Enhancing the surface finish of single point diamond turning
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Overview
Overview
abstract
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. 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. 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
R0 • 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.
