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Precision enhancement technologies for multi-axis ultra precision machining of freeform optical elements with variable micro/nano structures

: Uhlmann, E.; Kühne, S.; Jagodzinski, M.; Widemann, R.; Haskic, K.

American Society for Precision Engineering -ASPE-; Asian Society for Precision Engineering and Nanotechnology -ASPEN-:
ASPEN/ASPE Spring Topical Meeting on Manufacture and Metrology of Structured and Freeform Surfaces for Functional Applications 2017 : Hong Kong, China, 14-17 March 2017
Raleigh, NC: ASPE, 2017
ISBN: 978-1-5108-5354-6
Spring Topical Meeting on Manufacture and Metrology of Structured and Freeform Surfaces for Functional Applications <2017, Hong Kong>
Fraunhofer IPK ()

Aspherical and freeform optics receive uprising attention, due to their advantages in terms of aberrations and reduction of system volume, for example in spectroscopic applications. Surface structuring enables hybrid functionality and therefore a further reduction of system complexity. Manufacturing of metal optics and mirrors is established by ultra precision diamond machining. Possible technologies for freeform surface manufacturing are turning with dynamic axis machining, fast-tool techniques or surface shaping and planing. The machining of structured surfaces with variable groove profiles requires up to 5 degrees of freedom. Rotational movements of tools result in transversal displacements, caused by runout, off-axis position and process forces. Without knowledge of these displacements any rotational movement will cause wave front deviations of the optical element in the regime of low and mid spatial frequencies. In order to meet the requirements of diffractive VIS optics, the position of the tooltip in relation to the workpiece needs to be known within the nanometer range. The determination of tool displacements might be done directly by optical measurement, but is practically limited to spatial distances of approx. 1 µm. This can be reduced by the use of atomic force microscopy (AFM). Tool displacements can be indirectly obtained by the measurement of reference structures on plane samples. Shaping of test grooves with variable tool angles lead to deviations in height and lateral distance that can be measured by AFM. The measurement of those deviations enables a functional description of tool displacement in dependence of the angle. This function is used to determine a corrected tool path that compensates the tool off-axis error. AFM measurements on test samples with tool path correction show form deviations PV < 100 nm. A machine setup is essential with a high stiffness of the rotational and linear axes as well as a high positioning accuracy of all the axis. The machines “MMC900H” and “MMC1100” from LT Ultra-Precision Technology were used for this research. Especially nonlinear tool dislocations caused by process forces impede correct adjustment of tool positioning. Therefore, a high stiffness tilt and turn module is developed/presented that allows two additional rotational degrees of freedom. The position accuracy of each axis is ~1’’ with hydrostatic bearings that guarantee a rotation free of stick-slip and a stiffness >100 N/µm. A locating-floating bearing arrangement is used to reduce the impact of leverage forces. In order to achieve long term stabilities, several utilities as a high-precision temperature control and advantageous material selections are implemented. The control of the axis is fully integrated in the machine control and standard machine NC-codes can be used. Further enhancements of setup time will be accomplished by integrating an AFM in the manufacturing machine, allowing insitu measurements of reference structures and quality control. This research is part of the project “3D-Blaze” and is funded by the German Federal Ministry of Education and Research. The project partners are LT Ultra-Precision Technology GmbH, Carl Zeiss Jena GmbH and Technische Universität Berlin.