> 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

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