Search

In the traditional machining method, the surface quality will get worse during the surface figuring process while the surface figure will be destroyed after ultra-smooth polishing process. To obtain an ultra-precision optical surface, these two processes are conducted iteratively which is time-consuming and unstable. A new fabrication method combined with the magnetorheological finishing (MRF) and hydrodynamic effect polishing (HEP) technology was presented. The surface figure error can be quickly depressed by MRF due to its high deterministic figuring ability, while the surface quality will get worse due to the introduction of MRF polishing marks. The material removal of HEP depends on the chemical interaction between the nanoparticles and workpiece in elastic mode, and an ultrasmooth surface with sub-nanometer level can been easily achieved with the surface figure well maintained. The experiment results show that the MRF polishing marks are removed clearly and the surface figure is well maintained after HEP process. Combined with MRF, a quartz glass with initial figure accuracy of 0.415 λ RMS (λ = 632.8 nm), mid-spatial frequency and high spatial frequency roughness of 0.407 nm RMS and 0.525 nm RMS has been improved to figure accuracy of 0.05 λ RMS, mid-spatial frequency and high spatial frequency roughness of 0.268 nm RMS and 0.163 nm RMS. Power spectral density (PSD) analysis indicated that the surface spatial frequency error has all been greatly depressed. It demonstrates HEP is an effective ultra-smooth polishing method with good ability to maintain the initial surface figure. [ABSTRACT FROM AUTHOR]

The fabrication of high-quality freeform surfaces is based on ultra-precision diamond turning with fast tool servo (FTS) technology which allows direct machining of the freeform surfaces with sub-micrometric form accuracy and nanometric surface roughness. Surface roughness is an important factor in evaluating the performance of the optical freeform surfaces. This paper presents a theoretical and experimental analysis of surface generation in ultra-precision single-point diamond turning. In this model, we take into consideration the basic machining parameters as well as the relative vibration between the workpiece and the tool in both the cutting and feeding directions. Theoretical model is built to predicting the surface roughness of machined flat surface as well as freeform surfaces. A series of experiments have conducted and the results show good correlation between the theoretical model and the fabricated surfaces. [ABSTRACT FROM AUTHOR]

Abstract: Subsurface damage generated in manufacturing processes directly influences performances of optical elements, however, the rapid inspection of subsurface damage is unresolved for it is covered under surface. For the purpose of rapid, accurate, and non-destructive measurement of subsurface damage produced in optical grinding and lapping processes, a theoretical model of relationship between subsurface damage and surface roughness was established through investigating median and lateral crack system in brittle surface induced by sharp indenter, and contribution of elastic stress field to the median crack propagation was also considered in the loading cycle. With this model, subsurface damage depth can be predicted accurately via measuring surface roughness of manufactured optical elements. The subsurface damage depth and surface roughness of ground and lapped BK7 glass were measured by MRF (magnetorheological finishing) spot technique and contacting profilometer, respectively, in order to verify the validity of the relationship model. The results show that the relationship model is convenient and accurate in predicting the subsurface damage depth. And there exists monotone increasing non-linear correlation between subsurface damage depth and surface roughness ( value) in optical grinding and lapping processes. At last the relationship between subsurface damage depth and process conditions in grinding and lapping processes and material mechanical properties is discussed. [Copyright &y& Elsevier]

Hydrodynamic effect polishing (HEP), in which the material is removed in elastic region by chemical impact reaction, can effectively remove the surface/subsurface damage to get an ultrasmooth surface. However subsurface damage contains scratches and machining residual stress are hidden below the surface and can’t be easily detected. The subsurface/surface properties such as hardness and elastic modulus have direct relationship with the subsurface damage. Therefore, a nano-imprint method analysis of the optical subsurface quality was presented to evaluate the hydrodynamic effect polishing process effect. Polishing experiment was conducted on a conventional pad pre-polished silica glass. The surface before and after polishing process was firstly observed by atomic force microscope and ZYGO white-light interferometer. The observation shows the surface damage such as the scratches and bump structures are completely removed by HEP process, and the surface roughness rms value decreased to near 0.3 nm. To evaluate the subsurface damage, nano-imprint was conducted on the processed and unprocessed area under different load with different impressed depth. The imprint results show the processed surface become soft and easily be impressed, while the unprocessed surface becomes hard. The processed and unprocessed surface shows the same impress depth when impressed depth exceeded 200 nm. Analysis shows the surface becomes hard under the effect of the residual stress in subsurface damage which is generated in the traditional machining process with material removal in brittle or plastic mode. Material is removed in elastic mode by chemical impact reaction in HEP, and the surface/subsurface damage can be effectively removed. It demonstrates that HEP processed surface properties are much close to the bulk material. [ABSTRACT FROM AUTHOR]