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There are two ways to improve the precision of machine tools.

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There are two ways to improve the precision of machine tools.

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There are two ways to improve the precision of machine tools. One is to eliminate the possible source of error by improving the level of parts design, manufacture and assembly, known as error prevention (errorprevention). 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 makes the use of the method limited. The other is called error compensation (errorcompensation), usually by modifying the machining instructions of the machine tool, the error compensation is made to the machine tool, the ideal motion trajectory is achieved, and the soft upgrading of the precision of the machine tool is realized. Research shows that geometric errors and temperature induced errors account for 70% of the total error of machine tools, of which geometric errors are relatively stable and error compensation is easy. The compensation of geometric error of CNC machine tool can improve the processing level of the whole machinery industry, and it is of great significance to promote the progress of science and technology, improve our national defense ability, and greatly enhance the comprehensive national strength of our country.
 
1 causes of geometric error
 
It is generally believed that the geometric errors of CNC machine tools are caused by the following reasons:
 
The original manufacturing error of 1.1 machine tools
 
The error of machine tool motion caused by the geometric shape, surface quality and the position error of each other on the working surface of the machine tools is the main reason for the geometric error of the CNC machine tool.
 
Control system error of 1.2 machine tool
 
It includes servo error of machine tool shafting (contour following error) and numerical interpolation algorithm error.
 
1.3 thermal deformation error
 
Due to the internal heat source and thermal disturbance of the machine tool, the error caused by the thermal deformation of the machine tool structure is caused.
 
1.4 the error caused by the cutting load causing the deformation of the process system.
 
It includes errors caused by deformation of machine tools, tools, workpieces and fixtures. This error, also known as "let the knife", causes the shape distortion of the machining parts, especially when the thin wall workpiece is processed or the slender tool is used, the error is more serious.
 
Vibration error of 1.5 machine tools
 
In the cutting process, the CNC machine tool is more likely to fall into the unstable region because of the flexibility of the process and the variable process. It results in deterioration of surface quality and geometric error.
 
Test error of 1.6 detection system
 
It includes the following aspects:
 
(1) the error of the measurement sensor feedback system caused by the manufacturing error of the measuring sensor and its installation error on the machine tool;
 
(2) due to the errors in machine parts and mechanisms and the deformation in use, the errors of the measuring sensors are caused.
1.7 external interference error
 
Random errors caused by changes in environment and operating conditions.
 
1.8 other errors
 
Errors such as programming and operation errors.
 
The errors above can be classified into two categories according to their characteristics and properties: systematic error and random error.
 
The system error of NC machine tool is inherent error of machine tool, and it has repeatability. The geometric error of CNC machine tool is the main component and repeatability. By using this characteristic, the "off-line measurement" can be carried out, and the technology of "off-line detection - open loop compensation" can be used to modify and compensate it so that it can be reduced to achieve the purpose of strengthening the precision of the machine tools.
 
The random error is random. The method of "on-line detection - closed loop compensation" must be adopted to eliminate the effect of random error on the machining accuracy of machine tools. This method is strict and difficult to popularize for measuring instruments and measuring environment.

 

There are two ways to improve the precision of machine tools. One is to eliminate the possible source of error by improving the level of parts design, manufacture and assembly, known as error prevention (errorprevention). 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 makes the use of the method limited. The other is called error compensation
 
(errorcompensation), usually by modifying the machining instructions of the machine tool, the error compensation is made to the machine tool, the ideal motion trajectory is achieved, and the soft upgrading of the precision of the machine tool is realized. Research shows that geometric errors and temperature induced errors account for 70% of the total error of machine tools, of which geometric errors are relatively stable and error compensation is easy. The compensation of geometric error of CNC machine tool can improve the processing level of the whole machinery industry, and it is of great significance to promote the progress of science and technology, improve our national defense ability, and greatly enhance the comprehensive national strength of our country.
 
1 causes of geometric error
 
It is generally believed that the geometric errors of CNC machine tools are caused by the following reasons:
 
The original manufacturing error of 1.1 machine tools
 
The error of machine tool motion caused by the geometric shape, surface quality and the position error of each other on the working surface of the machine tools is the main reason for the geometric error of the CNC machine tool.
 
Control system error of 1.2 machine tool
 
It includes servo error of machine tool shafting (contour following error) and numerical interpolation algorithm error.
 
1.3 thermal deformation error
 
Due to the internal heat source and thermal disturbance of the machine tool, the error caused by the thermal deformation of the machine tool structure is caused.
 
