Seven methods for fault diagnosis of 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 with different functions. It is impossible to find a general diagnosis method suitable for all CNC machine tools and all types of faults. Here we only introduce some commonly used general methods, which are related to each other. In actual fault diagnosis, these methods should be used comprehensively.

1. Self-diagnostic function method. Although the modern numerical control system has not yet reached a high level of intelligence, it already has a strong self-diagnosis function. It can monitor the working status of the hardware and software of the CNC system at any time. Once an abnormality is found, an alarm message will be displayed on the CRT immediately or the approximate cause of the failure will be indicated with a light-emitting diode. Using the self-diagnosis function, the state 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 maintenance work today.

2. Functional program testing. The so-called functional program testing method is to compile the common functions and special functions of the CNC system, such as linear positioning, circular interpolation, helical cutting, canned cycles, user macro programs, etc., into a functional test program by manual programming or automatic programming. Input into the numerical control system, and then start the numerical control system to make it run, so as to check the accuracy and reliability of the machine tool to perform these functions, and then determine the possible cause of the failure. This method is used for checking the CNC machine tool that has been idle for a long time when it is turned on for the first time, and when the machine tool processing causes waste but no alarm, it is difficult to determine whether it is a programming error, an operation error, or the cause of the machine tool failure. method of judgment.

3. Isolation Act. The isolation method is to disconnect some control loops, so as to achieve the purpose of narrowing down the fault area. Example: In a machining center, in JOG mode, the feed is stable, but the automatic is abnormal. First of all, it is necessary to determine whether it is an NC fault or a servo system fault. First, disconnect the servo speed reference signal and use the battery voltage as the signal. The fault remains the same, indicating that there is no problem with the NC system. Further inspection was a malfunction of the Y-axis clamping device.

4. Local heating method. After long-term operation of the CNC system, the components will be aged, and the performance will be deteriorated. When they're not completely broken, the glitches can be sporadic. At this time, a hot hair dryer or an electric soldering iron can be used to locally heat up the suspected components and accelerate their aging, so as to completely expose the faulty components. Of course, when using this method, we must pay attention to the temperature parameters of the components, and do not bake the original good components.

5. percussion method. When the failure of the CNC system is sometimes absent, it is often possible to use the knocking method to check the location of the failure. This is because the CNC system is composed of multiple printed circuit boards, and each board has many solder joints, between boards or modules. They are connected through connectors and wires. Therefore, any virtual soldering or poor contact may cause malfunction. When the insulator is used to tap lightly on the suspected point of virtual welding and poor contact, the fault will definitely repeat. If it occurs, the tap is likely to be the fault.

6. Contrast method. When the CNC system manufacturer designs the printed circuit board, in order to facilitate adjustment and maintenance, a number of detection terminals are designed on the printed circuit board. 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 it is even possible to artificially create "faults" on normal printed circuit boards, such as disconnecting or short-circuiting, 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 of the printed circuit board when they are normal. Because CNC system manufacturers often do not provide information in this regard.

7. Diagnose using alarm number. Using the alarm number to diagnose faults is one of the main methods of fault diagnosis of CNC machine tools. If the machine tool breaks down and an alarm number is displayed on the CRT, the corresponding analysis and diagnosis should be carried out according to the content of the alarm number. Maintenance personnel can analyze the phenomenon pointed out by the alarm number, narrow the scope of inspection, and carry out inspection in a certain aspect with purpose. 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) Programmable control (5) Connection failure; (6) Abnormal temperature, pressure, liquid level, etc.; (7) The status of the travel switch (or proximity switch) is incorrect. In addition to the above commonly used fault inspection test methods, there are also pull-out method, voltage pull-off method, open-loop detection method and so on. Including the above-mentioned diagnostic methods, 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, and a comprehensive analysis of the fault can be carried out to gradually narrow the scope of the fault and eliminate the fault faster. . Once the faulty part is found, but there are no replacement parts on hand, it can be solved by transplanting and borrowing as an emergency measure.