External Fault Diagnosis and Maintenance of CNC Machine Tools

As the reliability of the modern numerical control system is getting higher and higher, the faults of the numerical control system itself are getting lower and lower, and most of the faults are caused by the non-system itself. Faults outside the system mainly refer to problems caused by the detection of switches, hydraulic components, pneumatic components, electrical actuators, and mechanical devices.

External faults of CNC equipment can be divided into soft faults and hard faults caused by external hardware damage. Soft faults are caused by improper handling or adjustments. These types of faults often occur during the early use of the equipment or during the adjustment period of the equipment user. For the CNC system, another easy-to-failure location is the servo unit. Because the movement of each axis is achieved by the servo unit controlling the servo motor to drive the ball screw. A rotary encoder is used for speed feedback and a grating scale is used for position feedback. The common fault areas are the rotary encoder and servo unit driver modules. There are also individual system confusions caused by power sources. Especially for those systems that have computer hard disks to save data. For example, the German Siemens System 840C.

Example 1: When a CNC lathe is just put into use, it must be returned to the reference point when the system restarts after the power is turned off. That is, when the axes are manually moved outside the non-interference zone, the axes are returned to the reference point. Otherwise, a collision may occur. Therefore, it is best to move the axis of the machine tool to a safe position after processing every day. There will be no problem after re-operation or power off.

Faults caused by external hardware operations are common faults in CNC repairs. It is generally caused by problems with the detection switch, hydraulic system, pneumatic system, electrical actuator, and mechanical device. Some of these types of faults can use the alarm information to find the cause of the fault. For general CNC systems, there are fault diagnosis functions or information alarms. Maintenance personnel can use this information to narrow the diagnosis. Although some faults have alarm information display, they do not reflect the true cause of the fault. At this time, it needs to be analyzed and solved according to the alarm information and fault phenomena.

Example 2: The SINUMERIK 840C system is used in a turning unit of our factory. The machine suddenly shuts down during work. The spindle temperature alarm is displayed. After a comparison check, the fault appeared on the temperature meter and the alarm disappeared after adjusting the peripheral circuit. It will return to normal after the new instrument is replaced.

Example 3: This is also the turning center. The CRT displays 9160 alarm when working.

"9160 NO PART WITH GRIPPER 1 CLOSED VERIFY V14-5". This means that the workpiece alarm was not picked up. However, in fact, the robot grasping the workpiece has picked up the workpiece, but it shows that the robot did not pick up the workpiece alarm. Check the PLC diagram. This fault is measured by the sensor switch. After checking the manipulator parts, the working stroke of the manipulator was not in place and the induction switch was not pressed completely. Then adjust the clamping force of the robot, this troubleshooting.

Example 4: A vertical machining center uses the FANUC-OM control system. The 414# and 410# alarms occur when the machine tool executes an X-axis rapid movement in automatic mode. This alarm is the speed control OFF and the X axis servo drive anomaly. Since this fault can be eliminated by restarting, it will be alarmed every time X-axis moves fast. After investigation, the servo motor power cord plug caused a phase-to-phase short circuit due to arc creep. After the trimming, this fault was eliminated.

Example 5: Improper operator operation is also an important cause of failure. For example, another CNC lathe using the 840C system at our factory is completely normal on the first day of work. However, it cannot be opened anyway at work on the second day. When the work mode is switched to the automatic mode, the alarm “EMPTYING SELECTED MOOE SELECTOR” ". After the workpiece has been machined, the spindle does not stop, the robotic arm picks up the workpiece, and then carefully checks that there is no fault in each part, but the position of a mode switch under automatic working conditions is wrong. Therefore, when some alarms with unknown causes appear, it is necessary to check the switch position in each working mode.
Some faults do not produce fault alarm information, but the action cannot be completed. At this time, it is necessary to analyze and judge according to the maintenance experience, the working principle of the machine tool, and the PLC operating status.

For the repair of CNC machine tools, it is important to find problems. Especially the external failure of CNC machine tools. Sometimes the diagnosis process is complicated, but once the problem is found, the solution is relatively simple. The following two principles should be followed for external fault diagnosis. First of all, it is necessary to master the working principle and operation sequence of the machine tool. Second, use PLC ladder diagrams. The status display function of the NC system or the off-board programmer monitors the operating status of the PLC. Generally, as long as the above principles are complied with, cautions will be taken. General NC faults will be eliminated in time.