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In production, it is often encountered that the machining accuracy of CNC machine tools is abnormal. Such faults are highly concealed and difficult to diagnose. There are five main reasons for this type of failure: {1} The machine feed unit has been changed or changed. {2}The zero offset [NULL OFFSET] of each axis of the machine is abnormal. {3} Axial backlash [BACK LASH] is abnormal. {4} The motor is running abnormally, that is, the electrical and control parts are abnormal. {5}Mechanical failures such as lead screws, bearings, couplings, etc. In addition, the preparation of the machining program, the selection of the tool and the human factors may also result in abnormal machining accuracy.
1. System parameters change or change system parameters mainly include machine feed unit, zero offset, backlash and so on. For example, SIMENS, FANUC number system, its feed units are metric and imperial. Some processes in the repair process of the machine tool often affect the change of zero offset and gap. After the fault is processed, it should be adjusted and modified in time. On the other hand, due to serious mechanical wear or loose connection, the measured value of the parameter may be changed. The parameters need to be modified accordingly to meet the machining accuracy requirements of the machine.
2. Abnormal machining accuracy due to mechanical failure A THM6350 vertical machining center with SIMENS 840D system. During the processing of the link mold, the Z-axis feed abnormality was suddenly found, resulting in a cutting error of at least 1 mm (Z-direction overcut). The investigation revealed that the fault suddenly occurred. When the machine tool is jogging, the MDI (manual data input mode) operation mode is normal for each axis, and the reference point is normal; there is no alarm prompt, and the possibility of hard fault in the electrical control part is excluded. According to the analysis, the following aspects should be checked one by one.
Check the machining 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. After visual, touching, and listening to the diagnosis of its motion state, it is found that 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 1X100, that is, every step of the change, the motor feeds 0.1 mm), and observe the movement of the Z-axis with the dial indicator. In the forward motion as the starting point after the one-way motion accuracy is kept normal, the actual distance of the Z-axis movement of the machine tool is d=dl=d2=d3....=0.1mm for each step of the pulser, indicating that the motor is running well and the positioning accuracy is also good. Returning to the actual movement displacement of the machine tool can be divided into four stages: 1 machine tool movement distance d1>d=0.1mm (slope is greater than 1); 2 is expressed as d1=0.1>d2>d3 (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 pulser given value (slope equals 1), and returns to the normal movement of the machine tool.
Regardless of how the backlash (parameter 1851) is compensated, the characteristic is that, besides the three stages can be compensated, the other sections still exist, especially the one stage seriously affects the machining accuracy of the machine tool. In the compensation, it is found that the larger the gap compensation, the larger the distance of the one-stage movement.
Analysis of the above inspections suggests that there are several possible reasons: First, the motor has an abnormality; second, there is a mechanical failure; 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 very obvious feeling of vacancy. Under normal circumstances, the bearings should be able to feel the orderly and smooth movement. After disassembly inspection, the bearing was indeed damaged and there was ball falling off. The machine returns to normal after replacement.
3. Machine tool electrical parameters are not optimized Motor operation is abnormal There is a Beijing-made vertical CNC milling machine equipped with SIMENS840D system. During the machining process, the X-axis accuracy was found to be abnormal. The inspection found that there was a certain gap in the X-axis and there was an instability when the motor was started. When the X-axis motor is touched by hand, the motor shake is more powerful. The stop is not obvious, especially in the jog mode. The analysis believes that there are two reasons for the failure, one is that the reverse gap of the lead screw is large; the other is that the X-axis motor works abnormally. The motor is commissioned using the parameter function of the SIMENS system. Firstly, the existing gap is compensated; the servo gain parameter and the pulse suppression function parameter are adjusted, the jitter of the X-axis motor is eliminated, and the machining accuracy of the machine tool returns to normal.
4. The machine position loop abnormality or control logic is not suitable for a TH61140 machining center. The system is FANUC18I, full closed loop control mode. During the machining process, the Y-axis accuracy of the machine tool was found to be abnormal, and the precision error was at least 0.006 mm and the maximum was 1.4 mm. During the inspection, the machine tool has set the G54 workpiece coordinate system as required. In the MDI (manual data input mode) mode, a program called “G00G90G54Y80F100; M30;” is run in the G54 coordinate system. The mechanical coordinate value displayed on the display after the standby bed is over is “-1046.605”, and the value is recorded. Then in the manual mode, the machine is jog to any other position, and the last block is run again in MDI mode. After the standby bed is stopped, it is found that the machine coordinate value of the machine is displayed as “-1046.992”, the same as the first time. The value after execution is 0.387 mm. In the same way, the Y-axis is jogged to a different position, and the value displayed on the display of the block is repeatedly executed. Using the dial gauge to carefully check the Y-axis, it is found that the actual error of the mechanical position is basically the same as the error displayed by the digital display, so that the cause of the fault is that the Y-axis repeat positioning error is too large. Checking the backlash and positioning accuracy of the Y-axis and re-compensating have no effect. Therefore, there is a problem with the grating scale and system parameters. But why is there such a large error, but there is no corresponding alarm message? Further inspection revealed that the secondary axis was a vertical axis, and the spindle case fell downward when the Y-axis was released, causing an error.
The plc logic control program of the machine tool has been modified, that is, when the Y-axis is released, the Y-axis is first loaded and then the Y-axis is released; and when clamping, the shaft is clamped first, then Y is The axis is enabled to be removed. The machine fault has been resolved after adjustment.
