Stylus instruments are widely used for surface characterization. It is well known that the size and shape of the stylus tip affects the measured surface geometry and parameters. In most cases, increasing the tip size decreases the measured Ra value because the enlarged tip does not come into contact with the bottom of sharp valleys. However, even when the tip can reach the bottom of valleys, the tip size can have a significant, and at times counter-intuitive, effect on the measured Ra value. In this paper we analyze the effect of stylus tip radius on the measurement of periodic roughness specimens having rectangular profiles with flat valleys. Rectangular profile roughness specimens function as Type C reference specimens described in the ASME B46-2009 and ISO 5436-1:2000 standards. For these profiles, the effect of tip radius on measured Ra is affected by the ratio of the peak width to valley width. If the width of a rectangular profile peak is larger than the profile valley, the measured Ra is smaller than the Ra of the real surface and decreases with increasing tip sizes. However, the opposite effect occurs if the observed width of a peak is smaller than the observed width of a valley. In this case, the measured Ra is larger than the Ra of the real surface and increases with increasing tip sizes. The effect of tip radius on Ra results in a systematic offset. This offset can be larger than the measurement uncertainty, even in cases where the tip radius is smaller than the low-pass digital cutoff filter used to suppress noise and tip radius effects. We will show results of theoretical analyses, simulations, and experiments. The results raise questions as to whether the measured surface texture parameters should be corrected for tip-size effects.
Conference Dates: March 21-23, 2012
Conference Location: Annecy, -1
Conference Title: 3rd International Conference on Surface Metrology
Pub Type: Conferences
surface roughness, roughness average, periodic roughness specimens, rectangular profile, stylus tip, stylus radius