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Fault and fault classification of CNC machine tools

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

CNC machine tools are the product of mechatronics, with advanced technology and complex structure. The faults of CNC machine tools are also diverse, different, and the causes of failures are generally more 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 the machine tool, this section roughly divides the fault of the CNC machine tool into the following categories according to the nature of the fault, the cause of the fault and the location of the fault.

1. Classification according to the nature of the fault of the CNC machine tool

(1) Systemic failure

This kind of failure refers to the inevitable failure of the machine tool or CNC system as long as certain conditions are met. For example, if the grid voltage is too high or too low, the system will generate an alarm of too high voltage or too low; When the amount of cutting is too large, an overload alarm will be generated.

For example, a CNC machine tool with a SINUMERIK810 system in the processing process, the system sometimes automatically shuts down the power, and after restarting, it can still work normally. According to the working principle of the system and the fault phenomenon, it is suspected that the cause of the fault is the fluctuation of the system power supply voltage, the 24V power supply on the power module of the measurement system is found to be about 22.3V, when the machine tool is machining, the voltage also fluctuates downward, especially when the amount of cutting is large, the voltage drop is large, sometimes close to 21V, then the system automatically shuts off the power, in order to solve this problem, replace the 24V power transformer with a large capacity to completely eliminate this fault.

(2) Random failures

(2) Random failures

This type of failure refers to the occasional failure of only one or two failures under the same conditions. It is not easy to artificially reproduce the same fault, and sometimes it is difficult to encounter it again for a long time. The analysis and diagnosis of such faults is difficult. In general, this kind of fault is often related to the loosening and dislocation of the mechanical structure, the drift of the working characteristics of some components in the CNC system, and the decline of the reliability of the electrical components of the machine tool.

For example, a CNC groove grinder, there are occasional problems during the machining process, and the position of the grinding groove changes, resulting in scrap. Analyze the working principle of this machine tool, first the measuring arm swings down to the clamping position of the workpiece during grinding processing, and then the workpiece begins to move, when the datum end face of the workpiece touches the measuring head, the numerical control device records the position data at this time, and then the measuring arm is lifted, and the processing program continues to run. According to the position data of the end face, the numerical control device grinds the groove at a certain distance from the end face, so the position of the groove is not accurate and has a great relationship with the accuracy of the measurement. Because it does not occur very often, it is difficult to observe the fault phenomenon. Therefore, according to the working principle of the machine tool, the inspection of the measuring head did not find any problems; When the rotation of the measuring arm is checked, it is found that the rotating axis is somewhat tight, and it may be that the measuring arm is sometimes not in place, resulting in measurement errors. The rotating shaft was taken apart and inspected and found to be severely worn, new spare parts were made, and this failure never occurred again after being replaced.

2. Classification according to the type of fault

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

2. Classification according to the type of fault

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

(1) Mechanical failure

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

For example, when a CNC hardening machine using SINUMERIK 810 system is turned on back to the reference point and takes the X-axis, the alarm 1680″ SERVOENABLETRAV. AXISX”, this alarm also appears when manually walking on the X-axis, check the servo device, and find that there is an overload alarm indication. According to the Siemens manual, the cause of this failure may be excessive mechanical load, problems with the servo control power supply, failure of the servo motor, etc. In line with the principle of mechanical first and then electrical, the X-axis slide table was first detected, and the X-axis slide table was manually rotated, and it was found that it was very heavy and the disk did not move, indicating that there was a problem with the mechanical part. The X-axis ball screw was removed and inspected, and it was found that the ball screw had been corroded, it turned out that the slide table was not well sealed, and the quenching liquid entered the ball screw, causing the corrosion of the ball screw, and the new ball screw was replaced and the fault was eliminated.

(2) Electrical faults

Electrical fault refers to the fault of the electrical control system, mainly including the fault of numerical control device, PLC controller, servo unit, CRT display, power module, machine tool control element and detection switch. This part of the fault is a common fault of CNC machine tools, which should be paid enough attention.