1.4 the error caused by the cutting load causing the deformation of the process system.
 
It includes errors caused by deformation of machine tools, tools, workpieces and fixtures. This error, also known as "let the knife", causes the shape distortion of the machining parts, especially when the thin wall workpiece is processed or the slender tool is used, the error is more serious.
 
Vibration error of 1.5 machine tools
 
In the cutting process, the CNC machine tool is more likely to fall into the unstable region because of the flexibility of the process and the variable process. It results in deterioration of surface quality and geometric error.
Test error of 1.6 detection system
 
It includes the following aspects:
 
(1) the error of the measurement sensor feedback system caused by the manufacturing error of the measuring sensor and its installation error on the machine tool;
(2) due to the errors in machine parts and mechanisms and the deformation in use, the errors of the measuring sensors are caused.
 
1.7 external interference error
 
Random errors caused by changes in environment and operating conditions.
 
1.8 other errors
 
Errors such as programming and operation errors.
 
The errors above can be classified into two categories according to their characteristics and properties: systematic error and random error.
The system error of NC machine tool is inherent error of machine tool, and it has repeatability. The geometric error of CNC machine tool is the main component and repeatability. By using this characteristic, the "off-line measurement" can be carried out, and the technology of "off-line detection - open loop compensation" can be used to modify and compensate it so that it can be reduced to achieve the purpose of strengthening the precision of the machine tools.
The random error is random. The method of "on-line detection - closed loop compensation" must be adopted to eliminate the effect of random error on the machining accuracy of machine tools. This method is strict and difficult to popularize for measuring instruments and measuring environment.

 2.3 relative error decomposition and synthetic compensation method

Most of the error measurement methods only get the relative comprehensive error, which can be used to decompose the single error of the machine tool. It is feasible to use error synthesis to compensate the error of machine tools. At present, domestic and foreign research on this area has made some progress.
 
In 2000, the doctoral student, ChenGuiquan, guided by Professor JunNi of Michigan University in the United States, made such an attempt, using TBB to measure the geometric error of three axis CNC machine tools at different temperatures, and set up a rapid temperature prediction and error compensation model, and made the error compensation. Christopher obtained the error information of the machine tool in 30 minutes by using the laser ball bar instrument (LBB) and established the error model. In the interval of 9 months, the error compensation results were evaluated 5 times. The results show that the accuracy of the machine tool can be improved by the method of software error compensation, and the accuracy can be kept for a long time. Unchanged.
 
The error synthesis method requires measuring the original errors of each axis of the machine tool. The more mature measurement method is the laser interferometer, and the measuring accuracy is high. The use of dual frequency laser interferometer for measuring errors takes a long time and requires a high level of debugging for operators. What's more, the error measurement environment is very high. It is often used in the measurement of three coordinate measuring machines, and is not suitable for production site operation. Relative error decomposition, synthetic compensation method, measurement method is relatively simple, one measure can obtain the whole circle of data information, at the same time, it can meet the accuracy of machine tool detection and machine tool evaluation. At present, there are many methods of error decomposition. Because of the different conditions of machine tools, it is difficult to find a suitable general mathematical model for error decomposition, and the original error terms that have the same influence on the measurement results can not be decomposed, and it is difficult to popularize the application. The direct compensation method of error generally takes the standard parts as the control to obtain the space vector error, makes the direct compensation, less intermediate link, and is closer to the practical situation of the machine tool. However, a large amount of information needs different standard parts, which is difficult to achieve, so the accuracy of compensation is limited.
 
In China, many research institutes and universities have also carried out research on machine tool error compensation in recent years. 1986 the Beijing Machine Tool Research Institute has carried out the research on the thermal error compensation of machine tools and the compensation of coordinate measuring machines. In 1997, Li Shu He of Tianjin University studied the modeling of machine tool error compensation and thermal error compensation. In 1998, Liu Youwu of Tianjin University, such as the multi-body system, established the error model of the machine tool, gave the measuring methods of 22 line, 14 wire and 9 line laser interferometer for geometric error. In 1999, they also studied the error compensation of the CNC machine tool in an all-round way, and got the gratifying fruit. In 1998, Yang Jianguo of Shanghai Jiao Tong University studied the thermal error compensation of lathe. From 1996 to 2000, under the support of National Natural Science Foundation and national 863 project, Huazhong University of Science and Technology has carried out research on the geometric error compensation of CNC machine tools and intelligent adaptive control based on the on-line cutting force identification, and some achievements have been achieved.
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