Machine tool accuracy faults are more difficult to repair in machine tool failures. The main performance is that the dimensional accuracy and positional accuracy of machine tool parts cannot meet the process requirements. The process of precision fault repair of CNC machine tools is more complicated than that of ordinary machine tools. Therefore, it is necessary to carry out comprehensive analysis from various aspects such as numerical control system, servo system, mechanical part and process plan, and propose a test plan. Then, through the analysis of the measured data, the fault location of the machine tool is diagnosed, and the accuracy detection is performed at the repaired fault location.
Machine tool failure
The author encountered a CNC lathe accuracy failure during work. When the machine tool is machined, the outer circle and the inner hole are not stable in size, and the out-of-range is about 2m. The axial dimension of the part also has a random error of about 0.5mm.
The numerical control lathe has a dimensional accuracy that is too poor and random, and the size distribution of the machined parts is relatively scattered. Analyze the process of the out-of-tolerance part. The parts are clamped using a hydraulic chuck. The end faces are machined first, the outer circle is machined, and the inner hole is machined.
2. Electrical system analysis
The machine uses the Siemens 802D CNC system, which is a semi-closed loop system. The CNC system did not generate alarms and information during the machining of the parts. There is no problem by checking the system parameters (pitch compensation, backlash, etc.) settings.
Servo motor and encoder detection: The disassembled servo motor detects the encoder at the end of the motor, and there is no fault in the connection part of the encoder; the encoder does not detect the contamination; the oscilloscope measures the encoder signal normally. The motor and encoder are normal.
3. Mechanical system analysis
The X-axis and Z-axis screw pairs of the CNC lathe were tested with good precision, and no axial turbulence and backlash were found.
Tool holder part detection: The CNC machine tool has a tool change action when processing the product; through the programming and processing of a batch of products using the tool change, it is found that the accuracy fault naturally disappears. Since the tool holder is also the part that produces the accuracy error, the dial gauge is used to detect the tool changer. It is found that the tool holder has an angular error, the positioning is inaccurate and it is a random error. The locking angle error of the tool holder will affect the machining accuracy of the outer and inner holes of the CNC lathe.
4. Tool holder repair
Through the disassembly and repair of the tool holder part, the reason for the inaccurate positioning of the tool holder was found. Product processing test after installation and recovery. The precision error of the processed product disappears and the fault is eliminated.
5 Conclusion
(1) The precision faults of CNC machine tools involve a wide range of parts. In the troubleshooting process, the diagnosis method from simple to complex and from electric to machine should be followed. From simple to complex, the workload can be avoided. From electricity to machine, the diagnosis and treatment of electrical faults do not require a lot of disintegration of the machine tool, and the electrical part fault diagnosis is complicated and easy to handle. The mechanical part is processed. It is cumbersome and the diagnosis is simple.
(2) CNC machine tool accuracy troubleshooting is a meticulous system work, which requires a series of diagnosis and testing, and finally the comprehensive diagnosis and test results to design a repair solution for CNC machine tools. After the machine tool's product trial production and accuracy inspection, the accuracy of the parts is qualified, and the machine tool's accuracy troubleshooting work is completed. (The author is from Beijing Erqi Rail Transportation Equipment Co., Ltd.)
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