Gd&t question

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  • Gd&t question

    Am I correct that if a Datum is on the leader line for tp of a 4 hole bolt pattern then the Datum would be a constructed circle from all 4 of those holes? Even though the leader line is only pointing to one of the 4 holes?

  • #2
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    • #3
      I would assume that it is the pattern.

      That doesn't mean that it is a circle constructed from the 4. It means that the datum is 4 features. 4 features like that can constrain rotation about the axis, as well as translation perp to axis. Unlike a circle.

      Remember to think about what it means for functional gaging, that is what GD&T represents. For your datum, you would have 4 pins all at the nominal location.

      So, to use as a datum with legacy dimensions, you would level and origin to the plane (assuming datum A is the normal plane to the holes), and then best fit 2D to the 4 holes.

      With Xact, apparently you can select all 4 holes and construct a feature set from them, and then define that feature set as the datum, and then use it in the FCF (at least, for newer versions of PC-DMIS).
      "This is my word... and as such is beyond contestation."


      • #4
        Thanks Vinni!


        • Schlag
          Schlag commented
          Editing a comment
          The 4X tells you its the pattern and not a " hole " .

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