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1 Causes of geometric errors
It is generally believed that the geometric errors of CNC machine tools are caused by the following reasons:
1.1 The original manufacturing error of the machine tool
Refers to the machine tool movement error caused by the geometrical shapes, surface quality, and the mutual position error between the components of the machine tool, which is the main reason for the geometric error of CNC machine tools.
1.2 Machine Control System Error
Including the servo axis error (profile following error), CNC interpolation algorithm error.
1.3 thermal deformation error
The error caused by thermal deformation of the machine tool due to the internal heat source of the machine tool and thermal disturbance of the environment.
1.4 The error caused by the deformation of the process system caused by the cutting load
Including errors caused by machine tool, tool, workpiece and fixture deformation. This kind of error is also known as "knife", which causes distortion of the shape of the machined part, especially when machining thin-walled parts or using slender tools.
1.5 Machine Vibration Error
During the cutting process, due to the flexibility of the process and the variety of processes, the numerically-controlled machine tool has a greater possibility of its operating state falling into an unstable region, thereby inducing a strong flutter. Deterioration of the surface quality of the machined workpiece and geometrical errors.
1.6 Test System Test Error
Including the following aspects:
(1) The error of the measurement sensor feedback system due to manufacturing error of the measurement sensor and installation error on the machine tool;
(2) Errors due to the measurement sensor due to machine part and mechanism errors and deformation during use.
1.7 External interference error
Random errors due to changes in the environment and operating conditions.
1.8 Other errors
Errors such as programming and operating errors.
The above error can be classified into two categories according to the characteristics and nature of the error: system error and random error.
The systematic error of the CNC machine tool is the inherent error of the machine tool and it is repeatable. The geometrical error of CNC machine tools is its main component and also has repeatability. Using this feature, it can be "offline measurement", which can be corrected and compensated by the "offline detection - open loop compensation" technology, so as to reduce the precision of the machine tool.
Random errors are random and must be "online detection - closed-loop compensation" method to eliminate the influence of random errors on the machining accuracy of the machine tool. This method has strict requirements for measurement instruments and measurement environments and is difficult to popularize.
2 geometric error compensation technology
For different types of errors, implementing error compensation can be divided into two categories. The random error compensation requires “online measurementâ€. The error detection device is directly installed on the machine tool. At the same time when the machine tool works, the error value of the corresponding position is measured in real time, and this error value is used to correct the machining instruction in real time. Random error compensation has no requirement on the error nature of the machine tool, and can simultaneously compensate for random errors and system errors of the machine tool. However, a complete set of high-precision measuring devices and other related equipment are needed, which are costly and the economic benefits are not good. The on-line measurement and compensation of temperature was carried out in [4] and it failed to reach practical application. The system error compensation is to detect the machine tool in advance with the corresponding instrument, that is, the “offline measurement†is used to obtain the error value of the command position of the working space of the machine tool as a function of the machine tool coordinates. When the machine tool works, according to the coordinates of the machining point, the corresponding error value is called for correction. The stability of the machine tool is required to ensure the certainty of the machine tool error so as to facilitate the correction. The accuracy of the machine tool after compensation depends on the repeatability of the machine tool and changes in the environmental conditions. In the normal case of CNC machine tools, the repeatability is much higher than its spatial integration error, so the system error compensation can effectively improve the accuracy of the machine tool, and even improve the accuracy of the machine tool. So far, there are many methods for compensating systematic errors at home and abroad, which can be divided into the following methods:
2.1 single error synthesis compensation method
This compensation method is based on the error synthesis formula as the theoretical basis, first through the direct measurement method to measure the machine tool's single original error value, the error synthesis formula to calculate the error component of the compensation point, so as to achieve the error compensation of the machine tool. The position error measurement of the coordinate measuring machine belongs to Leete. Using the triangular geometric relations, the representation method of the error of each coordinate axis of the machine tool is deduced, and the influence of the rotation angle is not considered. The earlier error compensation should be Professor Hocken, who measured the error of a large number of points in the working space within 16 hours for the three-dimensional coordinate measuring machine Moore5-Z(1). The effect of temperature was taken into account during this process. , and use the least square method to identify the error model parameters. Since the position signal of the machine tool movement is directly obtained from the laser interferometer, the influence of the angle and straightness error is taken into consideration, and satisfactory results are obtained. In 1985, G.Zhang successfully compensated the CMM. Flatness error of the table was measured, except that the value was slightly larger at the edge of the table, and the other was not more than 1 μm, which verified the reliability of the rigid body assumption. 21 errors measured using a laser interferometer and a level meter, error synthesis by linear coordinate transformation, and error compensation implemented. Measurement tests on the XY plane show that before compensation, the point where the error value is greater than 20 μm accounts for 20% of all the measurement points, and after the compensation, the error of the point not exceeding 20% ​​is greater than 2 μm, which proves that the precision has increased by nearly 10 times.
In addition to the error compensation of the coordinate measuring machine, the research on error compensation of CNC machine tools has also achieved certain results. In 1977, Professor Schultschik used vectorial methods to analyze the errors of various parts of the machine tool and its influence on the geometric accuracy, and laid the foundation for further research on the geometric errors of the machine tool. Ferreira and his collaborators also studied the method and obtained a general model of the geometric error of the machine tool, contributing to the single-error synthesis compensation method. J. Nietal further applied the method to online error compensation and obtained satisfactory results. Chenetal established 32 error models, 11 of which are related to the temperature and machine tool origin error parameters. The compensation test for the horizontal machining center shows that the accuracy is improved by 10 times. Eung-Suk Leaetal used almost the same measurement method as G. Zhang. He measured 21 errors in the three-dimensional Bridgeport milling machine and used the error synthesis method to obtain the error model. The compensated results were obtained using the laser interferometer and Renishaw's DBB respectively. The system has been tested to prove that the accuracy of the machine tool has been improved.
