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True Position of Feature to another Feature TP used as Datum

TSFee
Hello,

I have essentially a block that has Z+ plane as Datum A, X-axis is a midplane from a basic dimension, Datum B, Y-axis is a midplane from basic dimension, Datum C (practically on the CMM I am doing Plane, LineX, LineY, then intersection of lines for origin) with a hole that's True Position is relative to A|B|C, which gets defined as Datum D.
There's a second hole (technically a slot but the engineer said he's more concerned about the center point) that has a TP referenced to Datum A|D.
See my attached """""""drawing"""""""" (I'm not a drafter so I hope it makes sense).
Basically the engineer described it to me as saying the Datum D hole is positioned relative to the outer dimensions (and flat plane of A) of the device, then the second hole is "clocked" to the Datum D hole along those XY coordinates.

My question is the theoretical position of the slot based off the actual measured position of Datum D or the theoretical position? e.g. if Datum D THEO XY X=20 Y=80 | ACTL XY X=20.01 Y=80.01 is the X=40 Y =70 slot THEO dimensioned from the THEO or ACTL of Datum D? If I understand how true positioning works (I don't), the theo position of the slot needs to be in reference to the measured position of Datum D, because if you reference it to the Datum D theo you are adding the variance allowed in Datum D's position.


(I apologize this is a simple question but I'm an EET landed with the role of getting our CMM going again and spatial geometry was not my strength. We use OpenDMIS so I understand the specific program terminology is different but I've been figuring out how to program it by learning PC-DMIS from posts here then translating it to OpenDMIS. To save you that burden, answer as if I'm using PC-DMIS.)

Thank you!
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  • Thank you both of you, this helps confirm some of what I thought and clarifies others.

    I think where I am getting snagged is when trying to write the program and define the nominal of the slot feature. Since it's based on the Datum D actual I would need the program to change my origin to the Datum D, then define the slot nominal, then measure the actual, then tolerance it. This device has a handful of these individual Datum D/slot relationships so my program would require bouncing around coordinates to define the slot's nominal and recalling back to the -ABC- alignment DRF.

    This seemed cumbersome so I've been trying to leave it at the alignment coordinates, then the True Position function allows you to define up to 3 Datums to use as your coordinates if you don't want to use the existing DRF. The problem is if I do that the deviation values are different than just setting the coordinates to the Datum D.

    First picture is the DRF set exactly to the actual position of CIR_AL2_M (Datum D) and Slot2_Pt's nominal is exactly XY 90,-230 from Datum D, and the actual has some deviation. (looks good.)
    The report looks correct to me.

    Second picture the DRF is still set at -ABC- coordinates, which is shown by Cir_AL2_M (Datum D) being in it's nominal position, and an actual.
    The nominal of Slot2_Pt is obviously incorrect in reference to -ABC- but in correct position to the Datum D Actual.
    The TP report notes the Datums used it Datum: CIR_AL2_M (It doesn't show but the value used is Actual, not the theo).
    If it's Datuming off Datum D actual correctly as it's supposed to I should expect the exact same value.

    Do you guys see something in the numbers (regardless of the software) that could be the problem that I don't see? If the numbers all look correct to you guys I will chalk it up to the software and move on.

    Appreciate the help!



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  • Thank you both of you, this helps confirm some of what I thought and clarifies others.

    I think where I am getting snagged is when trying to write the program and define the nominal of the slot feature. Since it's based on the Datum D actual I would need the program to change my origin to the Datum D, then define the slot nominal, then measure the actual, then tolerance it. This device has a handful of these individual Datum D/slot relationships so my program would require bouncing around coordinates to define the slot's nominal and recalling back to the -ABC- alignment DRF.

    This seemed cumbersome so I've been trying to leave it at the alignment coordinates, then the True Position function allows you to define up to 3 Datums to use as your coordinates if you don't want to use the existing DRF. The problem is if I do that the deviation values are different than just setting the coordinates to the Datum D.

    First picture is the DRF set exactly to the actual position of CIR_AL2_M (Datum D) and Slot2_Pt's nominal is exactly XY 90,-230 from Datum D, and the actual has some deviation. (looks good.)
    The report looks correct to me.

    Second picture the DRF is still set at -ABC- coordinates, which is shown by Cir_AL2_M (Datum D) being in it's nominal position, and an actual.
    The nominal of Slot2_Pt is obviously incorrect in reference to -ABC- but in correct position to the Datum D Actual.
    The TP report notes the Datums used it Datum: CIR_AL2_M (It doesn't show but the value used is Actual, not the theo).
    If it's Datuming off Datum D actual correctly as it's supposed to I should expect the exact same value.

    Do you guys see something in the numbers (regardless of the software) that could be the problem that I don't see? If the numbers all look correct to you guys I will chalk it up to the software and move on.

    Appreciate the help!



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