Machine tools used for single point diamond turning have seen some changes in terms of technology advancements, greatly improving the quality of machined surfaces. The latest generation of SPDT machines is known to produce optical surfaces that are ideal for several visible wavelength applications.
This article talks optical surface specifications, where the specifications are based on performance parameters impacted by surface errors of related spatial frequencies.
Long spatial wavelength errors, also known as surface distortion or irregularity, impact the wavefront performance of the system. Surface scatter is caused by short spatial wavelength errors i.e., surface roughness. Generally, individual criteria are recognized for these two performance parameters.
Maréchal’s Criteria
Maréchal’s criteria are a classical specification for the performance of an optical system. It sets an acceptable value on diffraction focus intensity. It is logical to expect form errors to be on the order of system wavefront criterion despite the fact that the translation of system wavefront specifications to permissible surface form tolerances is specific to the individual system.
According to Maréchal, the normalized intensity of an accurate optical unit at its diffraction focus should be at least 0.8, and surfaces should have form error usually not greater than λ/14 rms to be acceptable. This is equal to a form error of below 45 nm rms at a 0.6328 µm wavelength.
Since scatter specification has a very high application sensitivity, the establishment of a criterion is difficult. Unlike polished surfaces, whose appropriateness is typically specified by only the roughness factor (e.g., rms or Ra), directly machined surfaces have the extra considerations of lay and periodicity.
One method for forming criteria for scattering intensity is the application of classical scalar scattering theory. Based on this theory, a roughness height tolerance can be formulated by its relationship to a total integrated scatter. Assuming a total integrated scatter of one-tenth of 1%, which is a value in line with the oscuration ratio typical of a 60/40 cosmetic surface quality, gives a required rms surface roughness specification of about λ/400. The result is a surface roughness of 16 Å rms at a wavelength of 0.6328 µm.
Design of SPDT Machine Tools
There have been many advances in the design of SPDT machine tools, which has allowed directly machined surfaces to attain the criterion required for visible wavelength applications. These developments can be grouped as enhancements in machine tool mechanical designs, servo/feedback control systems, and environmental (vibrational and thermal) applications.
The use of oil hydrostatic slides is considered to be the most critical advance in SPDT machine tool design. The existing generation of modular slide designs includes fully constrained symmetrical bearing cross-sections, consisting of higher stiffness and damping characteristics than earlier air bearing designs. The absence of roller bearings allows major reduction in short term slide motion errors and total improvements in slide straightness. The latest slide design applies an air isolated capstan drive unit to eliminate lead screw coupling effects.
In order to obtain directly machined surfaces suitable for visible wavelength applications, it is imperative to consider environmental factors. Existing designs focus on thermal effects by using large thermal mass synthetic “granite” machine bases, servo controlled environmental enclosures, active laser refractometry, aerostatic work holding spindle athermalization methods, and thermally insensitive metrology reference axes. Vibrations in machine tools have been decreasing in existing designs because of the use of vibration isolation, use of materials with high damping properties, and oil hydrostatic slide designs.
Conclusion
Advancements in servo control and feedback units also are said to be advantageous for improving the quality of SPDT surfaces produced. Distributed processing and the use of improved interpolation arrangements have greatly enhanced machine tool positioning precision. Using new generation laser interferometers, which have resolutions in the nanometer range, to act as feedback devices, has also refined machine tool positioning.
Several SPDT surfaces will be tested to reveal the effects of advancements in machine tools. It can be proved that these surfaces can be used for visible wavelength optical applications by displaying positive comparisons with the above criteria.
About Precitech
Precitech began operations in 1992, but continues the rich history of ultra-precision machine tool building dating back to 1962, when Pneumo Precision was founded. In October of 1997, the Pneumo ultra-precision machine tool division of Taylor Hobson (formerly Rank Taylor Hobson / Rank Pneumo) was merged with Precitech. The Precitech name was retained for this corporate entity and all offices and manufacturing facilities are now located at 44 Blackbrook Road in Keene, New Hampshire.
Our facility staffs approximately 100 talented individuals in a recently designed 60,000 Sq. Ft. building.
Precitech is a member of AMT (The Association of Manufacturing Technology) and has corporate affiliations with several professional societies and academic institutions such as Germany’s Research Community for Ultra Precision Technology at the Fraunhofer Institute, ASPE the American Society for Precision Engineering, and EUSPEN the European Society for Precision Engineering and Nanotechnology.
This information has been sourced, reviewed and adapted from materials provided by Precitech.
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