huangyhg

datum split
i have a base plate where the bottom of it is datum -a-.
there is a groove along the entire length of the bottom of the base plate.
can it be implied that datum -a- is the total bottom face, as if the groove is not there, or do i need to deal with it as two bottom faces?
whatever surfaces are implied or dimensioned with that datum indicating it, that is the surface (or plane) that the datum references.
the surfaces on both sides of the groove will be the datum -a-, unless there is some offset in your design.
the groove surface is separate and can have a separate datum referencing other features/dims.
a picture may be more useful.
chris
solidworks/pdmworks 08 3.1
autocad 08; catia v5
assuming you're working to asme y14.5m-1994 look at section 4.5.7.1 & figure 4-20.
if the groove is relatively narrow you may get by without making the distinction since 4.5.7.1 does say separate identification if the 'groove' in your case is of "of sufficient width".
another option is to show an extension line across the groove but this may not work for a narrow groove.
i've sometimes seen a note "2 surfaces" placed next to the datum id to clarify that the datum is derived from the 2 surfaces but i'm not sure this explicitly in the standard.
kenat,
place a phantom line across the groove to both surfaces which indicates that it is now considered one surface. that's all.
dave d.
dingy, i nearly suggested a phantom line and it's something i've done before but noticed that 14.5 says 'extension line'.
per asmm y14.2m-1992 section 2.8 extension lines are 'solid lines' (though with short visible gap from the part out line asme y14.5m-1994 1.7.1.4).
a phantom would probably work, but if you'r a stickler may not be correct.
also if it truely is a narrow groove then neither phantom or extension will work well unless you do a larger scale view or detail showing the groove.
kenat,
kinat:
i don't know where in 14.5 stating a extension line should be used and 1.7.1.4 reflects crossing dimension lines.
i would suggest that you go to page 172 and 173 fig. 6.20 and 6.21 where it does show a coplaner condition and a phantom line is used in both examples.
dave d.
dingy, the reference i gave in my first post is where it states to use extension lines, 4.5.7.1.
penultimate sentence "where appropriate, an extension line may be used to indicate a continuation of one datum feature across slots or obstructions."
the examples you give with phantom lines are talking about using profile tolerance for coplanar surfaces, not explicitly about simulation of a single datum plane. also, as has come up before, 1.1.4 means we can't overly rely on just figures.
i was just trying to share something i'd noticed.

kenat,
extension line
good to know you got shoes to wear when you find the floor."fff"> - robert hunter

some additional info on datums:
kenat is correct that 4.5.7.1, which specifically addresses this situation with respect to datum definition, says to use an extension line. he is further correct that y14.2m say that an extension line is solid.
however, dave is correct that the standard "shows" in figures 6.20 and 6.21 that a phantom line should be used.
my opinion in this is that you should defer to the section 1.1.4 that states that the figures are only intended as illustrations of the text. therefore the text in 4.5.7.1 is correct and the figures are wrong. though i would probably use a phantom line for clarity's sake regardless of how the text actually reads.
david
kenat:
4.5.7.1 does state an extension line and it does make sense. the figures i previously mentioned do not follow the standard but words do supersede drawings here. going to make a couple a changes in my training book.
got to give you a thanks on this one. hats off to you!
dave d.

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