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The HK grism was diamond machined on the Precision Engineering Research Lathe (PERL) at LLNL. Chipping of the grooves increased from moderate to severe as the cutting progressed resulting in excess scattered light and reduced diffraction efficiency. High magnification optical microscopy and SEM of the cutting edges indicated damage to the tool caused by wear.
A comparison of the outcomes of ZnSe gratings and grisms machined at LLNL indicated that chipping was minimal in low blaze angle cuts but moderate to severe with the blaze angle near 45° as in the HK grism. Vendor records showed that the (100) crystal planes of the diamond were aligned parallel to the tool shank. Therefore the (100) planes are closely aligned with the cutting edge in low blaze angle tools but 45° off in the HK tool. We believe that this misalignment of the cutting edge with the (100) crystal plane in the HK tool produced excessive tool wear resulting in the chipped grooves observed.
Technique for diamond machining large ZnSe grisms for the Rapid Infrared/Imager Spectrograph (RIMAS)
Previous fabrication of ZnSe grisms on the Precision Engineering Research Lathe (PERL II) at LLNL has demonstrated the importance of surface preparation, tool and fixture design, tight thermal control, and backup power sources for the machine. The biggest challenges in machining the RIMAS grisms are the large grooved area, which indicates long machining time, and the relatively steep blaze angle, which means that the grism wavefront error is much more sensitive to lathe metrology errors. Mitigating techniques are described.
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