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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.
  • Ah, thank you. I was worried that without checking this box the calculation for best fit was only performed for the initial part. Yes, there should not be an issue with unpredictable probe radius compensation in my pattern. Engineering has been "convinced" that there is something wrong with my program since some parts measure fine and others measure out. This is one of the more complex parts this shop casts, so it's possible there's enough variation in the process that some parts actually are slightly out. So, one more box I can check off on what's not wrong with my program. Thank you very much for the clarification.
  • Moving to 2023 we see this Iterate and repierce cad checkbox.

    The helpfile is trash.

    Why would the helpfile suggest enabling this option? This option seems only useful for no datums form/size only profile check, or some crazy organic surface.

    What do you all think about this glorious checkbox item resting there to make things more confusing and perhaps detrimental.
  • 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.
  • neil.challinor

    If Findnoms and only selected is on in a 2017 scan with a profile tolerance is that similar to 2023 using repiece or not using repierce?

    Sounds like when all deg of freedom are locked down it should be unchecked.
  • in reply to lovethepirk
    Yes, it's similar.
    Iterate & re-pierce is not available for Geometric Tolerances when the DRF is fully constrained.
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