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Reasons and solutions for abnormal machining accuracy of CNC machine tools

Author: ComeFrom: Date:2018/12/27 10:32:54 Hits:1184

Reasons and solutions for abnormal machining accuracy of CNC machine tools:

First, the cause of abnormal processing accuracy

The cause of abnormal machining accuracy is concealed and the diagnosis is difficult. Based on years of practical experience, the author summarizes five main reasons: the machine feed unit is changed or changed; the zero offset of the machine axes is abnormal; the axial back gap is abnormal; the motor running state is abnormal, that is, the electrical and control parts are abnormal. Mechanical failures such as lead screws, bearings, couplings and other components. In addition, the preparation of the machining program, the selection of the tool and the human factors may also result in abnormal machining accuracy. Metal processing WeChat, the content is good, it is worthy of attention.

  Second, the principle of fault diagnosis of CNC machine tools

1. The external internal CNC machine tool is a machine tool that integrates mechanical, hydraulic and electrical functions. Therefore, the occurrence of faults will be reflected by the three. Maintenance personnel should first check one by one from the outside to the inside, try to avoid unpacking and disassembly at will, otherwise it will expand the fault and make the machine lose precision and reduce performance.

2. First mechanical and electrical In general, mechanical faults are easier to detect, and the diagnosis of CNC system faults is more difficult. Before troubleshooting, first pay attention to the elimination of mechanical failures, often can achieve twice the result with half the effort.

3. After the static state, the machine can be energized by understanding, observing, testing, analyzing and confirming it as a non-destructive fault in the static state of the machine power failure; under the operating conditions, dynamic observation is performed. Verify and test to find faults.

For destructive failures, the danger must be removed before powering up.

4. Simple and complex, when there are multiple faults intertwined and concealed, when there is no way to start, you should solve the easy problem first, and then solve the difficult problem. Often, after a simple problem is solved, a difficult problem may become easier.


  Third, CNC machine tool fault diagnosis method

1. Intuitive method (Wang Wen asked) Ask - machine tool failure phenomenon, processing status, etc.; see - CRT alarm information, alarm indicator, capacitor and other components deformation smoke and burn, protector trip, etc.; listen - abnormal sound; Electrical components burnt and other odors; touch - heat, vibration, poor contact, etc. Metal processing WeChat, the content is good, it is worthy of attention.  

2. Parameter check method parameters are usually stored in RAM, sometimes the battery voltage is insufficient, the system will not be powered or external interference for a long time, the parameters will be lost or chaotic, and the relevant parameters should be checked and proofed according to the fault characteristics.

3. Isolation method Some faults, it is difficult to distinguish between the CNC part, or the servo system or mechanical part, often using the isolation method.

4. The same type of swap method replaces the suspected faulty template with the same function spare board, or exchanges the same template or unit with the same function.

  5. The function program test method writes some small programs for all the commands of G, M, S, T, and functions. When these faults are run when the fault is diagnosed, the function can be judged.  

Fourth、 processing accuracy abnormal fault diagnosis and processing examples

1. Mechanical failure leads to abnormal machining accuracy. Fault phenomenon: One SV-1000 vertical machining center adopts Frank system. During the processing of the connecting rod mold, the Z-axis feed abnormality was suddenly found, resulting in a cutting error of at least 1 mm (over-cutting in the Z direction).

Troubleshooting: The investigation found that the fault suddenly occurred. The machine tool is jogging. Under the manual input data mode operation, each axis runs normally, and the reference point is normal. There is no alarm prompt, and the possibility of hard fault in the electrical control part is excluded. The following aspects should be checked one by one. Metal processing WeChat, the content is good, it is worthy of attention.

Check the machining program block that is running when the machine accuracy is abnormal, especially the tool length compensation, proofreading and calculation of the machining coordinate system (G54-G59).

  In the jog mode, the Z-axis is repeatedly moved, and after visual, touching, and listening, the motion state is diagnosed, and the Z-direction motion noise is abnormal, especially the fast jog, and the noise is more obvious. Judging from this, there may be hidden dangers in the mechanical aspect.

  Check the machine's Z-axis accuracy. Move the Z-axis with the pulse generator by hand, (set the magnification to 1×100, that is, every step of the change, the motor feeds 0.1mm), and observe the movement of the Z-axis with the dial indicator. The positive movement as the starting point after the one-way motion remains normal, the actual distance of the Z-axis movement of the machine tool is d=d1=d2=d3=...=0.1mm for each step of the pulser, indicating that the motor runs well and the positioning accuracy is good. . The return to the actual movement displacement of the machine tool can be divided into four stages: (1) the machine tool 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 mechanism does not actually move, showing a standard backlash; (4) the machine tool movement distance is equal to the pulser's set value (slope equals 1), and returns to the normal motion of the machine. No matter how the backlash is compensated, the characteristic is that in addition to the (3) stage compensation, other sections still exist, especially the stage (1) seriously affects the machining accuracy of the machine tool. In the compensation, it is found that the larger the gap compensation, the greater the distance moved by the (1) phase. Metal processing WeChat, the content is good, it is worthy of attention.

