Theoretical And Technological Research On Optical Freeform Surface Of Single Point Diamond Ultra-precision Turning

Optical freeform surface elements show the superior optical performance to traditionaloptical ones and are widely applied in the field of aerospace, space optics, remote sensing,national defense military, precision instruments and other modern cutting-edge technologies,because of its special surface structure and optical imaging effect, which can significantlyreduce the system’s structure, reduce the system weight and improve the quality of imaging.But the optical surface quality and machining precision of freeform surface are difficult to beguaranteed due to the irregularity geometric characteristics. Currently, single point diamondultra-precision turning technology is widely considered as one effective method ofmachining optical freeform surface, and it can machine the optical components withnano-scale surface quality that meet the requirement of optical system in high efficiency,high precision and low cost of processing. To single point diamond ultra-precision turningtechnology, some theoretical problems are studied in processing of machining opticalfreeform surface in this paper.The choice of freeform surface machining way is determined by the kinematiccharacteristics of machining surface showed in machining process. So, a surface dynamicunfold analysis method is proposed in this paper, which is spread the surface and cuttingtrajectory under the space rectangular coordinate system X-θ-Z and analysis the movementcharacteristics of machine tool axes when turning processing to choose the suitableprocessing method.To avoid the collision interference between work-piece and tool, a new kind of diamondtool parameter selection model is established combined with the proposed machining surfacedynamic unfold analysis method. The researched tool parameters mainly include: clearanceangle, nose radius and included angle.In view of the cutting tool path problem, the commonly used methods, such as equallyangle method and equally arc method, cannot predict the machining accuracy. So, threekinds of optimal trajectory generation method are proposed in this paper, which are theequally angle and equally arc combination optimization method, the analytical mathematical optimization method, reverse and interpolation optimization method. Simulation resultsshow that equally angle and equally arc combination optimization method can greatlyimprove the surface precision, and the analytical mathematical optimization method and thereverse and interpolation optimization method can realize the excellent forecast and controlfor the surface precision. For the tool radius compensation, as well as accurate positioningthe tool location, a tool location calculation method based on space vector theorem isproposed in this paper in order to improve the generating precision of cutting trajectory.The clamping position error has important influence on the ultra-precision machiningaccuracy of optical freeform surface. The possible influencing factors in the process ofinstallation are studied and analyzed in this paper, and two mathematical error models fordifferent clamping methods, the off-axis installation and coaxial installation, are establishedand compared.In turning experiment section, the off-axis parabolic surface and sinusoidal ring surfaceare cutting by using the single point diamond ultra-precision turning technology, and thesurface roughness Ra and form accuracy PV of machined surface are measured. Themeasurement results show that the Ra can reach nano-scale and PV can reach micron orsub-micron, which are accorded with the quality requirement of optical surface.The processing performance of new materials polyetheretherketone(PEEK) is exploredin single-point diamond ultra-precision turning. Three kinds of PEEK materials, pure PEEK,glass fiber enhanced PEEK and carbon fiber enhanced PEEK, are processed plane cuttingand the Ra and PV of three planes both can achieve sub-micron level, which can demonstratethat the thermoplastic materials PEEK has good cutting process-ability, and the single pointdiamond ultra-precision turning technology is good for PEEK ultra-precision turningprocessing.