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Faults and fault classification of CNC machine tools

The phenomenon that a CNC machine tool loses all or part of its specified functions is called a CNC machine tool failure.

CNC machine tools are products of mechatronics, with advanced technology and complex structures. The faults of CNC machine tools are also diverse and different, and the causes of faults are generally complex, which brings many difficulties to the fault diagnosis and maintenance of CNC machine tools. In order to facilitate the fault analysis and diagnosis of machine tools, this section roughly divides the faults of CNC machine tools into the following categories according to factors such as the nature of the fault, the cause of the fault, and the location of the fault.

1. Classification according to the nature of faults that occur in CNC machine tools

(1) Systemic failure

This type of fault refers to the fault that will inevitably occur in the machine tool or CNC system as long as certain conditions are met. For example, if the power grid voltage is too high or too low, the system will generate an overvoltage alarm or too low alarm; when the cutting amount is too large, an overload alarm will be generated.

For example, during the processing of a CNC machine tool using the SINUMERIK810 system, the system sometimes automatically cuts off the power and shuts down. After restarting, it can still work normally. Based on the system working principle and fault phenomenon, it is suspected that the cause of the fault is the fluctuation of the system power supply voltage. I measured the 24V input power supply on the system power module and found that it was about 22.3V. When the machine tool is processing, this voltage also fluctuates downward, especially when the cutting volume is large. , the voltage drops greatly, sometimes approaching 21V. At this time, the system automatically cuts off the power and shuts down. In order to solve this problem, replace the 24V power transformer with a large capacity to completely eliminate this fault.

(2) Random failure

This type of failure refers to a failure that only occasionally occurs once or twice under the same conditions. It is not easy to reproduce the same fault artificially, and sometimes it is difficult to encounter it again for a long time. Analysis and diagnosis of this type of fault are relatively difficult. Generally speaking, this type of failure is often related to the looseness and dislocation of the mechanical structure, the drift of the working characteristics of some components in the CNC system, and the reduced reliability of the machine tool’s electrical components.

For example, in a CNC groove grinder, problems occasionally occur during the processing, and the position of the grinding groove changes, resulting in waste products. Analyzing the working principle of this machine tool, during grinding processing, first the measuring arm swings downward to the clamping position of the workpiece, and then the workpiece begins to move. When the reference end face of the workpiece contacts the measuring head, the CNC device records the position at this time data, then the measuring arm is raised and the machining program continues to run. The CNC device grinds the groove at a certain distance from the end face based on the position data of the end face, so the accuracy of the groove position is closely related to the accuracy of the measurement. Because it does not occur often, it is difficult to observe the malfunction. Therefore, according to the working principle of the machine tool, no problem was found when inspecting the measuring head; when inspecting the rotation of the measuring arm, it was found that the rotation axis was a little tight. It is possible that the measuring arm was not in position accurately sometimes, causing measurement errors. The rotating shaft was disassembled and inspected and found to be severely worn. New spare parts were made and replaced, and the fault never occurred again.

2. Classification by fault type

According to the type of machine tool fault, faults can be divided into mechanical faults and electrical faults.

(1) Mechanical failure

This type of failure mainly occurs in the host part of the machine tool, and can also be divided into mechanical component failure, hydraulic system failure, pneumatic system failure and lubrication system failure, etc.

For example, when a CNC quenching machine tool using the SINUMERIK 810 system is turned on to return to the reference point and move the X-axis, the alarm 1680 “SERVOENABLETRAV. AXISX” appears. This alarm also occurs when the X-axis is manually moved. Check the servo device and find an overload alarm indication. According to the Siemens instruction manual, the cause of this failure may be excessive mechanical load, problems with the servo control power supply, servo motor failure, etc. In line with the principle of mechanical first and then electrical, the X-axis slide was first tested and the X-axis slide was manually turned. It was found that it was very heavy and the slide did not move, indicating that there was a problem with the mechanical part. The X-axis ball screw was disassembled and inspected, and it was found that the ball screw was corroded. It turned out that the slide table was not properly sealed and the quenching fluid entered the ball screw, causing corrosion of the ball screw. The ball screw was replaced with a new one and the fault was eliminated.

