The accuracy of fast 3D topography measurements

One important factor in surface engineering is the evaluation of the surfac e topography, i.e. measurement and analysis. These measurements of the surf ace roughness are normally done by stylus instruments and 2D profiles due t o available standards. 3D surface topography evaluation is, however, in som e cases necessary and frequently used, a standard is under development, and newly developed measurement instruments are used. Specially interferometri c instruments are of interest here since measuring time is critical and the ir accuracy and comparability to stylus instruments must be evaluated. Ther efore, in this study a number of engineering surfaces have been measured an d evaluated: plateau-hones cylinder liners for engines, polished steel roll er for bearings, grounded and honed gear surfaces from gear boxes, and stee l sheet surfaces used in car bodies. By the use of a precise relocation tec hnique, an exact comparison was possible between an interferometric instrum ent (Wyko RST Plus) and a 3D stylus instrument (Somicronic 3D). To be able to judge and analyze differences between the instruments an Atomic Force Mi croscope (AFM) was used. The AFM is extremely accurate for this type of sur faces, but limited in its measuring range. The result shows a very good agreement between the instruments with deviati ons of approximately 5-20% depending on the parameter evaluated. The stylus in general gives lower values. The results from the AFM are generally foun d between the parameters of these instruments, and for soft surfaces, such as sheets or smooth surfaces, the interference instrument has a slightly be tter agreement to the AFM. This is due to the stylus tip geometry and the r elatively high contact pressure having difficulties measuring small feature s and also damaging softer surfaces. The result from an interferometric mea surement occasionally shows optically introduced artifacts caused by local surface slopes which exaggerate the topography. By knowing the degree of in fluence from these peculiarities it is still possible to use fast interfero metric instruments and to estimate its deviation from what would have been the result with a stylus instrument. (C) 2001 Elsevier Science Ltd. All rig hts reserved.