Chapter 2 Dimensioning and Tolerancing Symbology 33
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type of tolerance being specifi ed and the control
that is desired.
Datum feature references are always written
left to right with the primary datum shown fi rst.
See Figure 2-20. It is followed by the secondary
and tertiary datum feature references. Material
boundary modifi ers may be applied to the datum
feature references.
Placement of the Feature Control Frame
A properly composed feature control frame
must be shown in the CAD model or placed on
the drawing in the correct manner to ensure the
appropriate requirement is expressed. Correct
application will clearly indicate the feature or fea-
tures that are toleranced. Some tolerance types
may only be applied to surfaces and others only to
features of size, so correct application is essential
to avoid specifi cation of meaningless or incorrect
tolerances. Two tolerances, straightness and fl at-
ness, may be applied to surfaces or features of size.
These two tolerances when applied to surfaces have
a completely different meaning than associating
the same tolerances with a feature of size.
Application to Surfaces
Placing the feature control frame on an exten-
sion line from a surface in an orthographic view
indicates the specifi cation of a requirement that is
applicable to the surface. Feature control frames
may be placed on either side of extension lines.
The side of the extension line on which it is located
makes no difference. See Figure 2-21. Feature con-
trol frames are generally not shown on extension
lines in CAD models.
A leader may be used to connect a tolerance
specifi cation to a surface in orthographic views
and in CAD models. The use of a leader has exactly
the same effect as attaching the specifi cation to an
extension line. The leader may be extended from
either end of the feature control frame.
The leader terminates on the surface in CAD
models and ends with a dot on the surface. If a
leader is used in an orthographic view, it may ter-
minate with an arrowhead on a line representing
the profi le of the surface. In situations where no
edge view of the surface exists in an orthographic
drawing, the leader may terminate on the sur-
face using a dot as shown in the CAD example of
Figure 2-21.
Application to Features of Size
Tolerances may be applied on a feature of size
by placing the feature control frame adjacent to a
dimension or attaching it to the dimension line. See
Figure 2-22. When a feature control frame is placed
adjacent to a dimension value, care must be taken
that confusion does not exist regarding other dimen-
sions in the vicinity. The feature control frame is to
be clearly associated with only one dimension.
Tertiary
Secondary
Primary
Tolerance
Translation
modifier
Statistical tolerance modifier
Diameter symbol
(if applicable)
Material condition
modifier
Material boundary
modifier
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Figure 2-20. The desired tolerance specifi cation and
the type of feature being controlled determine when
a diameter symbol or material condition symbol is
required.
Application to surfaces
Or
Or
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Figure 2-21. The end of the feature control frame to
which a leader or extension line attaches does not
affect the interpretation of the requirements.