Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (24 page)

Sample dimensions.

The most important aspect of dimensions is that they are supposed to un-
ambiguously define the geometric characteristics of the object documented by
the drawing. In other words, if a manufacturer is given the drawing, the manufac-
turer should be able to create the object exactly as intended by the author of the
drawing. This seems simple enough, but in reality, this can be rather difficult for
some objects.

Figure 4.9 shows the details of some simple dimensions that could be
found on a typical drawing. The dimension starts with the extension lines near
the object. These are the lines that indicate what exact part of the object is being
dimensioned. They may “indicate to” edges of the part, the center of a hole, etc.
Next the extension line is met by the arrowhead of the dimension. The arrowhead
may point in either direction. The arrowhead is connected to the dimension line.
The dimension line extends for the length of the dimension, and then there is an
arrowhead at the other extension line.

Note that the American (ANSI) standard for dimensions breaks the dimen-
sion line to put the numerical value for the dimension in the “middle” of the di-
mension. However, the international standard (ISO) for dimensions does not
break the dimension line. Instead the numerical value is shown above the dimen-
sion line. These styles are shown in Figure 4.9.

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4.11.1 Dimension Values and Scale

Obviously the number or values shown in the dimension are very important. They
indicate the size required for the object. It is also important to understand the
concept of view scale for dimensions since the view scale indicates the relation-
ship between the object or geometry drawn on the drawing and the value shown
in the dimension. For instance, if the real object is 100 mm long, and the view
scale is 0.1, then the person making the drawing would make a line for showing
the object 10 mm long (since the scale is 1/10 or one-tenth). However, the dimen-
sion number shown on the drawing must still say 100.

When someone made a manual or “board” drawing (not using a CAD sys-
tem), they would use a scale. This was a special kind of ruler that had the mark-
ings at different scales (1/2, 1/4, 1/10, etc.). Then the person making the drawing
could just use these devices to make the drawing at a particular scale. The scale
would make sure that the object was being “compressed” or “expanded” cor-
rectly. Then the draftsman would just draw the numbers on the dimension. For
the example mentioned above, the person could write the dimension as “100”
even if the line drawn for the object was only 9.9 mm long (nearly 1/10 scale).

However, for CAD systems, this situation becomes more complicated. The
CAD software should use exact values for the lines that are drawn. The user can
indicate that a 100 mm object line is needed (at whatever scale). Then the user
can ask for a dimension to be drawn for that line, and it will automatically show
up as “100.” This will only work if the object is drawn very carefully and if the
user understands what the scale really means.

4.11.2 Dimension Values and Decimal Places

Besides showing the designer’s intention for the size of an object, the dimensions
on a drawing can also indicate information about the accuracy needed for the ob-
ject’s manufacture. For instance, a hole in a plate may be dimensioned at a certain
location with respect to the bottom and side edges of the plate. However, the hole
must be manufactured by some means, and the manufacturing process will have
some error with respect to the ultimate value shown in the drawing. Furthermore,
some processes will be more accurate than others; therefore the tolerance needs
to be indicated in the drawing so that an appropriate manufacturing process can
be selected.

If the hole is a critical item (perhaps the hole is for a shaft, and that shaft
has to line up with gears on a machine), then the hole might be drilled. Drilling is
generally a precise operation. If this hole is critical, then the dimension showing
the location of the whole would be shown with additional decimal places. The
decimal places are the number of digits shown after the decimal point (or
comma). If a dimension is for an integer value, such as 12 inches or 100 mm, then
greater accuracy is implied by showing these numbers in dimensions as 12.0,

Drawings and 2-D Design 97

12.00, or 12.000 inches or 100.0 mm. If a dimension already has decimal values
since they are fractional, then added zeros may or may not be needed. For in-
stance, 1.250 and 1.125 each indicate a tolerance to the thousandths place. The
accuracy being implied by the decimal places in the drawing is usually shown in
a special note near the Title Block (for example, “3 Decimal Places equals +/–
0.001 inch”).

This approach to indicating the needed accuracy of dimensions is only a
basic tolerance methodology. It simply indicates a general tolerance to the di-
mensions shown. There are, however, other types of critical geometric character-
istics. For example, 2 holes may need to be drilled in a accurately parallel fashion
(perhaps the holes are for 2 shafts that have gears that need to mesh). This more
sophisticated approach to controlling the accuracy of a part is an issue for what is
known as “GD&T” or Geometric Dimensioning and Tolerancing. This is men-
tioned in a little more detail below, but it is really beyond the scope of this work.

The use of decimal places for indicating basic accuracy creates somewhat
of a problem for CAD systems. As mentioned above, the values for dimensions
can actually be automatically calculated by the CAD system based on the precise
geometry that has been drawn with the software. If a rectangle is drawn by the
user to be 100.5 mm by 50.75 mm, then the CAD system is able to show the
dimension as 100.500 by 50.750 mm. However, this indicates a very accurate
tolerance (“to 3 decimal places”). Then the designer may have the CAD system
reduce the decimal places to a more typical tolerance of 1 decimal place. The
100.5 should be no problem, but the 50.75 has to be rounded. With the dimen-
sion shown as 50.8, the original intent of the dimension is lost. The original
rectangle was 50.75, and someone may now assume that the original CAD-
created rectangle was 50.8 (when in fact 50.75 was used). Whether a value is
rounded up or rounded down should be done according to a standard such as
ANSI Z210.1 (1973).

