【转帖】limit style tolerancing vs bilateral

yang686526

limit style tolerancing vs bilateral
we are in the process of writing drafting standards, and one of the debates is in the method of tolerancing.  the majority of our old drawings had ± tolerancing, with some limit style tolerances.  the group working on the standards is heading towards standardizing on limit style dimensioning only.  the reasoning is that when you are inspecting a part, you only care if you are outside the limits, so the nominal does not matter.  the debate comes on the question of design intent and the cad geometry.  a part with a .4998+.0001-.0005 tolerance has a different design intent than a .4999/.4993 toleranced part.
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they can bothn be modeled to the nominal .4998 on ug and pro/e, don't know about others. you just need to set your tolerances properly.
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ben loosli
cad/cam system analyst
ingersoll-rand
agree, but the question is if someone sees a drawing with the part toleranced as .4999/.4993, how are they supposed to know to model it as .4998 (design intent) instead of .4996 (middle value)?
i've had this argument before.  i like bilateral tolerancing for the same reason: it shows the intent better.
however...
most machinists/operators/inspectors do not care about your intent.  they also don't care much for mental math and would prefer the easy way out.
the fact remains that if your part doesn't function over the entire range of the tolerances, then something is wrong.
star for thetick, i agree.
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pugap,
   is it necessary to rigidly mandate this?  
   you should format the information for whoever is going to use it the most.  that standard changes depending on production levels, inspection requirements, and the need for engineering to monitor the process.
                     jhg
that's the crux of the debate.  the majority of the engineers are against the limit style, but the drive from the group developing the standards is to dimension prints to help manufacturing/inspection.
is the drafting/design dept under the control of manufacturing or engineering?  everywhere i've worked, it was an engineering department.  while consideration should be given to manufacturing methods, i feel that it is more important that the design be documented as engineered.
this seems to be boiling down to the same line of reasoning as
i agree with tick.
you can design for intent, but machinist will go for the nominal or mean. years ago, our company standarized on limit dimensions. we create our models at nominal. all of our machinists and vendors understand it and makes it easier to import into cam. i like bilateral in some cases, but we don't uses it unless a customer requires it ... it is rare.
chris
sr. mechanical designer, cad
solidworks 05 sp3.1 / pdmworks 05
if the intent of the _document_ is to preserve or communicate the design intent, then nominal +/- deviation style dimensioning is appropriate.
if the intent of the _document_ is to show what dimensions are to be measured, and what limits the measurements must lie within, then limit style dimensioning is appropriate.
yes, one of our customers maintains a separate set of inspection documents, with simplified geometry, and limits shown only for the dimensions to be measured, and with tabular space reserved for each item in a set of samples.  a copy of that drawing becomes the inspection record for that lot.
mike halloran
not speaking for
deangelo marine exhaust inc.
ft. lauderdale, fl, usa