huangyhg

profile and datum question
please read:
datum features are selected as required to provide the necessary framework for part verification. they go together to establish a drf. the location of these features may be established by non basic dimensions from other features and may also be sized by non basic dimensions.
that is intended to be a no to your question. surfaces located by basic dimensions must be referenced to a datum feature.
...but that can be by 'stacked' basic dimensions.
take a look at figure 6-19 & associated text in asme y14.5m-1994, i think it may help you with your first question.
in answer to your second question take a look at section 5.2.1.3 "it is necessary to identify features on a part to establish datums for dimensions locating true positions" also specific to surface profile see 6.5.4.
kenat,
jlang17,
in support of ringster's answer you may want to read pg 51 para 4.2.2.1 states that the drf consists of 3 mutually perpendicular planes that serve as the "origin" for related dimensions and measurements.
in support of kenat's answer on pg 81 para 5.2.2 in regard to "stack" dimensions, states that positional tolerancing uses basic dimensions for true position unlike unlike plus/minus locating dimensions which can accumulate tolerance.
designbiz

jlang17,
this question wasn't clear to me:
"can you locate a datum feature (with basic dimensions) from non-datum features?"
i don't believe you are using the correct terminology which makes the question awkward to me.
a datum feature is a feature of a part used to establish a datum. datums establish a datum reference frame (drf).
positional tolerance dimensions are basic and originate at the referenced datums.
if you are asking if non gd&t dimensioned features of a part can be dimensioned from other features rather than datum's, then i would obviously say yes. however if you are using a positional tolerance callout then that feature must be located with basic dimensions back to the drf.
hope i am not confusing the question however as you confidently expressed "yes", leads me to believe you may be
asking a different question that is obscured by using incorrect terminology.
i would be careful of the correctness of a callout just because it has been used repeatedly. i would always advise to reference the standard if something looks doubtful. i have seen countless incorrect gd&t callouts carried over from drawing to drawing largely due to ignorance of the standard.

designbiz

thanks for the replies. i'll try to explain what i'm asking a bit more clearly.
let's say i have a drf. primary a is a surface, secondary b is a hole and tertiary c is a hole. locating a is easy, it's the surface and would maybe only require a form control. locating b is where my question comes in. i can only reference a for perpendicularity, but to locate b in terms of x,y i have no datums to reference. b will need a positional tolerance so i need basic dimensions...but from where? not c, because c is established based on b. this is where i say yes, locate b from non-datum features, maybe the edges or centerlines....but i'm just making sure that's ok to do.
i think you are trying to make something relatively simple, complicated.
in your case as stated, the location of b does not need to be considered. it is the secondary datum feature, where ever it exists. the same goes for c.
yes, they will have a locating dimension and a tolerance on the part, but they do not need to be located with basic dimensions.
jlang17,
i believe i understand and this is a common question. your previous thread states that datums b and c are holes. i will take that as cylindrical holes. you do not locate b or c from the outside part profile. you can make b perpendicular to a (don't forget the diameter symbol in the fcf in front of the tolerance). this is all that is required. datum b is the origin not the outside profile. datum c (another hole) would be basically located from b with a tolerance of position fcf. you now have established your drf. you would have basic location dims from b and/or c to position the outside profile along with the profile fcf. you are starting with a plate which does not have an outside a boundary defined; locating hole b perpendicular to surface a; locating datum c with basic dims relative to b; locating other hole positions to drf a|b|c (just guessing at the datum precedent); and then defining and locating the outside part profile with basic dims for definition; and basic location dims back to the drf.
it may appear with a locating datum b from the outside profile with basic dims, as if you are locating datum b from an edge. you are not. with this callout you are locating the outside edges from the hole.
hope i got your question right.

designbiz

i know i'm over complicating this, i tend to do that...
ok, so on datum feature b i can forget any locating fcf and just use +/- dimensions?

as i mentioned above you normally have a "refinement" callout for a datum(e.g flatness, perpendicularity, parallelism). the perpendicular callout i suggest to use fulfills this requirement. you would use plus/minus for the hole size limits. no plus/minus location dims for datum b.
designbiz

ok, sorry i posted that before i read your last post. thank you very much. thinking in terms of located the edges from b, rather b from the edges helped me understand.

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