The causes of abnormal machining accuracy failures are highly concealed and difficult to diagnose. Today I have summarized the 4 diagnostic principles and 5 diagnostic methods for you. Do you know them all?
1. Causes of abnormal machining accuracy failures
Five main reasons: the feed unit of the machine tool is modified or changed; the zero offset of each axis of the machine tool is abnormal; the axial backlash is abnormal; the motor operating status is abnormal, that is, the electrical and control parts are abnormal; mechanical failure, such as screw, Bearings, couplings and other components. In addition, the preparation of processing programs, tool selection and human factors may also lead to abnormal processing accuracy.
2. CNC machine tool fault diagnosis principles
1. First look at the outside and then the inside. CNC machine tools are machine tools that integrate mechanics, hydraulics, and electricity, so the occurrence of faults will also be comprehensively reflected by these three. Maintenance personnel should first conduct inspections one by one from the outside to the inside, and try to avoid random unpacking and disassembly. Otherwise, the fault will be magnified, the machine tool will lose accuracy and performance will be reduced.
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 faults, which can often achieve twice the result with half the effort.
3. Static first, then dynamic. First, in the static state of the machine tool with power off, through understanding, observation, testing, and analysis, the machine tool can be powered on only after it is confirmed to be a non-destructive fault; under operating conditions, conduct dynamic observation, Inspect and test to find faults. For destructive faults, the danger must be eliminated before power can be turned on.
4. Simple first, then complex. When multiple faults are intertwined and covered up, and you have no idea where to start, you should solve the easy problems first and then the more difficult ones. Often after simple problems are solved, difficult problems may become easier.
3. CNC machine tool fault diagnosis methods
1. Intuitive method: (look, hear and ask) Ask - machine tool fault phenomena, processing conditions, etc.; look - CRT alarm information, alarm lights, capacitors and other components deformed, smoked and burned, protector tripping, etc.; listen - abnormality Sound; smell - the smell of burnt electrical components and other odors; touch - heat, vibration, poor contact, etc.
2. Parameter checking method: 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 calibrated according to the fault characteristics.
3. Isolation method: For some faults, it is difficult to distinguish whether they are caused by the CNC part, the servo system or the mechanical part. The isolation method is often used.
4. Similar replacement method uses a spare board with the same function to replace the suspected faulty module, or exchanges modules or units with the same function.
5. Functional program testing method: Write some small programs for all the instructions of G, M, S, and T functions. Run these programs when diagnosing faults to determine the lack of functions.
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(Image source: Angke Machine Tool)
4. Examples of fault diagnosis and processing of abnormal machining accuracy
1. Mechanical failure causes abnormal processing accuracy
Fault phenomenon: An SV-1000 vertical machining center uses Frank system. During the processing of the connecting rod mold, it was suddenly discovered that the Z-axis feed was abnormal, causing a cutting error of at least 1mm (overcutting in the Z direction).
Fault diagnosis: During the investigation, it was learned that the fault occurred suddenly. The machine tool is jogging, and each axis operates normally under manual data input mode, and returns to the reference point normally. There is no alarm prompt, and the possibility of a hard fault in the electrical control part is eliminated. The following aspects should be mainly checked one by one.
Check the processing program segment that is running when the machine tool accuracy is abnormal, especially the tool length compensation, calibration and calculation of the processing coordinate system (G54-G59).
In the jog mode, the Z-axis is repeatedly moved, and the motion status is diagnosed through sight, touch, and listening. It is found that the Z-direction motion noise is abnormal, especially in rapid jog, the noise is more obvious. Judging from this, there may be hidden dangers in machinery.
Check the Z-axis accuracy of the machine tool. Move the Z-axis with a hand-operated pulse generator (set the magnification to 1×100, that is, the motor feeds 0.1mm for each step), and observe the movement of the Z-axis with a dial indicator. After the one-way motion remains normal, it serves as the starting point of forward motion. Every time the pulser changes one step, the actual distance of the Z-axis movement of the machine tool is d=d<span lang="EN-US" style="font-size:10.5pt; mso-bidi-font-size:11.0pt;font-family:Calibri,sans-serif" ;mso-ascii-theme-font: minor-latin;mso-fareast-font-family:宋体;mso-fareast-theme- font:minor-fareast;mso-hansi-theme-font:minor-latin;mso-bidi-font-family:"times new roman";mso-bidi-theme-font:minor-bidi;mso-ansi-language: en-us;mso-fareast-language: zh-cn;mso-bidi-language:ar-sa;">1=d2=d3=……=0.1mm, indicating that the motor is running well and the positioning accuracy is also good. And return The change of the actual movement displacement of the machine tool can be divided into four stages: (1) The movement distance of the machine tool d1>d=0.1mm (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 the most standard backlash; (4) The machine tool movement distance is equal to the predetermined value of the pulser (the slope is equal to 1), and the machine tool returns to its normal motion. No matter how the The backlash is compensated, and its characteristics are: except for the (3) stage compensation, changes in other stages still exist, especially the (1) stage, which seriously affects the machining accuracy of the machine tool. During the compensation, it is found that the greater the backlash compensation , (1) The distance moved in stage is also larger.