2.2 Error Direct Compensation
This method requires precise measurement of the space vector error of the machine tool. The higher the compensation accuracy requirement, the more the measurement accuracy and the number of measurement points are required. However, it is impossible to fully know the error of any point in the measurement space, and the method of interpolation is used. The error component of the compensation point is obtained and the error correction is performed. This method requires establishing an absolute measurement coordinate system that is consistent with the compensation.
In 1981, Dufour and Groppetti measured the error of the working space of the machine tool under different load and temperature conditions to form an error vector matrix and obtain the error information of the machine tool. The error matrix is ​​stored in the computer for error compensation. Similar studies mainly include ACOkaforetal, which measures the relative error of multiple points on the standard reference part in the working space of the machine tool, uses the first point as the reference point, and then converts it into an absolute coordinate error. The interpolation method is used to compensate the error. The results show that Accuracy increased by 2 to 4 times. Hooman used a three-dimensional linear (LVTDS) measurement device to obtain 27-point error in the machine tool space (resolution 0.25 μm, repeatability 1 μm) and performed a similar job. Taking into account the influence of temperature, measurements were made at intervals of 1.2 hours and a total of eight measurements were performed. The temperature coefficient was corrected for the error compensation results. The disadvantage of this method is that the measurement workload is large, and more data is stored. At present, there are no completely suitable instruments, which limits the further application and development of this method.
2.3 Relative Error Decomposition and Synthetic Compensation
Most of the error measurement methods only get the relative comprehensive error. According to this, the single item error of the machine tool can be decomposed. Further using error synthesis methods, it is feasible to compensate for machine tool errors. At present, domestic and foreign research in this area has also made some progress.
In 2000, Chen Guiuan, a doctoral student under the direction of Professor Jun Ni of the University of Michigan in the United States, made an attempt to measure the geometric errors at different temperatures of a three-axis CNC machine using a ball barometer (TBB), and established a rapid temperature prediction and error compensation model. Error compensation was performed. Christopher used a laser ballbar (LBB) to obtain the error information of the machine tool within 30 minutes, established an error model, and evaluated the error compensation results five times in a 9-month time interval. The results showed that the software passed the software. The method of error compensation can improve the precision of the machine tool, and can keep the precision unchanged for a long time.
The error synthesis method requires the measurement of the original errors of each axis of the machine tool. The more mature measurement method is a laser interferometer with high measurement accuracy. Using a dual-frequency laser interferometer for error measurement takes a long time and requires a high level of debugging for the operator. What is more important is the high requirement for the error measurement environment. It is often used in the detection of coordinate measuring machines and is not suitable for on-site operation. The relative error decomposition and composition compensation method is relatively simple. A measurement can obtain the entire circle of data information, and at the same time, it can meet the accuracy of machine tool testing and machine tool evaluation. At present, there are also many error decomposition methods. Due to the different machine conditions, it is difficult to find a suitable general mathematical model for error decomposition, and the original error items that have the same influence on the measurement results cannot be decomposed and are difficult to popularize and apply. The direct error compensation method generally uses the standard part as a control to obtain the space vector error, performs direct compensation, and has fewer intermediate links and is closer to the practical situation of the machine tool. However, obtaining a large amount of information requires different standard parts and is difficult to achieve, so that the accuracy of compensation is limited.
In China, many research institutions and universities have also conducted research on machine tool error compensation in recent years. 1986 Beijing Machine Tool Research Institute carried out research on the compensation of thermal errors and the compensation of coordinate measuring machines. In 1997, Li Shuhe of Tianjin University conducted a study on the modeling of machine tool error compensation and thermal error compensation. In 1998, Liu Youwu of Tianjin University established a machine tool error model using a multi-body system, and gave a 22-, 14-, and 9-line laser interferometer measurement method for geometric errors. In 1999, they also performed error compensation for CNC machine tools. Comprehensive research has yielded gratifying results. In 1998, Yang Jianguo of Shanghai Jiaotong University conducted a study on the compensation of lathe thermal errors. From 1996 to 2000, under the support of the National Natural Science Foundation of China and the National 863 Program, Huazhong University of Science and Technology carried out research on geometrical error compensation of CNC machine tools and intelligent adaptive control based on online recognition of cutting forces, and achieved some results.
There are two ways to improve machine tool accuracy. One is to eliminate possible sources of error by increasing the level of part design, manufacture, and assembly, known as error prevention. On the one hand, this method is mainly restricted by the precision of the processing machine. On the other hand, the increase in the quality of the parts leads to the expansion of processing costs, which results in the use of the method being limited. Another kind of error compensation method (error compensation), usually by modifying the machining instructions of the machine tool, the error compensation of the machine tool, to achieve the ideal trajectory, to achieve a soft upgrade of the machine tool accuracy. Studies have shown that geometric errors and errors caused by temperature account for about 70% of the total error of the machine tool, where geometric errors are relatively stable and error compensation is easy. Compensating the geometrical error of CNC machine tools can improve the processing level of the entire machinery industry. It is of great significance to promote scientific and technological progress, improve China’s national defense capabilities, and then greatly increase China’s overall national strength.