Since shininess is one indication of surface smoothness, a photoelectric glossmeter was developed for possible use as a production-inspection device for determining the roughness of machine-finished surfaces between 100 and 500 micro-inches root-mean-square deviation from mean surface. Because these roughnesses are about one thousand times the smallest roughnesses that affect gloss, it is necessary to coat each machined surface tested with a thin film of light mineral oil which fills microscopic cracks and pores, but conforms to the larger surface irregularities. A near-grazing angle, 75°, was chosen for the measurement of gloss so that surfaces on which there are ridges of metal between adjacent tool cuts are rated low in gloss and, therefore, rough. Although the glossmeter essentially measures the fraction of the unshadowed surface which is nearly parallel to mean surface, the instrument resulting from the present development has proved to be a simple and useful device for making rapid comparisons of the roughnesses of surfaces machined with about the same tool feeds.
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