The causes of abnormal machining accuracy are highly concealed and difficult to diagnose. Today, I have summarized 4 major diagnostic principles and 5 major diagnostic methods. Let's see if you know them all.
1. Causes of abnormal machining accuracy
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 preparation of the machining program, the selection of tools and human factors may also lead to abnormal machining accuracy.
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, and analysis, confirm that it is a non-destructive fault, then power on the machine tool; under operating conditions, dynamic observation, inspection and testing are carried out to find faults. For destructive faults, the danger must be eliminated before powering 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. Fault diagnosis methods for CNC machine tools
1. Intuitive method: (look, smell, ask, and feel) Ask - machine tool fault phenomena, processing conditions, etc.; Look - CRT alarm information, alarm indicator light, deformation, smoke, and burnt components such as capacitors, tripping of protectors, etc.; Listen - abnormal sounds; Smell - burnt smell of electrical components and other odors; Touch - heat, vibration, poor contact, etc.
2. Parameter inspection method: Parameters are usually stored in RAM. Sometimes insufficient battery voltage, long-term system power failure, or external interference will cause parameter loss or confusion. 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. Isolation method is often used.
4. Same-type swap method Replace the suspected faulty template with a spare board with the same function, or exchange templates or units with the same function.
5. Functional program test method Write some small programs for all instructions of G, M, S, and T functions. When diagnosing faults, these programs can be run to determine the lack of functions.
(Image source: Angke Machine Tool)
IV. Diagnosis and treatment 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, causing 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 was inching, and each axis operated normally under the manual data input mode, and the reference point was returned normally, without any alarm prompts, and the possibility of hard faults in the electrical control part was ruled out. The following aspects should be checked one by one.
Check the machining program segment that is running when the machine tool accuracy is abnormal, especially the tool length compensation, and the calibration and calculation of the machining coordinate system (G54-G59).
In the inching mode, repeatedly move the Z axis, and diagnose its movement state through sight, touch, and hearing. It was found that the Z-axis movement noise was abnormal, especially the rapid inching, and the noise was more obvious. From this, it can be judged that there may be hidden dangers in the machinery.
Check the Z-axis accuracy of the machine tool. Use a hand-cranked pulse generator to move the Z axis (set its magnification to 1×100, that is, the motor feeds 0.1mm for each step), and use a dial indicator to observe the movement of the Z axis. After the unidirectional motion remains normal, the positive motion is taken as the starting point. For each step of the pulser, the actual distance d of the Z axis of the machine tool moves is d<span 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 runs well and the positioning accuracy is also good. The change in the actual movement 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 movement. No matter how the reverse clearance is compensated, its characteristics are: except for the compensation in stage (3), the changes in other stages are based on However, the gap exists, especially in stage (1), which seriously affects the machining accuracy of the machine tool. It was found during compensation that the larger the gap compensation, the larger the distance moved in stage (1).
Analysis of the above inspection shows 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 shows 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 machining accuracy caused by improper control logic
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, with the minimum accuracy error of 0.008mm and the maximum accuracy error 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 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 now "-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 execute the program segment repeatedly, but the values displayed on the display are different (unstable). 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. Therefore, 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 is there such a large error, 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 is first loaded, and then the X axis is released; when the X axis is clamped, the X axis is first clamped, 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 shakes 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 reverse clearance of the screw 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 movements are 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 process, 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 inside of the belt was damaged to varying degrees. It is obvious that it is 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, it was found that the X-axis accuracy was abnormal.
Fault diagnosis: Inspection found that there was a certain gap in the X-axis, and the motor was unstable when starting. When I touched the X-axis motor with my hand, I felt that the motor was pulling strongly, but it was not obvious when it stopped, especially in the inching mode. According to analysis, there are two reasons for the failure: 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 machining accuracy of the machine tool to normal.
