A Novel Turning Method And Its Device Development For Suppressing Scattering Effects

Fast tool servo (FTS) assisted diamond turning has been extensively regarded as avery promising technology for fabricating freeform optics and micro-structuredfunctional surfaces. During the turning process, the cutting tool will follow a spatialspiral trajectory with constant radial spacing, resulting in residual tool marks onmachined surfaces with strong periodic characteristics. Unfortunately, both theoreticalanalysis and experimental tests have demonstrated that these repetitive features wouldinduce undesired scattering effects, deteriorating optical performances of machinedsurfaces. To suppress the negative effects, certain post-processes such as polishingand so on should be employed for the optics to remove the periodic tool marks.However, it should significantly decrease the efficiency and increase the costs formanufacturing the optics, and it will be much harder to implement the post-processesfor micro-structured functional surfaces.To suppress the scattering effects during the turning process and accordingly decreasethe post-processing costs, a novel pseudo-random diamond turning (PRDT) method isproposed in this paper to actively modify micro-topographies of the machined surfacewithout loss of efficiency and accuracy. As for the PRDT method, micro-topographieson machined surfaces with certain statistical distribution pattern can be achieved bymeans of actively making the cutting tool follow spatial pseudo-random spiraltrajectories (SPRST), simultaneously, the periodic characteristics of the residual toolmarks can be removed. The main contributions of this thesis can be summarized asfollows:(a). The novel PRDT method and the corresponding tool path generation strategy areproposed. Considering geometric features and spatial positions of the cutting tool, theoptimal tool path generation strategy is proposed for two types of freeform optics,namely the ones with analytical representations and the ones with only point cloudrepresentations. With the constraints of the cutting tool dynamics, a series of pseudo-random sequences are generated, accordingly the pseudo-random tool path for thePRDT is proposed. The micro-topographies on machined surfaces during PRDTturning process is calculated by means of a numerical simulation model developed byD. Luo, validating the accuracy and the efficiency of the proposed tool pathgeneration strategy.(b). A novel totally decoupled FTS mechanism with2degree of freedom (2-DOF) isdeveloped. A unique Z-shaped flexure hinge with self-amplifying characteristic isdeveloped. Based on the unique characteristics of the proposed hinge and the differential motion principle (DMP), the totally decoupled2-DOF motion is achievedin principle. By means of compliance matrix modeling (CMM) method, the completecompliance model of the mechanism is constructed with consideration of flexuralfeatures of the linkages, and the motion characteristics of the mechanism are detailed.Based on Lagrange principle, the dynamics model of the mechanism is furtherdeveloped, and the dependence model of the natural frequencies on the structureparameters is established. Moreover, the potential complex nonlinear dynamicsbehaviors of the system induced by contact conditions of the actuators and the movingparts. The agreement of the analytical results and the finite element based resultsdemonstrate the efficiency of the proposed modeling method.(c). The prototype of the2-DOF-FTS is developed and investigated. The workingbandwidth, motion resolutions, strokes along two directions, hysteresis nonlinearitiesand practical coupling motions are investigated through open-loop testing. To enhanceperformances of the system, the control strategy for motions of the mechanism isdeveloped. Step responses and tool positioning performances are estimated. Flatsurfaces and droplet micro-structured surfaces are fabricated using conventional FTSbased turning and the proposed PRDT method on certain typical materials. Scatteringtests of the machined surfaces demonstrate the efficiency of the PRDT method forfabricating complex optics with scattering effect suppressions.