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True Position of Feature to another Feature TP used as Datum

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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  • The ACTL's of the 70 and 40 dimensions are used to calculate the true position of the slot. Actuals are always used for calculations. If you were to calculate it off the theos, then you would be measuring it to datums ABC, not AD.

    As a side note, you need to pick up all four outside edges and construct midlines for your B and C datums. You can't use one edge if it's defined as the midplane. This will change your true position values relative to datums B and C unless the sizes of datums B and C are perfectly nominal.
  • This is actually another question I have. Since Plane, Line, Line or Plane, Line, Point is slapping the top, and two edges it gets my device roughly aligned on those two edges but it doesn't define my midplanes and puts the trihedron in the corner of the part. Since Datum B and C are dimensions I can't actually make Datum B & C until I measure but I can't measure until a part is in alignment. But my part will never be in alignment until I align it to Datum B and C right? Or is just doing a manual->DCC iterative 3-2-1 close enough then you measure and then make Datum B and C from those measurements and we pretend that never happened? I feel like my cart is before the horse.
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  • This is actually another question I have. Since Plane, Line, Line or Plane, Line, Point is slapping the top, and two edges it gets my device roughly aligned on those two edges but it doesn't define my midplanes and puts the trihedron in the corner of the part. Since Datum B and C are dimensions I can't actually make Datum B & C until I measure but I can't measure until a part is in alignment. But my part will never be in alignment until I align it to Datum B and C right? Or is just doing a manual->DCC iterative 3-2-1 close enough then you measure and then make Datum B and C from those measurements and we pretend that never happened? I feel like my cart is before the horse.
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