Many of you may know that GD&T has been around for a long time (see an earlier blog entry about the history of this system). And like anything else that’s been around for a while, things sometimes change.The current American national standard for GD&T is maintained by the American Society of Mechanical Engineers (standard number Y14.5M-1994). Prior to 1994, the previous edition was dated from 1982 (and 1973 before that). And each time, there were a few things that were changed in the rules and symbols of GD&T.
So, if you look at the history of the standard, you might guess that it’s about time for another revision — and you would be correct! The Y14.5 committee has been hard at work for the past couple of years, and the next edition is in the review phase. (One sneak preview for you GD&T geeks: a new modifier to indicate unilateral profile tolerances.) It is projected that it will be released for general use in the first part of 2009.
As a side note, this underscores why it is important to always mention the specific tolerancing standard in the general notes of your prints, or even as part of your company’s standard title block. If this is not mentioned, then there may be confusion about the meaning of some callouts!