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Justification for GD&T
Geometric Dimensioning and Tolerancing (GD&T) has been
used since the 1940's in the United States. It was developed
to address the many problems that had been encountered over
the years as companies tried to describe their part geometry.
They realized that it was very difficult to describe how much
variation was allowed on their part and assembly geometry.
More importantly, they found that everybody that read their
drawings had a different interpretation of their Dimensioning
and Tolerancing specifications and the limits they created.
Companies have found that they had difficulty clearly
describing the size and form limits of individual part and
assembly features. For example, it wasn't clear from their
drawings how flat certain surfaces had to be, and in other
cases it wasn't clear exactly what the requirements were
for the size and form requirements for a hole.
They found that they had even greater difficulty describing
how much variation was allowed between features. For example,
it was even more difficult to understand how much a hole was
allowed to tilt relative to a surface, or how much variation
was allowed between related surfaces.
GD&T was developed specifically to address these problems
and eliminate the ambiguity that using the traditional Plus
and Minus Dimensioning and Tolerancing introduces. Its primary
purposes are to make the drawing specifications clear and
unambiguous, and to eliminate the problems, confusion, scrap,
rework and loss of profits that results.
Many companies ask us about their Dimensioning and
Tolerancing problems, and many of them want us to help
them understand why they are experiencing problems with
their parts and assemblies. In most of these cases, their
problems are a result of the confusion generated by using
Plus and Minus Dimensions and Tolerances to locate and
orient features. The majority of these problems can be
solved by proper application of GD&T and making sure
their staff and their suppliers are trained to understand
what it means.
The ASME Y14.5M-1994 Dimensioning and Tolerancing standard
is the result of many years of experience in industry and
research and development by various standards committees.
It is the latest in a long line of U.S. Dimensioning and
Tolerancing standards dating back to 1946. The standard
answers many of the questions that arise from using Plus
and Minus Dimensioning and Tolerancing to define Features
of Size, and it contains extensive material describing
how to use Geometric Dimensioning and Tolerancing to
describe the form of features and the relationships
between features.
Aside from cases where the process capabilities are
inadequate to meet the tolerancing specifications, the
vast majority of problems relating to out-of-spec parts
are a result of improper Dimensioning and Tolerancing
specifications. The majority of these cases are where
Plus and Minus Dimensioning and Tolerancing was used to
describe the relationship (orientation and/or location)
between features. There is simply too much ambiguity in
Plus and Minus Dimensioning and Tolerancing - it leads to
multiple interpretations of what the drawing specifications
mean, and there is simply no way to say which of these
interpretations are correct.
Perhaps the most pressing problem that GD&T corrects
is in clearly defining how Features of Size (such as holes
and pins) are located or oriented to other features on
the part.
We offer management presentations, helping to make it clear
why using GD&T is so much more useful and cost effective
that using Plus and Minus Dimensioning and Tolerancing.
We also offer a White Paper that presents the benefits of
using GD&T versus Plus and Minus Dimensioning and
Tolerancing, and it clearly explains why GD&T
is so important for you to adopt.
Please Contact Us to set up a management presentation or
for a complimentary copy of our White Paper.
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