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旧 2009-09-05, 10:41 AM   #1
huangyhg
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默认 fcf on reference dimensions

fcf on reference dimensions?
i recall coming across this before saying that there should not be a fcf on a dimension that is reference. i was trying to find where i saw it before and seem to not be able to find it. what is the rule with this?
i attached a sketch of what i'm talking about.
thanks for the help.
this is allowable if the part features were already defined, either in a separate drawing or elsewhere on this drawing. the fcf is locating the feature of one part to that of another. since these features have already been defined elsewhere, reference dimensions are appropriate.
good to know you got shoes to wear when you find the floor."fff"> - robert hunter

thanks a lot. i figured that was the case since i couldn't find it anywhere but for some reason it caught my attention. thanks again.
i'm not sure that this is allowable, even if the sizes were defined elsewhere. but i don't have any specific references to back that up.
in any case, i think it's a confusing practice at best. why would a feature's size tolerance and geometric tolerance be shown in different sections of the drawing? the same thing goes for the datum feature's size tolerance and its datum feature label.
evan janeshewski
axymetrix quality engineering inc.
it is not only permissible, but may be required in the case of serspec's attached sketch. the dimensions shown may be from the part were the tolerances were defined. the assembly should show the requirements at the assembly level, should the diameters not change during assembly then the dimensions used for the fcf should be reference. as alternative just use a fcf without a numerical dimension.
peter stockhausen
senior design analyst (checker)
infotech aerospace services
paragraph 1.4(n) of asme y14.5m-1994 states that "dimensions and tolerances apply only at the drawing level where they are specified." without the tolerances for feature sizes the variable portion of the positional tolerance and allowable datum shift resulting from the mmc modifiers cannot be determined.
if this part an in-separable assembly then it is a new part and it can (should) include each necessary dimension paragraph 1.4(c).
i know that this is not often the practice in design because assemblers claim that they cannot be held liable for sub-assembly "pass-thru" deviations... while the sub-assembly operation claims that the assembler introduced the deformation that caused the previously qualified feature specification to fail.
with in-separable assembly operations un-predictable things happen and when they do new specifications end-up on the assembly drawing despite all objections.
paul

the part is an in-separable assembly and the two components diameters are called out in their part level drawings. our policy here on assembly drawings like this is to always include the inner and outer diameters and lengths as reference on the final print. i just wasn't sure if since this particular part, or 3 parts, has a fcf associated with the dimensions, should those be regular dimensions instead of reference. this came up because they currently are regular diameter dimensions and they are all being changed to reference. just wanted to make sure that the change wouldn't be violating any standards.
i can certainly see this kind of thing being necessary for some assemblies. we have a lot of nominally coaxial items that have to be glued together at the assy level but don't have features to ensure alignment. to my mind this is a classic case where you might want to do this type of tolerancing.
kenat,
from what you described... i would say that it violates the standards that i cited... but politics in quality assessment typically intervene and the necessary size tolerances are omitted... until the part fails to function as predicted... then the critical specifications are added as i stated.
paul

quote:
if this part an in-separable assembly then it is a new part and it can (should) include each necessary dimension paragraph 1.4(c).
the key here is the word "necessary" and how you interpret it. if the feature was created on another drawing and no intent to remachine it is inherent in this drawing, then the dimension isn't really necessary for part definition.
i tend to agree with peterstock that it may be better to exclude the dimensions altogether, unless there are similar features that may cause confusion.

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

for an inseparable assemply, the od in the posted drawing would likey have changed from the as produced state. the new dimension should include that effect and not be reference. if control of the as assembled od not be important then a referene dim or no dim should be used.
peter stockhausen
senior design analyst (checker)
infotech aerospace services
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