Ph.D. Dissertation Defense by Kai Liu
Friday, October 7, 2005
(Dr. Shreyes N. Melkote, Chair)
" Process Modeling of Micro-cutting Including Strain Gradient Effect "
In mechanical micro-machining, the achievable accuracy/precision, and surface finish are greatly influenced by the plastic deformation of the workpiece during chip formation. However, the mechanisms that cause size effect and mismatch in surface finish prediction in micro-machining are not completely understood. At micrometer length scales of material removal, a steep strain gradient and a large amount of plastic deformation within the primary and secondary deformation zones are unique features of micro-cutting. Thus the focus of this thesis is to investigate the strain gradient strengthening effect at the micro/meso scales and its influence on the prediction of chip formation, specific cutting forces, as well as surface finish in micro-cutting.
A thermo-mechanical coupled finite element formulation based on commercially available software, ABAQUS @ /STANDARD, and incorporating strain gradient plasticity was first built to simulate orthogonal micro-cutting process. Three main factors, strain gradient effect, temperature effect and edge radius effect were analyzed quantitatively to study their relative contribution to size effect and dominant regions. Surface finish of micro-machining is found to be influenced by three major sources, the kinematics surface roughness, cutting edge roughness and the surface roughness due to the plastic deformation of the material being pushed to the side by the secondary cutting edge. The surface roughness due to plastic side flow is found to be important at very small values of feed and could explain the observed discrepancy between the theoretical and measured surface roughness in micro-cutting. A proposed semi-empirical model is shown to provide good prediction of surface finish in micro-cutting process.