The aim of this study is to investigate optimal air-driving fluid jet polishing (FJP) parameters by using Taguchi's method to improve surface roughness of N-BK7 optical glass on a machining center. An orthogonal array and the signal-to-noise (S/N) ratio are employed to determinate the optimal polishing parameters, and analysis of variance (ANOVA) is used to identify the main parameters that affect the surface roughness of the N-BK7 optical glass. An air-driving FJP tool is newly designed and fabricated to conduct experiments. To determinate the optimal air-driving FJP parameters, six polishing parameters, namely air pressure, impact angle, standoff distance, the abrasive material, abrasive concentration, and polishing time, are selected as the control factors of experiments. Based on the Taguchi's L₁₈ orthogonal array experimental results and the S/N ratio, the optimal parameters for the N-BK7 optical glass are found. These optimal parameters are to be as follows: an air pressure of 0.490 MPa, an impact angle of 40 deg, a standoff distance of 12 mm, the abrasive material of Al₂O₃, an abrasive concentration of 10 wt. %, and a polishing time of 30 min. The surface roughness of specimen is improved from R_a = 0.350 μm–0.032 μm by using the optimal air-driving FJP parameters. In addition, the determined optimal polishing parameters for the plane surface are applied to the surface finish of an N-BK7 spherical lens, and the surface roughness of the spherical lens can be improved from Ra = 0.421 μm to 0.202 μm within an area of 283.6 μm × 200 μm.

As the demand for optical glasses has increased, precision requirements for specific shapes, forms, surface textures, and sizes (miniaturization) have also increased. The standards and surface finishes applied to the reference mirrors used in measuring appliances are crucial. Hence, enhancements in figuring and surface finishing are indispensable to manufacturing industries. In this article, a novel self-propelled multi-jet abrasive fluid polishing technique is proposed for an ultra-precision polishing process in which a blade-less Tesla turbine was used as a prime mover. The turbine was characterized by high swirling velocity at the outlet; therefore, high levels of kinetic energy moving away from the turbine were used as polishing energy. Computational fluid dynamics (CFD) was also used to simulate the flow on the turbine blades. With a newly designed and manufactured polishing tool, the optimal polishing parameters for improving the surface roughness of crown optical glasses (N-BK7) were investigated. Taguchi's experimental approach, an L₁₈ orthogonal array, was employed to obtain the optimal process parameters. An analysis of variance (ANOVA) was also conducted to determine the significant factors. The surface roughness has been improved by approximately 94.44% from (R_a) 0.36 μm to (R_a) 0.02 μm. This study also presents a discussion on the influence of significant factors on improving surface roughness.