Diagnosis and treatment of abnormal machining accu

2022-08-26
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Diagnosis and treatment of abnormal machining accuracy of NC machine tools

abnormal machining accuracy of NC machine tools is often encountered in production. This kind of fault is hidden and difficult to diagnose. There are five main reasons for this kind of failure:

{1} the feed unit of the machine tool has been changed or changed

{2} the zero offset [null offset] of each axis of the machine tool is abnormal

The reverse clearance [back lash] of the

{3} axis is abnormal

{4} the motor operation status is abnormal, that is, the electrical and control parts are abnormal

{5} mechanical faults, such as lead screws, bearings, couplings and other components. In addition, the compilation of machining program, the selection of cutting tools and human factors may also lead to abnormal machining accuracy

1. system parameters change or change

system parameters are mainly from import to personal production, including machine tool feed unit, zero offset, reverse clearance, etc. For example, Simens and Fanuc CNC systems have two feed units, metric and imperial. Some treatments in the repair process of machine tools often affect the change of zero offset and clearance. Timely adjustment and modification should be made after fault treatment; On the other hand, serious mechanical wear or loose connections may also cause changes in the measured values of parameters, which requires corresponding modifications to meet the requirements of machine tool machining accuracy

2. Abnormal machining accuracy caused by mechanical failure

a thm6350 vertical machining center adopts Simens 840D system. In the process of machining the connecting rod die, it is suddenly found that the z-axis feeding is abnormal, resulting in a cutting error of at least 1 mm (Z-direction overcut). According to the investigation, the fault occurred suddenly. Under the inching and MDI (manual data input) operation mode of the machine tool, each axis operates normally and the return to the reference point is normal; No. 3. In the environment without vibration, there is no alarm prompt, and the possibility of hard failure of electrical control part is eliminated. According to the analysis, the following aspects should be checked one by one

[1] check the processing program segment that is running when the accuracy of the machine tool is abnormal, especially the tool length compensation, the calibration and calculation of the processing coordinate system (G54-G59)

[2] in the inching mode, move the Z axis repeatedly, and diagnose its motion state through viewing, touching and listening. It is found that the z-direction motion noise is abnormal, especially in the case of rapid inching, the noise is more obvious. Judging from this, there may be hidden dangers in the mechanical aspect

[3] check the z-axis accuracy of the machine tool. Move the z-axis with the hand-operated pulse generator (set its magnification to the position of 1x100, that is, the motor will feed 0.1mm every step of change), and observe the movement of the z-axis with the dial indicator. After the unidirectional motion accuracy remains normal, as the forward motion of the starting point, the actual distance of the z-axis motion of the machine tool d=dl=d2=d3= 0.1mm, indicating that the motor operates well and the positioning accuracy is also good. Returning to the change of the actual movement displacement of the machine tool can be divided into four stages: ① the movement distance of the machine tool d1>d=0.1mm (the slope is greater than 1); ② It shows that d1=0.1>d2>d3 (slope less than 1); ③ The machine tool mechanism does not actually move, showing the most standard reverse clearance; ④ The movement distance of the machine tool is equal to the given value of the pulser (the slope is equal to 1), and the normal movement of the machine tool is restored

no matter how to compensate the reverse clearance (parameter 1851), its characteristics are: except that the ③ stage can be compensated, other segment changes still exist, especially the ① stage seriously affects the machining accuracy of the machine tool. It is found in the compensation that the greater the gap compensation is, the greater the distance of stage ① movement is

after analyzing the above inspection, it is believed that there are several possible reasons: first, the motor is abnormal; Second, there are mechanical failures; Third, there is a gap in the lead screw. In order to further diagnose the fault, completely disconnect the motor and the lead screw, and check the motor and mechanical parts respectively. The inspection result is that the motor operates normally; In the diagnosis of the mechanical part, it was found that when the lead screw was turned by hand, there was a very obvious sense of vacancy at the beginning of the return movement. Under normal circumstances, the orderly and smooth movement of the bearing should be felt. After disassembly and inspection, it was found that the bearing was indeed damaged and the ball fell off. The machine tool returns to normal after replacement

The aluminum shovel breaks when cooking, the shovel breaks when digging, and the pickaxe is divided into two when digging are common

3. The electrical parameters of the machine tool are not optimized, and the motor runs abnormally

there is a vertical CNC milling machine made in Beijing, equipped with simens840d system. During the processing, the accuracy of x-axis is abnormal. It is found that there is a certain clearance in the x-axis and there is instability when the motor is started. When touching the x-axis motor with hands, I feel that the motor jitters severely, and the jitter is not obvious when stopping, especially in the inching mode. The analysis shows that there are two reasons for the failure: first, the reverse clearance of the lead screw is very large; Second, the x-axis motor works abnormally. Use the parameter function of Simens system to debug the motor. Firstly, the existing gap is compensated; Miao Wei, Minister of the Ministry of industry and information technology, and Xiao Yaqing, director of the state owned assets supervision and Administration Commission, were assigned as deputy team leaders to adjust the servo gain 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

4. the position loop of the machine tool is abnormal or the control logic is improper

a th61140 machining center, the system is fanuc18i, full closed-loop control mode. During the machining process, it is found that the y-axis accuracy of the machine tool is abnormal, with the minimum accuracy error of 0.006mm and the maximum accuracy error of 1.4mm. During the inspection, the machine tool has set the g54 workpiece coordinate system as required. In MDI (manual data input mode), run a program in g54 coordinate system, namely "g00g90g54y80f100; M30;", After the standby bed runs, the mechanical coordinate value displayed on the display is "-1046.605", and record this value. Then, in the manual mode, jog the machine tool to any other position, and run the last program segment in the MDI mode again. After the standby machine stops, it is found that the value of the machine tool's mechanical coordinate is "-1046.992", which is 0.387mm worse than the value after the first execution. In the same way, jog the y-axis to different positions and repeat the program. The value displayed on the display of this program segment is uncertain. Carefully check the y-axis with a dial indicator, and it is found that the actual error of the mechanical position is basically consistent with the error shown by the digital display. Therefore, it is considered that the fault reason is that the repeated positioning error of the y-axis is too large. Check the reverse clearance and positioning accuracy of Y-axis and make compensation again, but there is no effect. Therefore, it is suspected that there are problems with the grating ruler and system parameters. But why is there such a big error, but there is no corresponding alarm information? Further inspection found that the secondary axis was the axis in the vertical direction. When the Y axis was loosened, the headstock fell down, causing errors

the PLC logic control program of the machine tool is modified, that is, when the y-axis is loosened, the y-axis is enabled and loaded first, and then the y-axis is loosened; When clamping, clamp the axis first, and then remove the y-axis enable. After adjustment, the machine tool fault can be solved. (end)

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