A New Unified Approach to Surface Metrology

Abstract

Current standards for the assessment of surface texture are based on some form of height index, such as peak-to-valley or arithmetic average ( Ra). Although useful, these parameters take little or no account of the openness or closeness of the texture. In this paper, an additional parameter, the average wavelength, is introduced which is wavelength conscious and intended to supplement the information given by the Ra index. The authors show how this new parameter can be derived from arithmetic average/average slope and highlight the problem of small wavelength filtering and the importance of fully defined bandpass measurement. The average wavelength index, when considered together with the Ra value for a given meter cut-off, is useful in the control of surface roughness and could also be useful in the measurement of waviness, straightness and errors of form. Finally, in surfaces where the asymmetry of the waveform is important, a third parameter derived from the peak and valley distribution is introduced. This is essentially a measure of the asymmetry of the profile about the mean line and, consequently, is a measure of the skew.