Fault diagnosis methods commonly used in CNC machine tools

1. Understand the circumstances under which the fault occurs

In order to restore the machine tool faster when a failure occurs, first of all, it is necessary to correctly grasp the failure situation and handle it properly. Therefore, the failure situation should be confirmed according to the following contents.

(1) “When” the failure occurred

Ÿ Date and time when the failure occurred?

Y Does it happen at runtime? (How long does it take to run)

Y Happens when power is turned on?

Ÿ Did it occur during a thunderstorm, power outage, or disturbance to the power supply?

y multiple occurrences? (frequency of occurrence, several times/hour, several times/day, several times/month)

(2) Fault occurred after “what kind of operation”

Ÿ How does the CNC operate when a fault occurs?

Ÿ (JOG mode/memory (MEM) mode/MDI mode/remote operation mode (RMT)?)

Ÿ When the program is running…

1) Where does the program execute when a failure occurs?

2) Program number/sequence number?

3) What is the program?

4) Does it happen during axis movement?

5) Did it happen during M/S/T code execution?

6) Is the program being executed when the failure occurs?

Ÿ Does the same failure occur when the same operation is performed here? (Confirm the presence of the fault)

Ÿ Is there a failure during data input/output?

Ÿ When a fault related to the feed axis servo occurs:

1) Does the fault occur at both low-speed feed and high-speed feed?

2) Is it a fault that occurs when a certain axis moves?

Ÿ When a fault related to the spindle occurs, does the spindle run in the acceleration/deceleration state?

(3) Symptoms

Ÿ Is the screen display normal?

Ÿ What is displayed on the alarm screen?

Ÿ If the processing size is not accurate:

1) How big is the error?

2) Is the size of the position display screen correct?

3) Is the offset setting correct?

（4）About other information

Ÿ Whether there is any source of interference near the device: When the frequency of faults is low, consider the influence of external interference factors of the power supply voltage, and confirm whether other machine tools and welding machines are connected to the same power supply. If so, check the fault occurrence When, is there any device booting (or running). (Check of interference power supply)

Ÿ In terms of machine tools, have measures been taken against interference?

Ÿ Confirm the input voltage:

1) Has the voltage changed?

2) Is there any phase-to-phase voltage?

3) Is the standard voltage supplied?

2. Carry out fault diagnosis according to the alarm information

Now the self-diagnosis technology of the CNC system is more and more advanced, and many faults can be detected by the CNC system, and an alarm is generated and an alarm message is given. When the CNC machine tool breaks down, sometimes the alarm information will be displayed on the display, and sometimes there will be alarm indications on the CNC device, PLC device and drive device. At this time, the alarm information should be analyzed according to the “Manual”. In addition, the PLC program designed by the machine tool manufacturers is becoming more and more perfect, which can detect the faults of the machine tool and generate alarm information. Therefore, when an alarm occurs on the machine tool, it is necessary to pay attention to the research and analysis of the alarm information. For some faults, the cause of the fault can be judged according to the alarm information, so as to eliminate the fault.

For example, a CNC groove grinder using Siemens 810 system will generate No. 1 alarm display “BATTERYALARMPOWERSUPPLY” after starting up, which clearly indicates that the CNC system is powered off to protect the battery. Replace the battery when it is charged), reset the fault, and the machine tool returns to normal use.

3. Using PL(M)C status information to diagnose faults

Many numerical control systems have PLC input and output status display functions, such as the PLCSTATUS function under the DIAGNOSIS menu of the SIEMENS810 system, the PMC status display function under the DGNOSPARAM software menu of the FANUC0 system, and the MELDASL3 system DI-AGN menu of the Japanese MITSUBISHI company. PLC-I/F function, CHECKDATA function of Japan OKUMA system, etc. Using these functions, you can directly observe the instantaneous state of PLC input and output online. The online detection of these states is very useful for diagnosing many faults of CNC machine tools.

Some faults of CNC machine tools can be diagnosed by checking the PLC-related input and output status according to the fault phenomenon and the electrical schematic diagram of the machine tool.

