【转帖】inferred tolerance

yang686526

inferred tolerance
i have a component that is 123 +/-0.005 in width on witch there are some holes located on the centerline of part.
some are of the opinion that because these holes are shown on the centerline of the width with the applied tolerance, that the tolerance off of the centerline is +/-0.0025
there are others that believe that because there is no specific dimensional call out for the hole locations, that the default tolerance in the title block (+/-0.015) is the applicable tolerance for how far from the centerline these holes can deviate.
can anyone shed some light on exactly which opinion is correct, or if there is a different interpretation all together?
thanks
without an explicit tolerance on a dimension, the title block dimension is in effect.
don't rely on symmetry or anyone's questionable math skills.  add a dimension and tolerance.
centerlines without dimensions are poor practice for positioning features.  centerline of what?  inside?  outside?  other holes?
honesty may be the best policy, but insanity is a better defense.
in my mind, neither is true.  with no dimension to the hole, there is no tolerance unless you have a note that says "for all undefined dimensions, tolerance is yada yada yada."  of course, even in that case, there is still a question as to where to measure that dimension from.  the answer is simple when calling out the center of a cylinder (anywhere across the cylinder).  the answer is not so simple when looking at a square block (left side or right side?).  if you are saying that the centerline is the center between two edges, then that centerline moves as the material condition changes from part to part.  so, again, the question is "what are you supposed to measure to determine the tolerance?"  is it the distance of the edges to the hole, or is it the hole to the edges?  if the size is not what is expected, is it because the hole is out of place, or that the edges are out of place?
i think the simple solution is to add a dimension from one of the edges.  the better solution is to employ a positional tolerance.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
thr tick is spot on, as the brits say, there is no such thing as an assumed or implied symmetry from a centerline or a feature width. not dimensioned---not defined.
by the way, which witch put the holes on the part?
i agree with the others. for better clarification, leave the cl off the dwg and dim to all of the features. show only cl within the holes themselves.
to show a cl thru a part, can be a nightmare to measure for inspection & machinists, especially if a complicated part..
chris
solidworks 08 0.0/pdmworks 08
autocad 06
i agree as well. showing a feature on a centerline with no other dimensional controls means nothing. what you have is an incomplete print and open to interpretation.
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i agree with thetick.  assume nothing, if it's not explicitly stated on the drawing then it's open to misinterpretation which could be costly.
heckler   
sr. mechanical engineer
swx 2007 sp 4.0 & pro/e 2001
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this post contains no political overtones or undertones for that matter and in no way represents the poster's political agenda.
i too agree with tick and others except...
if there isn't a dimension to the holes and they are just shown on centerline then i'm not sure you can even justify invoking block tolerance.  
unless there's some note covering 'assumed centerlines' then the holes are undimensioned in that direcion, i.e. no requirement is given and the the drawing is incomplete.  so the holes can be anywhere (in that direction) and aren't out of tol.
if it's your drawing change it to properly define the holes as suggested above.
if it's someone elses drawing ask for clarification.
out of interest, what if any drawing standards are in place?  asme y14.5m-1994 or isos...  i think all of those posting above primarily use asme y14.5m-1994.  i don't know if there are any isos or similar that may say different but i doubt it.
kenat, probably the least qualified checker you'll ever meet...
hi all
my view on this is if you have holes located on a centre line then thats what it means the holes should be theoretically positioned there, however the positional tolerance of the holes needs to be specified otherwise as the tick and others say the position falls to the default tolerance block.
the holes shown on the centreline of a component may not be dimensioned because:-
lets say you have a square plate 100mm x 100mm and you wish to have a hole central in the plate, however the plate size may have a tolerance on the 100mm of +/- 2mm therefore if you dimension a centre of 50mm x 50mm you could well have your hole off centre, so if you show a hole on the centre lines or indeed on the intersection of the centre lines, whoever machines the hole as to measure the plate and find
the centre of it first, that said you still need a positional tolerance whether its the default tolerance block or a specified one.