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There are two ways to improve machine accuracy.
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 errorprevention. On the one hand, the method is mainly restricted by the precision of the processing machine, and on the other hand, the improvement of the quality of the parts leads to the expansion of the processing cost, so that the use of the method is limited. Another type of error compensation method (errorcompensation) usually compensates 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 overall 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.
1 Reasons for geometric error
It is generally believed that the geometric error of CNC machine tools is caused by the following reasons:
1.1 Original manufacturing error of the machine tool
It refers to the machine tool motion error caused by the geometric shape, surface quality and position error of the working surface of each component 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 error of the machine axis (contour following error), CNC interpolation algorithm error.
1.3 Thermal deformation error
The error caused by the thermal deformation of the machine structure due to the internal heat source of the machine tool and the environmental thermal disturbance.
1.4 Error caused by deformation of the process system caused by cutting load
This includes errors caused by machine tool, tool, workpiece and fixture deformation. This type of error is also referred to as “knife”, which causes distortion of the shape of the machined part, especially when machining thin-walled workpieces or using slender tools.
1.5 Machine tool vibration error
During the machining process, the numerical control 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, thereby arousing strong chatter. This leads to surface quality deterioration and geometrical errors in the machined workpiece.
1.6 Test System Test Error
Including 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 component 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 caused by 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 corrected and compensated by the technology of "offline detection & mdash; — open loop compensation" to reduce the machine precision.
The random error has randomness. The method of “online detection & mdash;— 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.
2 Geometric error compensation technology
For the different types of errors, the implementation of error compensation can be divided into two categories. The random error compensation requirement “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 machining instruction 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 using “offline measurement” as a function of the machine coordinate. 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. Therefore, 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 for systematic errors at home and abroad, which can be divided into the following methods:
2.1 Single-term error synthesis compensation method
This compensation method is based on the error synthesis formula. Firstly, the single original error value of the machine tool is measured by the direct measurement method, and the error component of the compensation point is calculated by the error synthesis formula, thereby realizing the error compensation for the machine tool. The position error measurement of the coordinate measuring machine belongs to Leete. 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 Moore5-Z(1), the error of a large number of points in the workspace was measured within 16 hours, and the influence of temperature was considered in the process. The error model parameters are 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 coordinate measuring machine. The flatness error of the workbench was measured, except that the value at the edge of the workbench was slightly larger, and the others did not exceed 1μm, 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 error value is greater than 20 & mu; m at all measurement points accounted for 20%, after compensation, the error of no more than 20% point is greater than 2 & mu; m, 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. Nietal further applied the method to online error compensation and obtained ideal results. Chenetal has established 32 error models, of which 11 are temperature and machine origin error parameters. The compensation test for horizontal machining center shows that the accuracy is increased by 10 times. Eung-SukLeaetal uses almost the same measurement method as G.Zhang. The 21 errors of the three-axis Bridgeport milling machine are measured. The error model is obtained by error synthesis. The compensated results are respectively laser interferometer and Renishaw DBB. The system was tested to prove that the machine accuracy was improved.
2.2 Error Direct Compensation Method
This method requires accurate measurement of the 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, and the interpolation method is used. The error component of the compensation point is obtained and error correction is performed. This method requires an absolute measurement coordinate system that is 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 to obtain machine error information. The error matrix is stored in a computer for error compensation. A similar study mainly includes ACOkaforetal. 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 method. It shows 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 & mu; m, repeatability 1 & mu; m), and a 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.
2.3 Relative error decomposition, 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 decomposed. 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, ChenGuiquan, a doctoral student directed by Professor JunNi of the University of Michigan in the United States, made such an attempt to measure the geometric error at different temperatures of a three-axis CNC machine using a ballbar instrument (TBB), and established a rapid temperature prediction and error compensation model. , error compensation was performed. Christopher used the laser ball bar (LBB) to obtain the error information of the machine in 30 minutes, and established an error model. The error compensation result was evaluated 5 times in the 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 constant 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 requires high error measurement environment, and is often used for the detection of CMMs. It is not suitable for on-site operation. Relative error decomposition, synthetic compensation method, the measurement method is relatively simple, one measurement can obtain the data information of the whole circumference, and at the same time 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 China, many research institutes and universities have also conducted research on machine tool error compensation in recent years. In 1986, Beijing Machine Tool Research Institute carried out research on compensation of thermal error of machine tools and compensation of coordinate measuring machines. In 1997, Li Shuhe of Tianjin University conducted research on modeling and thermal error compensation for machine tool 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 thermal error compensation of lathes. From 1996 to 2000, with 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 geometric error compensation of CNC machine tools and intelligent adaptive control based on online identification of cutting forces, and achieved some results.
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