Analysis of the reasons for the decrease in machining accuracy of CNC machine tools

The causes of abnormal machining accuracy faults are highly concealed and difficult to diagnose. Five main reasons are summarized: the feed unit of the machine tool has been changed or changed; the zero offset of each axis of the machine tool is abnormal; the axial backlash is abnormal; the motor Abnormal operating status, that is, abnormal electrical and control parts; mechanical failure, such as screw, bearing, coupling and other components. In addition, the preparation of machining programs, the selection of cutting tools and human factors may also lead to abnormal machining accuracy.

1. Causes of abnormal machining accuracy faults

The causes of abnormal machining accuracy faults are highly concealed and difficult to diagnose. Five main reasons are summarized: the feed unit of the machine tool has been changed or changed; the zero offset of each axis of the machine tool is abnormal; the axial backlash is abnormal; the motor Abnormal operating status, that is, abnormal electrical and control parts; mechanical failure, such as screw, bearing, coupling and other components. In addition, the preparation of machining programs, the selection of cutting tools and human factors may also lead to abnormal machining accuracy.

Second, the principle of fault diagnosis of CNC machine tools

1. First external and then internal CNC machine tools are machine tools that integrate machinery, hydraulic pressure and electricity, so the occurrence of failures will also be comprehensively reflected by these three. Maintenance personnel should check one by one from the outside to the inside, and try to avoid random unpacking and disassembly, otherwise it will expand the fault, cause the machine tool to lose precision and reduce performance.

2. Mechanical first, then electrical Generally speaking, mechanical faults are easier to detect, while the diagnosis of CNC system faults is more difficult. Before troubleshooting, first pay attention to eliminating mechanical failures, which can often achieve twice the result with half the effort.

3. Static first and then moving. First, in the static state of the machine tool power off, through understanding, observation, testing, analysis, and confirming that it is a non-destructive fault, the machine tool can be powered on; Inspection and testing to find faults. For destructive faults, the danger must be eliminated before powering on.

4. Simple first and then complex When multiple faults are intertwined and covered up, and it is impossible to start for a while, the easy problems should be solved first, and the more difficult problems should be solved later. Often after the simple problems are solved, the difficult problems may also become easy.

Three, CNC machine tool fault diagnosis method

1. Intuitive method: (see, hear, ask and cut) ask-the fault phenomenon of the machine tool, processing status, etc.; see-CRT alarm information, alarm indicator light, capacitor and other components are deformed, smoked and burnt, and the protector trips, etc.; listen-abnormal sound; smell - Burnt smell of electrical components and other peculiar smells; touching - heat, vibration, poor contact, etc.

2. Parameter inspection method: The parameters are usually stored in RAM. Sometimes the battery voltage is insufficient, the system is not powered on for a long time or external interference will cause the parameters to be lost or confused. The relevant parameters should be checked and corrected according to the fault characteristics.

3. Isolation method: For some faults, it is difficult to distinguish whether it is caused by the CNC part, the servo system or the mechanical part, and the isolation method is often used.

4. The same-kind swapping method replaces the suspected faulty template with a spare board with the same function, or exchanges the templates or units with the same function.

5. Functional program test method Write some small programs for all the instructions of G, M, S, T, functions, and run these programs when diagnosing faults to judge the lack of functions.

4. Example of fault diagnosis and treatment of abnormal machining accuracy

1. Mechanical failure leads to abnormal machining accuracy

Fault phenomenon: a SV-1000 vertical machining center, using the Frank system. During the process of processing the connecting rod mold, it was suddenly found that the feed of the Z axis was abnormal, resulting in a cutting error of at least 1 mm (overcut in the Z direction).

Fault Diagnosis: The investigation revealed that the fault occurred suddenly. The machine tool is jogging, and each axis is running normally under the manual data input mode, and the reference point return is normal, there is no alarm prompt, and the possibility of hard failure of the electrical control part is ruled out. The following aspects should be checked one by one.

Check the processing program segments that are running when the accuracy of the machine tool is abnormal, especially the tool length compensation, the calibration and calculation of the processing coordinate system (G54-G59).

In the jogging mode, move the Z axis repeatedly, and diagnose its motion status through sight, touch, and hearing. It is found that the Z-axis motion noise is abnormal, especially the rapid jogging, the noise is more obvious. Judging from this, there may be hidden dangers in the mechanical aspect.

Check the Z-axis accuracy of the machine tool. Use the manual pulse generator to move the Z-axis (set its magnification to 1×100 gear, that is, the motor feeds 0.1mm for each step change), and observe the movement of the Z-axis with the dial indicator. After the one-way movement remains normal, it is used as the starting point of forward movement. Every time the pulser changes one step, the actual distance of the Z-axis movement of the machine tool d=d1=d2=d3=...=0.1mm, indicating that the motor is running well and the positioning accuracy is also good. . As for the change of the actual movement displacement of the machine tool, it can be divided into four stages: (1) The movement distance of the machine tool d1>d=0.1mm (the slope is greater than 1); (2) It is shown as d1=0.1mm>d2>d3 ( The slope is less than 1); (3) The machine tool mechanism does not actually move, showing a standard backlash; (4) The movement distance of the machine tool is equal to the fixed value of the pulser (the slope is equal to 1), and the normal movement of the machine tool is restored. No matter how the backlash is compensated, its characteristics are: except for the (3) stage compensation, other changes still exist, especially the (1) stage seriously affects the machining accuracy of the machine tool. It is found in the compensation that the greater the gap compensation, the greater the distance moved in the (1) stage.

