Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (23 page)

The first three standard views already mentioned (Front, Right, and Top),
which are identified merely by their location on the drawing, are all going to be
assumed to be true views. If the face of the object shown in these views are not
parallel to the viewing plane, then other kinds of views need to be put on the
drawing to show accurate dimensions.

Although architectural or A/E/C drawings are not a major focus of this
book, these types of drawings will use views called plan
and
elevation. The plan
view is like a Top View; it views the construction from above (showing the plan
or layout of the site or project). The elevation view is like a Front View; it views
the construction from the side (as a person would face it), and it obviously would
show how high or elevated the objects on the site would be.

FIGURE
4.5

First angle and third angle projection symbols.

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4.9.3 Auxiliary Views

Probably the simplest type of view for showing part of an object at a nonstandard
angle is the auxiliary view. As shown in Figure 4.6, the auxiliary view is shown
as a projection or at a viewing angle that directly faces the part of the object that
needs to be defined. As mentioned earlier, usually the most important characteris-
tic of the view is that dimensions can be accurately shown for that part of the
object in the drawing. In this case, the auxiliary view is a “true” view (although
the standard views are often referred to as the “true views”).

4.9.4 Section Views

Another common type of view for more accurately defining the geometry of an
object is the section view. The section view is like a cutaway of the internal vol-
ume of a part or object. As can be seen in Figure 4.7, the section view uses cross-
hatching (the tightly spaced lines at an angle that fills a area of the view) to
indicate material of the part that has been cut into.

Sometimes the standard views (such as Front, Top, and Right) will have
cutaway sections (with crosshatching shown in these views), but generally they
are instead used as the “basis” for the section views. When they are used as the
basis for the section views, there are special lines drawn with letter identifiers and
arrows. In the Front View shown in Figure 4.7, the thick line with the letter A at
the beginning and end indicates what the section view is cutting. Arrowheads at
the beginning and end of the line show the direction the cut section is to be

FIGURE
4.6

Sample auxiliary view on a drawing.

Drawings and 2-D Design 91

FIGURE
4.7

Sample section view on a drawing.

viewed. The section view, in this case, is called Section A-A, and this is clearly
indicated by a note beneath the section view.

4.9.5 Detail Views

Another type of view for documenting a part in a drawing is the detail view. The
detail view is generally a magnification or clarification of a somewhat small fea-
ture. As shown in Figure 4.8, the area of interest is usually based on one of the
other views already in the drawing, and again, it is usually identified with a letter.

4.9.6 Isometric View

The last type of view presented for helping to define more complicated parts is
the isometric view. This view is expected to show the object more or less realisti-
cally. It shows the part at a more “arbitrary” viewing angle so that one is not lim-
ited to only seeing one particular surface or face at a time. The view in the upper
right corner of the drawing shown in Figure 4-1 is an isometric view.

The isometric view is generally not expected to show official dimensional
information. Instead, it is expected to allow the reader of the drawing to more
quickly visualize the object in the drawing. If the reader has just a basic idea of
what is in the drawing based on the isometric view, interpreting the remaining
views of the drawing is much more efficient. For very simple parts this not a very
significant advantage for the reader, but for more complicated parts it can be a
very large advantage.

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FIGURE
4.8

Sample detail view on a drawing.

Before the large scale adoption of the 3-D CAD systems, the isometric
view would rarely be expected on mechanical drawings used in basic manufac-
turing. It would only be found on drawing connected with service manuals, parts
manuals, training materials, etc. The use of the isometric view was limited be-
cause the creation of this view was very time consuming (particularly if no CAD
system at all is used). However, with 3-D models generally available for parts
and/or assemblies, the isometric view can usually be created automatically. These
isometric views can also have “hidden line removal” so that the part or object
looks even more realistic.

4.9.7 View Scale

An important concept with respect to views is scale. The scale of a view (any
view) is the relationship between the image or geometry shown on the drawing
and the actual object being documented. For example, if a plate is 100 mm long,
and that part’s lengthwise edge is shown on a view in the drawing that is at half
scale, then the line drawn of the plate shown on the drawing would be 50 mm
long. This 50 mm is the length of the line one would measure directly from the
hardcopy or print of the drawing (assuming it is printed according to the paper
size shown in the Title Block). In other words, the view scale is like making a
model of the object using the paper drawing and indicating how big or small that
model would be with respect to the real object.

It should be clear, then, that a large object (such as an airplane) would be
shown in a drawing with a very small scale, for example, 1/32 or one thirty-sec-
ond scale. In this case, one inch on the drawing would represent 32 inches on the

Drawings and 2-D Design 93

plane. For a mm drawing, perhaps a scale of 1/50 or one-fiftieth scale would be
used, and 1 mm on the drawing would represent 50 mm on the plane. Another
way of indicating view scale is in decimal form. For the 1/32 inch drawing, the
scale would 0.03125. For the 1/50 mm drawing, this would be 0.02. Yet another
way of indicating scale is to indicate a ratio using a colon (:). In this case, the two
scales in the examples would be 1:32 and 1:50 respectively.

