Cause analysis, diagnosis and solution of abnormal machining accuracy of CNC machine tools

1. Causes of abnormal machining accuracy

The causes of abnormal machining accuracy are highly concealed and difficult to diagnose. Based on years of practical experience, the author has summarized five main reasons: the machine tool feed unit has been modified or changed; the zero offset of each axis of the machine tool is abnormal; the axial reverse clearance is abnormal; the motor operation state is abnormal, that is, the electrical and control parts are abnormal; mechanical failures, such as screws, bearings, shaft couplings and other components. In addition, the compilation of machining programs, the selection of tools and human factors may also lead to abnormal machining accuracy. Metal processing WeChat, the content is good and worth paying attention to.

2. Principles of fault diagnosis of CNC machine tools

1. External first, then internal CNC machine tools are machine tools that integrate mechanical, hydraulic and electrical, so the occurrence of their faults will also be reflected by these three. Maintenance personnel should first check one by one from the outside to the inside, and try to avoid unsealing and disassembly at will, otherwise it will expand the fault, make the machine tool lose precision and reduce performance.

2. Mechanical first, then electrical Generally speaking, mechanical failures are easier to detect, while the diagnosis of CNC system failures 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, analysis, and confirming that it is a non-destructive fault, the machine tool can be powered on; under operating conditions, dynamic observation, inspection and testing are carried out 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, and it is difficult to start at the moment, the easy problems should be solved first, and then the more difficult problems. Often after the simple problems are solved, the difficult problems may also become easy.

3. CNC machine tool fault diagnosis method

1. Intuitive method (look, smell, ask, and feel) Ask - machine tool fault phenomenon, processing status, etc.; look - CRT alarm information, alarm indicator light, deformation, smoke, burning of capacitors and other components, protector tripping, etc.; listen - abnormal sound; smell - electrical component burnt smell and other odors; touch - heat, vibration, poor contact, etc. Metal processing WeChat, the content is good and worth paying attention to.

2. Parameter inspection method Parameters are usually stored in RAM. Sometimes the battery voltage is insufficient, the system is not powered 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 Some faults are 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. Same-type exchange method Replace the suspected faulty template with a spare board with the same function, or exchange 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, and functions. When running these programs when diagnosing faults, the missing functions can be determined.

4. Diagnosis and processing examples of abnormal machining accuracy

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

Fault diagnosis: During the investigation, it was learned that the fault occurred suddenly. The machine tool is inching. Under the manual data input mode, each axis runs normally and returns to the reference point normally. 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 and worth paying attention to.

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

In the jog mode, repeatedly move the Z axis, and diagnose its movement state through sight, touch and hearing. It is found that the Z-axis movement noise is abnormal, especially the fast jog, the noise is more obvious. From this, it can be judged that there may be hidden dangers in the machinery.

Check the accuracy of the Z axis of the machine tool. Move the Z axis with a hand-cranked pulse generator (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 a dial indicator. After the unidirectional motion remains normal, the positive motion as the starting point, each time the pulser changes one step, the actual distance of the machine tool Z axis movement d=d1=d2=d3=…=0.1mm, indicating that the motor is running well and the positioning accuracy is also good. The change in the actual motion displacement of the returning machine tool can be divided into four stages: (1) the machine tool movement distance d1>d=0.1mm (slope greater than 1); (2) it is shown as d1=0.1mm>d2>d3 (slope less than 1); (3) the machine tool mechanism does not actually move, showing the most standard reverse clearance; (4) the machine tool movement distance is equal to the pulser constant value (slope equals 1), and the machine tool returns to normal motion. No matter how the reverse clearance is compensated, its characteristics are: except for the compensation in stage (3), the changes in other stages still exist, especially the stage (1) 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 stage (1). Metal processing WeChat, the content is good and worth paying attention to.

Analyzing the above inspection, it is believed that there are several possible reasons: one is that the motor is abnormal, the second is that there is a mechanical fault, and the third is that there is a gap in the lead screw. In order to further diagnose the fault, the motor and the lead screw are completely disconnected, and the motor and mechanical parts are checked respectively. The inspection result is that the motor is running normally; in the diagnosis of the mechanical part, it is found that when the lead screw is turned by hand, there is a large sense of emptiness at the beginning of the return movement. Under normal circumstances, the orderly and smooth movement of the bearing should be felt.

