Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (27 page)

2-D CAD 113

sions can be automatically created by the CAD system. This leads to a significant
savings in time and effort.

Dimensions are usually created in the CAD system by selecting, picking,
or clicking on the geometric entities that already exist in the drawing (the CAD
system should have a method of highlighting or otherwise previewing what is be-
ing selected). In Figure 5.3, the center of the hole and the right edge were se-
lected. Subsequently, the CAD system usually creates or previews the dimension
for review by the user.

As simple as this dimension seems, there are a number of important issues
to be concerned with. First, one needs to be aware of the dimensioning standard.
The choices usually are the ANSI or ISO standard. The ANSI standard shows the
dimension in the middle of the dimension lines; the ISO standard has the dimen-
sion above an unbroken dimension line. The example in Figure 5.3 shows the
ISO standard dimension at the top. Usually the CAD system will have the user
decide which to use. There could also be a system-wide default that appears nor-
mally, but then the user can override it.

Second, one needs to be aware of the number of decimal places shown in
the drawing. This is the number of digits shown after the decimal point, and it
implies a degree of manufacturing accuracy. As with the dimensioning standard,
the user should be able to set a normal number of decimal places, and there could

FIGURE
5.3

Creating a dimension with a typical CAD system.

114 Chapter 5

also be a system-wide default. In Figure 5.3, the number of decimal places was
set to 0 by the user since there are no digits shown after the decimal point.

Third, one needs to be aware of the round-off inherent in the CAD system.
Once geometric entities (such as the hole and the edge in Figure 5.3) are created
in the mathematical model sense, and the user picks the entities for dimension-
ing, the CAD system must then decide how to “round-off” to the number of deci-
mal places selected. Particularly with inch drawings, this can be a significant
change in value. For example, if the “exact” mathematical model for the object
has a length of 3.875 in. (3–7/8 in.), and the user has selected 2 decimal places
for the dimension on this length, then the CAD system is actually going to show
3.88 in. as the dimension. Of course, most readers of this dimension (at least
readers accustomed to inch drawings) will figure that the designer really drew the
object to 3.875 in. But it would be wrong to show the dimension as 3.875 in.
instead of 3.88 in. on the drawing, since 3.875 would be 3 decimal places, and
that would mean that a higher level of precision would be expected in manu-
facturing, and an unnecessarily expensive manufacturing process would have to
be used.

Fourth, one needs to be aware of the application of any view scale in the
dimension. If the mathematical model is accurate, and the scale has been speci-
fied for the view that the dimension is placed in, the CAD system should auto-
matically account for the view scale. Unfortunately, not all geometric entities are
accurate in the mathematical model. For instance, if a company needs to show
something on a drawing that is actually designed by another company (such as a
supplier), then a drawing translation may have occurred. In this case, the sup-
plier’s CAD system creates data that is “read into” the user’s drawing. Since this
geometry may lose some accuracy in the translation process, or worse yet there is
a conflict between a paper space and a model space approach in the two CAD
systems, a dimension’s value created by letting the CAD system work with the
mathematical model may be wrong. At this point, the user needs to either correct
the mathematical model (by totally recreating the geometric entities) or create an
“out of scale” dimension. The “out of scale” option would be telling the CAD
system to ignore the mathematical model, and force it to show a desired value for
the dimension. This is a rather dangerous option, and it should be avoided when-
ever possible. Although the originating user may understand that this has been
done for a dimension, future users may not. Some CAD systems do make some
appearance change in the dimension to warn users that this has been done (per-
haps underlining or changing colors of the dimension).

Fifth, it is important to realize that there are geometric assumptions being
made by the creation of most dimensions. The dimension shown in the top of
Figure 5.3 obviously just shows the distance of the hole from the edge of the part.
However, the reader of the drawing may assume that the dimension is along a line
that is perpendicular to the edge. This is a good assumption, but what if that edge

2-D CAD 115

is not a perfectly vertical line in the mathematical model? Perhaps that edge leans
to the right; now the dimension shown in the middle of Figure 5.3 may mean a
perpendicular distance from the edge, or it might mean that the dimension is
along a perfectly horizontal line (and not perpendicular to the edge). Some CAD
systems will allow the user to make the distinction between the perpendicularity
and the horizontal orientation, but the user needs to realize that further dimen-
sions need to be created to carefully define how that edge is leaning.

Lastly, the CAD user needs to be aware that dimensions can have associa-
tivity, a term that was apparently invented by CAD software vendors to indicate
that one drawing or 3-D model can have a relationship to other entities. In the
case of 2-D CAD systems, associativity is likely to arise between a dimension
and the geometry that is selected for the dimension (such as the edge and the hole
in Figure 5.3). If this dimension is associative, then if the hole is moved to be
closer to or farther from the edge, then the value shown in the dimension will
automatically reflect that move. Often this level of associativity requires that the
2-D CAD data be based on a 3-D model of the object shown in the drawing. Fur-
ther complicating matters, keep in mind that if a 3-D model is the basis for the
drawing, selecting a line (such as the edge shown in the bottom of Figure 5.3)
may be making more geometric assumptions. There may be an edge at the front
and back of the part at the line’s location and by selecting one or the other edge,
different dimensions or associativity may result.

