Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (30 page)

Geometric entity Spline (or cubic spline)

Geometry defined by

Possible methods for creation
in the CAD system

X,Y coordinates for at least 4 points. On the spline or
“control” points depending on the system.
Bezier-curve formulation (control points approach):
x(t) = axt3
+ b

xt2

+ c

xt + x0

y(t) = ayt3
+ b

yt2

+ c

yt + y0

t is the distance along the spline

ax, bx, cx, ay, by, cy
are solved for based on the 4 points.
Enter the X- and Y-values directly for the through
points and/or control points.

Snap to existing geometry for the points.

of the spline. They indicate an influence or a direction that the spline must head
toward, but the spline does not touch them. Some CAD systems will allow these
control points to be used to create and/or modify splines. Also, due to the mathe-
matics of the spline, there must be at least 4 points indicated for the spline to be
created (either points on the spline or control points).

5.10 SPECIAL TECHNIQUES FOR 2-D CAD

Although the fundamentals of creating mechanical drawings explains much of
the functioning of a 2-D CAD system, there are many procedures, techniques, or
short cuts that the CAD system can provide that have no real analogy with the
manual drawing process. These special techniques are essential for making opti-
mal use of the CAD system.

5.10.1 Groupings

Probably the most common special technique in 2-D CAD systems is the logical
grouping of geometric entities. These groupings can then be manipulated as if
they were a single entity. These groupings may be called blocks, symbols,
clumps, or detail page items. A typical example of one of these groupings would
be an engine shown in a vehicle layout drawing. One advantage of this grouping
is that the engine can be moved to different locations on the drawing as a unit.
This is far more productive than having to select and/or move all the individual
entities in the engine (and their might be hundreds of lines, arcs, and splines).
This grouping can also be rotated as a unit to help position it within the 2-D de-
sign. Other common examples of these groupings would be drawing formats,
standard or catalog items such as nuts and bolts, and schematic symbols (hydrau-
lic, electrical, etc.).

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These groupings offer many advantages for users, but usually CAD users
need to first understand the concept of instance or instancing (for groupings as
well as even 3-D CAD techniques). An instance of something in a CAD system is
merely an intelligent copy of something that the software can track (if desired).
The instance is basically a copy made from some master item. If the master item
changes, then the CAD system can have all the instances change or update as
well. In many cases, the use of groupings is an exercise in using instances. The
CAD user may create a grouping and make it a master. Then the CAD user can
create many copies of that master so that they are all the same. This would be a
common activity for groupings used for standard or catalog items such as nuts
and bolts. Each copy of the grouping could be called an instance of the master.

What happens if the user no longer wants to treat the grouping as a unit? In
this case, the CAD system should provide a procedure or command to disassem-
ble or break or smash an instance. After this is done, the individual geometry en-
tities in the grouping can then be manipulated; the CAD system will no longer
consider it an instance of the master. If the instance needs to changed, but not
broken, then the CAD system should also provide a procedure to edit or modify
the grouping. Also, there may be an option to create instances of groupings that
are independent and not affected by future changes to the master geometry.

Some CAD systems will permit the groupings to be nested. A grouping is
nested if it is part of yet another grouping. Considering the engine example again,
there might be parts of the engine that, in turn, might be helpful to consider a
grouping as well (such as the cooling fan in front of the engine). If the CAD sys-
tem allows the engine grouping to use subgroupings such as the cooling fan, then
that system permits nesting. If the engine symbol is smashed, it would then ex-
pose its subgroupings. Then these subgroupings could again be smashed to even-
tually get back to the individual geometric entities (such as the lines, arcs,
splines, etc.).

Another important issue with respect to groupings concerns dimensions
that may be related to a grouping. Specifically, when groupings are scaled (com-
pressed or expanded to be smaller or larger), should the dimensions be scaled as
well? This usually depends on why the user is scaling the grouping. If the group-
ing is being scaled because the physical object is changing size, then usually the
user wants the dimension to show a new value. If the “smart paper” approach has
been followed properly, and dimensions are associated with the geometry in the
drawing properly, then when a grouping changes size, then the dimensions will
automatically change to reflect the new size of geometry. However, putting the
actual dimension in the grouping should be avoided since the grouping will have
a scale factor, and the view the grouping is in may also have a scale; now one has
a scale of a scale, and this can become difficult to manage.

In another situation, a grouping may be scaled just because the geometric
entities do not fit on the selected paper size. In this case, the physical object rep-

2-D CAD 131

resented in the drawing is not actually changing size. The value shown by the
dimensions should not change. The height of the letters and numbers in the di-
mension shown in the grouping may also need to be scaled to fit the new paper
size, as well.

