Modelling and analysis of material removal characteristics in computer controlled ultra-precision polishing

Abstract

Computer Controlled Ultra-precision Polishing (CCUP) based on fluid jet polishing (FJP) and bonnet polishing (BP) with multi-axis machining is an enabling technology which can fabricate ultra-precision freeform surfaces with sub-micrometre form accuracy and surface roughness in the nanometre range, especially for the fabrication of difficult-to-machine and ferrous materials, which are not amenable using other ultra-precision machining technologies such as single-point diamond turning and raster milling. However, CCUP is a complex process which relies heavily on the planning of the process steps in terms of the use of the polishing conditions and polishing fluid on specific materials being cut. It is found that the material removal analysis is clearly a fundamental element in achieving the corrective polishing and optimization of the polishing process. Nowadays, the acquisition of tool influence functions still depends largely on the expensive trial-and-error approach when new materials, new surface designs or new polishing parameters are used. Although studies on polishing mechanisms and nano-mechanics are still sparse, there is a need for methods and tools for modelling and simulation which can simulate and predict the effect of different polishing parameters on the material removal characteristics and surface generation in CCUP. To meet this need, this thesis describes a theoretical and experimental study of material removal characteristics and surface generation in CCUP. It is divided into three parts. Firstly, Taguchi trials were conducted to identify the optimal level of combination and the significance of the individual operational parameters in fluid jet polishing and bonnet polishing. Hence, a series of experiments was performed to investigate the effects of the process parameters on the material removal characteristics in fluid jet polishing (FJP) as well as the material removal mechanism in bonnet polishing. The results not only provide an important means for better understanding the effect of the factors affecting the polishing process, but also provide the basis for the establishment of theoretical models for the prediction and simulation of the material removal characteristics and surface generation in CCUP. Secondly, a comprehensive computational fluid dynamic (CFD)-based erosion model was developed by a combination of CFD simulation, erosion model, and experimental research so as to predict the material removal characteristics and surface generation in fluid jet polishing. In the computational fluid dynamic simulations, an Eulerian-Eulerian-Lagrangian method is used which treats the water and air as an Eulerian phase and the particles as Lagrangian particles. The coupled discrete phase model (DPM) and the volume of fluid (VOF) model are used to describe the multiphases in the FJP process. This CFD model also presents the application of the k-w model, together with the level set method, to describe the slurry/air interface in the commercial finite element analysis package FLUENT. After solving the stream flow characteristics, the motion of individual particles is also calculated in order to obtain more accurate predictions of the trajectories of individual particles impacting on the surface. The predicted results are used to integrate with the developed erosion model for the prediction of the material removal characteristics. Hence, the polishing path planning is determined based on the desired surface integrity of the optical surface to be generated using the data of the material removal characteristics. Finally, a theoretical model is built for predicting and simulating the surface generation in FJP. A series of spot and pattern polishing tests as well as simulation experiments by the theoretical model were conducted. The results not only show that the theoretical model predicts well for the surface generation under different polishing conditions but also helps to gain a better understanding of the polishing process in FJP. In the third part, a multi-scale theoretical model was established for predicting and characterizing the material removal characteristics and surface generation in bonnet polishing based on contact mechanics, kinematics theory and abrasive wear mechanism. Specifically, the pressure and velocity distributions are determined based on the kinematics theory and contact mechanics at the macro scale, and the pad topography which affects the contact ratio and hence the material removal rate at the micro scale, while the micro- or nano-sized abrasive particles scratch the surface at the nano scale. The model developed in this study attempts to capture much of the basic physics of the polishing process including the tool radius, polishing depth, head speed, precess angle, pad topography, polishing time, particle shape, slurry concentration, and the mechanical properties of the pad and workpiece. Experimental results show that the theoretical model predicts well that the material removal amount which increases with increasing precess angle and tool offset, depends linearly on the head speed and slurry concentration. A pattern test was also conducted to validate the surface generation model. The originality and significance of this research lies in the provision of CFD-based modelling of the material removal characteristics and surface generation for fluid jet polishing and multi-scale modelling of the material removal characteristics and surface generation for bonnet polishing. The successful development of the theoretical model helps greatly to make the CCUP process more predictive, so as to further optimize the manufacturing process for different work materials without the need for costly trial and error polishing tests. Moreover, this is the first of its kind in which the deterministic model has been successfully built which acquires much of the basic physics of the polishing process. This contributes significantly to form the theoretical basis for the better understanding of the polishing process.