【转帖】rue position of floating par

yang686526

true position of floating part
hi,
i have a dilema that i have not been able to resolve and am looking for some thoughts.  i am dimensioning a design that hase a captured component that is allowed to float (retained movement) to help alignment during assembly of the next part.  my customer is looking for a true position call out with variable data verification.  i can't figure out how to dimension it.  a standard true position will not show repeatability as the part moves by design within a range.  if i dimension the extreme travel points with a true position it is relying on set up variations in cluding pressure (it is a plastic assembly).  any thoughts?
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one could use positional tolerances without mmc and the movement of the centre of the feature could be confirmed using variable data.
if the feature is round, then there is a diametrical tolerance zone allowance.
if the feature is not round, then i would suggest positional at mmc with the term "boundary" below the feature control frame. now, it is the virtual condition boundary including orientation prevails over the centre of the feature. unfortunately, this is best confirmed with a checking fixture.
lastly, i hope that the customer does not want capability studies on positional tolerances. it should not be performed since the results are a bit "bogus".
hope this helps.
dave d.
true position controls the location of an axis within the tolerance zone. if the floating portion of the part is retained by fixed feature surrounding it (i am envisioning a floating pem fastener) then the true position callout in fact refers to the centerline axis of the whole component not just the floating center portion. the internal feature should have a tolerance of movement which is controlled by it's size and and the surrounding feature's size.
david
thank you for your thoughts gentlemen.  i agree the movement is controlled by the slot size and the pin size.  i have calculated total range of movement based on tolerances.  i have recommended the position be confirmed with a gauge in the past.  and yes, they are requesting capability studies.  where can i find information to support eliminating the cap studies on true positions?
the best way is an attribute gauge but the drawing must show the feature at the mmc mode so virtual condition size prevails.
i presented a paper to an asq group during a dinner meeting and this subject came up. i have about 3 pages on this subject and would love to post them in a pdf file so one can read it with acrobat reader but i don't know how to do it here.
all the ones that i have seen are bogus and as long as you state that each hole or feature has a ppk of 1.67, the customer will love you.
good luck on that one.
dave d.
dbalazs,
have you tried to apply the formula in the standard for floating fasteners?  if i understand your problem, that should provide a solution.  
thanks
ringman
i'm not familiar with the formula in the standard for floating fasteners.  can you direct me to it?
and yes 1.67 is my favorite number.
floating fastener info is in the standard on page 205 but i would again suggest it at mmc.it really is not a part of the standard but it is in appendix b for formulas.
hope this helps.
ps - the ppk must be above 1.67 on samples and above 1.33 in the production run in automotive today.
dave d.
let me see,now.......
do i correctly understand that positional tolerancing per asme y14.5 is being applied to the part drawing, but you dont know the formula for dimensioning? houston we have a proble.
you need to get a copy of the stadard and study it.
ringman