Think that Brad meant to say "positional tolerance zone within a
circle," not the square. This is the wonderful part of GD&T. Using the
square, or bilateral, approach, a vendor could theoretically provide
boards with annular ring breakouts, and say they were within spec. Not
so with GD&T! A drilled hole in a round pad would (theoretically) never
break out with the proper diameter and positional tolerance spec.
Obtain a copy of IPC-2615, which basically is a subset of the ASME
Y14.5M-1994 spec; the 325A is, like, old...
BTW, you're missing a datum in your geometric statement. You will need,
for holes not used as datums, 3 datums: primary, secondary, and
tertiary. So, for your example, a complete statement would be:
Position (symbol)
Diameter (symbol)
.005 (tolerance)
* Z (datum) * [primary datum, underside of board typically]
X (datum) [secondary]
Y (datum) [tertiary]
Otherwise the board house could drill a hole at an angle and it would
still be in spec! Gotta have that primary (z-axis) plane!
(doin' the Friday dance...!)
KRgds,
Roger
P.s.: John Sabo taught a course in GD&T for boards, anybody know if he's
still around (was at A-B in 2001)?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brad Saunders
Sent: Thursday, September 07, 2006 4:45 PM
To: [log in to unmask]
Subject: Re: [TN] Geometric Tolerance
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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