Some effects of stylus force on scratching surfaces

The calibration of the stylus type surface texture measuring instruments is usually carried out using a precision reference specimen of regular or random pattern of irregularities. These precision specimens are produced with hardened surfaces in order to minimise the screeching effect of the stylus on their certified values. Some of these specimens are made from glass, stainless steel or steel and nickel coated. The continual tracing on these surfaces is expected to damage their quality. An experimental investigation on one of these specimens having a sinusoidal pattern of irregularity has been carried out. The specimen was traced by a stylus of 2.5 μm and a measuring pressure of 1 mN several times on exactly the same position. Up to 350 traces were taken. From each trace a calibration constant for the measuring instrument was computed. Sets of three consecutive traces were taken and the average calibration constants computed for each set and the results plotted. It was found that the mean values of the calibration constants continually decrease. After tracing the specimen about 250 times there was a loss of more than 6% of its original value, and a considerably increased scatter within each set of results (about 10 times the scatter obtained from the first set of traces). The plots of the results as well as photographs of the surface of the specimen, taken using the Scanning Electron Microscope (SEM), to illustrate the size of the scratch due to stylus are given.