There are two ways to improve machine tool accuracy. One is to remove possible error sources by improving the level of part design, manufacturing and assembly, which is called error prevention. On the one hand, this method is mainly restricted by the precision of the machining machine, and on the other hand, the improvement of the quality of the parts leads to the expansion of the processing cost, which limits the use of this method to a certain extent. The other method is called error compensation. Usually, by modifying the machining instructions of the machine tool, the error compensation is performed on the machine tool to achieve the ideal motion trajectory and realize the soft upgrade of the machine tool accuracy. Studies have shown that geometric errors and errors caused by temperature account for about 70% of the overall errors of machine tools, among which geometric errors are relatively stable and easy to compensate for errors. Compensation for the geometric error of CNC machine tools can improve the processing level of the entire machinery industry, and is of great significance to promoting scientific and technological progress, improving our country's national defense capabilities, and then greatly enhancing our country's comprehensive national strength.
1 Causes of geometric errors
It is generally believed that the geometric error of CNC machine tools is caused by the following reasons:
1.1 The original manufacturing error of the machine tool
It refers to the motion error of the machine tool caused by the geometric shape, surface quality and position error between the working surfaces of the components of the machine tool, and is the main cause of the geometric error of the CNC machine tool.
1.2 Machine tool control system error
Including the servo error (contour following error) of the machine tool shaft system, and the error of the NC interpolation algorithm.
1.3 Thermal deformation error
The error caused by the thermal deformation of the machine tool structure due to the internal heat source of the machine tool and the 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 deformation of machine tools, tools, workpieces and fixtures. This kind of error is also called "tooth giving", which causes the shape distortion of the machined parts, especially when processing thin-walled workpieces or using slender tools, this error is more serious.
1.5 Vibration error of machine tool
During the cutting process, due to the flexibility of the process and the variability of the process, the operation state of the CNC machine tool is more likely to fall into the unstable area, thus arousing strong chatter. It leads to the deterioration of the surface quality and geometric shape error of the machined workpiece.
1.6 Test error of detection system
Including the following aspects:
(1) The error of the measurement sensor feedback system itself caused by the manufacturing error of the measurement sensor and its installation error on the machine tool;
(2) The error of the measurement sensor due to the error of the machine tool parts and mechanism and the deformation in use.
1.7 External interference error
Random errors due to changes in the environment and operating conditions.
1.8 Other errors
Such as errors caused by programming and operation errors.
The above errors can be classified into two categories according to the characteristics and nature of the errors: systematic errors and random errors.
The systematic error of CNC machine tools is the inherent error of the machine tool itself, which is repeatable. The geometric error of CNC machine tools is its main component and also has repeatability. Utilizing this feature, it can be "off-line measurement", and the technology of "off-line detection-open loop compensation" can be used to correct and compensate it, so as to reduce it and achieve the purpose of enhancing the accuracy of the machine tool.
Random errors are random, and the method of "online detection-closed-loop compensation" must be used to remove the influence of random errors on the machining accuracy of machine tools. This method has strict requirements on measuring instruments and measuring environments, and is difficult to popularize.
2 Geometric error compensation technology
According to different types of errors, the implementation of error compensation can be divided into two categories. Random error compensation requires "online measurement". The error detection device is directly installed on the machine tool, and the error value of the corresponding position is measured in real time while the machine tool is working, and the processing instruction is corrected in real time with this error value. The random error compensation has no requirement on the error nature of the machine tool, and can compensate the random error and systematic error of the machine tool at the same time. However, a complete set of high-precision measuring devices and other related equipment are required, and the cost is too high and the economic benefits are not good. Literature  carried out online temperature measurement and compensation, but failed to achieve practical application. System error compensation is to use corresponding instruments to test the machine tool in advance, that is, to obtain the error value of the command position of the machine tool work space through "off-line measurement", and use them as a function of the machine tool coordinates. When the machine tool is working, according to the coordinates of the processing point, the corresponding error value is called out for correction. The stability of the machine tool is required to ensure the certainty of the error of the machine tool for easy correction. The accuracy of the machine tool after compensation depends on the repeatability of the machine tool and the change of environmental conditions. Under normal circumstances, the repetition accuracy of CNC machine tools is much higher than its spatial comprehensive error, so the compensation of system errors can effectively improve the accuracy of the machine tool, and even improve the accuracy level of the machine tool. So far, there are many compensation methods for system 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. Firstly, the individual original error values of the machine tool are measured by the direct measurement method, and the error components of the compensation points are calculated by the error synthesis formula, so as to realize the error compensation of the machine tool. Leete is the one who measures the position error of the three-coordinate measuring machine. Using the triangular geometric relationship, he deduces the expression method of the error of each coordinate axis of the machine tool, without considering the influence of the rotation angle. It should be Professor Hocken who performed error compensation earlier. For the three-coordinate measuring machine of model Moore5-Z(1), within 16 hours, the error of a large number of points in the working space was measured, and the influence of temperature was considered in the process. , and the error model parameters were identified by the least square method. Since the position signal of the machine tool movement is obtained directly from the laser interferometer, the influence of angle and straightness errors is considered, and a satisfactory result is obtained. In 1985, G. Zhang successfully compensated the error of the three-coordinate measuring machine. The flatness error of the workbench was measured, except for a slightly larger value at the edge of the workbench, the others did not exceed 1 μm, which verified the reliability of the rigid body assumption. Using the 21 errors measured by the laser interferometer and the level meter, the errors are synthesized through the linear coordinate transformation, and the error compensation is implemented. The measurement test on the XY plane shows that before compensation, 20% of all measurement points have an error value greater than 20 μm, and after compensation, no more than 20% of the points have an error greater than 2 μm, which proves that the accuracy has increased by nearly 10 times.
