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A, resulting in machining accuracy Abnormality Cause <br> <br> cause of the abnormal machining accuracy failure concealed, diagnosis difficult, summarized five main reasons: tool feed unit has been altered or varied; individual machine axes The zero offset is abnormal; the axial backlash is abnormal; the motor is running abnormally, that is, the electrical and control parts are abnormal; the mechanical faults, such as screw, bearing, shaft coupling and other components. In addition, the preparation of machining programs, tool selection, and human factors may also lead to abnormal machining accuracy.
Second, the principle of fault diagnosis of CNC machine tools 1. First, external and internal CNC machine tools are mechanical, hydraulic, electrical as one of the machine tools, so the occurrence of the failure will be reflected by these three comprehensive. Maintenance personnel should conduct inspections one by one from the outside to the inside, try to avoid opening and disassembling as much as possible. Otherwise, they will expand the fault and cause the machine to lose accuracy and reduce performance.
2. Mechanical first, then electrical Generally speaking, mechanical faults are easier to detect, and the fault diagnosis of numerical control systems is more difficult. Before troubleshooting, first pay attention to the elimination of mechanical failure, often to achieve a multiplier effect.
3. After the static and dynamic operation, the machine tool can be energized after understanding, observing, testing, analyzing and confirming as a non-destructive failure in the static state of the power-off of the machine tool; under dynamic operating conditions, dynamic observation is performed. Inspection and testing to find faults. For destructive faults, the danger must be eliminated before power is applied.
4. Simple and complex first When there are many kinds of faults that are intertwined and covered, and you cannot start at once, you should solve the easy problems first, and then solve the more difficult problems. After simple problems are solved, difficult problems may also become easier.
Third, the numerical control machine tool fault diagnosis method 1. Intuitive method: (Wangwenqie) Q - machine tool fault phenomenon, processing conditions, etc.; see - CRT alarm information, alarm lights, capacitors and other components deformation smoke charred, protector tripping, etc. Listening - abnormal sound; smell - smell of electrical components and other odors; touch - fever, vibration, poor contact and so on.
2. Parameter check method: The parameters are usually stored in RAM. Sometimes the battery voltage is insufficient. The system will not be powered on for a long time or external interference will cause the parameters to be lost or confused. Check and correct the relevant parameters according to the characteristics of the fault.
3. Isolate method: Some faults, it is difficult to distinguish between the numerical control part, or the servo system or the mechanical part, often use the isolation method.
4. The same type of swap method replaces a template that is suspected to be defective with a spare board having the same function, or exchanges modules or units that have the same function.
5. Function program test method Write G, M, S, T, and all the functions of the instruction in some small programs. When these faults are diagnosed, you can determine the lack of function.
IV. Abnormal processing accuracy Fault diagnosis and processing examples 1. Mechanical faults cause abnormal machining accuracy Fault phenomenon: A SV-1000 vertical machining center adopts the Frank system. During machining of the connecting rod mold, an abnormal feed of the Z axis was suddenly found, resulting in a cutting error amount of at least 1 mm (cut in the Z direction).
Troubleshooting: In the investigation, it was learned that the failure occurred suddenly. When the machine tool is jogging, each axis runs normally under the manual data input mode operation, and the reference point is returned to normal. There is no alarm indication, and the possibility of hard failure of the electrical control part is eliminated. It should mainly examine the following aspects one by one.
Check the machining program block that is running when the machine's accuracy is abnormal, especially the tool length compensation, the proofreading and calculation of the machining coordinate system (G54-G59).
In the jog mode, the Z-axis is repeatedly moved, and after seeing, touching, and listening, the state of motion is diagnosed, and it is found that the Z-direction motion noise is abnormal, especially rapid jogging, and the noise is more pronounced. Judging from this, there may be hidden dangers in machinery.
Check the Z axis accuracy of the machine. Move the Z-axis by hand using a pulse generator (set the override to 1×100, ie, each step of the change, the motor feeds 0.1 mm), and observe the Z-axis movement with the dial indicator. After the unidirectional motion is maintained as normal, the positive movement of the starting point, the actual distance of the Z-axis movement of the machine tool is d=d1=d2=d3=...=0.1mm for each step change of the pulser, indicating that the motor runs well and the positioning accuracy is also good. . Returning to the actual movement displacement of the machine tool, it can be divided into four stages: (1) The machine movement distance d1>d=0.1mm (slope is greater than 1); (2) shows d1=0.1mm>d2>d3 ( The slope is less than 1); (3) The machine tool mechanism does not actually move, showing the most standard backlash; (4) The machine tool movement distance is equal to the value of the pulser fixed value (slope is equal to 1), and it returns to the normal movement of the machine tool. No matter how to compensate the backlash, the characteristic that it displays is: In addition to (3) phase compensation, other changes in each segment still exist, especially in (1) the stage seriously affects the machining accuracy of the machine tool. The compensation found that the greater the gap compensation, the greater the distance that (1) the stage moves.
Analysis of the above inspections suggests that there are several possible causes: First, there is an abnormality in the motor; second, there is a mechanical failure; and third, there is a gap in the screw. In order to further diagnose the fault, the motor and the screw are completely disengaged, and the motor and the mechanical part are inspected separately. The inspection result is that the motor is operating normally; in the diagnosis of the mechanical part, it was found that when the screw is moved by hand, there is a great sense of vacancy in the initial return movement. Under normal circumstances, it should be able to feel the orderly and smooth movement of the bearing.
