Measurements of curved surfaces: Asymmetric lens
The lens is measured on a CMM by scanning. For highest accuracy the tip radius of the probe was not compensated during the measurements. The lens has basically no datum surfaces which would help the creation of the coordinate system on the part and the points are simply collected as a cloud of points.
The measured points (X,Y,Z) and the IGES CAD model of the lens are imported into the SmartFit 3D package for evaluation.
The SmartFit 3D performs 3D best-fitting using variety of criteria such as Tolerance Envelope Fit (fit to a profile tolerance zone), Least Squares, Min-Max, RMS, The Most Material.
During the iterative fitting process the points are continuously projected to the CAD model compensating the tip radius of the probe every time. This eliminates the cosine error of the probe compensation.
The influence of outliers can be reduced or eliminated by using scale factors.
Such optimization is very difficult because of the very gradual curvature change and many local solutions. The SmartFit 3D has numerous tools helping to find the absolute minimum.
The results are shown by using whiskers or colour deviations map.
The results can be saved in a simple text file or in MS Excel.
The third picture is showing the results from a sphericity evaluation of an optical quality sphere: