Fault analysis and maintenance of parts of CNC machine tools
3. 1 Fault Inspection and Maintenance of Spindle Servo System of NC Machine Tool
With the rapid development of electronic industry, various speed-regulating drivers with high integration and advanced performance emerge one after another, which provides favorable conditions for the upgrading of CNC machine tools. However, for the reality that large and medium-sized enterprises can not completely transform the old CNC machine tools at present, it is still an arduous task to repair the old drive system on the maintenance front. Some faults encountered in the maintenance of CNC lathe with staggered contact and non-circulating reversible speed regulation drive system and their treatment methods.
1. Failure phenomenon: when 1.8m sleeper moves slightly, the panel swings back and forth.
Inspection: The ripple of the 20V DC regulated power supply of the drive control system is measured as 4V peak-to-peak, which greatly exceeds the specified range.
Analysis: In the amplification circuit of the control system, Qualcomm and low-pass filter can filter out various harmonic interference signals such as speed feedback, current feedback and voltage feedback, but they cannot filter out the harmonic components in the DC power supply circuit of the system itself. Because it exists in the whole system, these harmonics will block the amplifier and cause various abnormal phenomena in the system. In the inching state, due to the low motor speed, these harmonics have exceeded the voltage value when inching, causing the oscillation of the system and making the spindle disk swing back and forth. Once the harmonic signal is removed, the fault disappears immediately.
Solution: Replace the 100MF and 1000MF filter capacitors in the voltage board, and weld the new capacitors. After the ripple of only a few millivolts is detected, install the power board and start the trial operation to eliminate the fault.
2. Failure phenomenon: The 5m vertical lathe collided during the operation and processing, and the insurance was burned.
Inspection: It is found that the positive and negative groups of 5FC5FG and 5RG5RQ have no pulse output (see Figure 2). The measurement result shows that the IC7 inverter is damaged, and it is found that the output waveform of 1FG 1FC is much lower than other waveforms.
Analysis: The driving voltage of the main driving DC motor of the 5m vertical crane is provided by the anti-parallel rectifier circuit of the thyristor full-control bridge. Of the 12 trigger pulses, two have disappeared, and the amplitude of the other trigger pulse is one third shorter than that of other normal trigger pulses. When the gear clicked, I mistakenly thought that there was something wrong with the coupling of the hydraulic motor, but after a while, two more fuses were blown. In fact, the fuse had been burned twice before this fault, and it was only thought that it was caused by accidental power grid instability. Because the speed of 5m vertical lathe is low, although the SCR rectifier circuit is bridge rectifier, when the trigger pulse in the line is lost and the amplitude is small, the current will be discontinuous and the output voltage will be unstable, which will make the motor speed unstable. The opening sound of Kuang is actually a sign of unstable speed. Fuse failure caused by intermittent current may occur at any time after shutdown and normal operation.
Treatment: Replace the amplifier T 1 (the amplifier in another trigger circuit, with the function as T7 in Figure 2) and the inverter IC7, and troubleshoot.
3.2 Diagnosis of Initial Fault of Machine Tool PLC
Diagnosis of initial fault of machine tool PLC In order to protect the machine tool and facilitate maintenance, PLC has the ability to display and detect machine tool faults. Once a fault occurs, the maintenance personnel can determine the fault category according to the fault display number of the machine tool and eliminate the fault. However, in the actual processing process, we found that sometimes PLC displays several faults at the same time, which is a chain fault caused by a fault. After the initial fault is eliminated, other fault alarms disappear. However, judging from all the alarm faults displayed by the machine tool PLC, the maintenance personnel don't know which fault is the initial fault, so they can only check one by one, which increases the maintenance difficulty. Machine tool PLC initial fault diagnosis function, through PLC program, accurately judge the alarm number of initial fault. When maintaining, the initial fault is eliminated first, and other faults disappear automatically, which greatly facilitates the maintenance of machine tools and improves the rapidity and accuracy of machine tool maintenance. The PLC program designed according to the principle of initial fault diagnosis can not only detect and display every fault, but also automatically judge the most critical initial fault.