The emergence of CNC machine toolsMost of the faults are detected by the PLC device. The mechanism of PLC fault detection is to run the PLC ladder diagram (that is, the program) prepared by the machine tool manufacturer for a specific machine tool, and make logical judgments according to various input and output states. If a problem is found, an alarm is generated and an alarm message is generated on the display. Therefore, for some PLC faults that alarm, or some faults that do not alarm, you can diagnose the fault by analyzing the PLC ladder diagram, and use the ladder diagram display function of the NC system or the external programmer to track the operation of the ladder diagram online. Speed ​​and accuracy in diagnosing faults.

For example, an alarm 6025 “Dresser Arm Lower Time out” occurs on a CNC grinding machine, indicating that the dressing arm has fallen overtime. Check the status of the dresser and find that the dresser has dropped. There is no problem in manually lifting and dropping the dresser. According to the electrical schematic diagram, the fall of the dresser is detected by the position switch 2LS5, and the switch 2LS5 is connected to the input 12.5 of the PLC, as shown in Figure 2-5. Find the PLCSTATUS function under the DIAGNOSIS menu of the system, check the status of 12.5 online, and find that no matter whether the trimmer is falling or rising, the status of 12.5 is always “0”, indicating that the PLC has not received the trimmer in-position signal. Check the in-position switch 2LS5 and no problem is found. Check that the terminal level of 12.5 is “0”, indicating that there is no problem with the input port of the PLC. Finally, check the line connection and find that the power connection of the switch 2LS5 on the power terminal 34 is disconnected. After connecting the wire to the power supply, the machine fault disappeared.

4. Use PL(M)C program (ladder diagram) tracking method to diagnose faults

Most of the failures of CNC machine tools are checked out through PLC programs. Some faults can directly display the cause of the alarm on the screen, some have alarm information on the screen, but do not directly reflect the cause of the alarm, and some faults do not generate alarm information, but some actions are not executed. In the latter two cases, tracking the operation of the PLC ladder diagram is a very effective way to diagnose the fault. The FANUC0 system and the MITSUBISHI system itself have a ladder diagram display function, which can directly monitor the operation of the ladder diagram. Since the Siemens CNC system does not have a ladder diagram display function, for simple faults, it can display information through the status of the PLC according to the ladder diagram, monitor the status of related inputs, outputs, and flags, and track the operation of the program. For complex faults, programming must be used. device to track the operation of the ladder diagram.

For example, a CNC grinding machine using Siemens 810 system, the machine tool does not return to the reference point after starting up, and there is no fault display. Check the control panel and find that the indicator light of the falling indexing device is not on. For the sake of safety, as long as the indexing device does not fall, the feed axis of the machine tool cannot move. But check the indexing device, it has fallen without any problem. According to the electrical schematic diagram of the machine tool, as shown in the figure, the indexing device on the control panel of PLC output Q7.Lower the indicator light. Look at the PLC Ladder Diagram for this.

The ladder diagram of Q7.3 is in block 21 of PB12, as shown in Figure 2-7. Use a programmer to observe the operation of the ladder diagram online, and find that the flag bit F143.4 is not closed, resulting in no power to the output Q7.3. Flag bit F143.4 indicates workpiece indexing table

In the falling position, its control ladder diagram is in the 8 blocks of PB10, as shown in Figure 2-8. Check this part of the ladder diagram with a programmer, and find that because the contact of input 113.2 is not closed, F143.4 has no power. According to the electrical schematic diagram shown in Figure 2-9, the PLC input 113.2 is connected to the proximity switch 13PS2 that detects the fall of the workpiece indexing device. The indexing device was disassembled, and it was found that there was a problem with the mechanical device, which could not drive the mechanical device driving the proximity switch, so 113.2 could not be closed all the time. After the mechanical device was repaired, the machine tool returned to normal use.

5. Use machine data to maintain machine tools

Some failures of CNC machine tools are due to unreasonable machine data settings or need to be adjusted after the machine tool has been used for a period of time. When encountering such failures, modify the corresponding machine data appropriately to eliminate the failures.