After analyzing the above inspection, it is believed that there are several possible reasons: 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, completely disengage the motor and screw, and inspect the motor and mechanical parts respectively. The inspection result is that the motor is running normally; during the diagnosis of the mechanical part, it was found that when the screw is turned by hand, there is a large sense of vacancy at the beginning of the return movement. Under normal circumstances, you should be able to feel the bearing moving in an orderly and smooth manner.
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 replacement.
2. Improper control logic leads to abnormal processing 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 minimum accuracy error was 0.008mm and the maximum was 1.2mm. Fault diagnosis: During the inspection, the machine tool has set the G54 workpiece coordinate system as required. In the manual data input mode, run a program in the G54 coordinate system, namely "GOOG90G54X60.OY70.OF150; M30;". After the machine tool is finished running, the mechanical coordinate value displayed on the display is (X-axis) "-1025.243", record Lower this value. Then in manual mode, jog the machine tool to any other position, and run the program segment just now in manual data input mode again. After the machine tool stops, it is found that the machine tool coordinate value is displayed as "-1024.891", the same as the previous execution The difference between the latter values is 0.352mm. Follow the same method, move the X-axis jog to different positions, and execute this program segment 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 consistent with the error displayed by the numbers. Therefore, it is believed that the cause of the fault is excessive repeated positioning error of the X-axis. Check the backlash and positioning accuracy of the X-axis and re-compensate the error value, but it has no effect. Therefore, it is suspected that there is a problem with the grating ruler and system parameters. But why such a large error occurred, but no corresponding alarm message appeared. Further inspection found that this axis is a vertical axis, and when the X-axis is released, the spindle box falls downwards, causing the error.
Troubleshooting: The PLC logic control program of the machine tool has been modified, that is, when the X-axis is loosened, the X-axis is enabled and loaded first, and then the X-axis is loosened; when the X-axis is clamped, the X-axis is clamped first. After that, remove the enable. After adjustment, the machine tool fault was solved.
3. Machine tool position problems lead to abnormal processing accuracy
Fault phenomenon: A vertical CNC milling machine produced in Hangzhou, equipped with Beijing KND-10M system. During jogging or machining, an abnormality in the Z axis was found.
Fault diagnosis: The inspection found that the Z-axis moved up and down unevenly and made noise, and there was a certain gap. When the motor starts, there is unstable noise and uneven force in the upward movement of the Z-axis in the jog mode, and the motor feels shaking. When moving downward, the shaking is not so obvious; when it stops, there is no shaking. It is more obvious during the processing. Analysis believes that there are three reasons for the failure: first, the screw backlash is very large; second, the Z-axis motor is working abnormally; third, the pulley is damaged to the point of uneven stress. But one thing to note is that there is no jitter when stopping and the up and down motion is uneven, so the problem of abnormal motor operation can be eliminated. 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 elimination, the only remaining problem was the belt. When inspecting the belt, I found that this belt had just been replaced. However, when I carefully inspected the belt, I found that there were varying degrees of damage on the inner side of the belt. It was obviously caused by uneven force. , what is the cause? During the diagnosis, it was found that there was a problem with the placement of the motor, that is, the angular position of the clamping was asymmetrical, causing uneven stress.
Troubleshooting: Just reinstall the motor, align the angle, measure the distance (motor and Z-axis bearing), and make sure the belt (length) is even on both sides. In this way, the uneven movement of the Z-axis up and down and the noise and jitter are eliminated, and the Z-axis processing returns to normal.
4. The system parameters are not optimized and the motor operates 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. During the machine tool repair process, local processing often affects changes in zero offset and clearance. Timely adjustments and modifications should be made after the fault is resolved. On the other hand, serious mechanical wear or loose connections may also cause the measured parameter values to change. Changes in parameters require corresponding modifications to meet the requirements of machine tool processing accuracy.
Fault phenomenon: A vertical CNC milling machine produced in Hangzhou, equipped with Beijing KND-10M system. During the machining process, it was discovered that the X-axis accuracy was abnormal.
Fault diagnosis: The inspection found that there is a certain gap in the X-axis and the motor is unstable when starting. When you touch the X-axis motor with your hand, you feel that the motor pulls strongly, but the pull is not obvious when it stops, especially in the inching mode. Analysis believes that there are two reasons for the failure: first, the screw backlash is very large; second, the X-axis motor is working abnormally.
Troubleshooting: Use the parameter function of the KND-10M system to debug the motor. First, the existing gap is compensated, and then the servo system parameters and pulse suppression function parameters are adjusted. The jitter of the X-axis motor is eliminated, and the machining accuracy of the machine tool returns to normal.
