32 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
unequally offset relative to the basic profi le. This is
further explained in the profi le tolerances chapter.
The letter I inside a circle is the independency
symbol. Rule #1 of the standard requires that, by
default, a feature of size must have perfect form
at its maximum material condition. When it is
desired to take exception to that rule, the indepen-
dency symbol may be applied to the dimension
value.
An equilateral triangle pointing to the right
is the translation symbol. This symbol is used
with a datum feature reference to indicate that
the datum feature simulator may translate (move).
Application is explained in later chapters.
Profi le tolerances may be applied to extend all
around the profi le of the view to which the toler-
ance is applied. Multiple tolerance specifi cations
may be avoided on a view by using an all around
symbol. See Figure 2-17. The all around symbol is
a circle placed at the corner in the tolerance appli-
cation leader. It is similar to the all around symbol
used in welding specifi cations. The all over sym-
bol is two concentric circles located at the corner
in the leader. This indicates the profi le tolerance
applies all over all the surfaces of the part.
Feature Control Frames
Geometric tolerance requirements are speci-
fi ed in a feature control frame or in notes. A
feature control frame is a rectangle made up of
compartments containing symbology and numeri-
cal values to defi ne a tolerance requirement. Fea-
ture control frames may contain no more than a
tolerance symbol and a tolerance value, or they
may contain multiple lines of requirements that
include tolerance symbols, tolerance values,
modifi ers, and datum feature references. See Fig-
ure 2-18. The amount of information contained in
a feature control frame depends on the desired
level of control to be established.
The following segments of this chapter
describe the required composition for a feature
control frame. Determining the appropriate infor-
mation to include in a feature control frame is a
primary subject of this book, and the methods for
completing each type of specifi cation are described
in several chapters.
Composition of the Feature Control Frame
Every feature control frame follows the same
order of specifi cation, reading from left to right.
See Figure 2-19. The tolerance symbol (character-
istic) is always shown fi rst at the left end of the
feature control frame. It is followed by the toler-
ance value. Datum feature references, if required,
follow the tolerance value.
Words are generally not written inside a fea-
ture control frame. Some special applications do
exist, including the use of MAX, NONUNIFORM,
and BASIC, but symbols, numbers, and datum fea-
ture reference letters are generally adequate for
specifi cation of standardized tolerance controls.
The tolerance value in a feature control frame
may be no more than a number indicating the
size of the tolerance zone. It is also possible for
the tolerance value to include a diameter symbol,
a spherical diameter symbol, a material condition
modifi er, a statistical tolerance symbol, and a tan-
gent plane symbol. See Figure 2-19. When used,
the diameter symbol precedes the tolerance value.
It is required if the tolerance zone is round or
cylindrical. A spherical diameter symbol precedes
the tolerance value if the tolerance zone shape is
spherical. When a material condition modifi er is
shown, it follows the tolerance value. The tangent
plane modifi er follows the tolerance value when
it is used. (See Chapter 7.) The statistical tolerance
symbol follows the tolerance value and any mate-
rial condition modifi er that is included.
Datum feature references are required in all
feature control frames where the tolerance zone
has a required relationship to one or more datums.
The number of referenced datums depends on the
Goodheart-Willcox Publisher
Figure 2-18. A feature control frame may be composed
of one or more lines. Each line is called a segment in
ASME Y14.5.
Datum feature references
Tolerance value
2X
Tolerance symbol
(characteristic)
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Figure 2-19. Feature control frames are always read
from left to right.