Investigation of polishing quality of optical surfaces at small samples

Abstract

Flat elements are often used in optics. The most well-known are prisms, mirrors, and beam splitters. In order to produce high quality optical elements, the finishing process - polishing - is one of the most important processing steps. The purpose of this experiment was to determine the variation of two surface quality parameters, flat PV and waviness RMS, at different cutting rates. Blanks of optical glass BK7 were used for the experiments. Three parameters of cutting rates were changed: clamping force, polishing pad speed and polishing time. Each of these parameters had three values. At the lightest, medium and heaviest cutting rates, 6 flat surfaces were polished in each case for a total of 18 samples. The obtained measurement results were evaluated by the Shapiro-Wilk test. This is a normal distribution test that is used with a small number of tests. The smallest deviation from the flatness PV (1.3 μm) was obtained at the lowest cutting rates, when the lowest clamping force and rotation sped up. The parameters of this deviation almost doubled (up to 4.2 μm) when the polishing pad rotated at the maximum speed selected and under the maximum load. The surface waviness in RMS followed the same trend (the lowest obtained value was 0.3 mm, and maximum value was 1.2 μm). However, the obtained values increased more slowly with the cutting rates. Such variation in the results may be due to vibrations in the machining area.