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Justification for Tolerance Stackups
Every product manufactured today is subject to variation.
From the peaks and valleys of integrated circuits in the
microscopic regime, to the buttons on the cell phone in
your pocket, to the structures of bridges and the space
shuttle in the macroscopic regime, no product or part is
immune from variation. Understanding this variation and
quantifying its effect on the form, fit and function of
parts and assemblies is a crucial part of the mechanical
design process.
Tolerances limit the variation allowed for individual
feature geometry, such as form and size, and they limit
the geometric relationship between features, such as
orientation and location. Tolerance Analysis and Tolerance
Stackups are the tools and techniques used to understand
the cumulative effects of tolerances (accumulated variation),
and to ensure these cumulative effects are acceptable.
There are two methods used to specify tolerances:
traditional Plus and Minus Dimensioning and Tolerancing
and Geometric Dimension and Tolerancing (GD&T.)
The point of Tolerance Analysis is ultimately to prove
that a Dimensioning and Tolerancing scheme will work,
and the only way to precisely specify the geometric
requirements is through the use of GD&T. Although
Plus and Minus Dimensioning and Tolerancing is still
commonly used, and we explain how best to perform
Tolerance Stackups on parts and assemblies based on
Plus and Minus Dimensioning and Tolerancing, part of
our goal is to help our clients understand why GD&T
is a much better system for their business.
Tolerancing, Tolerance Analysis and Tolerance Stackups
have been around in one form or another for a long time.
Sometime in the past, it became necessary to determine
whether mating parts would fit together before they were
manufactured. A design team may have needed to know how
thin a part feature could become during manufacture, to
make sure the part would remain strong enough to work.
They may have needed to know how large a hole could be
and how far it could be from its nominal position to make
sure there was enough surface contact to properly distribute
the load from a fastener. It may have been important to
know that all the parts would be interchangeable. Perhaps
the manufacturing team needed to understand why the parts
in an assembly that met the drawing specifications did not
fit together at assembly. By performing Tolerance Analysis
and Tolerance Stackups, these and many other important
questions about the design can be answered. Indeed that
is why Tolerance Analysis and Tolerance Stackups are done -
to provide answers to questions. Tolerance Analysis and
Tolerance Stackups will help you to understand and solve
your tolerancing problems.
How can the designer determine if parts will fit together
at assembly? Better yet, how can the designer determine
if imperfect parts will fit together at assembly, as all
parts are imperfect? How much imperfection or variation
is allowable? Does it matter if a part is manufactured a
bit larger than nominal, and the mating part is manufactured
smaller than nominal? What if both parts are manufactured
on the small side, but mating holes in each part are slightly
tilted or out of position? What affects the performance of
the assembly more - variation in size or variation in
position? What happens to a feature on one part if a
surface on the mating part is tilted? These questions all
lead to a Tolerance Stackup.
Tolerance Analysis is part art and part science. All
Tolerance Analyses and Tolerance Stackups are truly
three-dimensional; the problem solver is forced to frame
the problem in such a manner as to facilitate a one-dimensional
solution. Simplification and idealization of the problem
is required.
The importance of a standardized approach to solving Tolerance
Analysis problems cannot be overstated. Equally important is
the need to communicate the results of a Tolerance Stackup.
Rarely, if ever, is a Tolerance Stackup done without the need
to share the results or to convince someone else to make a
change. Techniques must be used that help ensure that the
problem will be solved correctly and that the results will
be understood by all parties involved. Our training and
material addresses these needs and presents the techniques
for developing and formatting a standardized Tolerance
Stackup Report.
As stated above, Tolerance Analysis is an art, and it
requires practice to become an effective problem solver.
Using our techniques, our clients will be on the path to
understanding and effectively solving their tolerancing
problems.
Please Contact Us to set up a management presentation or
to discuss our Tolerance Stackup and Tolerance Analysis
products and services.
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