yang686526

dimensioning displaced state vs. free state of elastomer pro
the product of discussion is a rectangular silicone slab with sealing surfaces around the perimeter and inside thru-holes positioned about the interior. the part is installed in a housing whose mating surface perimeter is porportionally smaller than the part's perimeter. this produces some amount of displacement.
because the function of this elastomer part is only realized in a displaced or forced state, i have proposed that our drawings require a true position callout for the interior holes in this displaced state. formerly, true positions were shown only in a free state (aka: as manufactured).
forced state measurements would be taken by installing the part in a fixture detailed on the drawing.
initially, from a design standpoint, the delta from free state to forced state, assumes a stable material and manufacturing process.
my problem is how to convey different dimensional tolerances to manufacturing and to metrology within the same drawing. because of the above assumptions, i believe it would be unreasonable to assume that any free state dimensions dictated will always produce the more important forced state dimensions.
would it be best to include only the forced state dimensions on the engineering drawing and create a seperate manufacturing or processing drawing? is there an accepted methodology for including both on the engineering drawing? currently, my company does not produce seperate drawings for engineering and manufacturing.
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jszymkowski,
asme y14.5m-1994, section 6.8 discusses free state variation. this provides the logic needed to solve your problem on the drawing. this logic requires the fabricator and inspector to force your compressed state when they fabricate and inspect the part.
perhaps it would be easier if you worked out the uncompressed form that results in the correct compressed form.
does the form of your gaskets really need to be precise?
jhg