There are two ways to improve machine accuracy. One is to eliminate possible sources of error by increasing the level of part design, manufacturing, and assembly, known as error prevention. On the one hand, the method is mainly restricted by the precision of the processing machine. On the other hand, the improvement of the quality of the parts leads to the expansion of the processing cost, which results in certain restrictions on the use of the method. Another type of error compensation is usually used to compensate the machine tool by modifying the machining instructions of the machine tool to achieve an ideal motion trajectory and achieve a soft upgrade of machine tool accuracy. Studies have shown that geometric errors and temperature-induced errors account for about 70% of the total machine tool error, with geometric errors being relatively stable and error-prone. Compensation for the geometric error of CNC machine tools can improve the processing level of the entire machinery industry, and it is of great significance to promote scientific and technological progress, improve China's national defense capability, and then greatly enhance China's comprehensive national strength. The reason for the geometric error is generally believed that the geometric error of the CNC machine tool is caused by the following reasons: The original manufacturing error of the machine tool refers to the machine tool caused by the geometric shape, surface quality and position error between the working surfaces of the various components of the machine tool. Motion error is the main reason for the geometric error of CNC machine tools. The control system error of the machine tool includes the servo error of the machine axis (contour following error) and the error of the numerical control interpolation algorithm. The thermal deformation error is caused by the thermal deformation of the machine structure due to the internal heat source of the machine tool and the environmental thermal disturbance. Errors caused by deformation of the process system caused by cutting loads include errors caused by deformation of machine tools, tools, workpieces and fixtures. This type of error is also known as the "knife", which causes distortion of the shape of the machined part, especially when machining thin-walled workpieces or using slender tools. The vibration error of the machine tool during the machining process, the CNC machine tool has a greater possibility of falling into the unstable region due to the flexibility of the process and the change of the process, thus arousing strong chatter. This leads to surface quality deterioration and geometrical errors in the machined workpiece. The test error of the detection system includes the following aspects:
(1) The error of the measuring sensor feedback system itself due to the manufacturing error of the measuring sensor and its installation error on the machine tool;
(2) Errors in the measurement sensor due to machine part and mechanism errors and deformation during use. The external disturbance error is a random error caused by changes in the environment and operating conditions. Other errors such as errors due to programming and operational errors.
The above errors can be classified into two categories according to the characteristics and nature of the error: system error and random error.
The systematic error of CNC machine tools is inherently error of the machine tool and is repeatable. The geometrical error of CNC machine tools is a major component and is also repeatable. With this feature, it can be "offline measurement", and can be modified and compensated by the technique of "offline detection - open loop compensation" to reduce it and achieve the purpose of machine precision enhancement.
The random error is random. The method of “online detection-closed loop compensation†must be used to eliminate the influence of random error on the machining accuracy of the machine. This method is strict on the measuring instrument and measurement environment and difficult to promote. Geometric error compensation technology for error types can be divided into two categories. The random error compensation requires “on-line measurementâ€, and the error detection device is directly installed on the machine tool. While the machine tool is working, the error value of the corresponding position is measured in real time, and the machining command is corrected in real time by using the error value. Random error compensation has no requirement on the error nature of the machine tool, and can compensate the random error and system error of the machine tool at the same time. However, a complete set of high-precision measuring devices and other related equipment is required, which is too costly and has low economic efficiency. The literature conducted on-line measurement and compensation of temperature, which failed to reach practical application. The system error compensation is to detect the machine tool in advance with the corresponding instrument, that is, the error value of the machine tool space command position is obtained by "offline measurement", which is used as a function of the machine tool coordinates. When the machine is working, according to the coordinates of the machining point, the corresponding error value is called to correct. The stability of the machine tool is required to be good, and the certainty of the machine tool error is guaranteed to facilitate the correction. The accuracy of the compensated machine tool depends on the repeatability of the machine tool and changes in environmental conditions. Under normal circumstances, the repeatability of CNC machine tools is much higher than the spatial integrated error, so the compensation of system error can effectively improve the accuracy of the machine tool and even improve the accuracy level 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: Single-term error synthesis compensation method This compensation method is based on the error synthesis formula. Firstly, the direct measurement method is used to measure the various machine tools. The original error value of the item is calculated by the error synthesis formula to calculate the error component of the compensation point, thereby realizing the error compensation for the machine tool. The position error measurement of the coordinate measuring machine is called Lete. Using the triangular geometric relationship, the representation method of the coordinate axes of the machine tool is derived, and the influence of the corner is not considered. Earlier error compensation was given by Professor Hocken. For the Coordinator model Moore 5-Z(1), the error of a large number of points in the workspace was measured within 16 hours, taking into account the temperature during this process. Influence, and the error model parameters were identified by least squares method. Since the position signal of the machine tool movement is directly obtained from the laser interferometer, the influence of the angle and the straightness error is considered, and a satisfactory result is obtained. In 1985, G. Zhang successfully compensated the error of the coordinate measuring machine. The flatness error of the table was measured, except that the value at the edge of the table was slightly larger, and the others did not exceed 1 μm, which verified the reliability of the rigid body assumption. 