Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (31 page)

The next option that might be user-defined is a font. This is not necessarily
as useful to the CAD user as colors, but they may help with improved hardcopy
quality. For instance, if the standard or default appearance of the hidden line font
(should be short dashes) is not clear enough (perhaps the dashes are too close
together), then a better hidden line font could be created by the user. Fonts are
usually defined by a bit pattern. For instance, the CAD system may allow a re-
peating pattern using 16 ones and zeros. In this type of system, the hidden line
font might be represented by 1111000011110000. The system may also require
that this be specified in hexadecimal. The example bit pattern would be F0F0 in
hexadecimal. Keep in mind that the pattern is repeated over and over in a line, so
one needs to use a pattern that is continuing or symmetrical as desired.

The next option that might be user-defined is line weight. This also may not
be necessarily useful to the CAD user as a design is being created, but may be
used to enhance the appearance of the hardcopy. The only information that needs

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to be supplied is the how thick or “heavy” a line needs to be when printed. This
thickness will probably be specified by a measurement in inches or mm. At one
time, line weights may have been controlled by selecting different pens that a pen
plotter could use in making the lines. Some CAD systems may still refer to pen
mapping where different line weights (such as thin, medium, and thick) are given
a “virtual pen” number (such as pen 1, 2, and 3). This gives a CAD administrator
the opportunity to actually define the line weight by adjusting the printer (using
commands that the printer receives independently of the CAD system). In any
case, it is important that line weights make the drawings as readable as possible,
and it may be necessary to create a user-defined line weight to accomplish this.

5.10.4 Hiding and Showing

The next special technique can be referred to as hiding and showing. This is the
CAD system’s ability to temporarily remove parts of the drawing, and then re-
store them again at a later time. For example, if changes to a design are being
contemplated, but the previous design iteration still needs to be preserved, then
the old design information (geometric entities, dimensions, notes, etc.) can be
hidden. The user will usually just select the items to be hidden and then select the
command that hides the data.

The hidden data should be retained by the CAD drawing, even though it
will not appear on the monitor or in the hardcopy. Another example would be
having too much information shown on the drawing as the user is trying to design
a specific component or part. In order to see certain geometry more clearly, less
important entities in the drawing might be hidden temporarily. When the user
wishes to restore the information, the CAD system should have a method for in-
dicating what has been hidden already and then selecting the entities or informa-
tion that needs to be restored. Having the information re-appear (i.e., showing)
can also be considered an undo function for hiding.

One issue to keep in mind with hiding entities is that if the drawing is ex-
ported to another CAD system (through a neutral file such as DXF or IGES), the
neutral file may or may not handle this special technique. Even if the neutral file
is able to store the fact that entities are hidden, the other CAD system may not
handle it anyway. If there is uncertainty of whether this will cause problems, then
most likely the hidden entities should be removed before the export or translation
is performed. This may be referred to as a cleanup procedure for the drawing.

5.10.5 Selecting Techniques

Another important advantage of a CAD system is its ability to automatically rec-
ognize and alter a variety of items in the CAD drawing. Examples of these types
of items would be geometric entities vs. dimensions, notes vs. balloons, etc. An
example of the manipulation would be to change all the heights of all the letters
in notes (but not change the heights in balloons). However, this is usually accom-

2-D CAD 135

plished by first selecting or trapping all the particular items of interest, and then
making the manipulation. Therefore, it is important to understand and master the
special techniques for selecting the items.

One special technique would be intelligent recognition of the types of
items. In other words, if the user wants to select all dimensions, then the CAD
system may be able to simply recognize them all at once. This type of selection
can be referred to as filtering. For example, the user would select the Dimension
filter, and then have the system select everything that passes the filter. Some CAD
systems even allow a logical mixture of filters (i.e., select all Circles and
Splines). Another way to accomplish this type of selection may be to use a layer.
The user may put all dimensions on a dimension layer, and then the user would
have the system select everything on that layer (and thus select all dimensions for
alteration or manipulation).

Another special technique would be various uses of windowing. Window-
ing relates to how a pointing device (such as the mouse) is used to select items.
Most CAD systems would have the ability to have the device click in 2 locations
to form a box. And, usually, the system will select anything that is completely in
the box. This can be referred to as “trapping in.” Some CAD systems may offer
other options, such as using a circle instead of a box, or allowing the user to
sketch an arbitrary shape that eventually closes to form a selection area. Another
possibility is to allow selecting items that are only partially in the box area. Yet
another possibility is to select all the items that are outside of the box (i.e., “trap-
ping out”). If viewports are part of the CAD system, this allows for more possi-
bilities or sophistication in the select process. For instance, filtering and select
boxes can be combined with selecting items within the different views.

