Abstract: With the development of industrialization, CNC machine tool technology has been fully developed worldwide, and machine tool technology has also shown a diversified development trend, such as continuous improvement in high efficiency, high reliability, and high precision. This article will discuss the aspects of CNC machine tool machining accuracy.
Several thoughts on improving the machining accuracy of CNC machine tools
Abstract: With the development of industrialization, CNC machine tool technology has been fully developed worldwide, and machine tool technology has also shown a diversified development trend, such as continuous improvement in high efficiency, high reliability, and high precision. This article will discuss the aspects of CNC machine tool machining accuracy.
0 Introduction
In CNC machine tools, the expression of accuracy is mainly expressed by the size of the error. The specific accuracy is divided into two types, including static accuracy and dynamic accuracy. Among them, static accuracy refers to the detection of the machine tool when it is not working and not cutting. The main indicators are the geometric accuracy and positioning accuracy of the machine tool itself. This accuracy can only show the original accuracy of the machine tool for the detection of machine tool accuracy; dynamic accuracy, as the name implies, is the accuracy detected and achieved by the machine tool during the cutting process. The measurement of this accuracy value includes the original accuracy of the machine tool and the accuracy after the influence of the environment and process problems during the processing process, including the errors caused by the selected tools, workpieces, vibrations, etc. during the processing process. In the production process of machine tools, the dynamic accuracy of machine tools cannot be effectively controlled. What can be guaranteed is the static accuracy of machine tools and the accuracy of original manufacturing. The processing accuracy of CNC machine tools has many influencing factors, which will be the main content of the following discussion.
1. Factors affecting the processing accuracy of CNC machine tools
1.1 Errors of machine tools themselves A large amount of data and statistics show that more than 65.7% of machine tools cannot fully meet their relevant index standards during installation. During work, 90% of CNC machine tools are in an inaccurate working environment and state. This situation determines the importance of machine tool working status monitoring. The test of machine tool accuracy is a necessary basis for the guarantee of machine tool accuracy, which can better guarantee the processing accuracy of parts.
1.2 Temperature influence In addition to the accuracy error of the machine tool itself, the operation of the machine tool in the workshop environment will also be affected, including the temperature fluctuation of the workshop, the heating of the motor, the friction of loading and unloading, the influence of the medium, etc. These problems will have a certain degree of influence on the shape and accuracy of the machine tool. The temperature change of the machine tool will cause the loss of adjustment accuracy, which will affect the accuracy of the machine tool and the size and accuracy of the workpiece. At the same time, the temperature rise also changes the bearing clearance, which in turn affects the processing accuracy. On the other hand, the temperature rise makes the temperature distribution uneven, causing the relative position relationship between the parts or parts of the parts to change, thereby causing the displacement or distortion of the parts.
1.3 Reverse deviation The so-called reverse deviation refers to the error phenomenon caused by the reverse dead zone or reverse clearance caused by the coordinate axis during the transmission process in the operation of the CNC machine tool. It can also be called reverse clearance or lost momentum. For CNC machine tools using semi-closed loop servo systems, the existence of reverse deviation will affect the positioning accuracy and repeat positioning accuracy of the machine tool, thereby affecting the processing accuracy of the product.
1.4 Clearance error The transmission chain is required in the processing of CNC machine tools. The operation of the transmission chain will produce some gaps, which are easy to cause errors, especially when the machine tool does not move during the operation of the motor, which will cause the CNC machine tool to vibrate or have large errors.
2. Measures to improve the machining accuracy of CNC machine tools
2.1 Selection of machine tools Since the accuracy of machine tools themselves is also different, we need to pay attention to the selection of machine tool models and accuracy when selecting machine tools. At present, the inspection of the position accuracy of CNC machine tools usually adopts international standards ISO230-2 or national standards GB10931-89, etc. In the selection of machine tools, we must also pay attention to the standards, because different standards will also cause differences in accuracy.
2.2 Control of parts Machine tools with sliding bearings can use bearings with better wear resistance to ensure the working accuracy of the machine tools.
2.3 Control of workshop environment Reduce heat sources: focus on the speed, clearance adjustment and reasonable preload of the spindle bearings. For thrust bearings and tapered roller bearings, because of their poor working conditions and high heat generation, thrust angular contact ball bearings can be used instead when necessary to minimize the frictional heat generation of certain parts. Heat insulation: keep the heat source away from the spindle, such as isolating the motor and transmission, using separate transmission, etc. Heat dissipation: strengthen lubrication cooling, use oil cooling, air cooling and other methods to accelerate heat dissipation. Reduce the influence of thermal deformation: No matter what method is used, thermal deformation can only be reduced but it is difficult to completely eliminate it. Therefore, measures should also be taken to reduce the influence of thermal deformation.
2.4 Control of reverse deviation Since reverse deviation will cause the accuracy of the equipment to decrease, and as the machine tool is used for a longer time, the wear will increase, and the error will become larger and larger. This requires regular detection and compensation of reverse deviation during the application of the machine tool, so as to reduce the error as much as possible and improve the working accuracy of the machine tool.
2.5 Error compensation Error compensation refers to making corresponding records of the position of the fixed axis in the processing of CNC machine tools. In addition, the relevant recorded data is compared with the actual measurement results to understand the error value, and the reference point of the measurement is selected on the axis during operation, and the error value in operation is recorded and input into the relevant control system. In this way, the axis movement and error time at different points can be well controlled. If the number of measured points is larger, the error effect of the pitch compensation is more obvious. The premise of this error compensation technology is based on the coordinate system of the CNC machine tool. The important parameter for determining the coordinate system of the CNC machine tool is the reference point. Therefore, it is necessary to ensure that the error value of the selected reference point is zero.
2.6 Compensation of reverse clearance error Due to the influence of reverse clearance error in CNC machine tools, reverse clearance error must be fully valued in the design of CNC machine tools, and effective solutions must be taken. However, it is undeniable that the clearance exists, so what we need to do is to record the reverse clearance of each point during the operation of the machine tool through the pitch error compensation technology, and directly perform error compensation operation on the reverse motion through the control system of the CNC machine tool, so as to reduce the error by setting parameters and CNC system settings.
2.7 Improvement of technology The accuracy of machine tools is also constantly improving and improving with the development of science and technology. The accuracy of machine tools has progressed from the original micron level to the nanometer level. More research and development are needed, especially in bearing technology. Avoiding the hysteresis of bearing technology is one of the development problems. The hysteresis phenomenon is particularly important for the positioning accuracy. In the study, it was found that hydrostatic bearing technology can solve the hysteresis phenomenon of machinery, so it has been widely used in high-precision CNC machine tool processing.
3. Conclusion
In summary, there are many factors that affect the accuracy of CNC machine tools during machining. We need to make a comprehensive analysis of these factors and try to reduce errors as much as possible from multiple aspects to effectively improve machining accuracy.