Analyzing the above inspections, it is believed that there are several possible reasons: one is that the motor is abnormal, the other is that there is a mechanical failure, and the third is that there is a gap in the screw. In order to further diagnose the fault, the motor and the lead screw are completely disengaged, and the motor and the mechanical part are inspected separately. The result of the inspection is that the motor is running normally; in the diagnosis of the mechanical part, it is found that when the screw is turned by hand, there is a great sense of emptiness at the beginning of the return movement. Under normal circumstances, you should feel the orderly and smooth movement of the bearings.

Troubleshooting: After disassembly and inspection, it was found that the bearing was indeed damaged and the balls fell off. The machine returned to normal after the replacement.

2. Improper 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, it was found that the X-axis accuracy of the machine tool was abnormal, the accuracy error was as small as 0.008mm and as large as 1.2mm. Fault diagnosis: during inspection, the machine tool has been in accordance with It is required to set the G54 workpiece coordinate system. In the manual data input mode, run a program in the G54 coordinate system, that is, "GOOG90G54X60.OY70.OF150; M30;", after the machine tool runs, the mechanical coordinate value displayed on the display (X axis) "-1025.243", record down the value. Then in the manual mode, jog the machine tool to any other position, and run the program segment just now in the manual data input mode again. After the machine tool stops, it is found that the machine tool coordinate value is displayed as "-1024.891", which is the same as the previous execution. The difference between the final values ​​is 0.352mm. According to the same method, move the X-axis to different positions and execute the program repeatedly, but the values ​​displayed on the display are different (unstable). Carefully check the X-axis with a dial indicator, and find that the actual error of the mechanical position is basically the same as the error displayed by the numbers, so it is believed that the cause of the fault is that the repeated positioning error of the X-axis is too large. Check the backlash and positioning accuracy of the X-axis, and re-compensate the error value, but the result does not play any role. Therefore, it is suspected that there is a problem with the grating ruler and system parameters. But why there is such a large error, but there is no corresponding alarm message. Further inspection found that this axis is a vertical axis. When the X axis is released, the headstock falls down, causing an error.

Troubleshooting: The PLC logic control program of the machine tool has been modified, that is, when the X-axis is released, first enable the X-axis to load, and then release the X-axis; and when the X-axis is clamped, first clamp the X-axis After that, remove the enable. After the adjustment, the fault of the machine tool was solved.

3. The position of the machine tool leads to abnormal machining accuracy

Fault phenomenon: a vertical CNC milling machine made in Hangzhou, equipped with Beijing KND-10M system. During jogging or processing, the Z axis is found to be 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 is started, there is unstable noise and uneven force in the upward movement of the Z-axis in the jog mode, and it feels that the motor vibrates more violently; when it moves downward, the vibration is not so obvious; when it stops, it does not vibrate, It is more obvious during processing. According to the analysis, there are three reasons for the failure: one is that the backlash of the lead screw is large; the other is that the Z-axis motor works abnormally; the third is that the pulley is damaged to uneven force. But there is a problem to pay attention to. It does not vibrate when it stops, and the up and down motion is uneven, so the problem of abnormal motor operation can be ruled out. Therefore, the mechanical part is diagnosed first, and no abnormalities are found during the diagnostic test, which is within the tolerance. Using the rule of exclusion, the only problem left is the belt. When testing the belt, it was found that the belt had just been replaced, but when the belt was carefully inspected, it was found that the inner side of the belt was damaged to varying degrees, which was obviously caused by uneven force. , What is the reason? In the diagnosis, it was found that there was a problem with the placement of the motor, that is, the asymmetrical angular position of the clamping caused uneven force.

Troubleshooting: Just reinstall the motor, align the angle, measure the distance (motor and Z-axis bearing), and both sides (length) of the belt should be even. In this way, the uneven up and down movement of the Z-axis and the phenomenon of noise and jitter are eliminated, and the Z-axis processing returns to normal.

4. The system parameters are not optimized, and the motor runs abnormally

The system parameters that lead to abnormal machining accuracy mainly include machine tool feed unit, zero offset, backlash, etc. For example, the Frank CNC system has two feed units: metric and imperial. In the process of machine tool repair, local treatment often affects the change of zero offset and gap, and timely adjustment and modification should be made after the fault treatment is completed; In order to meet the requirements of machine tool machining accuracy, it is necessary to modify the parameters accordingly.

Fault phenomenon: a vertical CNC milling machine made in Hangzhou, equipped with Beijing KND-10M system. During the machining process, it was found that the X-axis precision was abnormal.

Fault diagnosis: The inspection found that there is a certain gap in the X-axis, and there is instability when the motor starts. When you touch the X-axis motor with your hands, you feel that the motor pulls strongly, but the pull is not obvious when it stops, especially in the jogging mode. According to the analysis, there are two reasons for the failure: one is that the backlash of the lead screw is large; the other is that the X-axis motor works abnormally.

Troubleshooting: Use the parameter function of the KND-10M system to debug the motor. Firstly, the existing gap is compensated, and then the parameters of the servo system and the pulse suppression function are adjusted, the vibration of the X-axis motor is eliminated, and the machining accuracy of the machine tool returns to normal.