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Readpoints and Alignments..Legacy/Xact

The Rook
I have a few questions and concerns I was hoping could get addressed. So thank you for your insight in advance.

A coworker and I were having a discussion about using the readpoint alignment to inspect a whole part. I said that is not good practice.

Anyway, he did so and inspected a part using just the readpoint. Numbers were coming out ok...so I was a bit confused.

Can you use the readpoint to inspect the whole part? He also said that you dont need to align to datums. That you can pick them up as normal features then when dimensioning something like true position you can just select those features.

This has really thrown me for a loop. This isn't really good practice...right? Shouldnt a basic alignment of the part be conducted after the readpoint to just solidify it a bit more? Then probe the same features again with a few more hits to refine it?

Is it necessary to do this? And one more thing, I've searched high...I've searched low, I still am not sure when folks talk about legacy or exactmeasure. How do I know what mode I'm in? How do I switch between?

Thanks for reading.
  • 1. a readpoint alignment is only supposed to be used as a starting point on the machine. it is not used to constrain 6 dof.
    2. if the part is very basic and you're only checking some very open toleranced features, you might be able to use a readpoint as a basic x,y,z locator, but it is the incorrect use of a readpoint
    3. you absolutely need to use the datums. he's wrong.
    4. yes, a basic 3,2,1 alignment if applicable should be used after a readpoint
    5. insert>dimension>LEGACY.
    TO ME, BIGWIG, there are some slight differences between xact and Legacy. all things being equal, if you build an alignment correctly, then XACT and LEGACY should provide the same result.
    XACT is convenient if you have a part with more than 3 datums as you don't have to build a separate alignment for each DRF

    Asking these questions is healthy, but it really sounds like you're not the only ROOK handling that CMM. please dm me and I can discuss some options to help you further.
  • The readpoint alignment should only be used to replace the manual alignment. A DCC alignment is still required.
  • Both guys above me gave great responses. I'd simply like to reiterate final point. Legacy dimensioning should report almost identically XactMeasure, assuming you've performed your alignment correctly (per the rules of your governing standard).
  • I thank everyone for their continuous contributions to my questions.
  • Someone please correct me if I'm wrong, but I was under the impression that using GEOTOL indeed does allow you to select datums from a dropdown without being aligned to those exact datums. Obviously the datums have to be measured and defined beforehand, but on a part with multiple datum structures (A|B|C, G|H|F, etc.) you don't have to be aligned to each one individually before dimensioning.
  • Someone please correct me if I'm wrong, but I was under the impression that using GEOTOL indeed does allow you to select datums from a dropdown without being aligned to those exact datums. Obviously the datums have to be measured and defined beforehand, but on a part with multiple datum structures (A|B|C, G|H|F, etc.) you don't have to be aligned to each one individually before dimensioning.

    Correct. They just need to be measured and defined.
  • in reply to bfire85
    BUT, there are (many) cases where NOT measuring features in the correct alignment can throw your numbers off.
    Easiest case to point out is:
    You have an emboss in your part, it has a generous tolerance, say position of 3mm. It is then defined as a datum for other features, specifically a surface profile. That emboss can be 1.5mm off location in any direction, but if your surface profile is 'tight' (say, 0.5 profile) to the emboss, and you measure them in some other alignment, that surface can in reality be in the correct relationship to the emboss, BUT, since you didn't use the emboss as the alignment to measure the surface, you can get bogus values due to probe comp issues among other things. BEST BET is to always measure to the datums used in the FCF for all features.
  • in reply to bfire85
    JFournier mentioned diimensioning to the datums in Geo Tol not physically probing the features in another alignment. If you are in some weird alignment when dimensioning and you select your datums and tell it to measure it via Datum Reference Frame, it will move the trihedron to that spot and change the trihedron colors. Probing the features not in the correct alignment, yes, can throw you out of whack especially relating to profile. I always try to probe in the correct alignment.
  • in reply to bfire85
    Good point D. Hoedeman, thanks for explaining!
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