Theoretical and experimental investigation of 3D-structured surface generation by computer controlled ultra-precision polishing

Abstract

Three-dimensional (3D) structured surfaces have been widely used in different applications such as self-adhesive sensors, compound lenses for holography in phonics products, bionic applications, bio-medical areas, etc. Although there have been some research studies on the manufacture and machining of 3D-structured surfaces, the research work is still far from complete. The current fabricating process, for instance, laser and etching, for 3D-structured surface generation are expensive and time-consuming. Computer Controlled Ultra-precision Polishing (CCUP) is capable of fabricating ultra-precision freeform surfaces with sub-micrometre form accuracy and surface roughness in the nanometre range. CCUP consists of Mechanical Polishing (MP) and Fluid Jet Polishing (FJP) processes. Although polishing is a historic machining process, research on the polishing mechanics and surface generation during the polishing process is still far from complete. Polishing is a multi-step process that involves a sequence of steps with different process and material parameters. “What parameters should be used for the next step is always a question asked by operators. They make their decisions by measuring the surface after each polishing step. Moreover, the achievement of desirable polished surfaces still depends largely on the expensive and time-consuming trial-and-error approach when new materials, new designs of 3D-structured surfaces or new machine tools are used. An appropriate modelling and simulation system can help to simulate and predict the effect of different factors on the surface generation in CCUP so as to improve the process by the reduction of cost and machining time. Most of the previous research work has mainly focused on the control of surface finish, material removal rate, and form errors of a surface polished in a single and particular polishing step. Research on the modelling and optimization of the surface generation of 3D-structured surfaces and how the surface generated in each step interacts mutually have received relatively little attention. As a result, further research work is necessary to better understand the surface generation mechanism for CCUP. The present study aims to conduct a theoretical and experimental investigation of the process of the surface generation of 3D-structured surfaces by using CCUP. A series of experiments was conducted to study the mechanisms and factors affecting the surface generation of 3D-structured surfaces by CCUP. The results of the experimental investigations show that it is technically feasible to generate 3D-structured surfaces by using CCUP. The generation of 3D-structured surfaces is mainly affected by the sequence of polishing steps, the polishing tool trajectories and the spacing between successive polishing. In the present study, it was found that there exists a limit on the size of the 3D-structured surfaces which depends on the spacing between successive polishing. Hence, a surface topography simulation model was established for the modelling of the surface generation and optimization of the polishing strategy in the CCUP of 3D-structured surfaces. The development of the model is based on a tool path generator for 3D-structured surfaces together with a surface topography simulation model as well as an optimization model for the polishing strategy. The tool path generator is used to generate the polishing tool path, based on the design of the 3D-structured surfaces, the polishing trajectory and the polishing parameters. The surface topography simulation model takes into account the cutting mechanics, material factors and kinematics characteristics of the polishing processes. The model allows the optimum polishing strategy to be determined. The performance of the model was evaluated through a series of simulation and polishing experiments. The 3D-structured surfaces generated were found to possess improved functional characteristics such as wettability. The surface topography simulation model was able to simulate the surface generated at each step in order to predict the final polished 3D-structured surfaces. It was also successfully verified through a series of simulation and practical polishing experiments. The results of the experiments demonstrate the capability of the surface topography simulation model to predict the pattern of 3D-structured surfaces by using CCUP. The success of this study not only contributes significantly to a better understanding of materials removal characteristics and the factors affecting surface generation but also leads to the development of a novel process, which is more cost-effective and less time-consuming than other machining processes, for the fabrication of 3D-structured surfaces with functional characteristics. As polishing is a multi-step process, a model-based simulation system for a process chain analysis has been developed to predict and simulate surface topography at each step of polishing and how the surface topography affects the steps afterwards. This can largely reduce the time for in-process inspection of polished surface after a single polishing step. The surface topography simulation model developed in this study makes the polishing process more predictable and this provides an important means for the optimization of process planning for the generation of 3D-structured surfaces by CCUP.