Hole group datum and order of precedence.

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  • Hole group datum and order of precedence.

    I'm looking for some advice in regards to the following image, I drew this up quickly so apologies if it is a little messy, it is in metric and I am working to BS8888. Here is my take on it:

    Datum [B] is controlled as a group, origin can be established at the centre of the group or any of the holes within the group defined by the theoretical exact dimensions? In this instance I believe datum [B] holes can be best fit then output to 0.025 true position from the centre of the group to each hole, this would effectively be 0.05 between pitch? Would the TED's be ignored in the pitch and over ruled by this, or would you say the pitch had to be held to 0.025 between holes as the [110] and [110] TED's illustrate?

    Datum [A][B] become a fully constraint datum system controlling all degrees of freedom, 3 by datum [A] and 3 by datum [B] part rotation is constraint by the TED's using a best fit? Datum [G] hole is controlled to [A][B] and fully defined?

    The 2 remaining holes are the ones I am not totally sure on. Would I be correct to say this is the correct datum precedence in regards to the TED's? Datum [A] 3 degrees of freedom, datum [G] would control 2 DOF becoming the origin point, then datum [B] as a group would control the last DOF being the rotation and orientation within their respective TED's? (best fit) I have read about creating hole groups as a feature set but I haven't used that method in alignments and unsure how they would behave.

    Hole group datum.jpg

    I would be appreciative of any helpful input or advice, thanks in advance..
    Attached Files
    Last edited by GEO3D; 02-24-2020, 08:37 AM.

  • #2
    Since B is just 2 holes wouldn't they really be +/- .0125 apart in a straight line distance ? They must be perp to A within .025 and a position of .025 to each other. Really this is a straight line distance since they aren't related to any other datums or rotations. I know its Monday....please help me out. What is a TED ?? ( Theoretic exact Dimension ? )
    Last edited by Schlag; 02-24-2020, 09:41 AM.


    • GEO3D
      GEO3D commented
      Editing a comment
      Hi Shlag, thanks for your input.

      My take is that the holes don't need any datums to define their position as their rotation is restricted by their TED's and in this scenario they make up one of the main datums for the rest of the part, and yes sorry it is theoretical exact dimensions for short . I also believe the diameter symbol after true position controls the bore to it's full depth in a cylindrical zone of 0.025 also controlling perpendicularity.

      +/- .0125 may also be too much tolerance I'm sure as the tolerance is held within an 0.025 diameter so it would only allow a 0.0125 hypotonuese of X + Y. I think it would become +/- 0.0125 if the diameter sign was absent in the tolerance frame ​​​​​​​

    • Schlag
      Schlag commented
      Editing a comment
      I get what your say but.... If they arent called to any other DATUMS and the hole pattern is the DATUM, what is the hypotenuse related to ? The " part " is really " squared up " to the hole and not the the other way around ? If an edge of the part was "B" and a 2 hole pattern was PERP to A and Squared to B I would agree but this hole pattern is called to itself. Based on your print, the holes are not located to the " outside " edges of the part. There must be some sort of profile callout that will relate the edges of the part to the hole patterns. This is telling you the edge is probably just clearance and the important features are the bolt pattern.

      Keep it simple. " TED " = BASIC
      Last edited by Schlag; 02-24-2020, 10:00 AM.

    • GEO3D
      GEO3D commented
      Editing a comment
      The [B] holes are called to [A] which is the face they are perpindicular to, [A][B] then becomes a fully defined datum system or so I believe?. Datum [A] would remain level and [B] holes would be pitched positional to themselves and square to [A] within true positition of 0.025 each hole. The outside faces would fall under the general drawing tolerance and would be positional to the main datums being Datum [A] for squareness and datum [B] holes for position.

      Sorry, the hypotuneuse I refer to is the X and Y error between holes, 0.0125 error in both X and Y would be outwith a 0.025 cylindrical tolerance zone. Totally agree in this instance, the outside faces are unimportant.

      I can't post the customers drawing due to data protection but this is similar to their design intent. The outer faces are less important, allthough there are details missing I was hoping it would be enough to get advice. Here is a little more detail, hope this makes it a little clearer:

      Say Datum [A] has a flatness tolerance of 0.010 microns. Part locates against [A] where the datum grouped holes need to be square and positionally good, the pitch and location of these holes fit on dowels and control the position of the part. The smaller dowel holes are for smaller parts which control targeting laser components. So orientation and hole position are key. Outside faces not so critical

      It's not always easy to paint a full picture with limited information, thanks again for your input.
      Last edited by GEO3D; 02-24-2020, 12:31 PM.

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