huangyhg

profile control of datum
please see the attached drawing.
i am using asme y14.5-2009, i think the '94 version would have "two surfaces" in place of my "2x", but anyway...
profile of a surface requires the true profile to be established with basic dimensions. i can't find anything in the standard that overrides this rule when the surface is a datum as i have shown. i don't want the .500 width to be basic, i just want those 2 edges to be within .005 and to establish datum b.
is it acceptable the way i have it?
for whatever reason, the back edges of the tabs are missing. the .500 dimension is the width of the tab.
i'm only familiar with -94 so can't say about the 2x and it's difficult to be sure as some of the lines & dimensions are missing but i think this is ok.
there are some very explicit examples in the 94 standard for using surface profile to control coplanarity and even as setting that as a datum. i'd assume 09 is the same so take another look if you haven't already.
you're setting the 'under side' of the tabs as your b datum. the .500 doesn't come into making the datum or not. the .500 (should maybe be 2x) is locating the top edge of the tab from datum b.
all other features in that orientation should be dimensioned (directly or indirectly) from b unless you have something funny going on.
kenat,
here's a better image.
i would do something to clarify the two surface profile segments that the control is intended for.
attached is one example but there are other ways to do it as well.
paul
paul, i'm sorry but i don't find your sketch a clarification, if anything it's more confusing to me.
jlang's repost appears pretty clear to me, very similar to figure 6-20 in the 94 spec. the phantom line makes it clear which 2 surfaces it applies to, although there should be a break between the phantom line & the geometry line.
i'm not sure about the "2x" but the op implies this is a 2009 thing.
kenat,

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