For example, a CNC grinding machine using Siemens’ siemens system found during grinding that sometimes the input tool compensation data does not change or the size reflected on the workpiece changes too small. According to the working principle of the machine tool, the Z-axis drives the grinding wheel to grind the workpiece radially during the grinding process, and the X-axis does not move when it is normal. ST1 />Move within 0.02mm , because the moving distance is small, it is possible that the backlash of the lead screw will affect the dimensional change.

When measuring the round-trip accuracy of the machine tool, it is found that when the X-axis is converted from forward to reverse, let it go 0.01mm with no change from the dial indicator; X axisThe same is true when switching from reverse to forward. Therefore, it is suspected that there is a problem with the backlash of the ball screw. After studying the system manual, it is found that the CNC system itself has a compensation function for the backlash of the ball screw. According to the data description, adjust the compensation value of the backlash of the machine tool data 2201 to restore the machine tool. normal work.

6. Single-step execution of the program to determine the fault point

Many CNC systems have the program single-step execution function, which is used when debugging the machining program. When there is a fault in the execution of the processing program, the fault point can be quickly confirmed by using the single-step execution program, so as to eliminate the fault.

For example, for a CNC grinding machine using Siemens 840D system, during the machine tool debugging, the foreign technicians clear the data of the CNC device, re-enter the machine data and programs, and then debug; The numerical control system of the processing program freezes and cannot perform any operations. After shutting down and restarting, it can still work, but it freezes again as soon as the program is executed. It is suspected that there is a problem with the processing program, but no problem has been detected, and this program has also been run before. When the program is executed with the single-step function, it is found that each crash occurs when the N220 of the subroutine L110 is executed. The content of the program N220 statement is G18D1, which calls tool compensation. Check the tool compensation data and find that it is 0, no data. According to the requirements of the machine tool, after the tool compensation value P1 is assigned a value of 10, the machining program of the machine tool is executed normally, and there is no crash anymore.

7. Visual observation method

Intuitive observation method is to use human hands, eyes, ears, nose and other sensory organs to find the cause of the fault. This method is very practical in maintenance.

For example, for a quenching machine tool, the Y axis does not move when it is turned on and returns to the reference point. After observing the fault phenomenon, it is found that when the Y axis is moved, the Y axis does not move, but the coordinate value of the Y axis on the screen changes normally. And observe that the Y-axis servo motor also rotates normally, so it is suspected that the coupling between the servo motor and the lead screw is damaged, disassemble and check that it is indeed damaged, and replace it with a new coupling to eliminate the fault. Another example is that when a CNC groove grinder is turned on, the No. 11 alarm sometimes occurs, indicating that the UMS identifier is wrong. It may be that the program stored in the UMS by the machine tool manufacturer is not available, or there is a problem in the process of calling. The cause of the fault may be a problem with the storage module or the UMS sub-module. The memory template was removed for inspection, and it was found that the connecting wires between A and B on the circuit board were corroded, and the contact was poor. After soldering these two points, and starting the test, the alarm did not appear again.

8. Measurement method

The measurement method is the basic method for diagnosing machine tool faults, and of course it is also a common method for diagnosing CNC machine tool faults. The measurement method is to use multimeters, oscilloscopes, logic testers and other instruments to measure electronic circuits.

For example, a cylindrical grinder using the Siemens system, when starting the grinding wheel, 7021 (GRINDING WHEEL SPEED) alarm, indicating that the speed of the grinding wheel is abnormal, and the observation of the grinding wheel finds that the speed is really slow. Analyzing the working principle of the machine tool, the grinding wheel spindle is controlled by Siemens servo module 6SN1123-1AA00, and the given speed is adjusted by a sliding rheostat. The sliding contact of this rheostat changes with the position of the diamond roller dresser, so as to ensure that the diameter of the grinding wheel becomes smaller by means of simulation, the given speed voltage increases, the speed of the grinding wheel increases, and the linear speed of the grinding wheel remains unchanged. The line connection is shown in Figure 2-10. Measure the voltage between terminals 56 and 14 of the analog given input of the servo module, and find that it is only about 2.6V. Because the given voltage is low, the speed of the grinding wheel is low. According to the principle analysis, R3 is inside the grinding machine, and its sliding contact changes with the diameter of the grinding wheel, because the working environment in the machine tool is harsh and easy to damage, and there is no problem in measuring R1 and R2, and the power supply voltage is also normal. For this reason, Ra was removed for inspection, and it was found that there was a lot of grinding fluid in the cable plug. After cleaning, the measured resistance value changed normally. After reinstalling, the machine tool fault was eliminated.

