Eric, You are correct on both counts. There are numerous tolerancing specifications. A well documented print will state which tolerancing specification was used. Since I am located in North America, it is the ANSI specification that I see called out the most. There is quite a bit of confusion about geometric tolerancing -- it is not as easy to understand as bilateral tolerancing. Jay Soderberg Here goes the opinion thing: They are mine and mine alone. Others may reasonably disagree. These opinions do not necessarily reflect the opinion of the ownership or management of this establishment. > ---------------------------------------------------------------- > > Subject: Re: TechNet Digest - 3 Jun 1999 - Special issue > Date: Fri, 4 Jun 1999 07:16:32 -0500 > From: Eric Christison <[log in to unmask]> > > Adeline, > > > > True position can be defined as the exact location of a feature as > > defined on the print (or some other manner) without taking any > > tolerances into account. A bilateral tolerance expresses the > maximum > > allowed variation as dimension based on an X-Y coordinate system. > > Geometric tolerancing expresses the tolerance as zone of allowed > > variation based on a circle of a defined diameter centered at the > true > > position. The diameter expresses the allowed variation. > > > > As Tom Burek pointed out, there are a number of variations possible > when > > using geometric tolerancing. Tom mentioned MMC (maximum material > > condition), but there are others also (LMC - least material > condition, > > RFS - regardless of feature size both come to mind). These > additional > > call outs change how the tolerance is applied. > > > > A good reference for all types of dimensioning is the publication > ANSI > > Y14.5M, Dimensioning and Tolerancing. > > > > Jay Soderberg > > > If you are working outside North America or dealing with customers > outside > North America you should refer to the ISO standard whose ref. number I > have > forgotten. However, if you are based in the UK refer to BS308 which is > compatible with the ISO spec and is an excellent document. > > It's important not to assume that ANSI drawing and tolerancing > definitions are > the same as ISO. > > However, if you're ever in any doubt it's best to ask the originator > of the > drawing. Twenty years of misunderstandings has taught me that many > people > (including those who use it) don't really understand this method of > tolerancing. That's why I, for one, don't use this method unless I > really have > to. > > Regards, > > > > > > Eric Christison >