(2) Electrical failure

Electrical faults refer to faults in the electrical control system, which mainly include faults in CNC devices, PLC controllers, servo units, CRT monitors, power modules, machine tool control components and detection switches. This part of the fault is a common fault of CNC machine tools and should be paid enough attention to.

3. Classify according to whether there is an alarm display after a failure of the CNC machine tool

According to whether there is an alarm display after the fault occurs, it can be divided into two categories: faults with alarm display and faults without alarm display.

(1) There is an alarm indicating a fault

This type of fault can be divided into two types: hardware alarm display and software alarm display.

1) Failure displayed by hardware alarm. Hardware alarm display usually refers to the alarm indication of the indicator light on each unit device. There are many indicator lights used to indicate fault locations in the CNC system, such as control system operation panels, CPU motherboards, servo control units, etc. Once these indicator lights in the CNC system indicate the fault status, the indicator lights on the corresponding parts will The meaning of the alarm can roughly determine the location and nature of the fault, which will undoubtedly bring great benefits to fault analysis and diagnosis. Therefore, maintenance personnel should pay attention to check whether the status of these indicator lights is normal during routine maintenance and fault repair.

2) Fault displayed by software alarm. Software alarm display usually refers to the alarm number and alarm information displayed on the display of the control system. Since the CNC system has a self-diagnosis function, once a fault is detected, it will be processed according to the level of the fault, and the alarm number and alarm information will be displayed on the display.

Software alarms can be divided into NC alarms and PLC alarms. The former is a fault alarm in the CNC part. You can use the alarm number to find the cause of the alarm and how to deal with it in the “CNC System Maintenance Manual” to determine the possible fault. The reason is; the alarm information of the latter PLC alarm comes from the alarm text prepared by the machine tool manufacturer, and most of them are fault alarms on the machine tool side. When encountering such faults, the fault can be diagnosed based on the alarm information or the PLC user program.

(2) Failure without alarm display

There is no hardware or software alarm display when this type of fault occurs, so it is difficult to analyze and diagnose. For faults that do not alarm, it is usually necessary to analyze the specific problems in detail. When encountering such problems, the fault must be analyzed and diagnosed based on the fault phenomenon, the working principle of the machine tool, the working principle of the CNC system, PLC ladder diagram and maintenance experience.

For example, a CNC quenching machine tool often automatically cuts off the power and shuts down. It can still work after being stopped for a while and then turned on again. After analyzing the working principle of the machine tool, the cause of this fault is generally the system protection function, so first check that the system’s power supply voltage is 24V, and there is no problem; when checking the system’s cooling device, it was found that the cooling fan filter was clogged and the problem occurred. It happened to be summer when the fault occurred. The system automatically shut down due to excessive temperature. The filter was replaced and the machine tool returned to normal use.

Another example is a CNC groove grinder using the German SINUMERIK 810 system. When automatically grinding the finished workpiece and dressing the grinding wheel, it drives the Z-axis of the grinding wheel to move upward. After stopping, the grinding wheel dresser does not dress the grinding wheel, but stops automatically. Loops, but there is no alarm indication on the screen. According to the working principle of the machine tool, when dressing the grinding wheel, coolant should be sprayed to cool the grinding wheel dresser. However, after observing the failure process many times, it was found that there was no cutting fluid spray. The cutting fluid solenoid valve control principle diagram is shown in the figure. When a fault occurs, use the PLC status display function of the CNC system to observe the output Q4.5 of the cutting fluid injection solenoid valve. Its status is “1”, which means there is no problem. According to the electrical The schematic diagram controls the solenoid valve through DC relay K45. Check the DC relay K45 and there is no problem. Then check the solenoid valve and find that there is voltage on the coil of the solenoid valve, indicating that the problem is with the solenoid valve. Replace the solenoid valve and machine tool The fault is eliminated.

4. Classification according to fault location

Machine tool faults can be divided into the following categories according to the location where they occur:

(1) Failure of the CNC device part

Faults in the CNC device can be divided into software faults and hardware faults.

1) Software failure. Some machine tool faults are caused by errors in processing programming, and some faults are caused by improper machine tool data settings. Such faults are software faults. As long as the cause of the fault is found and corrected, this type of fault will be eliminated.

2) Hardware failure. Some machine tool failures are due to problems with the control system hardware. Such failures must be replaced or repaired before the damaged components can be eliminated.

For example, a CNC punch machine fails and there is no display on the screen. Check the 24V input power of the power module of the machine tool control system. There is no problem. The NC-ON signal is also normal, but there is no 5V voltage on the power module, indicating that the power module is damaged. Repair it. Afterwards, the machine tool returned to normal use.

(2) Failure of the PLC part

Faults in the PLC part are also divided into two types: software and hardware faults.

1) Software failure. Due to problems with PLC user programming, failures can occur when certain conditions are met when the CNC machine tool is running. In addition, if the PLC user program is not well compiled, some machine tool side faults without alarms often occur, so the PLC user program must be compiled as complete as possible.

2) Hardware failure. Failures caused by problems with the PLC input and output modules are hardware failures. Sometimes individual input and output ports fail, the fault can be eliminated by modifying the PLC program and using a backup interface to replace the failed interface.

For example, in a CNC grinding machine using the German SIEMENS810 system, automatic processing cannot be performed continuously. After grinding a workpiece, the spindle grinding wheel does not return for dressing, and the automatic cycle is terminated. Analyze the working principle of the machine tool. The working status of the machine tool is set through the toggle switch on the machine tool operation panel. The toggle switch is connected to the input E7.0 of the PLC. Use the PLC status display function of the CNC system to check its status. But no matter how you turn the toggle switch, its status is always “0” and does not change. If you check the switch and find no problem, connect the connecting wire of the switch to the backup input interface E3.0 of the PLC. At this time, observe The change of this state normally follows the change of the toggle switch, and there is no problem. This proves that the input interface E7.0 of the PLC is damaged. Since there are no spare parts on hand, connect the toggle switch to the input interface E3.0 of the PLC. , and then use the programmer to change all E7.0 in the PLC program to E3.0, and then the machine tool returns to normal use.

(3) Servo system failure

Servo system failures are generally caused by problems with the servo control unit, servo motor, speed measuring device, encoder, etc.

For example: A CNC lathe uses the FANUC 0iTC system, and a 417 alarm occurs in the system. The alarm message is “SERVO ALARM: 2-TH AXIS PARAMETER INCORRECT”. Checking the servo system parameter settings reveals that parameter NO: 2023 has been modified to a negative value. (This parameter is the number of speed feedback pulses for one motor revolution). Modify this parameter and the system alarm will be cleared.

(4) Failure of the main part of the machine tool

Most of these failures are caused by external reasons, such as improper mechanical devices, problems with the hydraulic system, damaged inspection switches, and problems with the driving device. Due to various reasons, machine tool spindles, guide rails, screws, bearings, tool magazines, etc. may experience problems such as loss of accuracy, creeping, and overloading. These problems often cause alarms in the CNC system. Therefore, the fault judgment of CNC system is a comprehensive problem.

5. Classification according to the degree of damage caused by the fault

According to the degree of damage when a fault occurs, it is divided into destructive faults and non-destructive faults.

(1) Destructive failure

The occurrence of this type of fault will cause harm or damage to the operator or equipment, such as over-travel operation, speeding, component collision, etc.

After a destructive failure occurs, for example, a CNC lathe tool hits the workpiece during normal processing, causing heavy losses. After careful analysis, it is found that the return reference point is wrong, and the travel switch (stop) is found after careful analysis. The position coincides with the position of the electronic grid, which (occasionally) causes an extra electronic grid to be fed in the Z direction, causing a destructive failure in which the tool and workpiece collide. Move the position of the travel switch and the problem is solved satisfactorily.

(2) Non-destructive failure

The vast majority of faults in CNC machine tools belong to this type of fault. When a fault occurs, it will not cause any harm to the machine tool and the operator. Therefore, when diagnosing this type of fault, the fault can be reproduced, the fault phenomenon can be carefully observed, and the fault can be analyzed through the fault phenomenon. and diagnosis.

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