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How to solve the abnormal creeping of CNC machine tools?

In the process of driving the moving parts to run at low speed, the CNC machine tool feed system will have the moving parts unable to start at the beginning, and then suddenly accelerate after starting, and then pause, and then accelerate again. The moving parts will stop and jump, slow down and speed up in this cycle, which is called creeping. When it runs at high speed, the moving parts will vibrate obviously.

Creeping and vibration faults usually need to find problems in mechanical parts and feed servo systems. Because the creeping phenomenon of the CNC machine tool feed system at low speed often depends on the characteristics of the mechanical transmission parts, and the vibration phenomenon at high speed is usually related to the preload of the moving pair in the feed transmission chain. In addition, the creeping and vibration problems are closely related to the feed speed, so the speed loop and system parameters of the feed servo system should also be analyzed.

Inspection and elimination of mechanical component faults

If the cause of creeping and vibration is in the mechanical parts, first check the guide rail pair. Because the friction resistance of the moving parts mainly comes from the guide rail pair, if the dynamic and static friction coefficients of the guide rail pair are large and the difference is also large, it will easily cause creeping.

Although the guide rail pairs of CNC machine tools widely use rolling guides, hydrostatic guides or plastic guides, if they are not adjusted properly, they will still cause creeping or vibration. For hydrostatic guides, the focus should be on checking whether the static pressure is established; for plastic guides, check whether there are impurities or foreign matter that hinder the movement of the guide rail pair, and for rolling guides, check whether the preload is good.

Poor lubrication of the guide rail pair may also cause creeping problems. Sometimes the creeping phenomenon is simply caused by the poor lubrication of the guide rail pair. At this time, it is a very effective measure to use a guide rail lubricant with anti-climbing effect. This guide rail lubricant has polar additives that can form an oil film on the surface of the guide rail that is not easy to break, thereby improving the friction characteristics of the guide rail.

Secondly, check the feed transmission chain. In the feed system, the servo drive device must pass through the transmission chain composed of gears, screw nut pairs or other transmission pairs from the moving parts. Effectively improving the torsion and tension and compression stiffness of this transmission chain is very beneficial for improving motion accuracy and eliminating creep.

One of the reasons for the creep of moving parts is often due to the unsatisfactory pre-tightening or pre-tensioning of bearings, screw nut pairs and screws themselves. Too long a transmission chain, too small a transmission shaft diameter, and insufficient stiffness of the support and support seat are also factors that cannot be ignored in causing creep, so when checking, we must also consider whether these aspects are defective.

In addition, poor connection of the mechanical system, such as coupling damage, may also cause vibration and creep of the machine tool.

Inspection and elimination of feed servo system faults

If the cause of creep and vibration is in the feed servo system, it is necessary to check the relevant links in the servo system separately. Check the speed regulator, servo motor or tachometer generator, system interpolation accuracy, system gain, system parameter settings related to position control, whether the short-circuit bar setting on the speed control unit is correct, whether the gain potentiometer adjustment has deviations, and whether the circuit of the speed control unit is good, etc., and check and eliminate them one by one.

Speed ​​regulator detection

For speed regulator failure, mainly detect whether there are problems with the given signal, feedback signal and speed regulator itself. The given signal can be realized by detecting the analog signal VCMD sent to the speed regulator by the position deviation counter through D/A conversion. Whether this signal has vibration components can be observed by using an oscilloscope on the pins on the servo board. If there is a periodic vibration signal, there is no doubt that the machine vibration is correct, and there is no problem with the speed regulator, but the previous stage; then look for the problem in the D/A converter or deviation counter. If our measurement results do not have any periodic waveform of vibration, then the problem must be in the feedback signal and speed regulator.

Detection of speed motor feedback signal

The feedback signal and the given signal are exactly the same for the regulator. Therefore, the fluctuation of the feedback signal will inevitably cause the speed regulator to adjust in the opposite direction, thus causing the vibration of the machine tool. Since the machine tool is vibrating, it means that the speed of the machine tool is in violent oscillation. Of course, the waveform fed back by the speed generator must be turbulent. At this time, if there is an accurate ratio relationship between the vibration frequency of the machine tool and the speed of motor rotation, for example, the frequency of vibration is four times the frequency of the motor speed. At this time, we have to consider the problem of motor or tachometer generator failure.

Motor inspection

When the vibration frequency of the machine tool is in a certain ratio with the motor speed, we should first check whether the motor is faulty, check its carbon brushes, the surface condition of the commutator, and check the lubrication of the ball bearings.

In addition, the bad armature coil of the motor will also cause system vibration. This situation can be confirmed by measuring the no-load current of the motor. If the no-load current increases in proportion to the speed, it means that there is a short circuit inside the motor. When this fault occurs, the commutator should be cleaned first, the brushes should be checked, and then the measurement should be confirmed. If the fault phenomenon still exists, there may be a short circuit between the coil turns, and the motor should be repaired. If there is no problem, check the tachometer generator.

Detection of pulse encoder or tachometer generator

For the case of a bad pulse encoder or tachometer generator, the following method can be used for measurement and inspection. First, disconnect the position loop and speed loop, rotate the motor manually, and observe the voltage of the F/V converter on the printed circuit board of the speed control unit. If a waveform with a sudden drop in voltage appears, it means that the feedback component is bad.

A common problem in tachometer generators is that the carbon powder ground off by the carbon brush accumulates in the grooves between the commutator segments, causing a short circuit between the commutator segments of the tachometer generator. Once such a problem occurs, it will cause vibration.

Adjustment of system parameters

A closed-loop system may also cause system oscillation due to unreasonable parameter settings. The best way to eliminate oscillation is to reduce the amplification factor. In the FUNAC system, adjust RV1 and turn it counterclockwise. At this time, it can be seen that it will immediately improve significantly, but because the range of RV1 adjustment potentiometer is relatively small, sometimes it cannot be adjusted, and the only way is to change the short-circuit rod, that is, cut off the feedback resistance value, and reduce the amplification factor of the entire regulator.

Handling of external interference

For fixed interference, the waveforms of the F/V converter, current detection terminal and synchronization terminal can be checked to check whether there is interference, and corresponding measures can be taken. For accidental interference, it can only be avoided as much as possible through effective shielding, reliable grounding and other measures.

After using these methods, if the vibration cannot be completely eliminated, or even ineffective, it is necessary to consider replacing the speed regulator board or thoroughly checking the waveforms at all locations after replacing it.

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