Workpiece self-weight in precision optics manufacturing: compensation of workpiece deformations by a fluid bearing

Abstract

The effects, the extent and the importance of workpiece deformations, particularly lenses, caused by the weight of the workpiece itself, were examined in a previous paper1 . The considered deformations are in the single-digit to two-digit nanometer range. The investigation was carried out by FEM calculations. The conclusion of the previous aper was that a full-surface support of a workpiece in the processing of one surface presumably produces the best results. Furthermore, it was found that if the second functional surface is not to be touched in the process, a full contact lens mounting on its circumference is advisable. An alternative method for fixing precision lenses is therefore desirable. This can be accomplished in two steps. As a first step, the lens must be gripped at its periphery so that none of the optically functional surfaces of the lens is compromised. However, the complete circumference has to be fixated gaplessly because a punctual fixation has the disadvantage of deforming the lens surface asymmetrically. As a second step, the freely hanging lens surface should be supported to minimize deformation. An approach had to be found that supports the surface like a solid bearing but at the same time does not touch it. Therefore, the usage of an incompressible fluid as a hydrostatic bearing for full-surface support is pursued. For this purpose, the bottom side of the lens has to be stored on water. The results of the FEM simulation showed that with a fluid bearing the resulting deformations can be drastically reduced in comparison to a freely hanging surface. Furthermore, under the right conditions, a resulting deformation comparable to a full surface solid support can be achieved. The content of this paper is a test series under laboratory conditions for a first validation of the theoretical results. Therefore, a prototype model to test a lens fixation with a fluid bearing was developed and manufactured. The resulting deformations were measured with an interferometer and the effects are discussed.