Another example is that a CNC grinding machine cannot find the reference point on the Z axis. When the machine tool returns to the reference point, there is no problem when the X and Y axes return to the reference point. When the Z axis returns to the reference point, a pressure limit alarm occurs. Manually You can also walk back. Observe the process of the Z-axis returning to the reference point. After pressing the zero switch, the Z-axis decelerates, but does not stop until it reaches the limit. According to the principle analysis, it is possible that there is a problem with the zero pulse of the encoder. Check the zero pulse of the encoder with an oscilloscope, and it is indeed not. After purchasing a new encoder and replacing it, the machine tool works normally.

9. Interchange method to determine the fault point

For some system failures, because there are many factors involved, it is more complicated, and the exchange method can be used to quickly and accurately locate the failure point.

For example, when a CNC lathe fails, when the spindle rotates, an alarm No. 7006 appears, indicating that the spindle speed is out of tolerance. Observe that the spindle is indeed rotating, but the actual spindle speed is not displayed on the screen, so it is suspected that there is a problem with the spindle encoder. Swap the spindle encoder of this machine tool with the spindle encoder of another machine tool, and the other machine tool has alarm No. 7006, so it is determined that the spindle encoder is damaged.

Another example is that a CNC lathe suddenly loses power during normal processing, press the system start button: , the system cannot start, and none of the indicator lights on the panel are on. Measure the 5V DC power supply of the system power supply. When the start button is pressed, the voltage rises and then quickly drops to 0. Therefore, it is first suspected that there is a problem with the system power module, but if the backup power module is replaced, the fault remains, indicating that there is no problem with the power module. Continue to check and find that the connecting cable of the spindle encoder is damaged, and one line is connected with theThe ground is short-circuited, and the machine tool returns to normal use after treatment.

10. Principle analysis method

Principle analysis is the most basic method for troubleshooting. When other inspection methods are difficult to work, you can start from the working principle of the machine tool, check step by step, and finally find out the cause of the failure.

The above introduces 10 methods for diagnosing CNC machine tool failures. When diagnosing machine tool faults, these methods are often used comprehensively, and it is difficult to use a single method to be effective. This requires maintenance personnel to have certain maintenance experience, use diagnostic methods reasonably and comprehensively, so that machine tool failures can be eliminated as soon as possible.

In summary, CNC machine tools are the crystallization of the integration of modern high-tech machines, electricity, light, and gas, with complex electrical and intersecting pipelines. How to quickly find faults and hidden dangers and eliminate them in time? How can these expensive equipment be repaired? This may be a difficult problem for maintenance personnel. So how to solve this problem? I think we must do the following “five things”, read more materials, ask more questions and ask others, make more data records and summarize more, think more and think more, observe more and practice more.

1. Read more information

It is necessary to read more information about the CNC system. Each machine tool will be equipped with relevant materials of the CNC system used, such as operation manual, parameter manual, maintenance manual and so on. The purpose of reading more CNC system information is to understand the characteristics and functions of various CNC systems and PLC programmable controllers; to understand the alarms and troubleshooting methods of the CNC system; to understand the meaning of NC and PLC machine tool parameter settings; to understand PLC programming language; to understand the method of NC machining program programming; to understand the operation of the control panel and the contents of each menu; to understand the performance of the spindle and the tool feed motor and the characteristics of the drive, etc. There are often a lot of numerical control data, what do you think? The main point is to highlight the key points and find out the ins and outs. As a maintenance personnel, the key point is to understand the basic composition and structure of the CNC system, master the direction of the block diagram, power lines and signal lines, and flexibly use the maintenance functions provided by the CNC system. The meaning of the indicator light. Then, find out the setting method of the parameters and the meaning of each parameter and its adjustment method. Finally, learn about the preparation of NC machining programs and the meaning and use of various instructions. But the content of each part must be understood and mastered in key points.

