Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (42 page)

Figure 8.5 shows a datum-type of plane created by using the centerline of
the circular feature and then a midpoint of one of the vertical edges. This plane is
basically located at the center of the part.

8.3.3 Sketch Plane based on a coordinate system

Another common approach for sketch planes is the use of coordinate systems. A
coordinate system usually specifies the X-, Y-, and Z-directions (refer to Figure
8.6). Obviously, the X-, Y-, and Z-directions are all perpendicular to each other
(generally referred to as being orthogonal). Also, the Z-direction should always
be in a consistent direction based on the X- and Y-directions (if one draws X and
Y on a piece of paper in the “normal” orientation of X to the right and Y going
up, then the Z direction is out from the paper toward the viewer; this is dictated
by something called the “right-hand rule” or “right-handed coordinate systems”).

FIGURE
8.5

A datum or reference plane that can be used as a sketch plane.

Part Modeling 195

FIGURE
8.6

A coordinate system axis plane as a new feature sketch plane.

Usually, these coordinate systems are shown in the 3-D CAD system with
axis planes, as well. These axis planes are planes that are formed by the 2 lines of
the X-, Y-, and Z-directions. For instance, the XY plane on the coordinate system
would be the plane that contains the X- and Y-directions (extending infinitely in
each direction). The XZ plane contains the X- and Z-directions, and the YZ plane
contains the Y- and Z-directions. Clearly, these axis planes can be quite useful for
providing sketch planes. Often the X-, Y-, and Z-directions are the basis for many
features (these parts may be referred to as prismatic parts). Indeed, some 3-D
CAD systems (either by default or by standard user practice) always create a co-
ordinate system for parts (whether the user realizes it is there or not).

If a default coordinate system for a part does not provide the needed axis
planes for sketching a new feature, then the CAD system may allow new coordi-
nate systems to be created based on other geometry of the part. This is quite sim-
ilar to the creation of a datum or reference geometry; however, coordinate
systems are often better since they supply three potential sketch planes (the axis
planes) instead of just one plane (the datum plane). Figure 8.6 shows a coordinate
system that has been offset from a base coordinate system. The first one is for a
base feature that is already created. The second one (which is being shown as a
sketch plane for some new 2-D geometry) is offset from the first one by three
linear dimensions in the X-, Y-, and Z-directions. This is a quite useful technique
since it gives easily modifiable dimensions to control the new feature.

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8.3.4 Sketching for the Added Feature

Once the associative sketch plane has been successfully selected, the next step in
adding the feature is doing the sketch. As before, this involves creating 2-D ge-
ometry that forms the foundation for the new 3-D feature. Normally, this sketch-
ing is no different that the sketching that made the base feature or started the part
model. One uses the 2-D sketching tools or commands to create lines, arcs,
splines, etc. as necessary.

Normally the process of constraining and adding parametric or design in-
tent information is exactly the same as for the base feature, as well. However, this
process becomes a little more interesting since the new feature being created is
surrounded by existing 3-D geometry that can be used in interesting ways. For
instance, if one wants a new feature to start where another feature left off, or if
one wants an edge of the new feature to be perpendicular to the edge of another
feature, then this may be possible by relating or constraining the new 2-D geome-
try to the existing 3-D geometry. One needs to be careful, though, that the exist-
ing 3-D geometry is actually from the same part model that is being sketched on.
Otherwise, changes to the existing feature will not drive changes in the new
feature (ignoring for now how assembly models might accomplish this). Figure
8.7 shows how a 2-D point (shown as an asterisk) has been projected onto the
sketch plane by using an existing 3-D vertex. Since the 3-D vertex is from a pre-
ceding feature in the part history, changes to that 3-D vertex should automatically

FIGURE
8.7

Using an existing 3-D vertex for use with sketching 2-D geometry.

Part Modeling 197

update the sketch plane point; then the new feature being created will also adjust
automatically.

8.3.5 Making the New 3-D Feature

Following the three-step process once again, the next step after creating the 2-D
sketch geometry is actually making the new 3-D feature. As with the base fea-
ture, the common methods shown in Table 8.2 are very often used. One could use
an extrusion to make the 3-D feature prismatic (it sticks straight out); one could
use a revolve to make the 3-D feature revolute; one could use a sweep to make a
path-following 3-D feature, etc.

It is very important to notice, however, that indicating a method like “Ex-
trude” is not enough for added features. This is because the new feature is going
to operate on the existing part, so the CAD system needs to know how the new
feature is going to affect the existing feature. The obvious example is deciding if
the extrusion is a protrusion or a cutout. If the user indicates that the extrusion is
to protrude, then the CAD system joins the new 3-D surfaces to the existing part,
and it sticks out from the part as a protrusion. However, if the user indicates that
the extrusion is a cutout, then the CAD system makes the new 3-D surfaces cut
into the existing part. Table 8.3 lists the usual operations for new features acting
on existing part models.