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

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

According to whether there is an alarm display after the fault is generated, it can be divided into two categories: alarm display fault and no alarm display fault.

(1) There is an alarm display fault

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

1) Hardware alarm display of failure. The hardware alarm display usually refers to the alarm indication of the indicator light on each unit device. There are many indicators used to indicate the fault part in the numerical control system, such as the control system operation panel, CPU motherboard, servo control unit and other parts, once these indicators of the numerical control system indicate the fault state, according to the alarm meaning of the indicator light on the corresponding part, the location and nature of the fault can be roughly judged, which will undoubtedly bring great benefits to fault analysis and diagnosis. Therefore, maintenance personnel should pay attention to check whether the status of these indicators is normal during daily maintenance and fault repair.

2) The software alarm shows the fault. The software alarm display is usually the alarm number and alarm information displayed on the display of the index control system. Because the CNC system has a self-diagnosis function, once the fault is detected, it will be dealt with according to the level of the fault, and the alarm number and alarm information will be displayed on the display at the same time.

Software alarm can be divided into NC alarm and PLC alarm, the former is the fault alarm of the numerical control part, through the alarm number, find the cause of the alarm and how to deal with the content of the alarm in the “CNC system maintenance manual”, so as to determine the possible cause of the failure; The alarm information of the PLC alarm of the latter comes from the alarm text prepared by the machine tool manufacturer, most of which belong to the fault alarm on the machine tool side, and when encountering this kind of failure, the fault can be diagnosed according to the alarm information or the PLC user program.

(2) Failure without alarm display

This type of fault does not have any hardware or software alarm display, so it is difficult to analyze and diagnose. For faults without alarms, it is usually necessary to analyze the specific problems on a case-by-case basis. When encountering this kind of problem, it is necessary to analyze and diagnose the fault according to the fault phenomenon, the working principle of the machine tool, the working principle of the numerical control system, the PLC ladder diagram and the maintenance experience.

For example, a CNC quenching machine often automatically shuts down the power and turns it off, and it can still work when it is stopped for a while and then turned on. Analyzing the working principle of the machine tool, the reason for this failure is generally the system protection function, so first check that the power supply voltage of the system is 24V, and there is no problem; When inspecting the cooling device of the system, it was found that the cooling fan filter was blocked, and the failure happened to be summer, and the system automatically stopped because the temperature was too high, the filter was replaced, and the machine was restored to normal use.

Another example is a CNC groove grinder using the German SINUMERIK 810 system, when automatically grinding the finished parts and dressing the grinding wheel, the Z-axis of the grinding wheel is driven to move upward, and the grinding wheel dresser does not trim the grinding wheel after stopping, but stops the automatic cycle, but there is no alarm indication on the screen. According to the working principle of the machine tool, when dressing the grinding wheel, the coolant should be sprayed to cool the grinding wheel dresser, but after observing the process of failure several times, it was found that there was no cutting fluid injection. The control schematic diagram of the cutting fluid solenoid valve is shown in the figure, in the event of a failure, the PLC status display function of the numerical control system is used to observe and control the output of the cutting fluid injection solenoid valve Q4.5, its state is “1”, there is no problem, according to the electrical schematic diagram it is controlled by the DC relay K45, there is no problem with the DC relay K45, and then the solenoid valve is checked, and the voltage is found on the coil of the solenoid valve, indicating that the problem is in the solenoid valve, replace the solenoid valve, and the machine tool fault is eliminated.

4. Classification according to the location of the fault

According to the location of the machine tool failure, the fault can be divided into the following categories:

(1) Failure of the numerical control device

The failure of the CNC device part can be divided into software failure and hardware failure.

1) Software malfunction. Some machine failures are caused by errors in machining programming, and some failures are caused by improper machine data settings, which are software failures. As long as the cause of the fault is found and modified, this kind of fault will be eliminated.

