| Driven by space optical technology and civil optical technology,freeform optics,especially large-aperture freeform optics,is increasingly applied to modern space exploration and civil consumption.Due to low machining efficiency,fluctuations in side-feeding motion and redundant control points for large aperture optics,this paper combines the advantages of slow tool servo(STS)turning technology in freeform surface processing,studies the scientific problems in the process of large aperture freeform optics processing and realizes the high-efficiency and high-precision freeform surface processing methods to meet the needs of social development.This paper mainly deals with the key technologies and challenges in large aperture freeform optics processing,error analysis,tool geometry parameters calculation,tool path generation and surface treatment.1.Key technologies and error analysis of freeform optics turningThe key technologies of modern lathe to complete the freeform optics turning are spindle position servo technology,high dynamic response technology of linear axis and curve interpolation technology.Then,because the error sources of large aperture freeform optics processing technology are more complex,this paper comprehensively analyses the error models of large aperture freeform optics processing from four aspects: principle of STS,machine tool,diamond tool and processing process.The error model of large aperture freeform surface caused by the verticality of machine tool guideway is established,and the error model of tool edge waveness and tool noise radius is established.The influence of thermal deformation on the machining accuracy of large aperture freeform surface is analyzed from two aspects: the thermal deformation of machine tool spindle and diamond tool.Finally,the influence of material characteristics,clamping deformation,tool alignment error and dynamic balance error is analyzed from the aspect of processing technology,and the corresponding solutions are given respectively.2.Algorithms for geometric parameters calculation of freeform surface turning toolsThe range of included angle of diamond tool is directly related to the geometrical shape of freeform surface,and the accuracy of tool edge is directly related to the form error of machined surface.The radius of tool noise directly affects whether there will be "over-cutting" or not in the process of machining.However,the existing algorithms for calculating tool parameters have some shortcomings,such as inconvenience for computer programming and complexity of solving algorithms.Especially for solving the maximum tool noise radius,the existing algorithms have some limitations for using curvature radius to approximate the optimal tool noise radius.This paper synthesizes the simple characteristics of "cross-section curve method" and uses NURBS curve to represent cross-section curve.It not only satisfies the simple structure of the algorithm,but also uses a unified expression for cross-section curve,which is convenient for computer programming.In this paper,a NURBS curve inverse algorithm for freeform optics is adopted.The NURBS curve fitted by the control points and node vectors obtained by this algorithm has higher fitting accuracy than the current conventional algorithm,and can satisfy the accuracy of tool geometric parameters calculation.3.Tool path generation for diamond turning of large aperture freeform opticsTurning path generation is very important in freeform optics turning.It is not only directly related to the form accuracy of workpiece,but also related to the dynamic response,follow-up error and interpolation error of machine tool.Due to low machining efficiency,fluctuations in side-feeding motion and redundant control points for large aperture optics,this paper reports a novel adaptive tool path generation(ATPG)for STS diamond turning.In ATPG,the sampling intervals both in feeding and cutting direction are independently controlled according to interpolation error and cutting residual tolerance.A smooth curve is approximated to the side-feeding motion for reducing the fluctuations in feeding direction.A theoretical investigation for ATPG is conducted.The results indicate that the smooth curve produced by the spiral radius and revolution number can effectively suppress vibration of side-feeding motion,which can make the machining operated at a higher spindle speed.Comparison of surface generation of typical freeform surfaces with ATPG and commercial software DiffSys is conducted both theoretically and experimentally.The result demonstrates that the ATPG can effectively reduce the volume of control points,decrease the vibration of side-feeding motion and improve machining efficiency while surface quality is well maintained for large aperture freeform optics.4.Texture surface processing for freeform surface turningDue to tool wear and other reasons,the large aperture freeform optics surface is prone to dispersive phenomenon after turning.In this paper,firstly,the reasons of dispersion are analyzed,the technological method of eliminating the surface dispersive phenomenon of workpiece is considered from the point of view of process simplification and economic cost.The surface treatment process experiments of two groups of turning samples with feedrate of 5 μm/r and 8 μm/r are completed.The experimental results showed that the polishing process can effectively eliminate the obvious turning texture on the workpiece surface,thus eliminating the grating effect,and can also reduce the roughness of the workpiece surface to a certain extent.At the same time,the experiment also showed that this process has no obvious effect on improving the surface roughness caused by the workpiece material itself.Therefore,it is a necessary prerequisite for freeform surface processing to select uniform material.In this paper,the scientific problems of key thchnologies of large aperture freeform optics ultra-precision turning are studied.The research results can provide theoretical guidance and technical support for large aperture freeform optics machining. |
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