Evaluation of the error of the light beam incidence on concave surfaces in 3D roughness parameters using optical interferometry

Surface roughness parameters of mechanical components are sensitive to systematic errors, which can cause significant deviations in their values. These parameters are commonly used for manufacture controlling or even for tribological applications, such that their variations may lead to miscarried interpretations or mistaken conclusions. This investigation aims to analyze the error of light beam incidence, caused by eventual mal-positioning of the sample. Concave surfaces produced by boring and honing processes were evaluated by optical interferometry method. For this study, two samples of compacted graphite iron, extracted from a cylinder liner of a block of an internal combustion engine with a nominal diameter of 83 mm, were studied. The average and standard deviations values of chosen 3D roughness parameters - Sa, Sq, Sk, Spk, Svk, Spq, Svq, Smq, Sds, Ssc, and Sdq - were determined. The equipment utilized for the measurements was Talysurf CCI Lite, integrated with the software Talymap Platinum. Two procedures of measurement were realized for each type of surface. In the first, reflection angles of 0; 1.38 and 2.76° were imposed. Then, additional measurements were performed following the same principles, except that this time the reflection angle was corrected by the use of a shim, ensuring the perpendicular incidence of the light beam in the surface through evaluation of the fringe spreading during the measurement set-up. The bored surface was more sensitive to the effects of light beam incidence than those observed for honed one, confirmed by the general analysis of 3D images, by the variations in Sq parameter and those of Sk family. Within the parameters of family Sk, the robustness of Svk was remarkable for both investigated surface finishing. Together with this result, the Sp map analysis showed that the predominance of peaks in the bored surface was responsible by a more accentuated effect of light beam incidence, intensified to their greater values of Sq. The hybrid parameters were the most affected by the light beam incidence angle on the concave surface, suffering significant alterations for a reflection angle of just 1.38°. In addition, the correction of incident angle resulted in any significant divergence for any surface roughness parameter.