Investigation on the integrated approach to design and ultraprecision machining of freeform surfaced optics and its implementation perspectives

Abstract

Over the last two decades or so, high precision freeform surfaced components and devices have been drawing the increasing attention by the industry due to their potentials in fulfilling demands for various engineering and consumers applications, such as consumer electronics, biomedical engineering, ophthalmic optics, automotive, electro-optics, aerospace engineering and mobile communications. Meanwhile, ultraprecision manufacturing technology is becoming one of the most effective methods for manufacturing high precision freeform surfaced components with functional features. Therefore, scientific understanding of ultraprecision manufacturing for freeform surfaces is essential and much needed, particularly for robustly fulfilling the gaps between fundamentals, technological innovations and their industrial scale applications. This doctoral thesis is focused on investigating a NURBS (Non-Uniform Rational B-Splines) based integrated approach to design, manufacturing and assessment of freeform surfaced optics and its implementation and application perspectives. Therefore, this doctoral research objectively covers NURBS based modeling and analysis of freeform lenses combining with e-portal development for customization, virtual lens conception and ray tracing simulation assessment, NURBS based toolpath generation and analysis considering design for manufacturing, micro cutting mechanics and the ultraprecision process, dynamic cutting forces modelling, and freeform surface topography generation and characterization, further supported by simulations and experimental trials. The research reveals the integral process of design and manufacturing of freeform surfaced optics involves meticulous steps, including optic surface modeling and analysis, optic surface design, ultraprecision machining toolpath generation, simulation in both optical performance and machining cutting force on design for high precision manufacturing aspect, optic surface assessment in the digital mode, ultraprecision manufacturing physically, and quality assessment. Throughout the process, NURBS based modelling and analysis are the kernel, on which high precision is pursued and assured by the modelling/algorithms and the associated ultraprecision technology protocol. An integrated approach is developed and implemented through the web-based e-portal, for customized precision design and manufacturing of freeform surfaced varifocal lenses. The e-portal is specifically designed to meet the stringent demands of personalized mass customization, and to technically render a highly interactive and transparent experience of the lens design and manufacture for the lens users. By using Shiny and R-script programming for the e-portal development and combining COMSOL Multiphysics for the ray tracing simulation, the e-portal leverages open-source programming to provide the design responsiveness, manufacturing agility and accessibility. Furthermore, the integration of R-script and Shiny programming allows for advanced interactive information processing online, which also enables the e-portal driven ultraprecision manufacturing system for personalized freeform surface lenses. Cutting force is a pivotal parameter in the ultraprecision machining process. However, scant emphasis is placed on elucidating the nuances of cutting forces and the associated cutting dynamics in ultraprecision diamond turning of freeform surfaces particularly using fast and/or slow tool servo modes. Theoretical analysis on the cutting force and its modelling are carried out in the ultraprecision diamond turning of freeform surfaces, particularly considering constant variations of cutting forces along the freeform surface curvature and the increasingly stringent requirement on high precision optical surface finishing. The cutting forces modelling is based on further developing the improved Aktins model while taking account of the influence of shear angles varying constantly along the freeform surface machining. Based on the toolpath data of the cutting process at the freeform surface, the depth-of-cut of the surface, curvature variations, and shear angle variations throughout the process are meticulously analyzed. Subsequently, the cutting force modelling is developed to discern the nuances of the cutting motion by analyzing the cutting toolpath, and thus enabling the prediction of cutting forces variation during the cutting motions with a diamond cutting tool. Finally, an approach for examining the correlations between cutting forces and the surface texture, and surface texture aspect ratio is developed and further investigated, particularly against the functional performance of a freeform surface and its generation in ultraprecision machining. The investigation is also evaluated and validated by industrial application data. The analysis and characterization of the freeform surface is essentially mandatory for the ultraprecision manufacturing process due to its high-precision ‘deterministic manufacturing’ nature and the ability to producing the manufacturing outcome without any additional process. The machine tool trajectory is remained on the surface and can be observed with high-accuracy metrology equipment, which makes the surface topography characteristics containing more valuable information as required for optics surface performance. The above-mentioned surface assessment protocol is developed as a part of the integrated approach, in which the surface texture aspect ratio is investigated particularly the relationships between the surface texture height variation and lateral feature, the underlying micro cutting mechanics affecting their formation and generation in the process, and the resultant optical performance of the freeform surface. Nanometric surface measurement techniques and 3D surface parameters are further explored to quantify the surface texture aspect ratio and assess its correlation with the surface optical performance. Experimental results demonstrate there is a significant correlation between the higher surface texture aspect ratios and increased aberrations, leading to decreased optical quality. Controlling the surface texture aspect ratio during the machining process is crucial for achieving the optimal surface functional performance. The research results above contribute well to the understanding of how the surface texture aspect ratio affecting the performance of freeform surfaced optic components, and provide insights for design and manufacturing of high-performance optical components, although optimization work is further needed in optics surface functionality and optical system design.