Often encountered in the production of NC machine tool machining accuracy failure. This kind of fault concealment is strong and difficult to diagnose. The main causes of such failures are the following:

1) Machine feed unit is changed or changed 2) NULL OFFSET error of each axis of machine tool 3) Axial BACKLASH error 4) Motor operation status is abnormal, ie electrical and control part is faulty 5) In addition, the preparation of machining programs, tool selection, and human factors may also lead to abnormal machining accuracy.

1. The system parameter changes or changes The system parameters mainly include the machine feed unit, zero offset, backlash, and so on. For example, the SIEMENS and FANUC CNC systems have two feed systems, metric and imperial. Some processes in the machine repair process often affect the zero offset and gap changes. The fault handling should be adjusted and modified timely; on the other hand, due to the serious mechanical wear or loose connection, the actual measured value of the parameters may change. Need to make corresponding changes to the parameters in order to meet the requirements of machining precision.

2. Abnormal machining accuracy caused by mechanical failure A THM6350 horizontal machining center adopts FANUC0i-MA CNC system. Once in the process of milling the turbine blades, it was suddenly found that the Z-axis feed anomaly, causing a cutting error of at least 1 mm (Z-cut). The investigation learned that: The failure occurred suddenly. In the jog and MDI operation modes, the axes of the machine tool are normal, and the reference point is normal; without any alarm, the possibility of hard faults in the electrical control part is eliminated. According to the analysis, the following aspects should be examined one by one.

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

(2) In the jog mode, the Z-axis is repeatedly moved. After visual, haptic, and audible motions are diagnosed, it is found that the sound in the Z-direction movement is abnormal, especially in rapid jog, and the noise is more pronounced. Judging from this, there may be hidden dangers in machinery.

(3) Check the Z axis accuracy of the machine tool. Move the Z axis with the hand pulse generator, (set the MPG to a 1×100 gear, ie, each step of change, the motor feeds 0.1 mm), and observe the Z axis movement with the dial indicator. After the one-way movement accuracy is maintained as a normal point, the forward movement of the starting point, the actual distance of the Z-axis movement of the machine tool for each step of the manual movement d=d1=d2=d3...=0.1 mm, indicating that the motor runs well and the positioning accuracy is good. Return to the actual movement of the machine tool displacement changes, can be divided into four phases: 1 machine tool movement distance d1> d = 0.1mm (slope greater than 1); 2 showed as d = 0.1mm>; d2> d3 (slope less than 1 ); 3 machine tool institutions have not actually moved, showing the most standard backlash; 4 machine tool movement distance and hand set given value (slope is equal to 1), return to the normal movement of the machine tool.

No matter how the backlash (parameter 1851) is compensated, the characteristic that it displays is: Except for the compensation in the third stage, other changes in each stage still exist, especially the first stage seriously affects the machining accuracy of the machine tool. The compensation found that the greater the gap compensation, the greater the travel distance of the first segment.

Analysis of the above checks, CNC mechanics believe that there are several possible reasons: First, there are abnormal motor; Second, mechanical failure; Third, there is a certain gap. In order to further diagnose the fault, the motor and the screw are completely disengaged, and the motor and the mechanical part are inspected separately. The motor is operating normally; in the diagnosis of the mechanical part, it was found that when the screw is moved by hand, there is a very obvious sense of vacancy at the beginning of the return movement. Under normal circumstances, it should be able to feel the orderly and smooth movement of the bearing. After the inspection, it was found that the bearing had been damaged and a ball had fallen off. The machine will return to normal after replacement.

3. Machine tool electrical parameters are not optimized Motor operation is abnormal. A CNC vertical milling machine is equipped with a FANUC0-MJ CNC system. During machining, abnormal X-axis accuracy was found. The inspection revealed that there was a certain gap on the X axis and there was instability during motor startup. When the X-axis motor is touched by hand, the motor jitter is more serious, and it is not obvious when starting and stopping, and it is obvious under the JOG mode.

According to the analysis, there are two causes of the fault, one is that the mechanical backlash is large; the other is the abnormal operation of the X-axis motor. Use FANUC system parameter function to debug the motor. First, the existing gap is compensated; the servo gain parameter and the N-pulse suppression function parameter are adjusted; the vibration of the X-axis motor is eliminated and the machining accuracy of the machine tool returns to normal.

4. The machine position loop is abnormal or the control logic is not correct. A TH61140 boring and milling machine machining center, CNC system FANUC18i, full closed-loop control mode. During the machining process, the accuracy of the Y-axis of the machine tool was found to be abnormal, the accuracy error was at the minimum of 0.006mm, and the maximum error was 1.400mm. During the inspection, the machine tool had set the G54 workpiece coordinate system as required. In the MDI mode, run G90G54Y80F100; M30; with the G54 coordinate system. The machine coordinates displayed on the display after the operation of the standby bed are “-1046.605”. Record this value. Then, in the manual mode, the Y axis of the machine tool is jogged to any other position. The above statement is executed again in the MDI mode. After the standby bed is stopped, the mechanical coordinate value of the machine tool is displayed as “-1046.992”. The number of displayed values ​​after the execution is 0.387mm in comparison. According to the same method, the Y axis is jogged to a different position, and the statement is repeatedly executed, and the numerical indication is indefinite. Using the dial indicator to detect the Y-axis, it was found that the errors in the actual position of the mechanical position are basically the same as the errors displayed in the digital display. Therefore, it is considered that the reason for the fault is that the positioning error of the Y-axis is too large. The Y-axis backlash and positioning accuracy are carefully checked and recompensated for no effect. Therefore, there are questions about scales and system parameters. But why did such a large error occur and no corresponding alarm information appeared? Upon further inspection, it was found that the axis was a vertical axis. When the Y axis was released, the headstock fell downwards, resulting in an overshoot.

The PLC logic control program of the machine tool has been modified, that is, when the Y-axis is released, the Y-axis is enabled to be loaded, and then the Y-axis is released; when clamping, the shaft is clamped and then the Y is clamped. Shaft enable removal. Adjusted machine faults can be resolved.

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December 20, 2023