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Thread: True position to holes on not datum surfaces... is this allowed

  1. #1
    Associate Engineer
    Join Date
    Feb 2014

    True position to holes on not datum surfaces... is this allowed

    Hi All,

    I have just joined, mainly because I have a GD&T question which I need an expert answer to.

    I have a customer component drawing and there are 6 holes on the component with a control frame with datums ABC. The holes are constrained to the 3 datums (two planes and a centre line).
    The two plane datums are at 90 degrees to each other and the centre line is perpendicular to one plane, however, the holes are on a non datum surface which is not perpendicular or parallel to the two datum planes. Is this a correct method of geometrical tolerancing? And how would you measure the correct position of the hole if this is correct?

    Thanks for any feedback,


  2. #2
    Technical Fellow Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    We need to see a engineering drawing of the datum features and the features being controlled by the datums..

    If you can't post the original drawing, create a equivalent drawing showing the same type datum features..

  3. #3
    Project Engineer
    Join Date
    May 2011
    A general statement that might help... there's no requirement that a feature being toleranced with position in GD&T must be perpendicular to any of the datums. So if that is what caused the question then there is no problem with the callout.

  4. #4
    Associate Engineer
    Join Date
    Oct 2014

    I'm new to the forum and actually have a very similar question to this one. I have drawing snippets that I have attached.

    The holes are currently called out TP back to ABC, which is their design intent (the holes are mounting points for parts that fit relative to other parts that mount at ABC. think of this component as a bracket). Despite calling out TP to ABC, the print does not provide basic dimensions back to ABC, only basic dimensions to other features, as shown. We and our supplier use solid model files with all of our inspections. I had hoped the supplier would use the solid model to establish the basic dimensions back to ABC, and inspect to ABC. However, our suppliers never inspect to dimensions not on the print, so they just check the holes as dimensioned. I'm not sure that a "hole" even has basic dimensions to out-of-plane datums. I basically told them to establish a point at center break-out point of the hole, and then check the points XYZ back to ABC. However, there's not good way to put all those basic dimensions on a print.

    1) How should this be inspected per the current print?

    2) How can I dimension these features that are out-of-plane with the datums? (both the holes and the surface that are on compound angles)

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