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Thread: Datums affecting inspection of only limits of size

  1. #1
    Associate Engineer
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    Datums affecting inspection of only limits of size

    I work at a small company that does not have many people that understand Y14.5. Another individual and I have been having a debate on how datums are used, and I am trying to find third-party sources that are knowledgable on the subject that can chime in.

    For our drawings, we call out in our notes that we follow ASME Y14.5 (no specific version, I've been told it assumes "the latest version"). We do not have any feature control frames on our drawings, and only have limits of sizes called out to tolerance all our features. The other individual feels we need to include datums on our drawings to "inspect the same way every time". I believe he wants to use the datums to establish an orientation relationship between the features, like in 2.7.4. He also wants to be able to use the surfaces called out as datums as an origin for the limit of size, like an origin symbol would. I feel that, without feature control frames, or some additional note describing what our purpose is, the datums alone do not do anything. Can anyone expound on that, or if I am incorrect, please let me know what I'm missing? If you need additional information, please let me know. Thanks!
    Last edited by trouco2; 07-15-2015 at 10:11 AM.

  2. #2
    Administrator Kelly Bramble's Avatar
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    For our drawings, we call out in our notes that we follow ASME Y14.5 (no specific version, I've been told it assumes "the latest version"). that, or if I am incorrect, please let me know what I'm missing? If you need additional information, please let me know. Thanks!
    No - there is no industry standard that specifies that the most recent standard is to be used. Most correctly ~ the specific GD&T standard should be identified. To be fair... I doubt there will be a an interpretation issue based on the description of your engineering drawings.

    I believe he wants to use the datums to establish an orientation relationship between the features, like in 2.7.4.
    First, there is no para. 2.7.4 in the ASME Y14.5M-1994 standard... This is why we specify which standard...

    In ASME Y14.5-2009 para. 2.7.4 clearly identifies that there is no relationship control given by the limits of size and that features shown to be perpendicular, coaxial, or symmetrical to each other MUST be toleranced for location or orientation to avoid incomplete drawing requirements.

    I feel that, without feature control frames, or some additional note describing what our purpose is, the datums alone do not do anything.

    This is absolutely true.. There are no assumptions of relationships allowed. Specifying a datum feature does not automatically make the feature a reference zero or datum for other features. The specific relationship must be defined by appropriate methods.
    Last edited by Kelly Bramble; 07-16-2015 at 08:14 AM.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Project Engineer
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    Whether or not you use GD&T, every print must address the same four things (dimensionally speaking):
    form
    orientation
    location
    size

    If your or your colleague thinks that all of these things are perfectly clear without using datums, then keep it as is. But it's VERY difficult to do this! Even the simplest part such as a square block with a hole through it can have loopholes in interpretation without GD&T (or without written notes conveying the ideas that GD&T would control).

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