Another case of scaling a grouping would be to change unit systems. Per-
haps geometric entities and a grouping were originally created using the inch sys-
tem (obviously in a CAD system that uses the “paper space” system mentioned
earlier). Now, when an instance of this grouping is created in a drawing using
mm, the instance will be much too small. Instead of something being 10 inches
long, it will show up as only 10 mm long. To correct for this, the grouping can be
scaled by 25.4 either in a separate operation or as the grouping instance is actu-
ally being created. In this case, the geometric entities in the grouping need to
change by the factor 25.4, and the values shown in the dimensions should also be
scaled (i.e. the number 10 in a dimension meaning 10 inches should change to
254 for 254 mm).

Finally, CAD systems may allow for groupings to be imported and/or ex-
ported. Many computer software systems use these terms when information is
taken from one system to another. In this case, a grouping may be created in a
CAD drawing, but it may be useful in other CAD drawings. In order to get this
grouping to the other drawing, it may need to be exported using the CAD sys-
tem’s data management functions. Once the grouping is exported, it can then be
imported into the other CAD drawing. A set of standard groupings may also be
inherited as part of a template or prototype drawing that is referenced each time a
new drawing is created. Or, a entire customized library or catalog may be created
across the entire network. This functionality will often be accompanied by a set
of administrative functions that will allow someone to manage the library and
control revisions to the groupings included in the library.

5.10.2 Projections

The next special technique for 2-D CAD systems is called projections or project-
ing or 3-Viewing. This technique has to do with creating geometric entities in
different Views based on an originating view. For example, in Figure 5.10 there is
a Front View, a Top View, and a Right View that shows an object from different
viewing angles. Note that the Front View shows a square hole that has been
added to the object. This hole is shown in the Top View, but now is being added
to the Right and Isometric Views as hidden (using the hidden font which is a
short dashed line).

The projecting special technique assists with creating the lines in the other
views by the CAD system analyzing one view and using the known viewing an-
gles (how the object is supposed to be viewed in 3-D space) for other views.
Looking at Figure 5.10, since the Right View is looking at the object from the

132 Chapter 5

FIGURE
5.10
A “3-View” Drawing with a new feature in the Front View being “pro-
jected” to another view.

right side of the object (shown to the right of Front View using the U.S. standard
Third Angle projection system), then the location of the edges for the hole can be
properly located in the Right View using the CAD system’s projection capability.
The Front View does not indicate the depth of the hole, so the Top View had to be
used to tell the system where the hole starts and stops (a beginning and ending
projection plane). Sometimes projections are done more simply as infinite lines.
In this case, the projected geometry would be based on a point in the Front View,
and then a line that completely crosses the other view or views would be shown.
The CAD user may then “relimit” the line in the Right View manually to the
proper depth.

Keep in mind that the projection special technique may require that the
CAD system creates and manipulates views. Also, the CAD user will need to
make sure that the “smart paper” approach to creating entities is being followed.
The origins (where X=0 and Y=0 for the views) will probably have to be care-
fully defined for each view so that there is a consistent calculation between the
views. The user also needs to consider the impact of different view scales be-
tween the views (although the 3 most standard views—Front, Top, Right—would
be expected to be at the same scale).

2-D CAD 133

5.10.3 User-Defined Options

Most CAD systems have a number of special techniques or capabilities related to
user-defined options. These are options that allow the user to enhance, customize,
or even override the CAD system’s capabilities. Three of these options are men-
tioned here; some CAD systems will offer others. Also, in the next chapter more
information on CAD system customization is presented.

Most CAD systems will offer the ability to create user-defined colors,
fonts, and line weights. This allows the user to create a drawing that reflects spe-
cific company policies or just makes a more complicated drawing easier to create
and interpret. In terms of colors, the user will probably be given a chance to set
RGB levels or scales. R stands for red, G stands for green, and B stands for blue
(unlike pigments that use red, yellow, and blue as the primary colors; light
sources use red, green, and blue for primary colors). The scale for each of these
primary colors may go from 0 to 100 percent, or they may go from binary-based
values such as 0 to 15, or 0 to 255. Depending on the amount of each, a new color
can be created (most CAD systems should offer up to 255 ×
255
×
255 or about
16 million possible colors). Colors can be useful in visualizing information in a
drawing since different items can be color-coded (for instance, different details in
an assembly drawing can be different colors). Also, different entities such as di-
mensions, notes, balloons, etc. could be color-specific. Again, this could be quite
helpful in reading a drawing (at least if the electronic CAD drawing is viewed).
Obviously, one problem with using colors is that they generally will not be seen
by the reader or customer of the drawing. Most drawings are either printed or
scanned in a black-and-white format. Even if it is scanned with a color-enabled
file, most often a drawing will eventually be copied or printed in hardcopy form;
in this case the colors will be lost. Therefore, it is common practice to make sure
that a drawing is completely readable in the black-and-white format.