The application of CNC machine tools is becoming more and more extensive. It has good processing flexibility, high precision and high production efficiency, and has many advantages. However, as the technology is becoming more and more advanced and complex, the quality requirements of maintenance personnel are very high, and they are required to have deep professional knowledge and rich maintenance experience, so that they can be eliminated in time when the CNC machine tool breaks down.

　　The application of CNC machine tools is more and more extensive. Our company has dozens of CNC equipment, and there are many types of CNC systems. Over the past few years, some failures have occurred in these equipments. Through the analysis and treatment of these failures, we have gained certain experience. The following combines some typical examples to systematically analyze the faults of CNC machine tools for reference.

　　1. NC system failure

　　1. Hardware failure

　　Sometimes due to hardware damage of the NC system, the machine tool is shut down. For the diagnosis of this kind of fault, it is necessary to understand the working principle of the numerical control system and the function of each circuit board first, then analyze according to the fault phenomenon, and use the exchange method to accurately locate the fault point if conditions permit.

　　Example 1. A CNC machine tool adopting German Siemens SINUMERIK SYSTEM3, its PLC adopts S5─130W/B, once a failure occurs, the R parameter input through the PC function of the NC system does not work during processing, and the R in the processing program cannot be changed. The numeric value of the parameter. Through the analysis of the working principle and fault phenomena of the NC system, we believe that there is a problem with the main board of the PLC. After swapping with the main board of another machine tool, it is further determined to be a problem with the main board of the PLC. Repaired by a professional manufacturer, the fault was ruled out.

　　Example 2. Another machine tool also uses SINUMERIK SYSTEM 3 CNC system. The program number of its machining program cannot be entered, and automatic processing cannot be carried out. It was confirmed that there was a problem with the memory board of the NC system, and the fault was eliminated after maintenance.

　　Example 3: A CNC milling machine using TNC 155 from HEIDENHAIN, Germany, once failed, the system often crashed during work, and the machine parameters and programs were often lost when the power was cut off. After inspection, it was found that the main board of the NC system was bent and deformed. After straightening and fixing, the system returned to normal, and no similar failures occurred again.

　　2. Soft failure

　　Some failures of CNC machine tools are caused by the parameters of the NC system machine tools. Sometimes the parameters are changed or confused due to improper settings, and sometimes the parameters are changed or confused due to accidents. Such failures will naturally disappear as long as the parameters are adjusted. There are also some failures that cause the NC system to be in an endless loop due to accidental reasons. Sometimes such failures must be forced to start to restore the use of the system.

　　Example 1. A CNC lathe using the FANUC-OT system of Japan's FANUC company crashes every time it is turned on, and any normal operation does not work. Finally, the method of forced reset is adopted to clear all the system memory, and the system returns to normal. After re-entering the machine parameters, the machine can be used normally. This failure is caused by the confusion of machine parameters.

　　Example 2: A dedicated CNC milling machine, the NC system adopts Siemens SINUMERIK SYSTEM 3. During batch processing, the NC system displays No. 2 alarm "LIMIT SWITCH". This kind of failure is because the Y-axis stroke exceeds the limit value set by the software. Check There is no change in the program value. After careful observation of the fault phenomenon, when a fault occurs, the Y-axis coordinate displayed on the CRT is determined to have reached the software limit. After careful research, it is found that it is caused by the increase in the input of the compensation value. After properly adjusting the software limit setting, the fault occurs. to be excluded. This fault is caused by improper software limit setting.

　　Example 3: A CNC machine tool using Siemens SINUMERIK 810 had a problem once, and the system entered the AUTOMATIC state every time it was turned on, and could not perform any operations, and the system crashed. After forced startup, the system resumes normal operation. This failure is the NC system in an endless loop due to operator error or other reasons.

　　3. NC system failure caused by other reasons Sometimes the system failure may also be caused by the failure of the power supply or the failure of the buffer battery.

　　Example 1: A CNC machine tool using Siemens SINUMERIK SYSTEM 3 from Germany had a fault once. After the NC system was powered on, the CRT did not display. The inspection found that the LED on the left side of the "COUPLING MODULE" board on the NC system was shining, indicating a fault . After warm booting the PLC, the system works normally. But after a few days, the fault reappeared. After analyzing the flashing frequency of the LED, it was determined to be a battery fault. After replacing the battery, the fault was eliminated.

　　Example 2: A CNC machine tool using Siemens SINUMERIK 810, sometimes in the process of automatic processing, the system suddenly loses power, measured its 24V DC power supply, and found that it is only about 22V. When the grid voltage fluctuates downward, the voltage drops, resulting in The NC system takes protective measures and automatically cuts off the power. It was confirmed that the rectifier transformer was short-circuited between turns, resulting in insufficient capacity. After replacing the rectifier transformer with a new one, the fault was eliminated.

　　Example 3: Another CNC machine tool that also uses Siemens SINUMIK 810 has such a fault. When the system is powered on, the system starts self-test, and when the self-test is completed and enters the basic screen, the system powers off. After analysis and inspection, it was found that the X-axis brake coil was short-circuited to ground. After the system self-inspection, the servo condition is ready, and the brake is energized and released. The brake coil is powered by 24V power supply. Due to the short circuit of the coil to the ground, the 24V voltage drops instantly, and the NC system takes protective measures to automatically cut off the power.

　　Second, the failure of the servo system

　　Since the control core of the CNC system is to digitally control the feed part of the machine tool, and the feed is realized by the servo unit controlling the servo motor and driving the ball screw, the rotary encoder is used as the position feedback element to form a semi-closed loop position Control System. So the servo system plays a very important role in the CNC machine tool. The failure of the servo system is generally caused by problems in the servo control unit, servo motor, speed measuring motor, encoder, etc. Here are a few examples:

　　Example 1. The servo motor is damaged

　　A CNC lathe using SINUMERIK 810/T, once the turret broke down and the rotation was not in place. When the turret was rotating, alarm No. 6016 "SLIDE POWER PACK NO OPERATION" occurred. According to the analysis of the working principle and fault phenomenon, the turret rotation It is driven by a servo motor. As soon as the motor starts, the servo unit generates an overload alarm, cuts off the servo power supply, and feeds back to the NC system, displaying 6016 alarm. Checking the mechanical part and replacing the servo unit did not solve the problem. After replacing the servo motor, the fault was eliminated.

　　Example 2: An American CNC grinding machine using a DC servo system, when the E-axis moves, an "E AXIS EXECESSFOLLOWING ERROR" alarm is generated. Observe the fault occurrence process. When the E-axis is started, the E-axis starts to move, and the E-axis value displayed on the CRT Change, when the value changes to 14, it suddenly jumps to 471, so we think there is a problem with the feedback part, replace the position feedback board, and the fault is eliminated.

　　Example 3: On another CNC grinding machine, the E-axis dresser was out of control, and the E-axis could return to the reference point, but when it was automatically trimmed or semi-automatic, the movement speed was extremely fast until it hit the limit switch. Observing the process of the failure, it is found that when the limit switch is hit, the displayed coordinate value is much smaller than the actual value, which must be a problem of position feedback. But replacing the feedback board and encoder didn't solve the problem. After careful study, it was found that the E-axis dresser is driven by the Z-axis. Generally, when returning to the reference point, the E-axis is on the side of the Z-axis, and when trimming, the E-axis dresser is brought to the middle by the Z-axis. For this reason, we have done such an experiment, moved the E-axis dresser to the middle of the Z-axis, and then returned to the reference point. The E-axis feedback cable is broken and the contact is poor. The calibration line confirmed our judgment, found the breakpoint, welded and took anti-folding measures to restore the machine tool to work.

　　3. External failure

　　Due to the increasing variability of modern CNC systems, the failure rate is getting lower and lower, and failures rarely occur. Most of the failures are non-system failures and are caused by external factors.

　　1. Modern CNC equipment is a product of mechatronics, with a relatively complex structure, complete protection measures, and a very high degree of automation. Some failures are not caused by hardware damage, but are caused by improper operation, adjustment and handling. This type of failure occurs more frequently in the early stage of equipment use, when the operators and maintenance personnel are not particularly familiar with the equipment.

　　Example 1. When a CNC milling machine was first put into use, the rotary table often had the problem of non-rotation. After analyzing the working principle and processing process of the machine tool, it was found that this problem was related to the indexing device. Only the indexing device was in the The table can only rotate when it is in the initial position.

　　Example 2. Another CNC milling machine had a knife-breaking accident. After pressing the emergency stop button, a new knife was replaced, but the workbench did not rotate. Through PLC ladder diagram analysis, it was found that the tool-changing process was incorrect. The computer believed that the tool-changing process was not correct. End, other operations cannot be performed. After changing the tool again according to the correct program, the machine tool returns to normal.

　　Example 3. There are several CNC machine tools. When they are just put into use, sometimes accidents occur. After the operator presses the emergency stop button, the system is powered off and restarted. At this time, the machine tool does not return to the reference point and must be adjusted. Sometimes the shaft has to be turned manually into the non-interference zone. Later, I learned a lesson and changed the operation mode to manual after pressing the emergency stop button, released the emergency stop button, and restored the machine tool to the normal position. At this time, there will be no problems when the machine is operated or the power is turned off.

