Deformation compensation for milling curved blade parts
The detailed distribution information of surface machining deformation error of blade parts was obtained by the theory of material mechanics analysis, finite element analysis or measurement data analysis. Based on this, the original NC programming tool position is pre-corrected to compensate for the tool making error caused by the deformation of the tools and parts, so as to achieve the high-speed finishing of one pass.
I. Material mechanics theory analysis
Based on engineering mechanics and elastic mechanics theory, simplified model technology is used to establish the blade force model in a typical fixture structure, and elastic deformation analysis is performed to calculate the blade process stiffness. Graphically compare the process stiffness of the blades in various fixtures intuitively and clearly. It is convenient to judge the degree of deformation and the area with the largest deformation by the macro geometric dimensions such as blade length, width and thickness. Before the programming, the fixture structure is selected and optimized, and compensation measures are proposed to compensate for the loss of tool deformation accuracy to a certain extent.
II. Finite element analysis
Based on the calculation results of the finite element analysis, a deformation error distribution model of the machining surface of the workpiece is established, and the original NC programming tool position is modified to effectively compensate the machining deformation error.
The existence of residual stress on the surface layer of the workpiece severely affects its fatigue strength and serviceability, and the distortion caused by residual stress will also significantly reduce the accuracy of workpiece processing. In particular, it has a greater impact on the thin-walled structures commonly used in the aviation industry. How to accurately predict and control the residual stress and distortion of the surface of the workpiece, improve the integrity of the machining surface, and improve the accuracy of NC machining has always been an important research topic in the field of precision and ultra-precision cutting. Using the thermo-elastoplastic large-deformation finite element method, LIN et al. Simulated the distribution of the residual stress on the surface of NIP alloy ultra-precision cutting at different cutting speeds and cutting depths. It was found that the residual compressive stress first increased to a certain value and then began to decrease along the depth of the surface of the workpiece. The location where the maximum residual compressive stress appeared increased with the increase of the cutting depth. EL-AXIR studied the influence of material tensile strength, cutting speed, and feed rate on the distribution of residual stress on the surface layer of workpiece turning, and considered that the residual stress on the surface layer of the workpiece accorded with the polynomial function distribution along the depth direction. Using the blind hole drilling method for measuring residual stress, SRIDHAR et al. Analyzed the distribution of residual stress on the surface layer of the workpiece during milling of titanium alloy IMI-834. The research results show that for the range of cutting parameters selected, the residual stress on the surface of the workpiece is basically in the state of compressive stress. At the same time, the optimal heat treatment process temperature for eliminating residual stress without affecting the microstructure and mechanical properties of the material was determined.
I. Material mechanics theory analysis
Based on engineering mechanics and elastic mechanics theory, simplified model technology is used to establish the blade force model in a typical fixture structure, and elastic deformation analysis is performed to calculate the blade process stiffness. Graphically compare the process stiffness of the blades in various fixtures intuitively and clearly. It is convenient to judge the degree of deformation and the area with the largest deformation by the macro geometric dimensions such as blade length, width and thickness. Before the programming, the fixture structure is selected and optimized, and compensation measures are proposed to compensate for the loss of tool deformation accuracy to a certain extent.
II. Finite element analysis
Based on the calculation results of the finite element analysis, a deformation error distribution model of the machining surface of the workpiece is established, and the original NC programming tool position is modified to effectively compensate the machining deformation error.
