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Exploring Drafting
Dimensioning in CAD
As is the case with lettering a drawing,
dimensioning a drawing manually can be a
very time-consuming process. In CAD drafting,
dimensions can be created very quickly and
accurately with commands and drawing aids
such as object snaps. A number of common
dimensioning commands are typically available.
As discussed in Chapter 8, lettering is
known as text in CAD. Text is created with text
styles. In similar fashion, dimensions in CAD
are drawn with dimension styles. Typically, each
dimension style has a text style assigned to it.
There are also settings for the unit format. A
dimension style can be created for decimal inch,
fractional, metric, or dual dimensioning. When
creating a dimension style, it is important to
set the correct scale for dimensions in relation
to the drawing scale. This ensures that the
dimensions will be drawn to the proper size in
relation to object features.
Dimension styles also provide controls for
the extension line and dimension line format,
dimension spacing, text placement, and
tolerancing method. Dimensions can be placed
in a number of ways with CAD, depending on
the size of the feature and proximity to other
dimensions. For example, the dimension text
and dimension line can be placed inside or
outside the extension lines. Also, as with manual
drafting, dimensions may be placed using
unidirectional, aligned, or chain dimensioning.
Dimensions are usually placed on a separate
layer from other layers in the drawing. After
creating a dimension style, it can be set current
and used with various dimensioning commands.
There are normally different commands for
linear, aligned, angular, diameter, radius, and
leader dimensioning. See Figure 10-45. In linear
dimensioning, object points (such as corners or
center points) are selected for the extension line
origins. The placement of the dimension line and
text is then specified. In angular dimensioning,
the lines making up the angle are selected for the
extension line origins. When placing a diameter or
radius dimension, the circle or arc is first selected,
followed by locations for the leader line and text.
The diameter or radius symbol is generated
automatically, and the dimension value is calculated
automatically by the program. Leaders are drawn
in a similar manner. Points are selected for the
placement of the arrowhead, line, and text. When
creating a note, the text is entered manually. The
text can include a variety of symbols, such as
the counterbore or countersink symbol.
As is the case with text, special editing
functions are commonly available for modifying
dimensions. Editing commands may be used to
reposition dimension text, specify different text,
or change the dimension style.
Some CAD programs have special commands
for geometric dimensioning and tolerancing
applications. When this is the case, commands
are normally available for drawing datum
symbols, feature control frames, and geometric
characteristic symbols. When placing tolerance
dimensions, there are options for setting the
tolerance method, limits, and precision.
The same rules used in manual
dimensioning apply to dimensioning in CAD. To
save drawing time, create different dimension
styles based on the drafting discipline used.
As with other drawing conventions, dimension
styles should observe ASME, school, or
company standards.
CAD Application
Goodheart-Willcox Publisher
Figure 10-45. Different types of dimensioning are
associated with commands in CAD programs.
Copyright Goodheart-Willcox Co., Inc.