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Fit To Datums

parrottk
Can someone give a good explination of the difference in true position when the fit to datums check box is on or off?
When I have the check box on all holes match nominals perfectly. (NO DVIATION AT ALL).
Check box turned off shows deviation that matches the actuals in the measured features.
when the check box is on the part looks to good and questions arise in regards to the accuracy of the gage and or the programer "ME".
thanks for the help.
Parents

  • Are you saying that 4.1 is the first in a long time where, if you used the Datums (-A-B-C-) to align the part, you can actually recall not only the MMC for the feature but also the MMC for the Datums (-B- & -C-) as well without it shifting and turning the part in a seemingly unpredictable attempt to show Datum / feature relationship?




    Yes! 4.1 is the first and it does exactly that. It allows for virtual conditions on datums. And boy, do I use that! If you do a report, you can see all the gory details of how it wiggles to get the best "fit" of the functional gauge (meaning: the smallest TP).

    Be careful though: This may have been implemented correctly IF your datum has only 1 Virtual Condition. I am not 100% sure but it looks as if it has been done right.

    HOWEVER: I have many FCF on datums that have 2 VCs. PC-DMIS seems to add all of them up giving me a humongous bonus. I think that is wrong (see an earlier post regarding this issue). So I am still quite hesitant to use MMC on datums with more than 1 VC.

    My suggestion: try it and REALLY study the results. It may work if the TP is to a datum with just one VC.



    Jan.
Reply

  • Are you saying that 4.1 is the first in a long time where, if you used the Datums (-A-B-C-) to align the part, you can actually recall not only the MMC for the feature but also the MMC for the Datums (-B- & -C-) as well without it shifting and turning the part in a seemingly unpredictable attempt to show Datum / feature relationship?




    Yes! 4.1 is the first and it does exactly that. It allows for virtual conditions on datums. And boy, do I use that! If you do a report, you can see all the gory details of how it wiggles to get the best "fit" of the functional gauge (meaning: the smallest TP).

    Be careful though: This may have been implemented correctly IF your datum has only 1 Virtual Condition. I am not 100% sure but it looks as if it has been done right.

    HOWEVER: I have many FCF on datums that have 2 VCs. PC-DMIS seems to add all of them up giving me a humongous bonus. I think that is wrong (see an earlier post regarding this issue). So I am still quite hesitant to use MMC on datums with more than 1 VC.

    My suggestion: try it and REALLY study the results. It may work if the TP is to a datum with just one VC.



    Jan.
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