See more electrical diagrams. Every electrical component on the machine tool, such as contactors, relays, time relays, etc., as well as the input and output of PLC, should be marked on the electrical drawing one by one. To give a simple example, for example, KM 1 is the contactor started by the hydraulic pump motor M1. Generally, the whereabouts of its normally open and normally closed contacts are marked on the diagram. Therefore, the normally open or normally closed contact KM1 on the corresponding page can be indicated as the hydraulic pump motor is open. For the electrical diagram of a large CNC machine tool, there are tens of pages, or even hundreds of pages, and it takes a long time to understand the function of each component. Sometimes, one or two may

Still not sureThe function of this component should be read more, and then written after digesting. Therefore, the starting hydraulic pump motor M1 just mentioned should also be clearly marked which external output of the PLC drives the contactor KM1 to act, and the ins and outs should be clear. For some block diagrams in the electrical circuit diagram, for example, there is only one block diagram for each axis driver, as long as you understand a certain control condition (on-off situation), you can study and consider more detailed things when you have time. The electrical symbols of each country are different, so we must first understand them clearly. For the several thick PLC statement tables written by the manufacturer, you should also read more, master their programming language, and make Chinese annotations on the basis of understanding. This can greatly save the time for troubleshooting in the future. If you wait until a fault occurs to familiarize yourself with electrical diagrams and PLC statement tables, it will inevitably take a lot of time and often cause wrong judgments.

Look more at hydraulic and pneumatic diagrams. In CNC machine tools, hydraulic, pneumatic and electrical are interdependent and inseparable. When performing fault analysis, put the hydraulic, pneumatic and electrical drawings together for cross-reference reading, which can achieve twice the result with half the effort. For example, which solenoid valve is used to actuate the spindle locking tool? What are the corresponding PLC outputs and inputs? Mark on the diagram in a language that is easy for you to understand, so that you can go from electrical to mechanical action to the end, and at the same time, you should focus on understanding the parts that are closely related to electromechanical. Know both electricity and machine, mechatronics, and master a variety of skills, so that the ability to solve problems will be great.

Read more foreign language materials to improve your professional foreign language reading ability. The latest and most advanced numerical control technology in the world today is in the hands of foreigners, which means that its materials must be written in foreign languages. When translated into Chinese, the content starts to lag behind. If you don’t know foreign languages, especially English, you will not be able to understand a large amount of the latest and most advanced technical materials in foreign languages. Relying on translation alone is often not ideal. Reading the technical information in foreign languages ​​is difficult at the beginning, and there are many new words. After reading and memorizing, there are only so many professional words in common use, and it looks fluent in the future. A competent maintenance personnel must basically master language tools.

2. Ask more and ask others

If experts come to your factory to install and debug machine tools, it is the best thing for you to have the opportunity to participate. This is the second best learning opportunity, because you can get a lot of first-hand information and methods and skills of machine tool debugging. For example, after measuring the accuracy of each axis by laser, how to make electrical corrections, etc. If you don’t understand, you need to ask more questions. Through this period of time, there will be great gains, and many machine tool manufacturers can obtain internal materials and manuals for assembly and debugging (which are confidential to users). After the machine tool is put into formal production, it should also keep in close contact with machine tool manufacturers and experts. Through FAX and E-MAIL, it is very beneficial to inquire about ways to solve difficult machine tool failures and related materials, as well as special and dedicated spare parts. At the same time, agents of CNC systems, such as SIEMENS, FANCompanies such as UC should also maintain good relations. If you ask more questions, you can also get further information and related spare parts of the CNC system in time, and you can also have the opportunity to participate in special learning classes about the CNC system.