Principle of initial fault diagnosis: Take three faults as examples, in which three fault detection bits are set, namely R500.0, R5 10.0 and R520.0；; The three initial fault detection bits are R500.2, R5 10.2 and R520.2；; F 149. 1 is the system reset signal. In the initial state, there is no alarm, the fault detection bits are all "0", the initial fault detection bits are all "0", and the reset signal F 149. 1 is "0". Suppose the second fault appears first among the three faults. In the first cycle of program scanning, its corresponding fault detection bit R5 10.0 becomes "1", and the initial values of R500.2, R520.2 and F 149. 1 are "0", and the initial fault detection bit R5/kloc-0. In the second cycle of program scanning, R5 10.2 keeps "1", thus blocking R500. 1 and R520. 1. Even if another fault detection bit is "1", its initial fault detection bit cannot be changed to "65448". Through the control of this PLC program, the initial fault can be accurately judged from multiple faults occurring at the same time. In JCS0 18 CNC machine tools, many problems occur at the same time, such as tool change alarm and hydraulic alarm. During maintenance, first check the hydraulic control part, and then confirm that the fault is in the process of tool change. Only after inspection did we know that the power of tool changing was provided by hydraulic drive. In the design of PLC control program, in order to prevent more accidents, the hydraulic control is disconnected at the same time when the tool changing fault occurs, so two alarm messages will appear when the tool changing fault occurs. In order to follow the design idea of the original machine tool and send out the alarm information accurately, JCS0 18 CNC machine tool has added the function of checking the initial fault. According to the previous program analysis, the tool changing and hydraulic fault detection bits are R500.0 and R5 10.0 respectively, and the initial fault can be read from the initial fault detection bits R500.2 and R5 10.2. When the machine tool has a similar fault again, the initial fault can be quickly judged.
3.3 CNC equipment detection component failure and maintenance
The detection element is an important part of the servo system of CNC machine tools. Its function is to detect the displacement and speed of each control shaft and feed back the detected signal to form a closed-loop system. Measurement methods can be divided into direct measurement and indirect measurement: direct measurement refers to measuring the linear displacement of machine tools with linear detection elements, and the commonly used detection elements for direct measurement generally include linear inductosyn, measuring grating and magnetic ruler laser interferometer. Indirect measurement is to measure the linear displacement of machine tool with rotating detection elements. The commonly used detection elements for indirect measurement generally include: pulse encoder, resolver, circular inductosyn, circular grating and circular magnetic grating.
When the machine tool has the following failure phenomena, it should be considered whether it is caused by the failure of the detection element:
1. Mechanical vibration (when accelerating/decelerating):
(1) If the pulse encoder fails, check whether the terminal voltage of the feedback line on the speed unit drops at a certain point. If it drops, it means that the pulse encoder is bad, and replace the encoder.
(2) The cross coupling of the pulse encoder may be damaged, resulting in the shaft speed being out of sync with the detection speed. Replace the coupling.
(3) If the tachogenerator fails, repair and replace the tachometer.
2. Mechanical runaway (runaway):
If you check the position control unit and the speed control unit, please check:
(1) Whether the wiring of the pulse encoder is wrong, check whether the wiring of the encoder is positive feedback and whether phase A and phase B are connected reversely.
(2) If the pulse encoder coupling is damaged, replace the coupling.
(3) Check whether the terminal of tachogenerator is connected reversely and whether the excitation signal line is connected wrongly.
3. The spindle cannot be oriented or the orientation is not in place:
When checking the setting and adjustment of the direction control circuit, the direction plate and the spindle control printed circuit board, check whether the position detector (encoder) is faulty, and measure the output waveform of the encoder at this time.
4. Shaft vibration feed:
Check whether the motor coil is short-circuited, whether the mechanical feed screw is well connected with the motor, whether the whole servo system is stable, whether the pulse coding is good, whether the coupling connection is stable and reliable, and whether the tachometer is reliable.
Detection element is an extremely precise and easily damaged equipment, so we must pay attention to the following aspects in order to use and maintain it correctly.
1. It shall not be subjected to strong vibration and friction, so as not to damage the code wheel, and shall not be polluted by dust and oil, so as not to affect the output of normal signals.
2. The ambient temperature of the working environment should not exceed the standard, and the rated power supply voltage must be met to facilitate the normal work of the integrated circuit chip.
3. Ensure the normal resistance and capacitance of the feedback line and ensure the normal transmission of signals.
4. Prevent external power supply and noise interference, ensure good shielding and not affect the feedback signal.
5. The installation method should be correct, such as concentric alignment of the encoder connecting shaft to prevent the shaft from exceeding the allowable load to ensure its normal performance.
In a word, in the fault of CNC equipment, the failure rate of detection elements is relatively high. As long as the faults are used correctly, the maintenance is strengthened and the problems are analyzed deeply, the fault rate will be reduced, the faults will be solved quickly and the normal operation of the equipment will be ensured.
3.4 CNC machine tool machining accuracy abnormal fault and maintenance
In production, we often encounter faults with abnormal machining accuracy of CNC machine tools. This kind of fault is hidden and difficult to diagnose. There are five main reasons for this failure: (1) The feed unit of the machine tool has changed or changed. (2) The zero offset of each axis of the machine tool is abnormal. (3) Abnormal axial clearance. (4) The motor is not working normally, that is, the electrical and control parts are out of order. (5) Mechanical failures, such as screws, bearings, couplings and other parts. In addition, the preparation of machining program, the choice of cutting tools and human factors may also lead to abnormal machining accuracy.
1. System parameters change or change.
System parameters mainly include machine tool feed unit, zero offset, reverse clearance, etc. For example, Siemens and FANUC CNC systems have two feed units: metric and imperial. Some treatments in the process of machine tool repair often affect the change of zero offset and clearance, and should be adjusted and modified in time after fault treatment; On the other hand, due to serious mechanical wear or loose connection, the measured values of parameters may change, and the parameters need to be modified accordingly to meet the requirements of machining accuracy of machine tools.
2. Abnormal machining accuracy caused by mechanical failure
THM6350 horizontal machining center adopts FANUC 0i-MA numerical control system. During the milling process of steam turbine blades, it is suddenly found that the Z-axis feed is abnormal, resulting in a cutting error of at least 1mm (Z-direction overcutting). It is known from the investigation that the fault happened suddenly. In inching and MDI operation modes, each axis of the machine tool runs normally and the reference point is normal; There is no alarm prompt, which eliminates the possibility of hard failure of electrical control part. According to the analysis, we should check them one by one from the following aspects.
(1) Check the running machining program section when the accuracy of the machine tool is abnormal, especially the tool length compensation and the proofreading calculation of the machining coordinate system (G54~G59).
(2) In inching mode, the Z axis moves repeatedly. Through the diagnosis of its motion state by vision, touch and hearing, it is found that the motion sound in Z direction is abnormal, especially when inching rapidly, the noise is more obvious. Judging from this, there may be hidden dangers in machinery.
(3) Check the Z-axis accuracy of the machine tool. Move the Z-axis with a manual pulse generator (set the manual pulse ratio as 1× 100, that is, give 0. 1mm for every step of the motor), and observe the movement of the Z-axis with a dial indicator. For the forward motion as the starting point after the unidirectional motion accuracy keeps normal, the actual distance d = d1= d2 = d3 ... = 0.1mm. For the Z-axis motion of the machine tool with each step of manual pulse change, it shows that the motor runs well and the positioning accuracy is good. Back to the change of the actual movement displacement of the machine tool, it can be divided into four stages: ① machine tool D 1 > D = 0. 1 mm (the slope is greater than1); ② d = 0. 1mm > D2 > D3 (the slope is less than1); ③ There is no actual movement of the machine tool mechanism, showing the most standard reverse clearance; ④ When the moving distance of the machine tool is equal to the given value of hand pulse (the slope is equal to 1), the machine tool will resume normal movement.
No matter how to compensate the reverse clearance (parameter 185 1), its characteristic is that other changes still exist except the third stage, especially the first stage seriously affects the machining accuracy of the machine tool. It is found that the greater the clearance compensation, the greater the moving distance of the first segment.
Analysis of the above inspection shows that there may be the following reasons: First, the motor is abnormal; Second, there is a mechanical failure; Third, there is a certain gap. In order to further diagnose the fault, disconnect the motor and lead screw completely, and inspect the motor and mechanical parts respectively. The motor is running normally; In the diagnosis of mechanical parts, it is found that when the screw is driven by hand, there is a very obvious sense of emptiness at the beginning of the return movement. Under normal circumstances, you should be able to feel the orderly and smooth movement of the bearing. After disassembly and inspection, it was found that its bearing was really damaged and one ball fell off. The machine tool returned to normal after replacement.
3. The electrical parameters of the machine tool are not optimized, and the motor runs abnormally.
A CNC vertical milling machine, equipped with FANUC 0-MJ CNC system. In the process of machining, it is found that the accuracy of X axis is abnormal. It is found that there is a certain gap in the X axis, and the motor is unstable when starting. When touching the X-axis motor with your hand, you feel that the motor is shaking seriously, but it is not obvious when starting and stopping, especially in inching mode.
According to analysis, there are two reasons for failure. First, the mechanical reverse clearance is large; Second, the X-axis motor works abnormally. Using the parameter function of FANUC system, the motor is debugged. First, make up the existing gap; By adjusting the servo gain parameters and N pulse suppression function parameters, the jitter of the X-axis motor is eliminated and the machining accuracy of the machine tool returns to normal.
4. The machine tool position ring is abnormal or the control logic is incorrect.
A TH6 1 140 boring and milling machine machining center with FANUC 18i numerical control system and full closed-loop control mode. During the machining process, it was found that the Y-axis precision of the machine tool was abnormal. The minimum precision error was about 0.006mm, and the maximum error could reach1.400 mm. During the inspection, the machine tool had set the G54 workpiece coordinate system as required. In MDI mode, run a program in G54 coordinate system, namely "G90G54Y80F100; After M30 "machine runs, the mechanical coordinate value displayed on the display is"-1046.605 ",and record this value. Then move the Y axis of the machine tool to any other position in manual mode, and execute the above statement again in MDI mode. After the machine tool stopped, it was found that the numerical display value of machine tool mechanical coordinates was "-1046.992", which was 0.387mm different from the numerical display value after the first execution. In the same way, move the Y-axis to different positions, execute the statement repeatedly, and the indicated value of digital display is uncertain. Using a dial indicator to detect the Y axis, it is found that the actual error of the mechanical position is basically consistent with the error displayed by the digital display, so it is considered that the fault is caused by the excessive repeated positioning error of the Y axis. The Y-axis reverse clearance and positioning accuracy were carefully checked and compensated, but nothing worked. So it is suspected that there is something wrong with the grating ruler and system parameters, but why is there such a big error, but there is no corresponding alarm information? Further inspection found that the axis was vertical, and once the Y axis was loose, the headstock fell down, resulting in out-of-tolerance.
The PLC logic control program of machine tool is modified, that is, when the Y axis is loosened, the Y axis is enabled and loaded first, and then the Y axis is loosened; When clamping, first clamp the shaft, and then remove the Y axis. After adjustment, the machine tool fault was solved.
Four CNC machine tool maintenance
CNC system is the core component of CNC machine tools, therefore, the maintenance of CNC machine tools is mainly the maintenance of CNC system. After a long period of use, the performance of electronic components will age or even be damaged, especially some mechanical parts. In order to prolong the life and wear cycle of parts as much as possible, and prevent all kinds of faults, especially vicious accidents, it is necessary to carry out daily maintenance of CNC system. To sum up, we should pay attention to the following aspects.
(1) formulate rules and regulations for daily maintenance of CNC system.
According to the characteristics of each component, determine their own maintenance regulations. For example, clearly specify which parts need to be cleaned every day (for example, the input/output unit photoelectric reader of the numerical control system needs to be cleaned, whether the mechanical structure is well lubricated, and so on). ), and which parts should be checked or replaced regularly (for example, the brush and commutator of DC servo motor should be checked once a month).
(2) Open the doors of CNC cabinets and high-voltage cabinets as little as possible.
Because the air in the machining workshop generally contains oil mist, dust and even metal powder. Once it falls on the printed circuit or electrical components in the numerical control system, it is easy to cause the insulation resistance between components to drop, and even lead to the damage of components and printed circuits. In summer, some users open the door of the CNC cabinet to dissipate heat in order to overload the CNC system for a long time, which is an unacceptable method and will eventually lead to accelerated damage of the CNC system. The correct way is to reduce the external environment temperature of CNC system. Therefore, there should be a strict rule that unless necessary adjustments and maintenance are made, it is not allowed to open the cupboard door casually, let alone open the cupboard door when using it.
(3) Regularly clean the cooling and ventilation system of the numerical control cabinet.
Check whether the cooling fan on the CNC system cabinet works normally every day, and check whether the air duct filter is blocked every six months or every quarter according to the working environment. If the dust on the filter screen accumulates too much, it needs to be cleaned in time, otherwise the temperature in the cabinet of the numerical control system will be very high (generally not allowed to exceed 55℃), which will lead to overheating alarm or unreliable work of the numerical control system.
(4) Frequently monitor the grid voltage used by the numerical control system.
The numerical control system produced by FANUC allows the grid voltage to fluctuate within the range of 85% ~ 1 10% of the rated value. If it exceeds this range, the system will not work normally and even damage the electronic components inside the CNC system.
(5) Replace the battery regularly.
There are two kinds of memories in the numerical control system produced by FANUC:
(a) Bubble memory without battery maintenance.
(b) CMOS random access memory devices that require battery support. In order to save the stored contents when the numerical control system is not powered on, there is a rechargeable battery holding circuit inside. When the numerical control system is powered on, CMOS RAM is powered by a +5V power supply through a diode, and the rechargeable battery is charged. When the CNC system cuts off the power supply, it will be powered by the battery to maintain the information in CMOS RAM. Under normal circumstances, even if the battery is not out of order, it should be replaced once a year to ensure the normal operation of the system. In addition, it must be noted that battery replacement must be carried out under the power supply state of the numerical control system.
6. Maintenance of CNC system when it is not used for a long time
In order to improve the utilization rate of CNC system and reduce the faults of CNC system, CNC machine tools should be used at full load instead of idle for a long time. For some reason, when the CNC system is idle for a long time, in order to avoid damaging the CNC system, the following two points should be paid attention to:
(1) Always connect the power supply of the numerical control system, especially in rainy season with high ambient humidity. When the machine tool is locked (that is, when the servo motor does not turn), let the numerical control system idle. The heat of electrical components is used to dispel the moisture in the numerical control system to ensure the stability and reliability of electronic components. Practice has proved that frequent energization is an effective measure to reduce the failure rate in areas with high air humidity.
(2) If the CNC machine tool adopts DC feed servo drive and DC spindle servo drive, the brush should be taken out of the DC motor to avoid chemical corrosion of the commutator surface, which will worsen the commutation performance and even damage the whole motor.
Take the exam and contribute.
1 sheet, edited by Xie Fuchun. Numerical control programming technology. Beijing: Chemical Industry Press, 2004.
Two, edited by Luo Xun. Comprehensive training of numerical control machining technology. Beijing Machinery Industry Press 2003
3 CNC technical training series tutorial. Century Star CNC system programming \ operating instructions. Huazhong numerical control.2001
4. Tianjin Branch of National CNC Training Network. Numerical control programming. Beijing: Machinery Industry Press, 1997.
Express gratitude/gratitude
Four years of study and life are coming to an end in this season, but my life is just a comma, and I will face the beginning of another journey. Fourth, with the strong support of relatives and friends, I worked hard, but I also gained a lot. When the thesis is about to go to Fu Zi, I am full of thoughts and can't be calm for a long time. I admire great men and celebrities, but I am more eager to dedicate my respect and praise to an ordinary person, my mentor. I am not your best student, but you are my most respected teacher. Your rigorous academic attitude, profound knowledge, profound thoughts and broad horizons have created a good spiritual atmosphere for me. It is better to teach people to fish than to teach them to fish. Being in it and deeply influenced by it, I not only accepted brand-new ideas, set up grand academic goals, but also understood the basic way of thinking. From the topic selection to the writing guidance, through your careful guidance and understanding after thinking, I often feel that there is no doubt in the mountains and rivers, and there is another village.
Thanks to my parents, I can't get anything and I can't return it. My greatest wish is that you will be healthy and happy forever. When the paper is about to be completed, my mood can't be calm. How many respectable teachers, classmates and friends gave me wordless help from the beginning to the smooth completion of the thesis? Please accept my sincere thanks!
At the same time, I also thank the college for providing me with a good graduation design environment.
Finally, I would like to thank all my mentors, friends and classmates who helped me in my graduation project, as well as the authors of the works I quoted or referred to in my design.