Just trying to figure this out - when I dimension a circular feature I'll get one size but when adding a GD&T dimension for that same exact feature I get a different size altogether (example below). What's the reason for this?
DEFAULT is using the correct math, as defined by the standard you are working to - be that ASME Y14.5 or ISO 1101. Least Sqaures is what people are used to and can give more stable results where there are very few points or where there is "noisy" data, particularly if the amount of measurement uncertainty for your sensor is close to the tolerance value you are trying to verify. That doesn't necessarily mean that it gives "better" results, it is just more stable because it is an average - it depends on your definition of "better".
If you want to ensure that parts will assemble correctly and truly simulate how inspection using hard gauging would function, then measuring your features with dense point distributions that cover as much of the surface as possible and using constrained L2 math (DEFAULT for ASME Y14.5) would be the correct approach.
Good question. Why does Hexagon make things worse? I recall years ago the QC team (at one place I used worked at) was scratching their heads why when using Xact the results came out looking much better than customers who were using Calypso & Geopak/Mcosmos. Being a newbie in the CMM world I didn't know much what went on but wat I was told later that in the end when those guys programmed using legacy the results were within .0003 or so between Calypso & Geopak/Mcosmos. And from that point on they almost always programmed using legacy.
Legacy dimensioning, Calypso, Geopak/Mcosmos and many other, older CMM software packages all use least squares. That wasn't necessarily because it was right from a standards perspective, it was because it is a comparatively easy calculation to perform that is reasonably well understood and because NIST and the PTB offer certification for least squares math. As I frequently have to remind my kids - just because everyone else is doing something, it doesn't make it right.
In introducing the Geometric Tolerance command, Hexagon's intention was not to "make things worse", it was an attempt to give many of our users what they had been asking for - full compliance to standards. Some of the most common questions we are asked by new customers is "does your software comply to ASME / ISO" and "does your software support GD&T".
