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Geometric dimensioning and tolerancing
Geometric dimensioning and tolerancing (GD&T) methods are gaining greater
acceptance in the industry. The GD&T system uses a series of symbols and codes on
blueprints to communicate the characteristics of critical features that need to be
produced. This system, credited to Stanley Parker, was first developed in the 1940s in
Scotland and has developed into a detailed set of standards issued by both ANSI and ISO.
Geometric tolerancing is used to universally define the form, orientation, and location of
part features on blueprints.
This article is the first in a series that will discuss GD&T. This is an involved
subject and we welcome comments and input from our readers.
What is GD&T? We like to say it is a system that replaces words with symbols,
simplifying blueprints. Let's consider the challenge a designer faces when a series of
parts are required for a final assembly. Each part will be engineered to perform a
function and, most importantly, to assemble with a mating part.
What do we mean by the term tolerance? Two words can define tolerance--allowable
deviation. A tolerance is the amount of deviation from the exact size, allowed on a part.
Any part within the tolerance will still be functional. So why are tolerances used on part
prints? We are all aware that we cannot manufacture anything to an exact size. This is due
to the natural imperfections in the world including machine tools, part programs, tooling,
and, most importantly, all of us. As they say, if we could make a perfect part, the
quality control department could not verify its perfection because the gage blocks and
measuring equipment are not perfect either. So a plan is needed to allow the production
process to accept all the imperfections that are going to occur.
The challenge for a designer is to set part tolerances that are large enough to keep
manufacturing costs down and close enough to ensure that all parts will assemble with the
mating part, without resorting to a big hammer. It is easy for the designer to use a close
tolerance on features to reduce part dimension variables; however the part cost increases
dramatically as tolerances are reduced.
To begin our series on GD&T, let's look at the following sample print and review
the symbols found there.
The first thing to note is the part shape and main features (left). Features are the
surface of the piece or a physical part item such as a hole or a shoulder, as shown.
Next, find any datums. This print has two datums and they are noted with this symbol
(right). A datum is a reference point to the part. The datum clearly shows the starting
position for both the machining and inspection procedures.
 The latest datum symbol is the one shown above right, although many
drawings still have the symbol, shown to the left, for datum.
The print has many dimensions and numbers (right). Dimensions within boxes and with no
tolerances are called basic dimension--a dimension that does not have a tolerance, but
represents a theoretical exact size.
There is a tolerance listed for the center hole feature. Remember, a tolerance is an
allowable deviation from an exact size. The specified tolerance for the hole is 0.400 to
0.405.
The most complex symbol on this print is the feature control frame (right). This is a
series of information related to a specified part feature. This feature control frame
refers to the location of the hole. The symbols in this feature control frame are true
position (crosshair symbol), maximum material condition (circled M), tolerances, and
references to the datums. All of these symbols will be discussed in detail next month.
Until then, you can download a chart of the most common geometric tolerancing symbols
at http://www.CNC-training.com Please e-mail comments
to getsmart@cnc-training.com
Steve Rose is a manufacturing consultant and president of Rose Training Systems,
Solon, OH, which also offers Internet website development. Mr Rose can be reached by phone
440.542.3066; fax 440.542.3006; e-mail srose@cnc-training.com;
or on the web at http://www.cnc-training.com |
