CNC lathe maintenance servo feed system problem

1.1 Overtravel

When the feed motion exceeds the software limit set by the software or the hardware limit determined by the limit switch, an overtravel alarm will occur, and the alarm content will be displayed on the CRT. According to the CNC system manual, the fault can be eliminated. , lift the overtravel.

1.2 crawling

Generally due to factors such as poor lubrication of the feed chain, low gain of the servo system and excessive load. In particular, the coupling for servo and ball screw connection may cause the ball screw to rotate or the servo rotation to be unsynchronized due to the loose connection or the defects of the coupling itself, such as cracks. Fast and slow, causing creep.

1.3 Shaking

There is a phenomenon of turbulence during feeding. The possible causes are as follows: 1. The terminal is in poor contact, such as loose screws; 2. The position control signal is disturbed; 3. The speed measurement signal is unstable, such as the speed measuring device failure and speed feedback. Signal interference, etc. If the turbulence occurs at the moment of the forward and reverse movements, it is generally caused by the backlash of the feed train or the servo system gain being too large.

1.4 overload

When the load of the feed motion is too large, the parameter setting is wrong, the frequent positive and negative movements, and the lubrication condition of the feed transmission chain are poor, the overload fault will be caused. This fault is usually diagnosed by the machine itself and shows an overload, overheat or overcurrent alarm on the CRT display. At the same time, on the feed servo module, the indicator light or digital tube is used to display alarm information such as overload and over current of the drive unit.

1.5 Servo motor does not turn

This fault is caused when the speed, position control signal is not output, or the enable signal (ie, the servo enable signal, typically DC+24V relay coil voltage) is not turned on and the feed drive unit is faulty. At this time, it is necessary to measure whether the signal of the command output terminal of the numerical control device is normal, observe the I/O status through the CRT, analyze the PLC ladder diagram (or flow chart) of the machine tool to determine the starting condition of the feed axis, and observe whether lubrication, cooling, etc. Satisfy. If the feed drive unit fails, the exchange method can be used to determine whether the corresponding unit is faulty.

2 Typical case analysis of common faults in servo feed system

(1) A machining center equipped with the FANUC 7M system, during the feeding process, found that the Y-axis has vibration.

In order to determine the cause of the fault, the machine operation mode was placed in the manual mode, and the Y-axis feed was controlled by the hand pulse generator, and it was found that the Y-axis still had vibration. In this mode, after a long time of movement, the OVC alarm light on the Y-axis speed unit lights up. It is proved that the over-current alarm has occurred in the Y-axis servo drive. According to the above phenomenon, the possible reasons for the analysis are as follows:

1 motor overload is too heavy; 2 mechanical transmission system is poor; 3 position loop gain is too high; 4 servo motor is bad, and so on.

The maintenance method uses the interchange method to confirm that the cause of the fault is on the DC servo motor. When the Y-axis motor was removed, it was found that one of the two brushes had been blown, causing the armature current to be unbalanced and the motor output torque to be unbalanced. In addition, it was found that the bearing of the motor was also damaged, thereby causing vibration and overcurrent of the -axis. After replacing the motor bearings and brushes, the machine returns to normal.

(2) A machining center supporting the FANUC 6ME system. The speed of the shaft is unstable during the movement. During the movement to the stop, a large amplitude oscillation occurs at the stop position, and sometimes the positioning cannot be completed, and the machine must be turned off before it can be operated again.

Analysis and processing: Careful observation of the vibration of the machine tool, found that the X-axis oscillation frequency is low, and there is no abnormal sound. From the perspective of oscillation phenomenon, the fault phenomenon is related to the parameter setting of the closed-loop system, such as: the system gain setting is too high, and the integral time constant is set too large.

CNC lathe manufacturers

The parameter setting of the inspection system, the gain of the servo drive, and the adjustment of the integral time potentiometer are all in the proper range and are completely consistent with the adjustment before the fault. Therefore, it can be preliminarily judged that the oscillation of the shaft is independent of the setting and adjustment of the parameters. In order to further verify, the above parameters were re-adjusted and tested under the premise of recording the original adjustment value during maintenance, and it was found that the fault still existed, which proved the correctness of the judgment.


Post time: Aug-10-2018