Surface World December 2018 Surface World Magazine December 2018 - Page 40

www.vapour-degreasing.com TESTING & MEASUREMENT Profi le or roughness, which is it? David J Barnes: Group Technical Manager Why do we measure profi le ? The Parameters The primary reason for blasting a surface prior to coating is to provide a key for the coating to adhere to the surface. Blasting the surface increases the surface area giving the coating a larger surface area to stick to and hence theoretically better adhesion. Note this will also affect coverage rates. Blasting also “cleans” the surface enabling a closer bond between the coating molecules and the steel. The use of the terms Surface Profile and Surface Roughness are commonly, and often mistakenly, interchanged. As someone who is responsible for customer support, we often receive questions from customers who have confused profile and roughness or don’t understand the difference between them. In particular, blasting removes rust and mill scale from steel for new construction work and corrosion product and old paint coatings for repair work. Paint manufacturers will often, though not always, specify the profile depth required to ensure the coating system achieves the lifetime guarantee. It is essential, therefore, when specified, that the blast profile achieved is within the range specified by the coating manufacturer. Profiled steel surfaces are complex 3 dimensional structures, but fortunately only the depth is required for a profile measurement in the majority of cases. A profile is, mathematically, the line of intersection of a surface with a sectioning plane which is (ordinarily) perpendicular to the surface. It is a two-dimensional slice of the three-dimensional surface. Almost always profiles are measured across the surface in a direction perpendicular to the lay of the surface. The surface treatment specification therefore should describe the surface profile required, usually as an indication of the average amplitude achieved by the blast cleaning process. Several methods have been developed to measure or assess the distance between the peaks and troughs of blast cleaned surfaces. These have included comparator panels, special dial gauges and replica tapes. Surface Roughness is defined as a component of surface texture. It is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth. Roughness is typically considered to be the high-frequency, short- wavelength component of a measured surface. However, in practice it is often necessary to know both the amplitude and frequency to ensure that a surface is fit for a purpose. Ironically it is most commonly used as an indication of smoothness, i.e. for machined parts, such as those produced by turning. Surface roughness – also known as surface profile Ra – is a measurement of surface finish – it is topography at a scale that might be considered “texture” on the surface. Surface roughness is a quantitative calculation of the relative roughness of a linear profile or area, expressed as a single numeric parameter (Ra). Surface roughness measurement provides readings of many parameters; Fig 1. Profi led surfaces are complex Ra Rq Rt Rv, Rp Rpm Rz Rmax Roughness Average (Ra) Root Mean Square (RMS) Roughness Maximum Height of the Profile Rm Maximum Profile Valley Depth Maximum Profile Peak Height Average Maximum Profile Peak Height Average Maximum Height of the Profile Maximum Roughness Depth CONTINUED ON PAGE 40 38 DECEMBER 2018 read online: www.surfaceworld.com