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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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  • It must just be some form of courtesy or rule of thumb that I'm feeling. I feel like you should always put the rotational datum first.

    In my world, mostly turned and milled aerospace parts, the traditional alignment is a plane as primary, a turned diameter as secondary, and a timing hole as tertiary. Of course there are many other GD&T callouts used, but that is usually the most common one, which puts the third rotational datum last. Otherwise it would constrain translation degrees of freedom, which they don't want.

    It's really up to the engineer to use GD&T to represent how the part will be physically aligned in assembly.
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  • It must just be some form of courtesy or rule of thumb that I'm feeling. I feel like you should always put the rotational datum first.

    In my world, mostly turned and milled aerospace parts, the traditional alignment is a plane as primary, a turned diameter as secondary, and a timing hole as tertiary. Of course there are many other GD&T callouts used, but that is usually the most common one, which puts the third rotational datum last. Otherwise it would constrain translation degrees of freedom, which they don't want.

    It's really up to the engineer to use GD&T to represent how the part will be physically aligned in assembly.
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