　　2. Faults caused by external hardware damage

　　This type of failure is a common failure of CNC machine tools, and is generally caused by problems with detection switches, hydraulic systems, pneumatic systems, electrical actuators, and mechanical devices. Some faults can generate an alarm, and the cause of the fault can be found by reporting information.

　　Example 1. A CNC grinding machine, the CNC system adopts Siemens SINUMERIK SYSTEM 3, and a fault alarm F31 "SPINDLE COOLANT CIRCUIT" occurs, indicating that there is a problem with the spindle cooling system, but there is no problem when checking the cooling system. Check the PLC ladder diagram. This fault is caused by The flow detection switch B9.6 is detected. Check the switch and find that the switch is damaged. Replace the switch with a new one, and the fault disappears.

　　Example 2: A NC quenching machine tool using Siemens SINUMERIK 810 once appeared 6014 "FAULT LEVEL HARDENING LIQUID" and the machine tool could not work. The alarm information indicates that the quenching liquid level is not enough, check that the liquid level has far exceeded the minimum level, check the liquid level switch, and find that there is a problem with the liquid level switch, replace it with a new switch, and the fault is eliminated.

　　Although some faults have alarm information, they cannot reflect the root cause of the fault. At this time, it should be analyzed according to the alarm information and fault phenomenon.

　　Example 3: On a CNC grinding machine, when the E-axis returns to the reference point, the E-axis rotates but does not find the reference point, and keeps moving until the limit switch is pressed, and the NC system displays an alarm "EAXIS AT MAX.TRAVEL". According to the analysis of the fault phenomenon, there may be a problem with the zero point switch. It is confirmed that the non-contact zero point switch is damaged. Replace with a new switch and the fault is eliminated.

　　Example 4. A dedicated CNC milling machine breaks down during the batch processing of parts. Every time the parts have been processed and the Z-axis has not moved back in place, a failure occurs at this time, the processing program is interrupted, and the spindle stops. And display No. F97 alarm "SPINDLESPEED NOT OK STATION 2", indicating that there is a problem with the spindle, check the spindle system and there is no problem, other problems can also cause the spindle to stop, so we use an external programmer to monitor the running status of the PLC ladder diagram and find that The tool hydraulic clamping pressure detection switch F21.1 is disconnected instantly when a fault occurs. Its disconnection indicates that the clamping force of the milling cutter is not enough. For the sake of safety, the PLC stops the spindle. After inspection, it was found that the hydraulic pressure was unstable, and the hydraulic system was adjusted to make it stable, and the fault was eliminated.

　　There are also some faults that do not generate a fault alarm, but the action cannot be completed. At this time, the fault must be judged according to the maintenance experience, the working principle of the machine tool, and the operating status of the PLC.

　　Example 5. A CNC machine tool fails once, the load door cannot be closed, automatic processing cannot be performed, and there is no fault display. The load door is switched by the cylinder, and the load door is closed by the PLC output Q2.0 to control the solenoid valve Y2.0. Check the PLC with the PC function of the NC system

　　The status of Q2.0 is 1, but the solenoid valve is not powered. It turns out that the PLC output Q2.0 controls the solenoid valve Y2.0 through the intermediate relay. The damage of the intermediate relay caused this fault. Replace the new relay and the fault is eliminated.

　　Example 6: A CNC machine tool, the workbench does not rotate, and the NC system does not display a fault alarm. According to the action principle of the workbench, the first step of the workbench rotation should be to float the workbench pneumatically, use the external programmer to track the dynamic changes of the PLC ladder diagram, and find that the PLC signal has not been sent, continue to check according to this clue, and finally It was found that the initial position detection switches I9.7 and I10.6 of the second and third station indexing heads were out of sync, which resulted in the non-rotation of the workbench. It is further confirmed that the three-station dividing head has a mechanical misalignment, and the mechanical device is adjusted to synchronize with the two-station, so that the fault is eliminated.

　　Finding a problem is the first and most important step in solving it. Especially for external faults of CNC machine tools, sometimes the diagnosis process is more complicated. Once the problem is found, it is easier to solve it. For the diagnosis of external faults, we have summed up two points of experience. First, we should be proficient in the working principle and action sequence of the machine tool. Secondly, we must skillfully use the PLC ladder diagram provided by the manufacturer, use the status display function of the NC system or use an external programmer to monitor the operating status of the PLC, and determine the fault point according to the chain relationship of the ladder diagram. As long as the above two points are achieved, Generally, the external faults of CNC machine tools will be eliminated in time.