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Thread: GD&T Datum References

  1. #1
    Associate Engineer
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    Feb 2017
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    GD&T Datum References

    Hello, I am pretty new to GD&T so bear with me.

    My co-workers and I have been debating about this issue for a few days.

    If you open Example1.PDF, Datum C is measured from 1/2-13 Thread at 1.000". Datum C has a Surface Profile callout of .002 from Datum A & B. Example2.PDF shows that Datum C is measured straight from Datum B at 2.500". How does the 2 callout differ?

    My co-workers are telling me Example 1 does not make sense because Datum B & C dimensions the thread from their location and the Position calls for A,B, and C. Clarification on this would be great! Thank you!
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  2. #2
    Administrator Kelly Bramble's Avatar
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    First, Datum feature C is NOT measured from the threaded feature. Datum C is orientation and location verified from Datum's A and B.

    The threaded feature you mention is measured from an origin established from Datums, (A Primary, B Secondary, and C Tertiary).

    As far as dimensional verification in concerned a Basic dimension is not considered to be toleranced. The basic dimensions in both drawings are only used to locate the tolerance boundaries specified within the perspective Feature Control Frames.

    Both drawings are correct...
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
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    Feb 2017
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    Hi Kelly, from what you are saying, both drawings are interpreting the same information, but in different ways?

    Thank you for the info!

  4. #4
    Administrator Kelly Bramble's Avatar
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    Actually, both drawing are SPECIFYING the same thing, but in different ways.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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