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Surface Profile - Iterate and Repierce function

DaSalo
Can someone give me a rundown of what the iterate and repierce function does? I turn it on and all of a sudden my surface profile improves significantly. Is that real?

On a similar topic, has anyone else had trouble with vector least square best fit causing a best fit error? I have had several parts now that had profile dimensions set to vector best fit, which runs fine offline, that have given me best fit errors at the end of the program. If I hunt down the offending dimension and change it to least squares the error goes away immediately. I have also had a similar problem if I set the iterate and repierce function with too small of a tolerance. I'm using an .039 (1mm) ruby and have found I need to set the repierce tolerance to at least .025 to reliably run through the program without any best fit errors. This seems to be related to features with very large radiuses of contour (almost flat, but not quite). My assumption is that the math runs out to too many decimal places and it can't handle it.

Thanks for any advice. I am inspecting a part that is just one giant contour, no flats at all, and need to have all of my ducks in a row around how all of the surface profile options work.
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  • There are three different things being discussed here and it's important to distinguish between then.
    1. The Iterate and re-pierce option in best-fit alignments. As said, this is really only intended for manual devices (including portable arms and trackers) where it would not be practical to physically remeasure the part multiple times.
    2. The "FINDNOMS" option for scans. This requires a tolerance that is at least the size of your probe radius + the amount of surface deviation you expect to encounter. It is most useful where the physical part has large deviation from nominal or where a RELEARN scan is being performed, particularly when combined with "only selected" and "use best-fit". It takes the measured hits and re-pierces the CAD surface (along the hit's vector) in order to find the closest nominal point. Obviously, applying "only selected" surfaces and "use best-fit" helps to avoid piercing the wrong CAD surface.
    3. The iterate and re-pierce option for Geometric Tolerance profile commands with partially constrained datum reference frames. This functions in a similar way to the "FINDNOMS" method for scans except that it already knows the associated CAD surface because it "reads it" from the feature when obtaining the hits. When you have a partially constrained datum reference frame, the data is allowed to shift, rotate or some combination of both until it is optimized within the tolerance zone. That is why the help file recommends iterate and re-pierce is turned on and why it defaults to being on when available.
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  • There are three different things being discussed here and it's important to distinguish between then.
    1. The Iterate and re-pierce option in best-fit alignments. As said, this is really only intended for manual devices (including portable arms and trackers) where it would not be practical to physically remeasure the part multiple times.
    2. The "FINDNOMS" option for scans. This requires a tolerance that is at least the size of your probe radius + the amount of surface deviation you expect to encounter. It is most useful where the physical part has large deviation from nominal or where a RELEARN scan is being performed, particularly when combined with "only selected" and "use best-fit". It takes the measured hits and re-pierces the CAD surface (along the hit's vector) in order to find the closest nominal point. Obviously, applying "only selected" surfaces and "use best-fit" helps to avoid piercing the wrong CAD surface.
    3. The iterate and re-pierce option for Geometric Tolerance profile commands with partially constrained datum reference frames. This functions in a similar way to the "FINDNOMS" method for scans except that it already knows the associated CAD surface because it "reads it" from the feature when obtaining the hits. When you have a partially constrained datum reference frame, the data is allowed to shift, rotate or some combination of both until it is optimized within the tolerance zone. That is why the help file recommends iterate and re-pierce is turned on and why it defaults to being on when available.
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