Using a datum feature set (4 holes in this case)

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  • Using a datum feature set (4 holes in this case)

    Hello,

    I'm new to this and I came across a part (for the first time) that has datum feature A as a plane and B as 4 holes. Other features on the part have to be positioned within .015" with respect to -A-B-. How do I use the 4 holes as a datum in XACT measure? Can I use the plane (primary) and 4 holes (secondary) to find the location of the other features?

    Another thing I cant comprehend is that datum B (the 4 holes) are toleranced with a positional tolerance of .005 to datum A. (With no basic dimensions on the print). From my understanding, when holes are toleranced with a Position tolerance to a planar feature, its controlling the orientation to the planar datum (in this case). What about location of these holes? What am I missing here guys (besides years of GDT and cmm experience). I read an article (not sure when it was published) that focused on datum feature sets of holes and it didnt have a positive note for CMMs being able to measure a feature set's position.

    I hope this makes some sense. I will try and post a print once I figure out how to properly create a post.
    Last edited by Gus101; 11-07-2016, 04:12 AM.

  • #2
    As for the 4 hole Datum, you can make a Constructed Feature Set out of the 4 holes and then use that as a Datum.

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    • Gus101
      Gus101 commented
      Editing a comment
      I will try this. Thank you.

  • #3
    There are two things happening to the holes in the Position to A callout. The obvious first part is the orientation. The second, more important part, is that the holes are also being related to each other. Even if there are no basic on the print, they have a relationship to each other (usually designated by "CAD is Basic" or some such).

    The pattern of holes can then control rotation about A (as a simultaneous set of features), and the remaining translations (using the centroid of the group, generally).
    "This is my word... and as such is beyond contestation."

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    • Gus101
      Gus101 commented
      Editing a comment
      I see. So when I dimension the 3 holes to datum A I am only getting the result for orientation and not the relationship to each other, right?

      I forgot to mention that part profile is also being controlled on the print relative to -A-B- (I think I'm starting to see the light here. lol.)

    • VinniUSMC
      VinniUSMC commented
      Editing a comment
      The only reason to treat the holes as a pattern like this (since this is not a composite callout) is because they are being used as Datum B.

      The callout to A is not, by itself, creating a pattern callout. Individuallly, each hole to A is still just really an orientation control. However, because the 4 holes are Datum B, all 4 holes, simultaneously (thing of a gage with 4 fixed pins that fit in the holes) stop rotation and translation. Creating a feature set out of the 4 holes and defining that feature set as Datum B "should" use the pattern of holes properly to stop rotation and translation.

    • Gus101
      Gus101 commented
      Editing a comment
      Thank you for taking the time for explaining this concept. A million other inspectors couldn't (or chose not to) explain this to me at work....
      Last edited by Gus101; 11-08-2016, 07:20 PM.

  • #4
    Relax your brain and think about it this way...... Instead of the holes being dimensioned to the edge of the part, the edge of the part is dimensioned to the hole pattern. The hole pattern is the important feature on the part and the edges are not. There has to be some other callout for non-dimensioned features

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    • Gus101
      Gus101 commented
      Editing a comment
      Thanks for the therapy... lol

      Yes, everything else is pretty much controlled with profile of a surface relative to datum A (Plane) and B (4 holes).... I SEE THE LIGHT!!!

  • #5
    Thank you everyone for the quick response. The PCDMIS gods will reward you...
    Last edited by Gus101; 11-07-2016, 07:32 PM.

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