PRECISION POLISHING DYNAMICS: THE INFLUENCE OF PROCESS VIBRATIONS ON POLISHING RESULTS
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Abstract
The optical pitch polishing process has been used over 300 years to obtain high quality optical surface finish with little subsurface damage. A pitch tool consists of a metal platen coated with a layer of polishing pitch whereby pitch is a highly viscoelastic material. In polishing the workpiece is rubbed against the tool while abrasive slurry is supplied in between them. During polishing the workpiece is subjected to process vibrations, whereby be these vibrations are generated by the machine itself due to moving parts, or that are transmitted from the shop floor through the machine to the workpiece. To date, little is available in the public domain regarding the role of process induced vibrations on polishing outcomes. This research investigates such vibrations, how they transfer through the pitch layer on the tool, and ultimately how they affect the material removal rates and surface finishes obtainable on fused silica workpieces. Fundamental understandings with respect to the process vibration will reduce the heuristic nature of pitch polishing and generate deterministic polishing outcomes.Key findings include the following. The pitch selection has little influence on the magnitude or range of process vibrations transmitted through the tool to the workpiece in the 1 Hz to 16 kHz range. Within the same frequency bandwidth the recorded process vibrations are in the range of 0.2 to 10 nm and the main factors found to affect their magnitude include; the polishing machine itself, process speeds, and the use of passive damping materials in the tool construction. Material removal rates and surface finishes obtained on fused silica workpieces were found to be sensitive to the extent of the process vibrations. Up to 30% changes in the material removal rates were observed with increasing vibrational magnitudes. The higher level vibrations were also found to have a negative impact on the finishes obtained in the lower spatial domains. Additional testing on a specifically made test-bed demonstrated a linear correlation between the material removal rates and the vibrational power input. This relationship was further explored by adding external vibrational sources to an existing machine, and as expected the increased vibrational power resulted in 80% higher material removal rates. The results from this experimental work facilitated Dr. Keanini's development of a vibrational based material removal model. Additional polishing tests combined with surface topography analysis of both hard and soft pitch tools demonstrated the robustness of the proposed model to accommodate the influence of different pitch grades.The summary in general is that in pitch polishing the process vibrations are important to monitor and control for process optimization.