21 errors were measured using a laser interferometer and a level, error synthesis was performed by linear coordinate transformation, and error compensation was implemented. The measurement test on the XY plane shows that before the compensation, the point with the error value greater than 20μm at all measurement points accounts for 20%. After the compensation, the error of the point not exceeding 20% ​​is greater than 2μm, which proves that the accuracy is improved 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 the method of vector diagram to analyze the error of various parts of the machine tool and its influence on geometric precision, which laid the foundation for further research on machine geometry error. Ferreira and his collaborators also studied the method and derived a general model of machine geometry error, which contributed to the single-error synthesis compensation method. J. Ni et al further applied the method to online error compensation and obtained ideal results. Chen et al. established 32 error models, of which 11 are temperature and machine origin error parameters, and the compensation test for horizontal machining centers shows that the accuracy is increased by 10 times. Eung-Suk Lea et al used almost the same measurement method as G. Zhang, measured 21 errors of the three-coordinate Bridge port milling machine, and used the error synthesis method to obtain the error model. The compensated results were respectively laser interferometer. Tested with Renishaw's DBB system to demonstrate improved machine accuracy. Error direct compensation method This method requires accurate measurement of machine space vector error. The higher the compensation accuracy requirement, the more the measurement accuracy and the number of points to be measured, but it is impossible to know the error of any point in the measurement space in detail. The error component of the compensation point is obtained by the interpolation method, and the error correction is performed. This method requires the establishment of an absolute measurement coordinate system consistent with the compensation.
In 1981, Dufour and Groppetti measured the error of the working space point of the machine under different load and temperature conditions to form an error vector matrix and obtained machine error information. The error matrix is ​​stored in a computer for error compensation. A similar study mainly consists of ACOkafor et al. By measuring the relative error of multiple points on the standard reference piece in the working space of the machine tool, the first one is used as the reference point, and then converted into absolute coordinate error, and the error compensation is performed by interpolation. The results show that the accuracy is improved by 2 to 4 times. Hooman used a three-dimensional linear (LVTDS) measuring device to obtain a 27-point error in the machine space (resolution 0.25 μm, repeatability 1 μm), and similar work was performed. Taking into account the influence of temperature, the measurement was performed once every 1.2 hours, and the total measurement was performed 8 times. The temperature compensation coefficient was corrected for the error compensation result. The shortcoming of this method is that the measurement workload is large and the data is stored. At present, there is no completely suitable instrument, which limits the further application and development of the method. Relative Error Decomposition and Synthetic Compensation Method Most error measurement methods only obtain a relative comprehensive error, from which the single item error of the machine tool can be obtained. Further use of the error synthesis method is feasible for machine tool error compensation. At present, domestic and foreign research on this aspect has also made some progress.
In 2000, Chen Guiquan, a doctoral student directed by Professor Jun Ni of the University of Michigan in the United States, made an attempt to measure the geometric error at different temperatures of a three-axis CNC machine using a ballbar instrument (TBB), establishing a rapid temperature prediction and error. The compensation model was compensated for. Using a laser ball bar (LBB), Christopher obtained the error information of the machine in 30 minutes and established an error model. The error compensation result was evaluated five times in a 9-month interval. The results showed that the software was passed. The error compensation method 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 the various axes of the machine tool. The more mature measurement method is the laser interferometer, and the measurement accuracy is high. The error measurement with the dual-frequency laser interferometer takes a long time and requires high level of debugging for the operator. More importantly, it has high requirements for the error measurement environment. It is often used for the detection of CMMs and is not suitable for production site operations. Relative error decomposition, synthetic compensation method, the measurement method is relatively simple, one measurement can obtain the data information of the whole circumference, and can meet the machine tool precision detection and machine tool evaluation. At present, there are also many methods of error decomposition. Due to the different machine conditions, it is difficult to find a suitable general mathematical model for error decomposition, and the original error term with the same influence on the measurement results cannot be decomposed, and it is difficult to popularize and apply. The direct compensation method of error generally obtains the space vector error by comparing the standard parts, and performs direct compensation, which reduces the intermediate link 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 implement, so that the compensation accuracy is limited.
In summary: the error compensation of CNC machine tools, error measurement is the key, the error model is the basis. Through the compensation of the error, the accuracy of the machine tool can be effectively improved, and the contribution to the improvement of the manufacturing industry in China can be made.
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