The existence of residual stress on the surface layer of the workpiece severely affects its fatigue strength and serviceability, and the distortion caused by residual stress will also significantly reduce the accuracy of workpiece processing. In particular, it has a greater impact on the thin-walled structures commonly used in the aviation industry. How to accurately predict and control the residual stress and distortion of the surface of the workpiece, improve the integrity of the machining surface, and improve the accuracy of NC machining has always been an important research topic in the field of precision and ultra-precision cutting. Using the thermo-elastoplastic large-deformation finite element method, LIN et al. Simulated the distribution of the residual stress on the surface of NIP alloy ultra-precision cutting at different cutting speeds and cutting depths. It was found that the residual compressive stress first increased to a certain value and then began to decrease along the depth of the surface of the workpiece. The location where the maximum residual compressive stress appeared increased with the increase of the cutting depth. EL-AXIR studied the influence of material tensile strength, cutting speed, and feed rate on the distribution of residual stress on the surface layer of workpiece turning, and considered that the residual stress on the surface layer of the workpiece accorded with the polynomial function distribution along the depth direction. Using the blind hole drilling method for measuring residual stress, SRIDHAR et al. Analyzed the distribution of residual stress on the surface layer of the workpiece during milling of titanium alloy IMI-834. The research results show that for the range of cutting parameters selected, the residual stress on the surface of the workpiece is basically in the state of compressive stress. At the same time, the optimal heat treatment process temperature for eliminating residual stress without affecting the microstructure and mechanical properties of the material was determined.
III. Measurement data analysis
The material mechanics theoretical analysis method and finite element analysis method are used to predict the blade deformation error. The accuracy of the prediction is closely related to the cutting force model and the processing technology model. The measurement data analysis method is to measure the completed blade test piece by a coordinate measuring machine, and compensate the processing error of the blade by analyzing the detection result. The data analysis method is ex post analysis, while the material mechanics analysis method and finite element analysis method are ex ante predictions. Comparatively speaking, the measurement data analysis method is more expensive. The measurement data analysis method is to measure and analyze the blade specimens. Therefore, the number of specimens is very important. Generally, a batch of 3 to 5 blades is better. In addition, test piece processing also requires process stability. If the process is unstable, the deformation of the processed specimen will be irregular, and the deformation of the blade cannot be accurately analyzed from the measured data. The measurement data analysis method uses a three-dimensional coordinate measuring machine to measure the blades that have been processed. By analyzing the measurement data, the deformation error rule of the blades is obtained. Then, the CAD model is modified according to the deformation of the blade, that is, the CAD model of the blade is anti-deformed. Then rewrite the NC code through the modified CAD model to process the blade.
Three pieces of secondary rotor blades of a certain type of engine were numerically processed (torsion deformation error was not compensated), and the torsional errors calculated after measuring 8 cross sections of the blade by a measuring machine. The distribution trend of the blade cross-section torsional error (uncompensated) of the three test pieces is consistent, indicating that the processing technology system is stable. Without error compensation, the maximum torsional error is 39.758 ', which exceeds the requirement of "maximum torsional error not exceeding ± 12'" allowed in the drawing. Based on the average of the cross-section torsional errors of the three blade specimens, the CAD model of the blade machining process was used to compensate the blade's reverse deformation error. That is, each section of the blade is rotated based on the theoretical position: -3.126667 ',-5.936667', -9.453333 ', -17.525', -26.36817 ', -33.3512', -36.0071 ', -38.0152', and then reshape the blade profile. Then write the NC program according to the new CAD model, and reprocess 3 blades. The reprocessed blades are inspected and processed by a measuring machine. The maximum torsional error is 11.5326 ', which meets the requirements of the drawings. It can be further compensated as required to make the torsional error smaller.
After the CNC machining of 3 test pieces of curved blades of a certain type of engine rotor (the bending deformation error is not compensated) is completed. After measuring the 9 cross-sections of the blade with a CMM and processing the measurement data, the bending deformation error distribution.
Based on the average of the offsets of the cross-sections of the three curved blade test pieces, the CAD model of the blade machining process was used to compensate the blade's reverse deformation error. That is, each section of the blade is translated based on the theoretical position: 0.02543MM, 0.04526MM, 0.07026MM, 0.15101MM, 0.18391MM, 0.16234MM, 0.12243MM, 0.09541MM, 0.0833MM, the blade profile is reshaped. Then write the NC program according to the new model and reprocess the three-piece curved blade. After measuring and processing the re-machined curved blades. The maximum offset is -0.04214MM, which meets the drawing requirements. It can be further compensated as required to make the bending error smaller.
