In most CNC machining plants, machine tools operate 24 hours a day, and some older machines often experience various malfunctions. Based on the experience of repairing machine tools in the factory, this article summarizes three examples of maintenance to analyze the problems that often occur in CNC machine tools.
Fault of machine tool overload alarm
Fault phenomenon: A CNC vertical machining center equipped with FANUC-0M system often experiences overload alarms during machining, with alarm number 434. The manifestation is that the Z-axis motor current is too high, the motor heats up, and the alarm disappears after stopping for about 40 minutes. After working for a while, the same type of alarm occurs again.
Fault analysis and inspection: After inspection, the electrical servo system has no faults, and it is estimated that it is caused by excessive load and inability to move. In order to distinguish whether it is an electrical fault or a mechanical fault, the Z-axis motor was disconnected from the machinery and the fault no longer occurred during operation. This confirms that it is caused by mechanical screws or excessively tight moving parts.
Fault handling: After adjusting the anti loosening nut of the Z-axis screw, the effect was not significant. Later, the Z-axis guide rail insert was adjusted, and the machine load was significantly reduced. This fault was eliminated.
Driver malfunction
Fault phenomenon: A machining center equipped with FANUC0M system (injection mold), after the machine tool is started and runs in automatic mode, the CRT displays an alarm number 401. Analysis and processing process: The meaning of FANUCOM’s 401 alarm is “the VRDY signal of the axis servo drive is disconnected, that is, the drive is not ready”.
Fault analysis and inspection: Based on the meaning of the fault and the actual configuration of the servo feed system on the machine tool, the maintenance was carried out in the following order for inspection and confirmation.
(1) Measuring the auxiliary control voltage on the driver control board, it was found that ± 24V and ± 15V were abnormal.
(2) Check that the servo drive power supply ACl00V and ACl8V are both normal.
(3) Check the servo drive of the L/M/N axis and find that the status indicator lights PRDY and VRDY of the drive are not on.
Based on the above inspection, it can be preliminarily determined that the fault is related to the control power supply of the driver.
Upon careful inspection of the input power supply, it was found that the resistance of the input power fuse on the X-axis servo driver was greater than 2M Ω, far exceeding the specified value.
Fault handling: After replacing the fuse, the DC auxiliary voltage was measured again, and ± 24V and ± 15V returned to normal. The status indicator lights PRDY and VRDY returned to normal. The machine tool was restarted, and the 401 alarm disappeared.
Axis displacement fault
Fault phenomenon: The XHK716 vertical (injection mold) machining center produced by Beijing First Machine Tool Factory experiences movement of the X-axis when it reaches a fixed position, and the machine tool does not sound an alarm.
Fault analysis and inspection: Shaft movement may be caused by abnormal speed or position rings. Firstly, check that the speed loop, tachometer, motor, driver, and connecting cables are functioning properly. The X-axis of the machine tool uses an induction synchronizer as the measuring ruler. The excitation sine and cosine signals, amplifier, fixed ruler, and sliding ruler are all normal during inspection. However, there are obvious signs of wear on the signal cable that moves with the workbench. The measurement of the cable shows intermittent phenomena, causing the X-axis of the machine tool to move.
Troubleshooting: Replace the cable and troubleshoot.
Of course, there may not only be these three points, but there are also many more, which may be more detailed than this. However, we are still learning and accumulating. On the future road, we will pay attention to the maintenance of machine tools, reduce the failure rate of machine tools, improve production efficiency, and ensure product quality.
