【转帖】datum consumption

yang686526

datum consumption
not sure if i need to post a picture so i will try to describe this as best as i can.
i have a hole pattern in a stamping located with a positional callout (mmc) to three datums:
a = plane, surface of part
b = central pilot diameter (mmc)
c = hole, not contained in hole pattern (mmc)
when the stamped part is used to produce the final part, a rivet is orbitally formed into datum c. after all other parts are added to finalize the assembly, the rivet cannot be reached to use as a datum feature.
so in the assembly i have specified the position relative to two datums (a and b as defined above).
the other engineer in the group thinks that i have to have a third datum to "clock" the hole pattern. in general i think she is correct, otherwise the pattern can technically rotate around datum b. however the higher level drawing does prevent this to a certain extent. the problem is that i do not really have a valid feature that i can use for a tertiary datum that makes functional sense (i don't want to pick a bogus datum just for the sake of doing so).
any ideas (do i really need a tertiary datum)? thanks!
if i understand correctly you are inspecting at the next level after assembly.
i thought dimensioning only applies at the drawing level it is specified on.  asme y14.5 1.4(n)
quote:
dimensions and tolerances apply only at the drawing level where they are specified.  a dimension specified for a given feature on one level of drawing (for example, a detail drawing) is not mandatory for that feature at any other level (for example, an assembly drawing).
so inspecting it after it is assembled for compliance with dimensions given on a lower level drawing is bogus isn't it?
id think its, 鈥榗locking鈥?should be inspected before assembly.
did i understand correctly, does this help?
kenat, probably the least qualified checker you'll ever meet...
i think you hit the nail on the head kenat. we are currently debating this because it is company policy for us to check each other's drawings - this creates serious problems since some are less versed in gd&t but function as checkers.
i understand where she is coming from but i really don't want to add a tertiary datum that i know to be bogus and will probably create problems.
i'd probably say keep the tertiary datum if orientation of the whole pattern wrt the datum c hole is significant.
however this should be detailed on the part detail drawing & inspected on the part before it is assembled, so the fact the rivet fills the hole/hole is otherwise in acessible is irelevant.
or do you have a detailed assembly or something?  even so if the dimensioning is given on a view of the unassembled plate i'd think this was how it should be inspected, but now i'm pushing the limits of my knowledge.
or is the datum so that parts are assembled to the whole pattern in the correct orientation, i'm starting to get confused.
as to the company policy, while any check is probably better than none there's a massive difference between regular peer checking and getting detailed checking from knowledgable senior designer or dedicated checkers.  see
the stamping drawing is correct. you do need a tertiary datum for orienting the pattern about the secondary datum. i like that fact that both the tolerances and the datums are at mmc so you will be able to fixture it. you will now control the pattern at the stamping operation on the shop floor.
now the assembly drawing.
why would you want to place positional tolerances on the assembly drawing when the pattern was produced at the stamping operation? i guess that there must be a reason and if there is a reason, you do need a tertiary datum for orientation but now you have a problem if you do not use the same tertiary as the stamping.   
dave d.
i should have mentioned that we typically use the assembly drawing as sort of a data sheet to our customers. i am currently trying to change this but it is a slow process that has met with a lot of resistance.
so the positional tolerance is typically included on the final assembly drawing because our customers use this for fit-up.
i have been thinking about it, i don't think the tertiary datum on the drawing brings value to the assembly drawing since it is controlled earlier. i think it will only confuse matters since the mating part almost never includes a tertiary datum (or positional tolerance at all in most cases).
thanks for you help and opinions, it is good to get other points of view.
joebk:
here is another thought.
the stamping drawing absolutely needs the 3 datums. this will include the feature to feature position and also the pattern position relative to datum hole b and tertiary hole c.
on the final assembly drawing, show the positional tolerances at mmc with only a reference datum a. in other words, we have a hole to hole positional tolerance and perpendicular to datum a. this is called the frtzf per asme page 95. this would reflect the features within a pattern and is conducive to assembly of the mating parts.
dave d.
dave, you must have been looking over my shoulder this morning. i convinced my colleague to stick with two datums (a and b at mmc) since the pilot diameter on the assembly is functional. thanks for the feedback!
joebk:
i only mentioned referencing the positional tolerances at mmc to  datum a. this excludes orientation of the pattern which is on the stamping drawing.
this application is conducive to a situation where we have 4 pins on a fixed location that have to assemble in the 4 holes of your part.
usually, when one has a positional tolerance at frtzf referencing the primary datum, the value is a bit less than the positional tolerances to datums a, b & c.
dave d.