Micro-mechanical machining using diamond tools is advantageous to other micro-manufacturing techniques as it can create complicated geometric features with high precision, and it is applicable to a wide range of materials including polymers and metals. The effectiveness of manufacturing micro-scale features can be enhanced by understanding the process and interactions between the tools and materials.
Investigation of material properties with respect to the machining process is especially important when the material has unique mechanical properties such as polymeric carbon nanotube (CNT) nanocomposites with aligned CNTs and when the process itself is a unique such as an elliptical vibration machining (EVM) process. This study has sought to develop a micro indenter-scriber system for investigation of mechanical machining characteristics of the injection molded polymeric CNT nanocomposites, identifying parameters to model the micro scribing process. The study also aimed to develop a versatile EVM system with motion amplification without limiting the operation frequency to the natural frequencies of the system. The developed system was used to investigate its range of motion and effects of vibration frequency to reduce machining forces.
A custom micro indenter-scriber system was developed, capable of 3-axis nanometric displacements and 3-axis force sensing, along with a Berkovich diamond tool with known geometry. Polystyrene based multi-walled CNT – polystyrene (MWCNT-PS) composite samples at varying CNT concentrations (0, 0.5, 2.0 and 5.0 wt.%) were prepared through micro injection molding procedures to align CNTs in the composite. Using the custom indenter-scriber system, indentation and scribing experiments were performed to identify the indentation hardness, modulus of elasticity and scribing forces in varying concentrations and orientations of CNTs.
Indentation experiments have found that CNT composite samples have increased hardness with small addition of CNTs (i.e. 0.5 wt.%); however, adding more CNTs (i.e. 5.0 wt.%) would result in decreased hardness. Scribing forces were revealed to have similar trends, showing the highest scribing forces at 0.5 wt.% of CNT concentrations but less at 5.0 wt.%. The orientation of CNTs also affected the forces, as higher forces were observed when scribing perpendicular to the CNT flow direction than scribing parallel to them. A micro scribing force model was proposed to predict forces from given material properties, then three unknown parameters, namely the shearing coefficient, the plowing coefficient and the adhesion friction coefficient were identified from experimental scribing tests. A relationship was established between three parameters with respect to the material properties, which would enable prediction of scribing forces from material properties.
To minimize forces in micro machining, a versatile EVM system was developed which consist of a diamond tool, two piezoelectric actuators, flexure joints and levers capable of amplifying vibrational motions. A mathematical toolpath generation algorithm was developed and compensated through experiments. Using the EVM system, experiments were performed at varying frequencies (i.e. 0, 1, 10 and 50 Hz) to measure the vertical and horizontal forces. It was found that increasing the vibration frequencies reduced the forces. Its surface pattern generation ability was also tested at a relatively high feed rate (300 µm/s) to examine whether the system can accurately fabricate dimples of 5 and 20 µm depths.
Understanding micro scribing force model of polymeric CNT nanocomposites provides fundamental understanding in the relationship of cutting forces and material properties, while the development of new EVM system and understanding of its mechanism provides a basis for optimization of machining parameters of more accurate, precise and efficient micro mechanical machining. This knowledge will be applicable not only to conventional materials but to hard-to-cut materials such as glass, ceramics and polymeric CNT nanocomposites, providing accurate and economic methods to fabricate micro channels for biomedical applications, micro thermal exchanger, micro electromagnetic interference shield, micro optical lens and various functional surfaces with geometrically complex features.