in a Complete Manner
I knew you would get
around to tolerancing
[“CAD templates and otherwise specified tolerances,” Precision Matters, February 2019, p. 58], a subject
dear to my heart!
It’s been said that there are two reasons for
tolerances: to make parts producible and to
make sure they work. Your approach in the February issue validates the first, but I’d like to see
you add something about the second.
First some key concepts:
1. Tolerance is what people put on the drawing,
Variation is what God and the process put into
the parts. (In cases of dispute, guess who wins?)
2. Tolerancing for a mass-produced part is different from tolerancing for a single or individually produced part.
3. Variation refers to di;erences between
mass-produced parts, while deviation refers to
di;erences between a single part and the designer’s purest intent.
Part of my role is providing “real” tolerances,
which is how far from the perfect depiction in
the CAD drawing or model a part can be and
still function. Though modern manufacturing
processes often provide parts with only tiny deviations, there is sometimes a delicate balance
between small tolerances (expensive but ensuring function) and large tolerances (cheaper but
may not work as well). We look for a sweet zone
where an available or low-cost process will limit
variation to a level that ensures the function of
You didn’t mention the ASME Y14.5 standard
that provides guidance in tolerancing. Basic rules,
such as every dimension must have a tolerance
(in the 1994-2009 revisions), which changed to
every feature must have a tolerance (in the 2018
revision), help define how a drawing should ex-
press product requirements. The change from
“dimension” to “feature” between the 2009 and
2018 revisions is significant as it draws a distinc-
tion between tolerancing lines on the drawing
and tolerancing features on the part.
It is advantageous for a designer to know what
processes are available and their relative costs,
but allowing the variation of the available processes to determine the drawing tolerances can
be a serious mistake. It can lead to parts that
don’t function or that need rework or repair to
meet basic functional requirements. The other
side of the coin is, of course, the specification
of tight tolerances (just to be sure) that drive
part production to processes, which can’t even
be spelled without a lot of dollar signs. (Let it
never be said that tight tolerances can’t be met;
it’s only a matter of how much needs to be spent
to achieve them.)
The link shown in Figure
1c (see photo) need not
be produced by bending
a piece of material 90 degrees at two places. It is also unclear from the design whether the four holes are to be made in a
flat blank that is then bent or if the functional requirements can be met by making holes in a bent
or extruded channel. (I sometimes refer to the
di;erence as “burn and bend” versus “bend and
burn.”) If tight tolerances are required between
holes that are separated by multiple bends, the
bend and burn process can produce a part with
less deviation (or a bunch of parts with less variation) than the burn and bend method.
The taper of the casting is also of interest. That
feature may be designed without taper, for a
perfect fit for the link. The taper may be added later
by a casting supplier as it is needed for the casting process. It then makes a big di;erence if they
make the taper by adding or subtracting material.
Tolerance analysis can be performed to es-
tablish tolerances required for function. Often
referred to as a tolerance stack or stackup, the
intent is to understand what the e;ects of part
variation will be on an assembly before parts are
produced. A timely tolerance analysis may even
reveal that the production costs of a product
will be too much to justify its production.
Tolerance analysis can range from adding minimum and maximum dimensions (worst case,
one dimensional) to the use of sophisticated
software that includes the e;ects of varying
form, orientation, location, fastener clearance,
and even part flexibility. Whatever level of analysis is used, it is better to do something than to
just cross your fingers.
Mike F. Matusky
Old School Is Cool
Editor’s Note: The February 2019 edition of the
“Fabricating Update” e-newsletter referenced a
blog entry from Senior Editor Tim Heston titled
“Get to know a metal fabrication guru.” The blog
entry covered a recent visit to Metcam, a precision custom metal fabricator north of Atlanta,
and a conversation with Ken Roostee, shop plan-ner and an experienced metal fabricator. The
blog generated the following comments.
Too funny to see a story about Ken Roostee.
Kenny and I used to work together in Las Vegas.
I too am one of those old-school gurus you
talk about with 40 years in the trade. When we
are gone, it will change the game.
Fabrication Technologies Inc.
Very nice article on Kenny Roostee.
Some of those skills seem to be a dying art,
and it is hard to get young folks involved to learn
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FEBRUARY 2019 | VOL. 49 NO. 2
How real-time machine data