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Datum Precedence

KyleB
Hi all,

Today's conversation is datum precedence.

Should this be A|C|B and can it be A|B|C? (according to asme y14.5)

My understanding is that it SHOULD be A|C|B and that A|B|C is wrong - but it has now turned into a topic of debate as it cannot be found in the standard.
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  • I'm not certain, but I think A|C|B and A|B|C accomplish the same thing on that part. As Matt says, Datum A constraines 4 DOFs. B and C can only each constrain one DOF not already constrained by Datum A.

    Either way, their use of datums A|B|C, in the order they defined, are what I'm accustomed to on round parts.

    From ISO 5459 :
    The primary datum imposes orientation constraints on the secondary and tertiary datums, the secondary datum imposes an orientation constraint on the tertiary datum.

    In this case, I would say A|C|B is not right...IMO
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  • I'm not certain, but I think A|C|B and A|B|C accomplish the same thing on that part. As Matt says, Datum A constraines 4 DOFs. B and C can only each constrain one DOF not already constrained by Datum A.

    Either way, their use of datums A|B|C, in the order they defined, are what I'm accustomed to on round parts.

    From ISO 5459 :
    The primary datum imposes orientation constraints on the secondary and tertiary datums, the secondary datum imposes an orientation constraint on the tertiary datum.

    In this case, I would say A|C|B is not right...IMO
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