  Analysis of the above inspections suggests that there are several possible reasons: First, the motor has an abnormality, second, there is a mechanical failure, and third, there is a gap in the lead 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 moved by hand, the return movement initially has a large feeling of vacancy. Under normal circumstances, you should be able to feel the orderly and smooth movement of the bearing.

Trouble-shooting: After disassembly inspection, the bearing was found to be damaged and there was ball falling off. The machine returns to normal after replacement.

2. Inappropriate 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 small at 0.008 mm, which is 1.2 mm. Fault diagnosis: During inspection, the machine tool has been followed. The G54 workpiece coordinate system is required to be set. In the manual input data mode operation, run a program in G54 coordinate system that is “GOOG90G54X60.OY70.OF150; M30;”, the mechanical coordinate value displayed on the display after the standby bed is running (X axis) “-1025.243”, record The value is below. Then, in the manual mode, the machine is jog to any other position, and the block is run again under the manual input data mode operation. After the standby bed is stopped, it is found that the machine coordinate value is displayed as “-1024.891”, the same as the previous execution. After the comparison, the numerical difference is 0.352 mm. In the same way, the X-axis is moved to a different position, and the block is repeatedly executed, and the values ??displayed on the display are different (unstable). Metal processing WeChat, the content is good, it is worthy of attention. Using the dial indicator to carefully check the X-axis, it is found that the actual error of the mechanical position is basically the same as the error displayed by the number, so that the cause of the fault is that the X-axis repeat positioning error is too large. Check the backlash and positioning accuracy of the X-axis and recompensate the error value. As a result, it does not have any effect. Therefore, there is a problem with the grating scale and system parameters. But why did such a large error occur, but the corresponding alarm information did not appear to be further checked and found that the axis is the axis in the vertical direction. When the X axis is released, the headstock is dropped downward, causing an error.

Fault handling: The PLC logic control program of the machine tool has been modified. When the X axis is released, the X axis is enabled to be loaded first, and then the X axis is released. When the X axis is clamped, the X axis is clamped first. Immediately, remove the enable. The machine fault has been resolved after adjustment.

3. Machine position problem leads to abnormal machining accuracy

Fault phenomenon: A vertical CNC milling machine from Hangzhou is equipped with Beijing KND-10M system. During the jog or machining process, the Z-axis was found to be abnormal.

Fault diagnosis: The inspection found that the Z-axis moves unevenly and is noisy, and there is a certain gap. When the motor starts, there is unstable noise and uneven force in the Z-axis motion in the jog mode, and the motor shake is felt to be more powerful; when moving downward, there is no such jitter; when it stops, it does not shake. It is more obvious during the processing. The analysis believes that 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 is working abnormally; the third is that the pulley is damaged to uneven force. However, there is a problem to be noted that when the stop is not shaken, the up and down motion is not uniform, so the problem that the motor works abnormally can be ruled out. Metal processing WeChat, the content is good, it is worthy of attention. Therefore, the mechanical part is diagnosed first, and no abnormality is found during the diagnostic test, within the tolerance. Using the rule of exclusion, the only problem with the belt was left. When the belt was detected, it was found that the belt had just been changed, but when the belt was carefully examined, it was found that the inside of the belt was damaged to varying degrees, which was obviously caused by uneven force. What caused it? In 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, resulting in uneven force.

Trouble-shooting: As long as the motor is reassembled, the angle is aligned, and the distance (motor and Z-axis bearing) is measured, and both sides (length) of the belt should be uniform. In this way, the Z-axis moves unevenly up and down, and noise and jitter are eliminated, and the Z-axis processing returns to normal.

4. The system parameters are not optimized and the motor is running abnormally.

The system parameters that cause abnormal machining accuracy mainly include the machine feed unit, zero offset, backlash and so on. For example, Frank CNC system, its feed units are metric and imperial. In the process of machine tool repair, the local offset often affects the zero offset and gap change. After the fault is processed, it should be adjusted and modified in time. On the other hand, the measured value may be caused due to severe mechanical wear or loose connection. The changes need to be modified accordingly to meet the machining accuracy requirements of the machine.

Fault phenomenon: A vertical CNC milling machine from Hangzhou is equipped with Beijing KND-10M system. During the machining process, the X-axis accuracy was found to be abnormal.

Fault diagnosis: Check that there is a certain gap in the X axis, and there is instability when the motor starts. When the X-axis motor is touched by hand, it feels that the motor pulls more severely, and the pulling is not obvious when stopping, especially in the jog mode. The analysis believes that there are two reasons for the failure: First, the screw has a large backlash; second, 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 servo system parameters and the 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.

Through the discussion on the fault diagnosis and treatment methods of abnormal machining precision of CNC machine tools, the practical problems and solutions encountered in the practice production are summarized, which enriches the teaching content and penetrates into the production practice, which not only allows the students to master the problem-solving problem. The method, more importantly, is to cultivate students' practical ability, use knowledge analysis and problem-solving ability to achieve the goal of education and teaching in technical schools, and make students become practical talents to adapt to market demand.

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