BS EN ISO 21920-2:2022 pdf free download – Geometrical product specifications (GPS)一Surface texture: Profile Part 2: Terms, definitions and surface texture parameters.
D.5 Determination of the parameters Rpq, Rvq and Rmq
Three nonlinear effects can be present in the material probability curve shown in Figure 23 (see 4.5.1.7) for measured profile data from a two-process surface. These effects shall be eliminated by limiting the fitted portions of the material probability curve, using only the statistically sound, Gaussian portions of the material probability curve, excluding a number of influences.
In Figure 23 (see 4.5.1.7), the nonlinear effects originate from:
— debris or outlying hills in the data (scale-limited profile) (key G);
— deep scratches or outlying dales in the data (scale-limited profile) (key H);
— an unstable region (curvature) introduced at the plateau-to-dale transition point based on the combination of two distributions (key I).
These exclusions are intended to keep the parameters more stable for repeated measurements of a given surface.
Figure 28 (see 4.5.4.4) shows a profile with its corresponding material probability curve and its plateau and dale regions and the parts of the profile that define the two regions. The profile has a hill that is outlying and the figure shows how it does not influence the parameters. Figure 28 (see 4.5.4.4) also shows how the bottom parts of the deepest dales, which will vary significantly depending on where the measurements are made on a surface, are disregarded when determining the parameters.
D.6.3 Estimation of plateau to dale transition
Determine the asymptotes of the conic section (key reference D in Figure D.2). Bisect the asymptotes with a line (key reference E in Figure D.2). The intersection of this line with the conic section serves as an initial estimate of the plateau to dale transition (see Figure D.2).
NOTE Graphically the bisector line (key reference E) might appear to be at an improper angle (see Figure D.2). This is because of the different scaling of the two axes in Figure D.2.
D.6.4 Determination of UPL and LVL
The second derivative is computed at each point of the material probability curve, starting at the transition point key F and working upward to the plateau region and downward to the dale region.
The second derivative at each point is computed using a “window” of 0,05 standard deviations ( ±0,025 s around the point at which the derivative is to be recorded). See the middle of Figure D13. For the dale region and the plateau region individually:
— find 25 % of the number of points to one side of the transition point (key F) and call this value i
— working out from transition point (key F), the standard deviation s1 is computed for the second derivative values using i points on one side.