Fault handling: After disassembly and inspection, it was found that the bearing was indeed damaged and the ball fell off. After replacement, the machine tool returned to normal.

2. Abnormal processing accuracy caused by improper control logic

Fault phenomenon: A machining center produced by a Shanghai machine tool manufacturer, the system is Frank. During the processing, 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 of 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 this value. Then in 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 0.352mm different from the value after the last execution. In the same way, move the X-axis to different positions, and repeatedly execute the program segment, but the values displayed on the display are different (unstable). Metal processing WeChat, the content is good and worth paying attention to. Use a dial indicator to carefully check the X-axis and find that the actual error of the mechanical position is basically consistent with the error displayed by the digital display, 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 reverse clearance and positioning accuracy of the X-axis, and re-compensate its error value, but it has no effect. Therefore, it is suspected that there are problems with the grating ruler and system parameters. But why such a large error occurs, but no corresponding alarm information appears? Further inspection found that this axis is a vertical axis. 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, the X-axis enable is loaded first, and then the X-axis is released; when the X-axis is clamped, the X-axis is clamped first, and then the enable is removed. After adjustment, the machine tool failure was solved.

3. Abnormal machining accuracy caused by machine tool position problem

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

Fault diagnosis: Inspection found that the Z-axis moved up and down unevenly and with noise, and there was a certain gap. When the motor starts, there is unstable noise and uneven force when the Z axis moves upward in the inching mode, and the motor feels that it is shaking more severely; when it moves downward, it does not shake so obviously; when it stops, it does not shake, which is more obvious during the processing. According to analysis, there are three reasons for the failure: one is that the screw reverse clearance is large; the second is that the Z axis motor works abnormally; the third is that the pulley is damaged to uneven force. But there is one problem to note, that is, there is no shaking when it stops, and the up and down movement is uneven, so the problem of abnormal motor operation can be ruled out. Metal processing WeChat, the content is good and worth paying attention to. Therefore, the mechanical part is diagnosed first, and no abnormality is found during the diagnostic test, which is within the tolerance. Using the rule of elimination, the only problem left is the belt. When testing the belt, it was found that the belt had just been replaced not long ago, but when carefully testing the belt, it was found that the inner side of the belt was damaged to varying degrees. It is 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 asymmetric angle position of the clamping caused uneven force.

Fault handling: Just reinstall the motor, align the angle, measure the distance (bearings of the motor and the Z axis), and the two sides (length) of the belt should be uniform. In this way, the uneven up and down movement of the Z axis and the noise and jitter phenomenon 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 cause abnormal processing accuracy mainly include machine tool feed units, zero offset, reverse clearance, etc. For example, the Frank CNC system has two feed units, metric and imperial. During the repair of the machine tool, local processing often affects the change of zero offset and clearance. After the fault is handled, timely adjustments and modifications should be made; on the other hand, due to severe mechanical wear or loose connection positions, the actual measured value of the parameters may also change. The parameters need to be modified accordingly to meet the requirements of machine tool processing accuracy.

Fault phenomenon: A vertical CNC milling machine produced in Hangzhou, equipped with a Beijing KND-10M system. During the processing, the X-axis accuracy was found to be abnormal.

Fault diagnosis: Inspection found that there was a certain gap in the X-axis, and the motor was unstable when starting. When touching the X-axis motor with your hand, you feel that the motor is pulling more strongly, but it is not obvious when it stops, especially in the inching mode. According to analysis, there are two reasons for the fault: one is that the screw backlash is large; the other is that the X-axis motor is working abnormally.

Fault handling: Use the parameter function of the KND-10M system to debug the motor. First, compensate for the existing gap, then adjust the servo system parameters and pulse suppression function parameters, eliminate the jitter of the X-axis motor, and restore the machine tool processing accuracy to normal.

Through the discussion of the diagnosis and treatment methods of abnormal machining accuracy of CNC machine tools, the actual problems and solutions encountered in the internship production are summarized, the teaching content is enriched and infiltrated into production practice, not only allowing students to master the methods of solving problems, but more importantly, cultivating students' practical operation ability, the ability to use knowledge to analyze and solve problems, achieving the goal of technical school education and teaching, and making students become practical talents that meet market needs.