huangyhg

http://www.eng-tips.com/viewthread....d=259057&page=1

fsincox (Aerospace)

15 Nov 09 14:00
I am interested in hearing the different points of view on your philosophy to GD&T.
As new draftsman we had always been told "you can't use bolt circles, only co-ordinates" and "don't dimension from centerlines, only edges" I suspect these are a lot like the caliper guys of today. When I was first trained in Y14.5-1982 in 1987 I found it a very liberating. The philosophy I was told was if it did not violate the basic rules or is not prohibited by the standard it was OK, Notes on drawings were not desired because of language barriers, but in extreme cases, you may need to supplement with a note to explain what you intend. The sense I got was it was a tool kit to be used and the simplistic examples in the text were just that, period. The book certainly did not explore the limits of what can be done it was more of a universal language that would be built upon as languages do. English, for example, has had words like computer and geometric dimensioning and tolerancing added to it (my MS word still thinks tolerancing is not a word or is misspelled). We all know it is a word.
In my first job AGDT (After GD&T Training) I worked with a lot of machines and a lot of dowels patterns that people always wanted located to unimportant edges ("the from the edge guys", always 2 dowels in case some don't know), I said: "fine, we have this new tool called composite position tolerancing that was perfect for that". Eventually, the question was asked: "now, can we put an orientation on the centerline between the dowels to refine for orientation". "Not in the ANSI world", I said, "this is explicitly prohibited", if we were ISO well life would have been easy. I was told the committee was working on just that issue, and, the restatement of secondary datums in a composite position tolerance would do just that. We were also instructed that since the standard did not actually show it we may want to add a flagnote to explain what we meant. This practice is part of the standard now and since Y14.5-1994 an accepted practice, some apparently argued it was implied before in the 82, but, it was not explicitly shown.
Out here in the real world we do not always have time to wait for the politicians to make decisions, the job has a deadline and we need tools to do the job. MMC and LMC (also a new concept at the time) are good valid tools, adding it to profile tolerances to get the job done foe the heavy hitters is great, but why take it away from the poor little radius, Is it really because it is hard to measure? Life isn't always easy.
There's lots I want to get into with guys who are interested in exploring ideas.
Like:
To circle "E" or to circle "I", that is the question?
Why not true position of a surface instead of profile?
Why perpendicular and parallel, not just orientation?
Is a feature defined by a radius really different than the same one defined as diameter?
Doesn't anyone out there use the dreaded ISO and like it?
How can rule #1 not be a violation of all the logic all we are trained in as engineers and assume the worst case, as ISO does, by the way. Must we cling to our calipers in one hand and our concept that we will someday actually produced that perfect feature at MMC in the other? (When I am asked by the shop to accept an oversize shaft is it more perfect, then?)
Anyway thanks, if you bothered to read this far, I guess I will get off my soapbox for now to give someone else a chance.

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