locally heat up the suspected components to accelerate their aging so as to completely expose the faulty components. Of course, when using this method, you must pay attention to the temperature parameters of the components and do not burn out the components that were originally good.

5. Knocking method When the faults in the CNC system appear to be intermittent, the knocking method can often be used to check the location of the fault. This is because the CNC system is composed of multiple printed circuit boards, each of which has many solder joints, and the boards or modules are connected by connectors and wires. Therefore, any cold soldering or poor contact may cause a fault. When the suspected cold soldering and poor contact are gently tapped with an insulating object, the fault will definitely recur. If it occurs, the knocked point is likely to be the faulty part.

6. Comparison method When designing printed circuit boards, CNC system manufacturers design multiple detection terminals on the printed circuit boards for the convenience of adjustment and maintenance. Users can also use these terminals to compare and measure the difference between a normal printed circuit board and a faulty printed circuit board. The voltage and waveform of these measurement terminals can be detected to analyze the cause and location of the fault. Sometimes, a "fault" can even be artificially created on a normal printed circuit board, such as disconnecting the connection or short circuit, unplugging components, etc., to determine the cause of the real fault. For this reason, maintenance personnel should accumulate the correct waveform and voltage values of key parts or fault-prone parts on the printed circuit board when they are normal. Because CNC system manufacturers often do not provide relevant information in this regard.

7. Diagnosis using alarm numbers Fault diagnosis using alarm numbers is one of the main methods of fault diagnosis for CNC machine tools. If a machine tool fails and an alarm number is displayed on the CRT, the first thing to do is to perform corresponding analysis and diagnosis based on the content of the alarm number. Maintenance personnel can analyze the phenomenon indicated by the alarm number, narrow the scope of inspection, and purposefully conduct inspections in a certain aspect. The alarm number (error code) generally includes the following fault (or error) information: (1) Programming error or operation error; (2) Memory is not working properly; (3) Servo system failure; (4) PLC failure; (5) Connection failure; (6) Abnormal temperature, pressure, liquid level, etc.; (7) Incorrect status of travel switch (or proximity switch). In addition to the above commonly used fault inspection and testing methods, there are also board pulling method, voltage deviation method, open loop detection 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 and applied flexibly at the same time. Only by conducting a comprehensive analysis of the fault can the fault scope be gradually narrowed and the fault can be eliminated quickly. Once the fault location is found, but there is no spare part on hand, the transplantation and borrowing method can be used as an emergency measure to solve it.