Enhancing the surface finish of single point diamond turning .

摘要

Ultra precision single point diamond turning (SPDT) is a machining process used to produce optical grade surfaces in a wide range of materials. Aluminum is of primary interest as a workpiece material because it is easily diamond turnable, highly reflective and corrosion resistant. The cutting tool used is made from a single crystal diamond honed to a very sharp cutting edge. The machines used in this process are extremely precise and stiff. The nature of the cutting parameters used in SPDT changes the process physics substantially over conventional machining. The underlying reason relates to the relative size of the uncut chip thickness and the cutting edge radius of the tool in comparison to the grain size of the workpiece. When performing SPDT, there is a functional limit to the achievable surface finish. This is predominately due to material side flow and the opening up of material defects. Thus the machined' surfaces have to undergo post processing operations like lapping or polishing, which increase cost and production time. Thus, the objective of this study was to improve the surface finish of the SPDT process to minimize the amount of post processing.;A special tool with a flat secondary edge was then developed to address the remaining side flow issue for planar surfaces. The combination of thermo-mechanically produced ultra fine grained material with the special tool provided a substantial reduction in surface roughness from values typically reported at 3nm [Roblee, 2007] Ra to 0.75nm Ra. In addition to this the use of the custom designed tool can improve the productivity associated with machining a flat face by a factor of one hundred times by allowing the feed rate to be increased while still achieving the desired surface finish. v;The approach involved addressing the ratio between the tool cutting edge radius and the microstructure. Realizing the limitations associated with sharpening a diamond tool further, efforts have been made to mechanically or thermo-mechanically induce dislocations into the workpiece to refine the microstructure and in so doing enhance machinability. As dislocations act as a point of defect, it is observed that higher dislocation density offers less side flow and leads to better surface roughness.