Study of miniature features generated by backside patterned texturing in precision diamond machining

• Main Author: Ahmed Syed Adnan
• Other Authors: Yeo Swee Hock
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Manufacturing::Product design
• Online Access: http://hdl.handle.net/10356/73729
• Institution: Nanyang Technological University

Structured and functional surfaces are key components in many advanced technological applications, such as electronics, information technology, energy, optics and tribology. A gamut of products has emerged where the surfaces are specially textured or engineered for a particular function. These textured surfaces rely on the control of surface characteristics to obtain the desired functional performance. Various techniques and methods have been used to develop micro-scale functional surfaces. For example, ultraprecision single point diamond turning (SPDT) and its associated processes, such as fast tool servo (FTS) and slow slide servo (SSS) have been used to fabricate various types of micro-sized surface textures. However, these techniques have limitation to produce surface features at submicron scale. In addition, the existing associated methods of SPDT require a complex tool path programming for the tool-workpiece synchronized motion for asymmetric and freeform surface generation. This thesis attempts to address the limitations of SPDT and its associated specialized techniques and to propose a potential alternative: backside patterned texturing (BPT) for submicron to micro-sized surface features generation on the diamond machined surface. The proposed novel technique of BPT utilizes the pre-fabricated macro-features on the backside of work material, and thereafter the front side is face turned with a single point diamond tool. The pre-fabricated patterns divide the entire workpiece into thick and thin sections. Unlike existing texturing methods of SPDT, BPT produces textured surfaces from submicron to micro-scale and without any external electromechanical system for synchronized tool-spindle motion or vibration-assisted machining. The diamond machining induces residual stresses at the surface and subsurface of the machined surface. The workpiece attached to the machine remains flat due to the displacement constraints provided by workpiece holding mechanism like a vacuum chuck. Upon removal of the workpiece from the machine, the induced residual stresses in the machined workpiece achieve a new state of equilibrium and consequently, the deformation of the overall newly formed surface is taken place. The thinner section of the workpiece experiences relatively larger deformation, which leads to the development of the associated texture on the front side of machined workpiece. The machining experiments are conducted for different backside pattern geometries and workpiece thicknesses. The effects of backside patterns, workpiece thickness and cutting speed on the texture formation are investigated. To demonstrate the efficacy of the method, various types of freeform surface textures, such as an array of convex shapes bumps, waterdrop, spiral and cylindrical freeform surfaces are fabricated. The novelty of the proposed technique of BPT demands the explanation of the surface deformation phenomenon during texturing process. Therefore, along with the experiments, a mathematical formulation and a finite element (FE) model are developed to predict the surface deformation in BPT. The mathematical formulation requires some preliminary machining tests for an arbitrary type of surface texture, which gives an estimation of the source residual stresses in the plastically deformed layer of the machined workpiece. The known source residual stresses are then taken as an input to predict the surface deformation for the subsequent axisymmetic and doubly symmetric texture geometries produced with similar machining conditions. The simulation results were compared with the experiments that showed a good agreement and validated the proposed methodology. Finally, a finite element (FE) model is developed to simulate the surface deformation in the surface texture produced by BPT. The FE model was based on the arbitrary Lagrangian-Eulerian (ALE) mesh motion scheme to simulate diamond turning (DT) in order to extract the machining-induced residual stress (RS) profile as a function of workpiece thickness. Subsequently, the estimated RS profile is mapped to 2D and 3D FE models to predict the relative surface deformation. The simulated results were found promising compared to the experiments and the similar trends of surface deformation were obtained for various thicknesses of the workpiece material.