If all the views in a drawing are at the same scale (which is somewhat typi-
cal), then only a single value of scale needs to be indicated on the drawing, and
this value can be put into the Title Block. Often though, section views, detail
views, etc. are shown at a different value of scale (usually larger scale to show
more detail). In this case, there should be a note below these special views to
indicate the scale for these views. In these cases, the scale shown in the Title
Block becomes more like a predominant scale; meaning that most of the views in
the drawing are at that scale, but not necessarily all of them.

Keep in mind that the dimensions shown in the drawing are not really re-
lated to the view scale (as far as the drawing reader is concerned). The dimension
on the drawing for the 100 mm plate example should be shown as 100 mm, be-
cause that is how long the real plate is supposed to be. As discussed later, how-
ever, for CAD users it is important to know the relationship between the view
scale and the dimensional values.

In terms of what value of view scale should be used, there are no specific
rules. It depends on the complexity of the object between documented and the
paper sizes available. If only smaller-sized drawings can be printed, then perhaps
the smaller drawing sizes (B and C for inch; A2 and A3 for mm) should be used.
In this case, smaller scales would be used to “shrink” the objects image enough to
fit on the paper. If larger drawings can be used, then perhaps larger drawing sizes
could then be used. Of course, if the drawing contains a great number of edges,
lines, circles, notes, etc. then a smaller paper size with the small scale will simply
not work (the drawing will not be readable). In this case, the larger paper size, or
perhaps using multiple sheets of the larger scale will be required.

4.10 “OBJECT” LINES

Obviously the object (part, assembly, product) documented by the drawing will
be shown in the views of the drawings. The lines drawn in the views that are
showing the object can be called object lines. They may also be referred to as line
work, geometry or visible and dashed lines.

As shown in Figure 4.1, there are solid lines (not broken at all) and dashed
lines (broken). The solid lines are usually edges of the object that are visible
(from the viewing angle of the view). The dashed lines are edges of the object
that are hidden from sight (again from the viewing angle of that particular view).
These dashed lines are then called hidden lines. In CAD systems, these appear-

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ances of lines (solid, dashed, etc.) are usually referred to as fonts or line fonts.
The solid font indicates a solid line type; the hidden font indicates the hidden or
dashed line type, etc. Keep in mind, however, that some fonts are not actually for
the object, but rather for indicating the center of a line (this is the centerline font).

The main task for the object lines is to help the reader visualize the view of
the physical object. Beyond the line type being visible or hidden, the next visual
cue is called line weight. Line weight is a term that indicates how thick and dark
the line appears. A heavy line weight indicates that the line is drawn thick and
dark. A lighter line weight indicates a thinner and lighter line. Often the use of
line weight is limited to 2 or 3 levels such as light, medium, and heavy; or they
may be referred to as thin, medium, and thick.

The solid lines (indicating visible edges) are generally thick or heavy lines.
They are supposed to stand out clearly. Then the hidden lines and other lesser
indications are generally thin or light line weights. As mentioned earlier, the
point is to assist the reader in visualizing the object.

It is important to realize that the standard drawing is somewhat of a simpli-
fication versus the actual 3-D object. For instance, the Front View in a drawing
shows some lines that are more than one edge. The object may have an edge fac-
ing the reader which is shown as a solid line; but, then there is another edge at
that line that is coincident or on top of the first line. This is not really an issue for
the manual creation of drawings, but when the drawing is created electronically,
and particularly from a 3-D model, this can be an important consideration. The
CAD system may regard them as the same line, or the CAD system may regard
them as two distinct entities. If the CAD system does treat them as a single line,
then usually there is some tolerance or error to indicate how close the edges must
be before the line is merged. Unfortunately, this means that a very small rotation
of the 3-D model can be incorrectly documented in the drawing if the tolerance is
not acceptable.

4.11 DIMENSIONS

Referring to Figure 4.9, some parts of the views show numbers with arrows.
These are called dimensions. They are meant to indicate the actual size of the
object being documented by the drawing.

Notice that the views in Figure 4.9 do not show all of the possible dimen-
sions for the object in every view. Instead, in each view, certain dimensions are
shown that make the most sense for that particular view. For instance, the Front
View shows the width and height of the object, while the Top View shows the
depth. Of course, the depth could also be shown as a dimension in the Right
View, but there is no need to show this parameter more than once in the drawing
(repeating it would be considered confusing and a source of potential redundancy
and error).

Drawings and 2-D Design 95

FIGURE
4.9