An empirical survey on the influence of machining parameters on tool wear in diamond turning of large single crystal silicon optics

The research described in this paper is a continuation of the collaborative efforts by Lawrence Livermore National Laboratory (LLNL), Schafer Corporation and TRW to develop a process for single point diamond turning (SPDT) of large single crystal silicon (SCSi) optical substrates on the Large Optic Diamond Turning Machine (LODTM). The principal challenge to obtaining long track lengths in SCSi has been to identify a set of machining parameters which yield a process that provides both low and predictable tool wear. Identifying such a process for SCSi has proven to be a formidable task because multiple crystallographic orientations with a range of hardness values are encountered when machining conical and annular optical substrates. The LODTM cutting program can compensate for tool wear if it is predictable. However, if the tool wear is not predictable then the figured area of the optical substrate may have unacceptably high error that can not be removed by post-polishing. The emphasis of this survey was limited to elucidating the influence of cutting parameters on the tool wear. We present two preliminary models that can be used to predict tool wear over the parameter space investigated. During the past two and one-half years a series of three evolutionary investigations were performed. The first investigation, the Parameter Assessment Study (PAS), was designed to survey fundamental machining parameters and assess their influence on tool wear [1]. The results of the PAS were used as a point-of-departure for designing the second investigation, the Parameter Selection Study (PSS). The goal of the PSS was to explore the trends identified in the PAS in more detail, to determine if the experimental results obtained in the PAS could be repeated on a different diamond turning machine (DTM), and to select a more optimal set of machining parameters that could be used in subsequent investigations such as the Fluid Down-Select Study (FDS). The goal of the FDS was to compare the performance of water, polyalkaline glycol and HT200 as a cutting fluid for SCSi, and select a single baseline cutting fluid from this set for use in future SCSi SPDT efforts. Synopses of the experimental work and results of the PAS and PSS are now presented