Considering all these caveats for creating a simple dimension between a
hole and an line, it is clear that properly creating and documenting a design with
2-D CAD can still be demanding. However, if one spends the time needed to cre-
ate accurate geometry in the mathematical modeling sense, then there should be
no problem getting accurate and productive results when creating the dimensions.
And, users should not consider getting a complete understanding of the 2-D CAD
system as waste of time when moving to a 3-D CAD system. The 3-D CAD sys-
tems still require that concepts (such as dimensions in a 2-D plane) be used to
create 3-D geometry.

Finally, dimensions in the 2-D CAD system may be able to take advantage
of GD&T. Usually this takes the form of adding Feature Control Symbols (FCS)
to dimensions. These added notations and symbols expand the meaning of the
dimension by showing such things as datums (basically flat surfaces on the part
that dimensions are expected to start from), tolerances and tolerance zones (how
accurate a dimension must be in a more accurate sense than the decimal places
mentioned earlier), and more. If a dimension is drawn to a particular hole, and the
hole needs to be more accurate in the horizontal direction as opposed to the verti-
cal direction, then the user needs to indicate that by having the CAD system cre-
ate the proper symbols. Note that the concept of associativity also needs to be
applied to this system (in both the 2-D CAD only sense as well as the 3-D model
to 2-D drawing sense).

116 Chapter 5

5.6 VIEWS AND VIEWPORTS

As already mentioned, views are a very important concept in drawings. They pro-
vide a standardized means of interpreting the three-dimensional character of an
object through two-dimensional information. Furthermore, some CAD systems
use viewports to handle these views. This section covers some important implica-
tions of views for the CAD system. Some of these concepts will not be relevant if
the viewports are not used.

5.6.1 Viewport Clipping

Probably the most obvious ramification of the use of viewports in a CAD system
is the idea of clipping. Clipping means that even though the user has selected
mathematical values for geometric entities (such as lines or arcs) that can fit on
the drawing’s overall size, the entities (or parts of them) are not shown. They are
cut off or clipped at the border of the viewport. Figure 5.1 shows some geometry
that is clipped. The solid lines forming rectangles are the borders or edges of the
views.

Although clipping may seem detrimental, it can be of value if used prop-
erly. Clipping can assure that the different views are properly “contained” and
segregated (which is usually how the drawing needs to appear anyway). If a De-
tail View is created (a view that “zooms in” on a small area to show greater de-
tail), then the user can just select a segment of a larger view and the CAD system
will automatically clip all the unneeded geometry. Figure 5.2 shows this situation.

5.6.2 Viewing Angles

If the CAD system can create projections, then the views need to have defined
viewing angles. These angles specify the precise direction at which someone is
looking at the object shown by the drawing. The angles indicate the direction by
three rotations (about the X, Y, and Z axis), or the components of a 3-D vector.
The angles are standard values for the standard views (such as Front View is 0,0,0
or no rotation; Top View is 90,0,0 or rotate 90 degrees about the X axis to see the
top, and Right View is 0,-90,0 or rotate 90 degrees about the Y-axis to see the
right side of the object; the Isometric View would be more general such as
45,-35,-30).

If an arbitrary view is created for a drawing (one that looks at the object in
some oblique or ad hoc direction), then viewing angles may be entered for these
views in some CAD systems. Of course, the viewing angle is important for view-
ing faces of objects that are “true.” In this case, if the viewing angle is true, then
dimensions created automatically will be correct. If the viewing angles are not
true, and do not view a face directly (or along a vector normal to the surface),

2-D CAD 117

then dimensions created by the CAD system will be skewed and the values may
not be what is desired.

5.6.3 View Origins

In addition to specifying the boundaries, scale, and viewing angles of views, us-
ers need to be careful to specify and manage the origin of views. An origin is the
location where X is 0, and Y is 0. For a CAD system that does not use viewports,
then there is probably just one origin for the entire drawing, so there would prob-
ably be little problem in managing it. When there are viewports, then there can be
an origin for each view, and more care needs to be taken with them.

It is important to keep the viewport origins aligned. In Figure 5.1, these
origins are indicated by the X-Y symbols. Notice that they are in alignment verti-
cally and horizontally. This alignment is best understood with respect to the real
object being shown in the drawing. The real object in 3-D space has only one
origin where X and Y and Z are all zero (refer to Figure 5.4 where the origin is
shown as an X-Y-Z coordinate system). When one views the object from the
Front direction (along the Z axis) and the Right direction (along the X axis), one
can see that origin for the Y axis (where Y is zero) is going to be at the same
height in both directions. Therefore, the 2-D drawing views should show the Y
direction starting from the same location (i.e. the 2-D view origins should be