Sometimes it seems unnecessary to have to indicate whether the new fea-
ture is to protrude or cutout since the CAD system often will often prompt the

TABLE
8.3

Some Operations that Added Features Can Perform

Operation Description

Protruding This operation means that the new feature is going to add volume to the

existing part. After extruding, revolving, etc., the new 2-D geometry

forms its own 3-D geometry, and then the system combines the 2 vol-

umes and figures out how they connect together to create a new single

volume.

Cutting This operation means that the new feature is going to remove volume

from the existing part. After extruding, revolving, etc., the new 2-D ge-

ometry forms its own 3-D geometry, and then the CAD system takes

away the new volume from the existing volume to create a new single

volume.

Intersecting This operation means that the new feature is going to be a junction. After

extruding, revolving, etc., the new 2-D geometry forms its own 3-D ge-

ometry, and then the CAD system will create a new volume that is

made up of the space that is shared by both new volume and the exist-

ing volume.

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user to indicate a direction to extrude. If one makes this direction away from the
existing part, then one would think it will always be a protrusion. If one makes
the direction into the existing part, then it should be a cutout. However, there are
some cases, where the new extrusion will interfere with other features, and it will
not be clear what the user intended based on the direction of the extrude alone. In
Figure 8.8, for example, a new rectangular feature is shown being created from
the sketch plane coordinate system shown in Figure 8.7. On the left, the new fea-
ture is extruded as a protrusion; on the right, the new feature is extruded as a cut.
The direction alone would not be enough to let one know what the designer
intended.

8.3.6 Surface Issues for New Features

Although indicating whether a new feature is a protrude, cutout, or intersection is
very often all that is needed to create the desired feature, there are some underly-
ing surface issues that need to be considered. This deeper understanding will al-
low the user to figure out why certain 3-D feature operations are successful,
while others fail.

FIGURE
8.8
Example of different features using the same extrude direction (left is pro-
trude; right it cutout).

Part Modeling 199

Recall that one of the major misconceptions about 3-D modeling (even
when it is called solids modeling) is that the 3-D models are not solid at all. In-
stead, they are a construction of surfaces that meet at their edges. This process of
having the edges meet is often called stitching. For instance, a cylindrical 3-D
model has a cylindrical surface that is the body of the model, and then there are
two circular surfaces that represent the ends. The edges of the circular surfaces
(which are circles) are considered stitched or sewn up by the CAD system. The
system deems them this way since the edges of the surfaces are found to be coin-
cident within a very small amount (this small distance is usually called the basic
part tolerance). Figure 8.9 shows the surfaces that would be stitched to create a
rectangular solid.

8.3.6.1 “Stitching” for New Features

This issue of stitching can often be ignored since the CAD system takes care of
this automatically. But to really master 3-D modeling, it is important to think
about this issue at times. For example, Figure 8.10 shows a cylindrical feature

FIGURE
8.9

The surfaces that would stitch to form a cube-like solid.

200 Chapter 8

FIGURE
8.10
undefined).

Cylindrical feature grazes the block’s top surface (stitching may be

that has been added to the top of a block (using the block as the base feature).
This was done by sketching a circle that was precisely tangent to the top edge of
the block. In this case of the sketch circle’s tangency, there is just one point (one
local X- and Y-value) on that face that is both part of the straightedge at the top of
the block and the circle. Now think about how that cylinder is extruded across
the top surface of the block. It means that there is an exact line (infinitely thin)
that just splits the top surface of the block (this edge is shown highlighted in
Figure 8.10).

Now, if the circle had been sketched down into the block, then there would
be more interference between the cylinder and the block, and the CAD system
would find that there is not an exact line shared between the block and the cylin-
der. Instead there would be a finite region of the top of the block that can be used
to create a new top surface. The top surface of the block would simply have a
rectangular region whose edges would be dictated by the location of the cylinder.
This new top surface of the block would contain both the flat top surface of the
block and also the cylindrical area. This would be a new surface whose edges can
then be stitched to the boundary of the cylinder that intersects into the block. Re-
fer to Figure 8.11.

Part Modeling 201

FIGURE
8.11

Cylindrical feature interferes with block (stitching clearly defined).

Of course, if the cylinder were located above the block, then there would be
no interference or shared space between the block and the cylinder. In this case,
there would be no effect on the surfaces of either feature. The part model would
simply contain two totally separate volumes (which is sometimes the desired
result).

Now, returning to the case where the cylinder is exactly tangent or just
touching the block, note that there is no shared volume, but yet they still touch.
Should the CAD system consider this another case of two separate volumes? Or
should it consider the two volumes joined together like Figure 8.11? The answer
is not clear. In this sort of case, the CAD system may issue messages, warnings,
or errors to the user. Or, the CAD system may simply make the two features
closer together or farther apart by a small amount to force a solution to the prob-
lem. In either case, it really is not a good idea to have the CAD system making
assumptions about what the designer really means.