External faults of CNC equipment can be divided into soft faults and hard faults caused by external hardware damage. Soft faults refer to faults caused by improper operation, adjustment and handling, which often occur in the early stages of equipment use or during the adjustment period of equipment users. For CNC systems, another prone area to malfunction is the servo unit. Due to the fact that the movement of each axis is achieved by the servo unit controlling the servo motor to drive the ball screw. Use a rotary encoder for speed feedback and a grating ruler for position feedback. The areas that are generally prone to malfunctions are the drive modules of the rotary encoder and servo unit. There are also some system disruptions caused by power supply issues. Especially for systems that have computer hard drives to store data. For example, the Siemens 840C system from Germany.
Example 1: When a CNC lathe is first put into use and restarted after the system is powered off, it must return to the reference point, that is, when manually moving each axis outside the non-interference zone, then return each axis to the reference point. Otherwise, a collision may occur. So, after processing every day, it is important to move the axis of the machine tool to a safe position. At this point, there will be no problem after operation or power off.
Malfunctions caused by external hardware operations are common faults in CNC repairs. It is usually caused by problems with detection switches, hydraulic systems, pneumatic systems, electrical actuators, and mechanical devices. Some of these types of faults can be traced back to the cause through alarm information. For general CNC systems, there are fault diagnosis functions or information alarms. Maintenance personnel can use these information methods to narrow down the scope of diagnosis. Some faults may have alarm information displayed, but they cannot reflect the true cause of the fault. At this point, it is necessary to analyze and solve the problem based on the alarm information and fault symptoms.
Example 2: The turning unit uses the SINUMERIK840C system. The machine suddenly stops working. Display spindle temperature alarm. After comparative inspection, the fault occurred on the temperature instrument, and the alarm disappeared after adjusting the peripheral circuit. Immediately replace the instrument panel and restore normal operation.
Example 3: For the same turning center, the CRT displays an alarm of 9160 "9160 NO PART WITH GRIPPER 1 CLOSED VERITY V14-5" during operation. This refers to the alarm for not picking up the workpiece. But in reality, the robotic arm that grabs the workpiece has already picked it up, but it displays an alarm indicating that the robotic arm has not picked up the workpiece. Refer to the PLC diagram, this fault is caused by measuring the induction switch. After investigation of the mechanical arm, it was found that the working stroke of the mechanical arm was not in place, which was caused by incomplete pressing of the induction switch. Then adjust the clamping force of the manipulator, and the fault is eliminated.
Example 4: A vertical machining center adopts FANUC-OM control system. When the machine tool moves rapidly on the X-axis in automatic mode, alarms 414 # and 410 # appear. This alarm is for speed control OFF and X-axis servo drive abnormality. Due to the fact that this fault can be eliminated by restarting, but an alarm is triggered every time the X-axis moves rapidly. After investigation, it was found that the power cord plug of the servo motor caused a phase to phase short circuit due to arc crawling. After repair, this fault was resolved.
Example 5: Improper operation by the operator is also an important cause of malfunction. If another CNC lathe using the 840C system works normally during the day, but cannot be turned on the next day, it will alarm "EMPTYING SELECTED MOOE SELECTOR" as soon as it switches to automatic mode. After processing the finished product, the spindle kept moving and the robotic arm went to grab the workpiece. Later, after careful inspection, all parts were found to be fine, but the position of a mode switch under automatic working conditions was incorrect. So, when some alarms with unknown causes of faults occur, it is necessary to check the switch positions under each working mode.
There are still some faults that do not generate fault alarm information, but only fail to complete the action. In this case, it is necessary to analyze and judge based on maintenance experience, the working principle of the machine tool, and the operation status of the PLC.
For the repair of CNC machine tools, it is important to identify problems. Especially external faults of CNC machine tools. Sometimes the diagnostic process can be complex, but once the problem is identified, it is relatively easy to solve. The diagnosis of external faults should follow the following two principles. Firstly, it is necessary to master the working principle and action sequence of the machine tool proficiently. Secondly, one should be able to utilize PLC ladder diagrams. The status display function of the NC system or the monitoring of the operating status of the PLC by an external programmer. Generally, as long as the above principles are followed with caution, general CNC faults will be promptly eliminated
