Hi,
Roberto's description is correct. To take that to its conclusion,
consider that the radius of the circle is the hypotenuse of a 45/45/90
right triangle. With a TP = .009" (assuming this is TP diameter), the
radius of the circle is .0045". Applying the old a^2 + b^2 = c^2, and
knowing that a = b, you get sqrt(.0045^2/2) or .00318, which equals
your plus/minus xy tolerance.
Hope this helps...Andy
______________________________ Reply Separator _________________________________
Subject: Re: [TN] True Position
Author: Roberto Torres <[log in to unmask]> at smtplink-hadco
Date: 06/02/99 3:00 PM
Mr. R Hamilton;
A while back on one of the sigs I subscribe to, John Laur had this
explanation.
True position is a way of defining a tolerance as a circle as opposed to a
square.
The easiest way to describe this is to have you draw a square that
represents a
0.006" x and y tolerance. Now draw a circle inside the square that is
tangent to the four
sides. Color the area outside the circle but inside the square, IE the four
corners of the square.
The colored area is outside the 0.006" true position tolerance but is inside
the x and y tolerance.
Since you are drilling a round hole into a round pad this makes sense. The
0.006 is what is considered
allowable on old designs for whatever reason, in circuit test, auto
insertion. The front to back
registration is just what it implies. When all is said and done how far off
are the centers of your pads
from front to back. This could have a lot to do with controlling annular
ring.
This is the best explanation I have found (thanks John Laur) all holds up
except we use a .007" tolerance
at most the place I have worked.
As far as I can see this makes a circle .006" in diameter, so good x and y
locations in your case could
fall any where within that circle.
Note:
This varies according to the tolerance zone specified on the Fabrication
drawing. Positional tolerance
zone or Bonus Tolerance Based on Maximum Material Concept.
Geometry note: A point is inside a circle if the distance form the center is
< then the radius.
Sorry for the length, I hope this helps.
Roberto Torres C.I.D.
Matsushita Avionics Systems Corporation
-----Original Message-----
From: Richard Hamilton [mailto:[log in to unmask]]
Sent: Wednesday, June 02, 1999 2:08 PM
To: [log in to unmask]
Subject: [TN] True Position
Hello all,
I am sure this one will stretch the memory!! I used to know
the specifics,
but when you don't use something, well, it kinda goes away!
We have a hole position tolerance of .009"TP (True
Position). Can anyone
tell me how to convert this tolerance into an X-Y location
value? Like,
based on the zero datum you are allowed a maximum of .003" x
.003" off the
defined location. Or, you are allowed a maximum of .006" in
any direction
combination which represents the .009"TP.
The only number I am using from actual is the .009"TP.
Taking that tolerance
and defining the location values allowed is what I am having
a tough time
finding out.
Thanks to anyone who can save me here!
Richard Hamilton
Clemar Mfg. / Rain Bird
[log in to unmask]
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