270
Exploring Drafting
(angle), and position (location) are needed to
design products.
Geometric dimensioning and tolerancing
(GD&T) is a standard system devised to
control interpretation of the form, profi le,
orientation, location, and runout of features on
drawings. This type of tolerancing provides the
necessary precision for the most economical
manufacture of parts and interchangeable parts.
Different dimensioning standards exist and a
common drawing language must be applied
because components for specialized products
are typically manufactured in a number of
locations and often in different countries.
The GD&T system provides an international
language that standardizes the dimensioning
and tolerancing process.
GD&T uses geometric characteristic symbols
to specify and explain form and positional
tolerances, Figure  10-41. These symbols relate
to such variables as the form of an object, the
profi le or outline of an object, the orientation of
features, the location of features, and the runout
of surfaces or relationship of features to an axis.
Geometric tolerances are often used to
specify the location or relationship of features
that originate from a datum. A datum is an
exact point, axis, plane, or surface from which
features of a part are located. Datums are
identifi ed by symbols. The symbol consists of
a reference letter enclosed in a box attached
to a triangle, Figure  10-42. Datum feature
symbols are positioned next to the datum
point or surface, on an extension line, or under
a dimension, along with a note.
Characteristic symbols, datums, and other
geometric tolerance specifi cations are often
indicated in a feature control frame. A feature
control frame is a rectangular compartment that
contains a divided series of symbols identifying
geometric tolerance. See Figure  10-43. The
characteristic symbol is typically given fi rst
and is followed by the allowable tolerance
and a datum reference letter. (Datums are also
identifi ed by squares containing a reference
letter.) Feature control frames may be shown
along with the dimension or attached to a
dimension line or extension line.
General Tolerances
Not every dimension needs to be toleranced
directly. General tolerances allow the drafter
to apply a tolerance for all dimensions that
do not have specifi ed tolerances. General
tolerances can be included in the title block or
in a note, table, or separate document. Unless
otherwise specifi ed, general tolerances are
applied to all applicable dimensions.
General tolerances can be provided for
fractional, decimal, angular, and metric toler-
ances, Figure 10-40. Multiple decimal tolerances
can be specifi ed depending on the precision
requirements of the drawing. One- and two-
place decimal tolerances are used for standard
precision requirements. Three- and four-place
decimal tolerances are used for higher precision
applications. Angular tolerances can be specifi ed
in degrees or decimal degrees. Metric toler-
ances only apply to metric dimensions.
Geometric Dimensioning
and Tolerancing (GD&T)
The design of parts for many complex
applications requires a highly precise system
of specifying dimensions and tolerances on
drawings. Machining processes have improved
to the point where highly accurate defi nitions of
tolerances for form (shape and size), orientation
Goodheart-Willcox Publisher
Figure 10-40. General tolerances can be found in the
tolerance block section of the title block.
Copyright Goodheart-Willcox Co., Inc.