This article takes five actual cases from the production line to describe the diagnosis method and maintenance process of the common faults of CNC machine tools, and provides readers with the train of thought and methods for the maintenance of CNC machine tools.
With the increasing popularity of CNC machine tools, the maintenance of CNC machine tools has also received increasing attention. The characteristics of CNC machine tools themselves determine that the maintenance of CNC machine tools is rather complicated and the requirements for maintenance personnel are relatively high. When a CNC machine tool fails, how to repair it at the first time is a problem faced by various companies. In this article, five typical maintenance examples of CNC machine tools are introduced for readers to use for reference.
1 axis fault Fault phenomenon: XHK716 vertical machining center is produced by Beijing No. 1 Machine Tool Factory. The X-axis moves when it reaches a certain fixed position. The machine tool does not alarm.
Fault analysis and inspection: Axis kinking may be caused by an abnormal speed loop or position loop. First check that the speed loop, tachometer, motor, driver, and connecting cable are normal. The X-axis of the machine tool uses an inductive synchronizer as a measuring rule. The excitation sine and cosine signals, amplifiers, scales, and sliders are all normal. However, the signal cable that moves with the table shows signs of wear. The measurement of the cable is sometimes broken. The phenomenon continues causing the X axis of the machine to oscillate.
Troubleshooting: Replace the cable troubleshooting.
2 Feed axis failure due to mechanical crawl Symptom: When a machining center is running, a clear mechanical crawling fault occurs during the displacement of the worktable in the Y-axis direction. When the fault occurs, the system does not alarm.
Fault analysis and inspection: When the fault occurs, the system does not report an alarm, and at the same time observe that the rate of the Y-axis displacement pulse digital quantity displayed by the CRT is uniform (observed by comparing the rate of change of the digital pulse of the X-axis and Z-axis displacement pulses). The influence of system software parameters and hardware control circuit faults can be eliminated. Since the fault occurs in the Y-axis direction, the fault location can be determined using the exchange method. By switching the servo control unit, the fault does not transfer, so the fault location should be on the side of the Y-axis servo motor and the screw drive chain. In order to distinguish the motor failure, the elastic coupling between the motor and the ball screw can be disassembled, and the motor can be individually checked for power. The inspection results showed that there was no vibration when the motor was running. It was obvious that the fault was in the mechanical transmission part. Remove the elastic coupling and use a wrench to turn the ball screw for feel inspection. Through the inspection of the hand, the presence of this kind of jitter failure is felt, and this abnormal phenomenon is present in the full stroke range of the screw. The ball screw was checked and the ball screw bearing was found to be damaged.
Troubleshooting: After replacing the new bearing with the same model, troubleshooting.
3 Machine overload alarm failure Symptoms: A CNC vertical machining center supporting FANUC-0M system often has an overload alarm during processing. The alarm number is 434. The manifestation is that the Z axis motor current is too high, the motor heats, and the alarm disappears after about 40 minutes. Then the alarm disappears. After working for a while, similar alarms appeared again.
Failure analysis and inspection: After checking the electrical servo system is not faulty, it is estimated that the load is too heavy to be fixed. In order to distinguish between an electrical fault and a mechanical fault, the Z-axis motor is removed and mechanically disengaged and the fault no longer occurs during operation. It was confirmed that the mechanical screw or the moving part is too tight.
Troubleshooting: Adjust Z-axis screw locknut, the effect is not obvious, and later adjust the Z-axis guide rails, machine load significantly reduced, the failure to eliminate.
4 Vibration failure Fault phenomenon: After a CNC lathe adopting the FANUC 0T CNC system, as soon as the Z-axis moves, there will be severe oscillations. The CNC will not alarm and the machine tool will not work properly.
Failure analysis and inspection: After careful observation and inspection, it was found that the machine's Z-axis moves in a small range (about 2.5mm) and works normally. The movement is smooth and without vibration: However, once the machine exceeds the above range, the machine tool vibrates violently.
According to the analysis of this phenomenon, the position control part of the system and the servo driver itself should be free of faults, and the initial judgment is that the fault is on the position detection device, ie, the pulse encoder.
Considering that the machine tool is a semi-closed-loop structure, it was confirmed by replacing the motor during maintenance that the cause of the fault was caused by a defect in the pulse encoder.
In order to understand in depth the root cause of the failure, the following analyses and tests were performed during maintenance.
(1) When the main circuit of the servo driver is de-energized, manually rotate the motor shaft and check the system display. It is found that regardless of the forward rotation or reverse rotation of the motor, the actual position value can be correctly displayed on the system display, indicating that the position encoder A, B, *A, *B signal output is correct.
(2) As the Z axis screw pitch of this machine tool is 5mm, as long as the Z axis moves about 2mm, it will vibrate. Therefore, the cause of the malfunction may be related to the actual position of the motor rotor, that is, the pulse position encoder detection signal C1, C2, Poor C4 and C8 signals.
Based on the above analysis, considering that the Z axis can normally move about 2.5 mm, which is equivalent to the fact that the motor actually rotates by 180°, therefore, it is further determined that the fault is the C8 in the rotor position detection signal.
According to the same method as above, after removing the pulse encoder, according to the connection requirements of the encoder, after adding DC5V to the pins N/T and J/K, rotate the encoder shaft and use the multimeter to measure C1, C2, and C4. C8, it is found that there is no change in the state of C8, and it is confirmed that there is a malfunction in the encoder rotor position detection signal C8. Further inspection revealed that the C8 output driver integrated circuit inside the encoder has been damaged.
Troubleshooting: After replacing the integrated circuit, re-install the encoder, and adjust the rotor angle in the same way as in the previous example, the machine will return to normal.
5 drive failure Fault phenomenon: A machining center supporting the FANUC 0M system. After the machine is started, the CRT displays the alarm number 401 in the automatic mode. Analysis and Processing: FANUC OM has an alarm of No. 401 which means "the VRDY signal of the axis servo driver is off, that is, the driver is not ready".
Fault analysis and inspection: According to the meaning of the fault and the actual configuration of the servo feed system on the machine tool, the maintenance and inspection were performed in the following order.
(1) Check the L/M/N axis servo driver and find that the driver status indicators PRDY and VRDY are not lit.
(2) Check that the servo driver power supply ACl00V, ACl8V are normal.
(3) Measure the auxiliary control voltage on the driver's control board and find that ±24V and ±15V are abnormal.
Based on the above checks, it can be initially determined that the fault is related to the control power of the driver.
Carefully check the input power and find that the input power fuse resistance on the X-axis servo drive is greater than 2 MΩ, far exceeding the specified value.
Troubleshooting: After replacing the fuse, measure the DC auxiliary voltage again, ±24V, ±15V return to normal, the status indicators PRDY, VRDY all return to normal, re-run the machine, alarm 401 disappears.