In addition to the error compensation of coordinate measuring machines, the research on error compensation of CNC machine tools has also achieved certain results. In 1977, Professor Schultschik used the vector diagram method to analyze the errors of various parts of the machine tool and their influence on the geometric accuracy, which laid the foundation for further research on the geometric errors of machine tools. Ferreira and his collaborators have also studied this method, obtained a general model of machine tool geometric errors, and made a contribution to the single error composite compensation method. J.Nietal further applied this method to online error compensation and obtained relatively ideal results. Chenetal established a 32-item error model, of which 11 extra items are related to temperature and machine tool origin error parameters. The compensation test on the horizontal machining center shows that the accuracy is increased by 10 times. Eung-SukLeaetal used almost the same measurement method as G. Zhang to measure 21 errors of the three-coordinate Bridgeport milling machine, and obtained the error model by using the error synthesis method. The compensated results were respectively used by laser interferometer and Renishaw's DBB The system was tested and proved that the accuracy of the machine tool was improved.
2.2 Error direct compensation method
This method requires accurate measurement of the machine tool space vector error. The higher the compensation accuracy, the more the measurement accuracy and the number of measurement points. However, it is impossible to know the error of any point in the measurement space in detail, and the method of interpolation is used Obtain the error component of the compensation point and perform error correction. This method requires the establishment of a measurement coordinate system consistent with the compensation.
In 1981, Dufour and Groppetti measured the errors of machine tool workspace points under different load and temperature conditions to form an error vector matrix and obtain machine tool error information. The error matrix is stored in the computer for error compensation. Similar research mainly includes ACOkaforetal. By measuring the relative errors of multiple points on the standard reference part in the machine tool workspace, the first one is used as the reference point, and then converted into coordinate errors, and error compensation is performed by interpolation. The results show that the accuracy Increased by 2 to 4 times. Hooman used a three-dimensional linear (LVTDS) measuring device to obtain the error of 27 points in the machine tool space (resolution 0.25 μm, repeatability 1 μm), and carried out similar work. Further considering the influence of temperature, the measurements were taken every 1.2 hours, and a total of 8 measurements were taken, and the temperature coefficient was corrected for the error compensation results. The disadvantage of this method is that the measurement workload is large and the storage data is large. At present, there is no completely suitable instrument, which also limits the further application and development of this method.
2.3 Relative error decomposition and composite compensation method
Most error measurement methods only get the relative comprehensive error, which can be decomposed to obtain the single error of the machine tool. It is feasible to compensate machine tool errors by further using the method of error synthesis. At present, some progress has been made in research in this area at home and abroad.
In 2000, Chen Guiquan, a doctoral student under the guidance of Professor JunNi of the University of Michigan in the United States, made such an attempt, using a ballbar (TBB) to measure the geometric errors of three-axis CNC machine tools at different temperatures, and established a rapid temperature prediction and error compensation model. , with error compensation. Christopher used the 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 5 times within a time interval of 9 months. The results showed that through the software The method of error compensation can improve the accuracy of the machine tool and keep the accuracy 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 the laser interferometer, which has high measurement accuracy. It takes a long time to measure the error with a dual-frequency laser interferometer, and requires a high level of debugging by the operator. More importantly, it has high requirements on the error measurement environment. It is often used in the detection of three-coordinate measuring machines, and it is not suitable for on-site production operations. Compared with the error decomposition and composite compensation method, the measurement method is relatively simple, and the data information of the entire circumference can be obtained in one measurement, and at the same time, it can meet the inspection of machine tool accuracy and machine tool evaluation. At present, there are many error decomposition methods. Due to the different machine tools, it is difficult to find a suitable general mathematical model for error decomposition, and the original error items that have the same impact on the measurement results cannot be decomposed, and it is difficult to popularize and apply. The direct compensation method of the error generally uses the standard part as a comparison to obtain the space vector error for direct compensation, which reduces the intermediate links and is closer to the practical situation of the machine tool. However, obtaining a large amount of information requires different standard parts, which is difficult to achieve, so that the compensation accuracy is limited.
In China, many research institutions and universities have also carried out research on machine tool error compensation in recent years. In 1986, the Beijing Machine Tool Research Institute carried out research on the compensation of thermal errors of machine tools and the compensation of coordinate measuring machines. In 1997, Li Shuhe of Tianjin University carried out modeling of machine tool error compensation and research on thermal error compensation. In 1998, Liu Youwu of Tianjin University established the error model of the machine tool using a multi-body system, and gave the 22-line, 14-line, and 9-line laser interferometer measurement methods for geometric errors. In 1999, they also carried out error compensation for CNC machine tools. Comprehensive research has achieved gratifying results. In 1998, Yang Jianguo of Shanghai Jiaotong University carried out research on thermal error compensation of lathes. From 1996 to 2000, with the support of the Natural Science Foundation and the 863 Program, Huazhong University of Science and Technology carried out research on geometric error compensation of CNC machine tools and intelligent adaptive control based on online identification of cutting forces, and achieved some results.