4.11.3 Arc and Angular Dimensions

So far, linear dimensions (dimensions that indicate lengths) have been discussed.
There are other kinds of dimensions, however. The most common would be for
circular geometry and angles.

Figure 4.10 shows examples of the circular and angular dimensions. Note
that a special symbol that looks like a zero with a line through it indicates a value
to be a diameter of a circular feature. Also, note that an R is used to indicate a
radial dimension. This is common for features known as fillets (pronounced “fill-
its”). Fillets are regions where two intersecting planes of material are blended to-
gether. They may be the result of the mold used to make a single part; they may
be the result of machining; or, they may be the result of welding two segments
together.

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FIGURE
4.10

Examples of arc and angular dimensions.

Also, note that there are lines that connect from the dimension values to the
dimension lines. This extra type of line is known as a “leader line” or just a
“leader.”

4.11.4 GD&T

As mentioned above, there are sophisticated approaches to indicating the allow-
able errors or tolerance to the object being documented by the drawing. This is
called GD&T or Geometric Dimensioning and Tolerancing or just Geometric
Tolerancing. GD&T involves an entirely new set of standard symbols and notes
for the dimensions and data shown on the drawing. Only a few examples are
shown here; consult a standard such as ASME Y14.5 or www.asme.org for more
information.

The most obvious indicator of the presence of GD&T on a drawing is a
Feature Control Symbol or FCS. As shown in Figure 4.11, the FCS is a set of
symbols and values in a box that is connected to or “associate” with a dimension.
The letters such as -A-, -B-, or -C- are used to specify a datum. The datum is like
the reference or grounding point for the dimensions. They refer to surfaces of the
part that are considered a base for the remaining dimensions.

Besides the GD&T symbols, tolerances of dimensions can be indicated by
over/under conditions. In this case, the dimension value is shown at a maximum
permissible high and low value. Figure 4.11 shows some dimensions with this
approach as well.

In general, drawings are not supposed to be physically measured (using the
hardcopy). The system of dimensions shown on the drawing is to solely indicate
the size of the object being documented (so measuring is unnecessary). To rein-
force this idea, a note on the drawing will generally state “NOT TO SCALE”
(even if the drawing is quite accurate in terms of scale). This “never measure”
approach is more problematic with the CAD data. If a user has carefully created a

Drawings and 2-D Design 99

FIGURE
4.11

Simple examples of GD&T symbols and over/under conditions.

2-D CAD model (or a 3-D model for that matter), then there really should be no
problem with measuring the model (using the CAD system) to obtain some extra
geometric properties or the designer’s intent. However, this measuring of the
model will not indicate any tolerance information; at this point, only the GD&T
symbols, the decimal places, or other notation can really reflect the designer’s
intentions for tolerancing.

4.12 NOTES

An important source of information on drawings is notes. Notes are just instances
of written text on the drawing. Some notes are very general, such as “NOT TO
SCALE” or “SPECIAL PROJECT.” Other notes may be very specific to the part
or assembly shown on the drawing. For instance, “USE LUBRICANT X345
WHEN ALIGNING ITEMS 22 AND 23.” Notes can be short like these exam-
ples, or they may even fill the entire view of the drawing (i.e. a “word”).

Notes are often standardized as a numbered list in the upper left hand cor-
ner of the drawing.

4.13 BALLOONS

Another important indication on some types of drawings is balloons or bubbles or
item numbers. As shown in Figure 4.12, balloons are numbers in a circle with an

100 Chapter 4

FIGURE
4.12

Balloons in an assembly drawing.

arrow pointing to a part in the drawing. This arrow that points from the number to
the object is known as a leader.

The numbers in the balloons usually correspond to an item number in a list
that appears on an assembly type of drawing. The assignment of specific numbers
is found in a parts list (shown on the drawing) or in a Bill of Material (BOM).
These items numbers could also be shown in the drawing with underlines (in-
stead of balloons)

4.14 CENTERLINES

A special line type is used for various circular features such as holes. In this case,
a centerline is used to locate the center of the feature. As shown in Figure 4.13,
the centerline is a font using thin line weight that has a long solid section with
occasional breaks to a shorter line. The centerline is useful in quickly identifying
that some lines are associated with a hole or other circular feature. Figure 4.13
also shows a sort of “bull’s-eye” that locates the center of a hole within its “true
view” (viewed directly along the axis of the hole). The lines of this type of cen-
terline are often used as extension lines to a dimension to locate the hole.

Drawings and 2-D Design 101

FIGURE
4.13

Centerlines in a typical drawing.