Recent advancements in machine tools for single point diamond turning have considerably enhanced the quality of machined surfaces available. The latest SPDT machines produce optical surfaces suited for visible wavelength applications. This article covers the quality of present SPDT surfaces and compares them with criteria for visible wavelength surface specifications.
Criteria for optical surface specifications are application related. These specifications are related to performance parameters impacted by surface errors of associated spatial frequencies. Long spatial wavelength errors, normally referred to as surface distortion or irregularity, influences system wavefront performance. Surface scatter is accounted to short spatial wavelength errors such as surface roughness. Separate criteria are normally established for these two performance parameters.
Marechal’s Criteria is one typical specification for the performance of an optical system. Based on this criteria, a permissible value is fixed on diffraction focus intensity. Even though translating system wavefront specifications is specific to the individual system, one can expect form errors on the order of system wavefront criterion.
Based on Marechal’s suggestion that a well corrected optical system has a normalized intensity at its diffraction focus of at least 0.8, surfaces must have a form error no larger than λ/14 rms to be compliant. It is a challenge to establish a criterion for scatter specification due to its high application sensitivity. In contrast to polished surfaces, whose suitability is defined only by roughness, such as Ra or rms, directly machined surfaces have added considerations of lay and periodicity.
Classical Scalar Scattering Theory
One method for establishing criteria for scattering intensity is applying classical scalar scattering theory. Using this theory, a roughness height tolerance can be specified by its relationship to total integrated scatter. For a total integrated scatter of 1/10th of 1%, a value consistent with the obscuration ratio typical of 60/40 cosmetic surface quality, then the rms surface roughness specification required would be around λ/400. This is equal to a surface roughness of 16 Å rms at 0.6328 µm wavelength.
Advancements in SPDT Machine Tools
Advancements in SPDT machine tools have directly allowed machined surfaces to attain these criteria for visible wavelength applications. It is possible to classify these advancements as enhancements in machine tool mechanical designs, environmental (thermal and vibrational) considerations, and servo/feedback control systems.
The key advancement in SPDT machine tool design is using oil hydrostatic slides. The present generation of modular slide designs includes completely constrained symmetrical bearing cross-sections, with higher damping characteristics and stiffness than prior air bearing designs. Eliminating roller bearings allows considerable reduction in short term slide motion errors and overall slide straightness improvements. The latest slide design makes use of an air-isolated capstan drive system to remove lead screw coupling effects.
In order to obtain directly machined surfaces suited to visible wavelength applications, environmental consideration are critical. Present designs address thermal effects by using large thermal mass synthetic "granite" machine bases, servo controlled environmental enclosures, active laser refractometry, aerostatic work holding spindle athermalization techniques and thermally insensitive metrology reference axes. In present-day designs, machine tool vibrations have been reduced by using vibration isolation, oil hydrostatic slide designs, and using materials with high damping properties.
Enhancements in servo control and feedback systems have enabled the production of better SPDT surfaces. Distributed processing, and the use of improved interpolation schemes, have considerably improved machine tool positioning accuracy. Machine tool positioning has also been improved due to the use of next-generation laser interferometers, with resolutions in the nanometer range, as feedback devices.
Several SPDT surfaces will be studied to show the effects of these machine tool advancements. By showing favorable comparisons with the above criteria, these surfaces show that they can be used for visible wavelength optical applications.
This information has been sourced, reviewed and adapted from materials provided by Precitech.
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