Once drawn, the sketch can be modified in a variety of ways.
The geometry can be altered in a limited way, and the constraints can also
be changed. Edit options are listed by selecting the Edit button
along the right side of the window. The button next to the option must be
selected for any of these operations to function as described.
Geometry Editing
Any geometric entity can be deleted simply by selecting it, after having
chosed the Delete option from the Edit menu. Deleting an
end point
of a line segment deletes any
segments containing it, and analogously
for deletion of an end point
of an arc.
The center point
of a circle can be deleted
without the circle itself
being deleted, however.
This computes the intersection of a pair of lines
or line segments, and
inserts a point at the intersection.
By placing two points on a pre-existing
line or circle,
a line segment or
arc is created in place of the
line or circle.
A single point is added at the
selected position of a
segment, line,
arc, or circle.
This splits an arc or
segment into two arcs or segments,
respectively.
It causes the circle
to be replaced by an arc
whose start and end vertices
are identical, and it places a point on a
line.
This substitutes the complement of the selected
arc for the arc
itself.
Constraint Editing
The value of any dimensioned constraint
can be selected and modified using
the popup window that appears when such a constraint is
selected and the
Edit button on the menu bar along the right side of the window is
selected. Note that no other menu button may be depressed simultaneously
when constraint values are being edited.
A dimensioned constraint can
be deleted by selecting it after choosing
Delete in the Edit menu bar.
The location of the label of a
dimensioned constraint
can be moved using the
left mouse button to select
and reposition the label. In addition,
the angle constraint between
lines and
segments can be moved to any of the
sectors formed by the pair of entities by using the middle
mouse button to select
the constraint label.
To delete a non-dimensional constraint,
the Unconstrain option must be
selected. The region of interest is selected
using the SHIFT method,
and the geometric entities in
that area are listed in a popup menu. After selecting one of the geometries
involved in the constraint to be deleted, a new popup menu is displayed, which
contains the constraints in which the chosen geometric element is involved.
The desired constraint can then be deleted by choosing it from this menu.
In this section we work through a simple example, which demonstrates some
of the input and editing operations described above. As a first step, we
will create a rectangle. Begin by inputting four
line segments which are
connected to make a quadrilateral. Then select
Distance from the
Constraint menu, and insert a distance dimension along each
edge. Notice that some arbitrary unit of measure is used by the
program. Now select Angle,
and insert a dimension in any one
interior angle. Remember that you can
Delete any erroneous geometric input or
constraints
as you go. Your figure should look something like this:
Figure 1:
Select the Edit option,
and modify the lengths of the sides so
that two non-adjacent segments
are each 400 units long, and so that the other two
segments are
300 units long. Modify the angle
so that it is exactly 90 degrees.
When the dimension values are altered, the dimensions are highlighted in
the display, but the value displayed is not yet changed.
Once the segment lengths and the
angle are set to the desired values, select
Regenerate in the upper right hand corner of the window. The message
region at the bottom of the input screen should indicate that the constraint
solver is working. When it finishes, your picture should look something
like this, with arbitrary
orientation:
Figure 2:
Suppose that we want one corner to be rounded. This can be accomplished
in many ways. One way is to Cut the
two segments adjacent to the
corner to be rounded. Now delete
the corner point, and the two line segments
adjacent to it. This also automatically
deletes
the constraints attached to the corner.
Add distance constraints along the
new edges of 350 and 250 units. Your
drawing should look like this now:
Figure 3:
Click Regenerate. The constraint solver should return the message
that the sketch is underconstrained.
We need to add a 90 degree
angle constraint to each of the remaining
two unconstrained interior corners. After adding those
constraints, Regenerate successfully constructs the rectangle with
a missing corner:
Figure 4:
The round in the corner should be an arc
which is tangent to the two
segments
forming the open corner. As a first attempt, insert an
arc containing the
two end vertices of the two edges, and assign a
tangency constraint between
each edge and the arc. Click Regenerate. The constraint solver
should respond that the sketch is
overconstrained. In fact, if even one
tangency constraint is put in,
the sketch is overconstrained.
Delete the
arc.
This also deletes the
constraints associated with the arc. As a
second attempt, make an auxiliary circle
near the corner which
is large enough to touch both edges. Assign a
tangency constraint between
the circle and each of the two
segments, and Regenerate.
The constraint solver is successful, but the sketch has a
circle in the corner where we
only want an arc:
Figure 5:
To obtain the desired arc, we now input an
arc using the two end
points of the
segments,
and position the third point so that it
lies on the auxiliary circle.
Greater accuracy can be obtained by determining the
radius of the auxiliary
circle and making the
radius
of the arc match. The auxiliary
circle can now
be deleted, and the remaining
arc
will be tangent to the two segments,
as desired.
Figure 6:
