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

Gus101
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.
Parents
  • 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).
Reply
  • 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).
Children
  • in reply to VinniUSMC
    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.)
  • in reply to VinniUSMC
    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.
  • in reply to VinniUSMC
    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....
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