Hello Don,
The others were right on target however I love this method so much I thought I'd reply.  
simply put the diameter tolerance is equal to the diagonal of the ole +/- tol zone; the square.  The bonus comes in understanding the MMC, usually the small side of the hole size callout (maximum material); the diametric tolerance zone is still the larger size; hence bonus area.  What is even greater is understanding the datum callouts.  If more folks followed this procedure the NPTH associated with PTH would be much easier to deliver.  It really is great, don't even get me going on profile tolerance; it's the best thing since LPI mask... 
Get a copy of Y14.5 and have a blast!
Sincerely,
Boston Brad
  ----- Original Message ----- 
  From: Donald Kyle<mailto:[log in to unmask]> 
  To: [log in to unmask]<mailto:[log in to unmask]> 
  Sent: Monday, July 10, 2006 11:09 AM
  Subject: [TN] 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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