Ross,

I agree with Ed, keep it simple.  If you believe that GDT is required to properly specify your product by all means use it, BUT make sure your designers and your receiving departments understand how it works.  You wouldn't believe some of the horror stories that fabricators can relate about poorly prepared GDT drawings.

Interpretation of the drawings is not difficult if they are properly prepared.  The preparation at the design end is significantly more difficult.  Determining the proper minimum and maximum material conditions can require significant effort.  In my opinion, transforming an orthographic tolerancing scheme to GDT without reexamining the tolerancing zones will not gain you any benefit.

Don Vischulis

Ed Cosper wrote:

> Ross,
>
> One thing I have learned over the years is keep it simple.  Put what you need to put on the print and always try and be as specific as possible. I think you will find that most board shops have a very good understanding GDT.  Another thing you should consider is your receiving inspection area.  Don't forget, that whatever you put on your drawing, your own personnel will have to work with as well.  Just my thoughts...
>
> Ed Cosper
> Director Quality Assurance and Engineering
> Graphic Electronics Inc.
> Tulsa OK
>
> ----------
> From:  Ross A. Eskite[SMTP:[log in to unmask]]
> Sent:  Monday, April 13, 1998 3:36 PM
> To:  [log in to unmask]
> Subject:  [TN] Geometric dimensioning & tolerancing
>
> Greetings-
>
> We are currently in the midst of a lively discussion
> in our CAD department over the pros and cons of
> geometric dimensioning and tolerancing (GDT) when
> used on PWB fabrication drawings.  From what I've seen:
>
> Pros:
>   More accurate (potentially)
>   Provides maximum producibility in achievement of
>     desired fit (when competently applied)
>   "Encouraged" by IPC
>
> Cons:
>   Poorly understood by many draftsmen
>   Poorly understood by some board shops (this bothers me)
>   Takes longer to produce a drawing
>   Many consider it "overkill"
>
> I'd like some feedback from other designers, board shops,
> ME's, etc., on this issue.  Has anyone had any compellingly
> positive or negative experiences with GDT?  Would you write
> it into *your* PWB CAD process?
>
> TIA,
> --
> Ross Eskite
> "Though I fervently recite the Mass in perfect Latin to the cannibals,
> somehow I always end up in the big black pot with the carrots" - me

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