Chapter 7 Computer-Aided Drafting and Design
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geometric constraints include the horizontal,
vertical, parallel, perpendicular, tangent, equal,
concentric, collinear, and coincident constraints.
The equal constraint is used to constrain two
objects to the same size or length. For example,
if a circle has a diameter of 1″ and a second circle
has a different diameter, an equal constraint
can be assigned to constrain the two circles
to the same diameter. If the size of one circle
is changed, the other circle is changed to the
same size. Referring to Figure 7-28A, parallel
constraints have been applied to the vertical
lines on the left and right sides of the sketch.
The lines will remain parallel regardless of
changes made to the lines or to other sketch
geometry. In addition, the lines have equal
constraints to keep them the same length if
they are edited. Notice that each sketch entity
can have one or more geometric constraints.
The display of constraints can be turned off if
it becomes diffi cult to see or select the sketch
geometry.
A dimension is another type of constraint
and is used to control the size and location
of sketch geometry. See Figure 7-28B. When
creating a sketch, it is usually desirable to
draw each object to an approximate “rough”
size without specifying the exact value. Then,
dimensions can be added to constrain each
object size to an exact value. There are different
dimension tools used to place linear, angular,
radius, and diameter dimensions.
In most cases, a sketch is fully constrained
by adding the appropriate geometric constraints
and dimensions before using the sketch to
create a feature. This is referred to as “fully
defi ning” a sketch. See Figure  7-29. When a
sketch is fully defi ned, all objects in the
sketch are constrained and cannot be freely
moved or changed in size. In some programs,
a message at the bottom of the screen provides
an indication of whether the sketch is under-
defi ned or fully defi ned. The display color of
the geometry provides another indication of
whether the geometry is fully defi ned. For
example, unconstrained sketch geometry is
displayed blue. When the geometry becomes
fully defi ned, the color changes to black.
can be drawn to serve as the mirror axis. See
Figure 7-27.
Adding Constraints
Once the sketch has been created, the next
step is to constrain the sketch. Constraining
sketch geometry serves the purpose of preserving
geometric and dimensional relationships when
the sketch is edited or when a feature created
from the sketch is edited. Sketches are con-
strained by adding geometric constraints and
dimensions. A geometric constraint, usually
referred to as a constraint or a relation, controls
a geometric relationship between objects. For
example, a parallel constraint can be used to
constrain two lines so that they are always
parallel to each other.
Geometric constraints can be added in one
of two ways. By default, the program adds
geometric constraints automatically as you
create sketch geometry. For example, if you
draw a straight line in a horizontal direction, a
horizontal constraint is automatically added.
The horizontal constraint ensures that the line
will remain horizontal if the sketch is edited.
Geometric constraints can also be added
manually by the user after creating the sketch
geometry. There are different types of
constraints for controlling specifi c relationships.
Each constraint is represented by an icon in
the sketch. See Figure 7-28. Common types of
Original Sketch
Geometry
Sketch after
Mirroring
Mirror axis
(construction line)
Goodheart-Willcox Publisher
Figure 7-27. Mirroring a sketch of a symmetrical object.
The mirror axis can be drawn as a construction line.
Copyright Goodheart-Willcox Co., Inc.