2) Hardware failure. Some machine failures are due to problems with the control system hardware, which must be rectified after the replacement of the damaged component or repair.

For example, a CNC punch fails, the screen is not displayed, check the 24V power supply 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, and the machine tool will return to normal use after repair.

(2) Failure of the PLC part

The failure of the PLC part is also divided into two types: software and hardware failure.

1) Software malfunction. Due to the problem of PLC user programming, failure can occur when certain conditions are met when the CNC machine tool is running. In addition, the PLC user program is not good, and there are often some machine tool side faults without alarm, so the PLC user program should be prepared as well as possible.

2) Hardware failure. The failure caused by the problem of the PLC input output module is a hardware failure. Sometimes individual input and output ports fail, and the fault can be eliminated by modifying the PLC program and replacing the faulty interface with a spare interface.

For example, a CNC grinding machine using the German SIEMENS810 system, automatic processing can not be carried out continuously, after grinding a workpiece, the spindle grinding wheel does not return for dressing, and the automatic cycle is stopped. Analyze the working principle of the machine tool, the working state of the machine tool is set by the toggle switch on the machine tool operation panel, the toggle switch is connected to the output of the PLC E7.0, the PLC status display function of the numerical control system is used to check its status, but no matter how to flip the toggle switch, its state has been “0”, no change, and the check switch has not found any problems, The connecting wire of this switch is connected to the spare transmission interface E3.0 of PLC, and then the change of this state is observed, and the change of the toggle switch is followed normally, and there is no problem, so that it is proved that the transmission interface E7.0 of PLC is damaged, because there is no spare parts at hand, the toggle switch is connected to the transmission interface of E3.0 of PLC, and then all E7.0 in the PLC program is changed to E3.0 by the programmer, and the machine tool resumes normal use at this time.

(3) Servo system failure

The failure of the servo system is generally caused by problems with the servo control unit, servo motor, speed measuring device, encoder, etc.

FOR EXAMPLE, IF A CNC LATHE USES THE FANUC 0iTC SYSTEM, AND THE SYSTEM DISPLAYS A 417 ALARM, AND THE ALARM MESSAGE IS “SERVO ALARM:2-TH AXIS PARAMETER INCORRECT”, CHECK THE SERVO SYSTEM PARAMETER SETTINGS AND FIND THAT THE PARAMETER NO:2023 HAS BEEN MODIFIED TO A NEGATIVE VALUE. (This parameter is the number of speed feedback pulses of one revolution of the motor). If this parameter is modified, the system alarm will be cleared.

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

Most of these failures are caused by external causes, such as inadequate mechanics, problems with hydraulic systems, broken inspection switches, and problems with drives. Machine tool spindles, guide rails, lead screws, bearings, tool magazines, etc., due to various reasons, there will be problems such as loss of accuracy, crawling, overload, etc. These problems often cause alarms in the CNC system. Therefore, the fault judgment of the CNC system is a comprehensive problem.

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

According to the degree of damage at the time of the failure, it is divided into destructive failure and non-destructive failure.

(1) Destructive failures

The occurrence of such faults can cause injury or damage to the operator or equipment, such as overtravel, speeding, component collisions, etc.

After the occurrence of destructive failure, for example, a CNC lathe in the case of normal machining, the tool hits the workpiece, causing significant losses, after careful analysis, it is found that the return reference point is wrong, and the careful analysis finds that the position of the travel switch (gear block) coincides with the position of the electronic grid, (occasionally) causes an extra electronic grid in the Z direction to feed, thereby causing the destructive fault of the tool workpiece colliding. Move the position of the travel switch, and the problem is solved satisfactorily.

(2) Non-destructive failures

The vast majority of faults of CNC machine tools belong to this kind of fault, and the failure will not cause any harm to the machine tool and the operator, so when diagnosing this kind of fault, the fault can be reproduced, and the fault phenomenon can be carefully observed, and the fault can be analyzed and diagnosed through the fault phenomenon.

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