A good example of asymmetric machining tolerances is when one part needs
to fit inside another part.
Suppose an axle has to go through a sleeve bearing, or suppose a flange
has to bolt into a recessed port on a vacuum chamber...
If the nominal flange size is 5 inches, and you want precise alignment
with clearance of about 5 mils, you might specify the flange as 5.000 +0
-0.002 and then specify the port-recess on the vacuum chamber as 5.004
-0 + 0.002. You could end up with pieces "right on" in which case you
have 4 mils clearance. You could end up with each at the maximum
deviation and have 8 mils clearance. If you had said 5.000 +-.002 and
5.004 +-0.002, then both pieces could end up 5.002, and I know from
embarrassing experience that a 5.002 flange will not go into a 5.002
hole.
This experience was actually a trick played on me when I was a graduate
student and I was learning from a master machinist. I used the
symmetric tolerances. The machinist knew what I was designing, and he
intentionally made the flange and the port-recess right at 5.002 inches
each. When the flange wouldn't fit, and I accused him of sloppy
machining, he got out the micrometers and showed me that the machining
was just at the maximum tolerance I had specified. He then said, "Today
you have learned something about specifying tolerances." Then he took
the flange, shaved off a few mills, and said, "Now it will fit. Don't
make that mistake again."
He was an excellent machinist who typically made things within a
tenth-mil of the given dimension even if tolerances were looser than
that. It didn't even take him extra time to do very precise machining.
He was also an excellent teacher.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu