Investigations of machining optical surfaces into brittle materials using an ultraprecision machine tool are presented. The newly developed ultra precision NIONmachine is evaluated to gain a good appreciation of its operating performance. Themachining accuracy capability of this machine is established by careful measurementof its; motion accuracy, thermal and dimensional stability and loop stiffness.Corroboration of these measurements are provided by assessment of surfaces whichwere produced in soft "easily machined" metal materials. It was found that surfacessmooth to -1 nm Ra could be produced on the NION machine and with a form errorof less than 100 nm P-V. The main source of figure error, approximately 80 nm, wasfound to be caused by the synchronous axial error motion of the workhead spindle.Other elements of the machine, including thermal effects, incurred less than 25 nm ofadditional figure error.Assessment of the diamond turning process for the producing optical surfaces made ina number of important optical materials, which are ostensibly brittle, were undertaken.Turning tests were carried out to establish the relative difficulty for machining opticalsurfaces in these materials and to define the most important parameters which affectthe attained surface quality. Assessment of the produced surfaces was based on theirroughness quality, surface morphology and residual stress condition. It was found thatdiamond tool edge quality degraded with total cut distance. Tool cut distance wasfound to be a major influence on achievable material removal rate before micro-fractures became present at the surface. Surface quality and residual stress conditionwere also greatly influenced by the overall tool cut distance.Diamond grinding trials were also carried out using the NION machine tool. Thesegrinding trials were carried out using a mode of grinding which permits complexshape optical surfaces to be produced. Various grinding technologies were employedto establish the optimum methods. Selected grinding trials were carried out toestablish the dominate parameters affecting the optical quality. Assessment of themachined surfaces was in regard of their surface roughness, residual stress andseverity of sub-surface micro cracking. It was found that grinding wheel specificationwas a major influence on surface quality and sub-surface damage. The level ofresidual stress associated with "ductile" mode grinding was not found to prohibit itsapplication toward the direct manufacture of optical elements. Selection of grinding parameters which ensured the grain depth of cut, GDOC,.,allowed glass surfaces to be parameter did not exceed the materials critical depth, d,ground to 1-2 nm Ra. These ground glass surfaces appeared free of any surfacefractures. Sub-surface assessmentsdid however reveal small levels of micro-fractureshidden below the surface.Discussion of both machining processes is provided. Available material removal ratesfor each process is given when cutting a number of important optical materials.Conclusions regarding the production of both Infrared and visible wavelength opticsusing the NION machine tool are provided. Recommendations for future work toimprove both; the understanding of the processes and the effectiveness of applying theprocesses are suggested.
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