5.10.6 Undo and Redo

Another special technique for CAD systems is being able to undo and redo oper-
ations with a CAD drawing. The undo simply undoes the change or command of
the user. The redo redoes the operation. This capability is generally provided due
to a journal file. This file keeps track of all the operations that the user is making.
The CAD system reads this file to know what to undo and/or redo.

Keep in mind, however, that CAD systems may have limitations on this
journal process. For instance, the journal may be “cleared” every time the user
makes certain operations (such as saving the file, deleting view ports, etc.). In
other systems, there may be a limit on the number of steps that will be journaled,
and thus the number of steps that can be undone or redone may be limited.

5.10.7 Related Geometry Creation

Once some geometric entities (such as lines, arcs, etc.) are created, the CAD sys-
tem can create new entities based on these existing entities. This is possible since

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TABLE
5.7

Special Techniques for Creating Related Geometry

Special

technique Description

Trimming +
This involves altering geometry so that they touch. If a line is sup-

extending
posed to end at another line or a circle, then the system can find the

intersection. If the line in question overshoots the other line, then it

is trimmed to the intersection. If the line in undershoots, then it is

“extended” to end at the intersection.

Dividing If a line is too long, and it needs to stop at a specific point or intersec-

tion, but the user also wants the “remainder” of the line left as well,

then this technique will perform this operation by making a single

entity into 2 separate new pieces (using the intersection as the point

of division).

Offsetting This technique creates new geometry at a specific distance away from

existing geometry. This usually can make use of lines, arcs, circles,

etc. After existing geometry is selected, the user needs to enter the

offset distance, how many copies, as well as the direction to offset

(there will be two directions away from the existing geometry).
Merging This technique would be the opposite of “Dividing” mentioned above.

In this case, 2 separate geometric entities are merged into a single

new entity. Of course, the entities must already be “just touching”

in some manner. For instance, if 2 lines are going to merge, they

should already have the same line mathematical definition (having

the same slope and sharing a point).

Making corner This technique does a “Trim/Extend” type of operation mentioned

above, but it is done to 2 lines, and the result is a corner. One can

imagine 2 lines that intersect in the shape of a cross, and then the

system cuts off 2 pieces to leave the corner.

Filleting This technique creates a new arc where 2 lines intersect. This is a

very, very common operation in mechanical drawing since most

parts with features that join have some sort of transition from one

feature to the next (they aren’t really a totally sharp corner). See

Figure 5.8. The user will usually just select the 2 lines and then en-

ter the radius for the arc that is then tangent to both lines.
Patterns This technique would create a pattern or array of a specific entity or

entities. For instance, a bolt circle is a common mechanical design

technique where 2 components (such as a pipe and a flange) are

held together with a set of bolts in a circular pattern. The system

creates a bolt circle by having the user select one bolt that is in the

correct location and size, and then picking the origin of the bolt cir-

cle and the number of bolts needed. The system would then draw a

series of bolts whose center lies on the bolt circle’s center. Another

common pattern arrangement would be a rectangular pattern.

2-D CAD 137

the CAD system can perform mathematical operations on the entities. For in-
stance, the CAD system can quickly determine the intersection of 2 straight lines
(assuming they are not parallel). Then this intersection point can be used to trim
or extend lines to make them meet at this exact point. So the user can create 2
lines that are supposed to touch, but only do it roughly, and then have the CAD
system make the precise correction. Other possible special techniques would be
taking 2 lines that cross and cutting them so that a corner is formed, creating new
lines or arcs that are “offset” a given distance from existing lines or arcs, or mak-
ing patterns or arrays of existing geometry. Table 5.7 lists some of these special
techniques.

Keep in mind that these techniques will not work well if the “smart paper”
paradigm is not carefully followed. It is the “underlying” mathematics of the ge-
ometry that makes these special techniques work. So, if lines are not drawn in the
proper scale, in the proper view, at the proper angles, etc., then these special tech-
niques may not work.

5.11 ENABLING 2-D DESIGN METHODOLOGIES

In the previous chapter, an overall philosophy or design methodology was pre-
sented based on drawings. This involved a hierarchy of designers and types of
drawings (such as layouts, detail drawings, assembly drawings, etc.). The 2-D
CAD system can be a powerful enabler or enhancer for this methodology.

For instance, the detail drawings can be automatically extracted from lay-
outs. Recall that the layout drawing has the overall locations of items in the de-
sign. The layout designer determines the basic envelope or region of space that a
specific smaller part (or detail) is supposed to fit into. So, if the layout designer
exports this envelope or boundary to a detail designer (perhaps as a grouping
symbol, block, clump, etc.), then this detailer can be pretty certain that his or her
new drawing’s part will fit into the overall design (assuming the layout designer
does not alter the overall layout without relaying the change to the detailer).