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

The causes of abnormal machining accuracy faults are highly covert and difficult to diagnose. Five main reasons can be summarized: the feed unit of the machine tool has been modified or changed; Abnormal zero offset of each axis of the machine tool; Abnormal axial reverse clearance; Abnormal motor operation status, namely abnormal electrical and control parts; Mechanical failures, such as screws, bearings, couplings, and other components. In addition, the programming of machining programs, the selection of cutting tools, and human factors may also lead to abnormal machining accuracy. 1、 The causes of abnormal machining accuracy faults are highly covert and difficult to diagnose. Five main reasons can be summarized: the feed unit of the machine tool has been modified or changed; Abnormal zero offset of each axis of the machine tool; Abnormal axial reverse clearance; Abnormal motor operation status, namely abnormal electrical and control parts; Mechanical failures, such as screws, bearings, couplings, and other components. In addition, the programming of machining programs, the selection of cutting tools, and human factors may also lead to abnormal machining accuracy. 2、 Principle of fault diagnosis for CNC machine tools: 1. Starting from the external and then from the internal. CNC machine tools are a combination of mechanical, hydraulic, and electrical components, so the occurrence of their faults will also be reflected comprehensively by these three factors. Maintenance personnel should first inspect one by one from the outside to the inside, and try to avoid randomly opening and disassembling, otherwise it will expand the fault, cause the machine tool to lose accuracy, and reduce performance. Generally speaking, mechanical faults are easier to detect, while diagnosing faults in CNC systems 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. In the static state of the machine tool when it is powered off, after understanding, observing, testing, analyzing, and confirming that it is a non-destructive fault, the machine tool can be powered on; Under operating conditions, conduct dynamic observation, inspection, and testing to identify faults. For destructive faults, the danger must be eliminated before power can be applied. 4. When multiple faults are intertwined and covered up, and there is no way to start at the moment, the easy problems should be solved first, and the more difficult problems should be solved later. Often, after solving simple problems, more difficult ones may also become easier. 3、 Numerical control machine tool fault diagnosis method 1. Intuitive method: (observing, smelling, questioning, cutting) asking about the phenomenon of machine tool faults, processing conditions, etc; Check the - CRT alarm information, alarm indicator lights, deformation, smoking, burning of capacitors and other components, and tripping of protective devices; Abnormal sound during listening; Smell - Electrical components have a burnt smell and other unpleasant odors; Touch heating, vibration, poor contact, etc. 2. Parameter inspection method: Parameters are usually stored in RAM. Sometimes, insufficient battery voltage, long-term power failure of the system, or external interference can cause parameter loss or confusion. Relevant parameters should be checked and calibrated based on fault characteristics. 3. Isolation method: For some faults that are difficult to distinguish between the CNC part, servo system, or mechanical part, isolation method is often used. 4. Same type swapping method: Replace the suspected faulty template with a backup board with the same function, or exchange templates or units with the same function. 5. The functional program testing method involves writing some small programs with all the instructions of G, M, S, T, and other functions. When diagnosing faults, these programs can be run to determine the lack of functionality. 4、 Example of Diagnosis and Handling of Abnormal Machining Precision Fault 1. Mechanical Fault Causing Abnormal Machining Precision Fault Phenomenon: An SV-1000 Vertical Machining Center Using Frank System. During the machining of the connecting rod mold, it was suddenly discovered that the Z-axis feed was abnormal, resulting in a cutting error of at least 1mm (over cutting in the Z-direction). Fault diagnosis: During the investigation, it was found that the fault occurred suddenly. The machine tool is in jog mode, and under manual input of data, all axes operate normally and return to the reference point without any alarm prompts. The possibility of hard faults in the electrical control part has been ruled out. The following aspects should be checked one by one. Check the machining program segments that are running when the accuracy of the machine tool is abnormal, especially for tool length compensation, calibration and calculation of the machining coordinate system (G54-G59). Under the jog mode, the Z-axis is repeatedly moved, and after visual, tactile, and auditory diagnosis of its movement status, it is found that the Z-axis movement noise is abnormal, especially when quickly jog, the noise is more pronounced. Based on this, there may be hidden dangers in the mechanical aspect. Check the Z-axis accuracy of the machine tool. Move the Z-axis using a hand cranked pulse generator (set its magnification to 1) × At a gear of 100, that is, for each step of change, the motor feeds 0.1mm, and observe the movement of the Z-axis with a dial gauge. After maintaining normal unidirectional motion as the starting point for forward motion, with each change in the pulse generator, the actual distance of the Z-axis movement of the machine tool d=d1=d2=d3=...=0.1mm indicates that the motor is running well and the positioning accuracy is also good. When it comes to the actual movement displacement of the machine tool, it can be divided into four stages: (1) the machine tool movement distance d1>d=0.1mm (slope greater than 1); (2) Manifested as d1=0.1mm>d2>d3 (slope less than 1); (3) The machine tool mechanism did not actually move and exhibited the most standard reverse clearance; (4) The movement distance of the machine tool is equal to the set value of the pulse generator (with a slope of 1), and it returns to normal movement of the machine tool. No matter how the reverse clearance is compensated, its characteristic is that, except for the compensation in stage (3), the changes in other stages still exist, especially in stage (1), which seriously affects the machining accuracy of the machine tool. During the compensation process, it was found that the larger the gap compensation, the greater the distance traveled during stage (1). Analyzing the above inspection, it is believed that there are several possible reasons: firstly, there is an abnormality in the motor, secondly, there is a mechanical malfunction, and thirdly, there is a gap in the screw. To further diagnose the fault, completely disconnect the motor and screw, and inspect the motor and mechanical parts separately. The inspection result shows that the motor is running normally; In the diagnosis of the mechanical part, it was found that there was a significant gap in the initial movement when manually turning the lead screw. Under normal circumstances, it should be possible to feel the orderly and smooth movement of the bearings. Fault handling: After disassembly and inspection, it was found that the bearing was indeed damaged and there were ball bearings falling off. After replacement, the machine tool returned to normal. 