Greetings! Good to hear a little GDT rag on here every once in a while.

Don't know why but "Position" is a often misunderstood by folks.  In
it's simplest form (as you've described) it's really very simple.

Position = Theoretical (nominal) position of a feature (center of a
hole, for example)

Diameter (.005) = the "zone" for which that feature must reside. The
theoretical center of the hole (radius) is allowed to travel ANY
direction .0025 from theoretical center, defined by datums A,B,C or X &
Y in your case. The concept utilizes a circular tolerance zone as
opposed to a square tolerance zone.

If you guys are going to be using this on drawing specs or just
interpreting outside specs, you really need to get everyone on board
with some formal GDT training.

My explanation above is paraphrased from some past experience that I've
had with the subject.  Short answer -- long subject -- hope it helps.

-- Kelly
________________________________________

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Donald Kyle
Sent: Monday, July 10, 2006 8:10 AM
To: [log in to unmask]
Subject: [DC] Geometric Tolerance

We are having a little debate here as to the interpretation of a
Geometric statement evolving the tolerance of holes on a PCB.

The geometric statement reads from left to right:
Position (symbol)
Diameter (symbol)
.005 (tolerance)
X (datum)
Y (datum)

Some say that this statement means a positional tolerance zone within a
circle.

Others say that this is just another way of stating a bilateral
tolerance zone within a square.

How do you interpret this?

If you have IPC-D-325A you can read about this on page 28 and
appendix B for more information.

Because I don't know how to insert a geometric symbol for POSITION
and DIAMETER you will need to use your imagination here.

  Geometric      Geometric       Tolerance       Datum
  Symbol for     Symbol for                      Reference
  Position       Diameter
        __             __                   .005                 X
Y


Thanks for your time


Donald Kyle C.I.D.+

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