Troubleshooting: After disassembling inspection, it was found that the bearing was damaged and there was a ball drop. The machine will return to normal after replacement.
2. Inadequate control logic leads to abnormal machining accuracy Fault phenomenon: A machining center produced by a Shanghai machine tool manufacturer, the system is Frank. During the machining process, the X-axis accuracy of the machine tool is found to be abnormal, and the accuracy error is a minimum of 0.008 mm and a maximum of 1.2 mm. Fault diagnosis: In the inspection, the machine tool has set the G54 workpiece coordinate system as required. In the manual input data mode operation, the G54 coordinate system runs a program that is "GOOG90G54X60.OY70.OF150; M30;", the machine coordinate value displayed on the display after the standby bed operation is (X axis) "-1025.243", record Under this value. Then, in the manual mode, the machine tool is moved to any other position, and the previous program segment is again operated in the manual input data mode operation. After the standby bed is stopped, the machine coordinate value is displayed as “-1024.891â€, which is the same as the previous execution. The difference between the values ​​is 0.352mm. In the same way, move the X-axis jog to a different position and execute the program repeatedly. The displayed value on the display is different (unstable). The X-axis was carefully checked with a dial indicator. It was found that the actual error of the mechanical position is basically the same as the error shown by the figure. Therefore, the reason for the fault is that the X-axis repeated positioning error is too large. The X-axis backlash and positioning accuracy are checked to re-compensate for the error and the result does not have any effect. Therefore, there are problems with the scale and system parameters. But why did such a large error occur, but no corresponding alarm message appeared? Further inspection revealed that the axis was a vertical axis. When the X axis was released, the headstock fell downward, causing an error.
Troubleshooting: The PLC logic control program of the machine tool has been modified, that is, when the X axis is released, the X axis is enabled to load, and then the X axis is released; and when the X axis is clamped, the X axis is clamped first. Immediately afterwards, remove the enabler. Adjusted machine faults can be resolved.
3. Machine position problems lead to abnormal machining accuracy Fault phenomenon: A vertical CNC milling machine made in Hangzhou is equipped with a Beijing KND-10M system. In the jogging or machining process, the Z axis is abnormal.
Fault diagnosis: The inspection found that the Z axis moves up and down unevenly and with noise, and there is a certain gap. When the motor starts, there is unstable noise and uneven force in the Z-axis movement in the jog mode, and it feels that the motor shakes more violently; when it moves down, there is no jitter so obvious; when it stops, it does not shake, In the processing process is more obvious. According to the analysis, there are three reasons for the failure: one is that the backlash of the screw is large; the second is that the Z-axis motor is working abnormally; and the third is that the pulley is damaged to the uneven force. However, there is a problem to be noted is that when the stop does not shake, up and down movement is not uniform, so the problem of abnormal motor operation can be ruled out. Therefore, the mechanical part is diagnosed first, and no abnormality is found during the diagnostic test, within the tolerance. Using the rule of exclusion, only the belt was the problem. When the belt was inspected, it was discovered that the belt had just been changed. However, when the belt was inspected carefully, it was found that there were different degrees of damage inside the belt. Obviously, the force was uneven. What caused the problem? In the diagnosis, it was found that there was a problem with the placement of the motor, that is, the angular position of the clip was not uniform due to the angular position.
Troubleshooting: As long as the motor is reinstalled, align the angle, and measure the distance (motor and Z-axis bearings), both sides (length) of the belt should be uniform. In this way, the Z-axis moves up and down unevenly and noise and jitter are eliminated, and the Z-axis machining returns to normal.
4. The system parameters are not optimized. Abnormal motor operation leads to abnormal machining accuracy The system parameters mainly include the machine feed unit, zero offset, and backlash. For example, Frank's CNC system has two feed units, metric and imperial. In the process of machine tool repair, the zero offset and the change of the gap are often affected for the local processing. After the fault processing is completed, timely adjustments and modifications should be made. On the other hand, due to serious mechanical wear or loose connection points, the actual measured values ​​may also be caused. The changes need to make corresponding changes to the parameters in order to meet the requirements of machining precision.
Symptom: A vertical CNC milling machine manufactured in Hangzhou is equipped with a Beijing KND-10M system. During machining, abnormal X-axis accuracy was found.
Fault diagnosis: Check that there is a certain gap on the X axis, and there is an unstable phenomenon when the motor starts. When you touch the X-axis motor with your hand, you feel that the motor pulls more severely, and the pull is not obvious when you stop, especially in the jog mode. According to the analysis, there are two reasons for the failure: First, the backlash of the screw is large; second, the X-axis motor is working abnormally.
Troubleshooting: Use the KND-10M system parameter function to debug the motor. First, the existing gap is compensated, and then servo system parameters and pulse suppression function parameters are adjusted. The vibration of the X-axis motor is eliminated, and the machining accuracy of the machine tool returns to normal.
Analysis of the Causes of Decreased Precision of CNC Machine Tools
The causes of abnormal machining accuracy faults are highly concealed and difficult to diagnose. There are five main reasons: the machine feed unit is changed or changed; the zero offset of each axis of the machine tool is abnormal; the axial backlash is abnormal; the motor Abnormal operation status, ie abnormal electrical and control parts; mechanical faults, such as screw, bearings, couplings and other components. In addition, the preparation of machining programs, tool selection, and human factors may also lead to abnormal machining accuracy.