2. Improper control logic leading to abnormal machining accuracy fault phenomenon: A machining center produced by a Shanghai machine tool manufacturer has a Frank system. During the machining process, it was found that the X-axis accuracy of the machine tool was abnormal, with a minimum accuracy error of 0.008mm and a maximum accuracy error of 1.2mm. Fault diagnosis: During inspection, the machine tool has already set the G54 workpiece coordinate system as required. Under manual data input mode, run a program in the G54 coordinate system, i.e. "GOOG90G54X60. OY70. OF150; M30;". After the standby bed runs, the mechanical coordinate value displayed on the display is (X-axis) "-1025.243". Record this value. Then, in manual mode, move the machine tool to any other position and run the previous program segment again in manual data input mode. After the machine tool stops, it is found that the coordinate value of the machine tool is displayed as "-1024.891", which is 0.352mm different from the value after the previous execution. Using the same method, move the X-axis to different positions and repeatedly execute the program segment, The values displayed on the monitor are all different (unstable). Careful inspection of the X-axis using a dial gauge revealed that the actual error in the mechanical position was consistent with the error displayed by the numbers, indicating that the cause of the malfunction was excessive repeated positioning error on the X-axis. Checking the reverse clearance and positioning accuracy of the X-axis and compensating for its error value again did not have any effect. Therefore, it is suspected that there are issues with the grating ruler and system parameters. But why did such a large error occur without corresponding alarm messages? Further inspection revealed that this axis is vertical, and when the X-axis is released, the spindle box falls down, causing the error. Fault handling: The PLC logic control program of the machine tool has been modified, that is, when the X-axis is released, enable the loading of the X-axis first, and then release the X-axis; When clamping the X-axis, first clamp the X-axis and then remove the enable. After adjustment, the machine tool malfunction was resolved. 3. Abnormal machining accuracy caused by machine position issues: A vertical CNC milling machine produced in Hangzhou, equipped with the Beijing KND-10M system. During jogging or machining, Z-axis abnormalities were found. Fault diagnosis: Upon inspection, it was found that the Z-axis moves unevenly up and down with noise, and there is a certain gap. When the motor is started, there is unstable noise and uneven force distribution in the Z-axis upward movement under the jog mode, and it feels that the motor shakes quite severely; When moving downwards, there is no such obvious shaking; When stopped, there is no shaking, which is more noticeable during the machining process. Analysis suggests that there are three reasons for the malfunction: firstly, the reverse clearance of the screw is very large; The second is the abnormal operation of the Z-axis motor; The third issue is that the pulley is damaged to uneven force distribution. But one thing to note is that there is no shaking when stopping and uneven up and down movement, so the problem of abnormal motor operation can be ruled out. Therefore, first diagnose the mechanical part, and no abnormalities were found during the diagnostic testing process, within the tolerance. Using the exclusion rule, the only remaining issue is with the belt. When inspecting the belt, it was found that it had just been replaced. However, when carefully inspecting the belt, it was found that there was varying degrees of damage on the inside of the belt, which was clearly caused by uneven force. What was the reason for this? In the diagnosis, it was found that there was a problem with the placement of the motor, that is, the angle and position of the clamping were asymmetric, resulting in uneven force. Fault handling: Just reinstall the motor, align it with the angle, measure the distance (between the motor and the Z-axis bearing), and ensure that the length on both sides of the belt is even. In this way, the uneven up and down movement of the Z-axis, as well as noise and vibration, are eliminated, and the Z-axis machining returns to normal. 4. System parameters not optimized, abnormal motor operation leading to abnormal machining accuracy. System parameters mainly include machine feed unit, zero offset, reverse clearance, etc. For example, the Frank CNC system has two types of feed units: metric and imperial. In the process of machine tool repair, local handling often affects the changes in zero offset and clearance. After the fault is resolved, timely adjustments and modifications should be made; On the other hand, severe mechanical wear or loose connection positions may also cause changes in the measured values of parameters, and corresponding modifications are needed to meet the requirements of machine tool machining accuracy. Fault phenomenon: A vertical CNC milling machine produced in Hangzhou, equipped with the Beijing KND-10M system. During the machining process, it was found that the X-axis accuracy was abnormal. Fault diagnosis: Upon inspection, it was found that there is a certain gap in the X-axis and there is an unstable phenomenon when the motor starts. When touching the X-axis motor with your hand, you feel that the motor is pulling quite hard, but when it stops, the pulling is not obvious, especially in the jog mode. Analysis suggests that there are two reasons for the malfunction: firstly, there is a large backlash between the lead screws; The second issue is the abnormal operation of the X-axis motor. Fault handling: Utilize the parameter function of the KND-10M system to debug the motor. Firstly, compensate for the existing gaps, then adjust the servo system parameters and pulse suppression function parameters, eliminate the vibration of the X-axis motor, and restore the machining accuracy of the machine tool to normal.