After a failure occurs, the operator of the CNC machine tool should be asked about the whole process of the failure in detail. Don’t just ask, or just ask casually. In this way, the correct on-site information is often not obtained, resulting in wrong judgments and making the problem complicated. , prolonging the repair time of the machine tool. Therefore, ask more, ask in detail, understand the whole process of the fault (beginning, middle, end), what alarm signal was generated, what was operated at that time, what was touched, what was changed, and what was the external environment? On the basis of fully investigating the site and mastering the first-hand materials, the fault problem should be listed correctly, and half of the problem has actually been solved, and then analyzed and solved. For experienced operators, they are familiar with machine tool operation, processing procedures, and common diseases of machine tools. Close cooperation with them is very beneficial for rapid troubleshooting.

When other maintenance personnel are repairing the machine tool and you are not there, after they come back, you should also ask more, what happened just now? How was he ruled out? Ask him to describe his method of elimination. This is also a better learning opportunity. Learn other people’s correct troubleshooting skills and methods, especially learn from experienced maintenance personnel, learn their skills and improve your own level.

3. Make more data records and make more summaries

In the process of daily maintenance of CNC machine tools, it is necessary to record various relevant parameters, focusing on recording various relevant parameters after the machine tool is adjusted, such as NC machine parameters, PLC machine parameters, PLC programs, and spindles and servos. Motor current, voltage, speed and other data. Also write down the status of the relays and contactors in the electric cabinet when they are energized and formally processed (closed or disconnected), as well as the status of all input and output LED light-emitting diodes of the PLC (bright and dark, flashing), or record the status on the screen. PLC status X (input bit), Y (output bit) is 0 or 1. Recording it in this way is of great benefit for future analysis and judgment of faults.

Take a notebook with you, and record the failures that occur every day, especially the failures that have occurred, and how to eliminate them one by one. People’s brains tend to forget after a long time. “A good memory is not as good as a bad pen.” The faults of CNC machine tools often appear repeatedly, and these faults often occur. Just check how to solve them at that time, and the fault can be eliminated in a few minutes, which is fast and good. If the company has established equipment repair records, after troubleshooting, record the troubleshooting methods and bind them into a book for archiving. In this way, a complete historical file of a CNC machine tool is available.

4. Think more

Be more thoughtful and broaden your horizons. When repairing CNC machine tools, they are often not calm enough, fail to comprehensively analyze the faults, and dig into the horns. There was a fault, the Y axis of WERNER machining center stopped suddenly during machining, The alarm of dirty Y-axis grating ruler appeared on the screen many times. At that time, the grating ruler and the grating head were cleaned twice, but the machine still stopped. It took a few days without a solution, and finally found the real reason. The reason is that there is a problem with the wire between the Y-axis grating head and the amplifier.

Because the snake tube is bent for a long time when the Y axis moves, one of the position feedback lines breaks at a certain position and the machine tool stops. At that time, we only paid attention to the static state, ignoring the dynamic state. There was an open circuit alarm of the control loop, but it did not attract enough attention. Therefore, all the alarms and failures that occurred should be listed, and comprehensive judgment and screening should be performed to predict the greatest possibility of failure and then eliminate it. “The mountain is poor and the river is full of doubts and no way, and the willows are dark and the flowers are bright and there is another village”, Duosi, pointed out the direction for you.

5. More observation and more practice

For maintenance personnel, they must be bold and careful, and dare to do it. You can only talk, but if you don’t do it, you can’t repair the CNC machine tool. But do it after you are familiar with the situation, don’t be blind, otherwise it will expand the fault, cause an accident, and the consequences will be disastrous. At the same time, you must be good at hands-on. First of all, you must be familiar with the operation panel of the machine tool and the contents of each menu on the machine, so that you can operate freely. At the same time, we must make full use of the self-diagnosis technology of CNC machine tools to quickly deal with and solve faults. Now the more development of numerical control technology, its self-diagnosis ability is getting stronger and stronger.

Although the faults of CNC machine tools are complex and ever-changing, as long as they are taken seriously, they will be able to rely on their own strength to make good use, repair and manage the CNC machine tools.