Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (32 page)

Another enabler for the 2-D design methodology is using full-scale CAD
drawings. Although it would be quite impractical to design a building using paper
that is at full scale (since the paper would be as big as the building), the CAD
system has no problem having the user design in a mathematical space as big as a
building. This approach has the advantage that any time a geometric entity is cre-
ated or an entity is measured, the result is correct in the context of the overall
design. If the CAD user needs a beam that is 10 m long, then a line can be entered
with a length of 10,000 mm and the resulting line will accurately reflect the
length of the real beam. Of course, if the CAD system does not support viewports
and view scale, then this “full scale” approach is always available (since every-
thing in the CAD drawing would be
at “full scale” in a mathematical sense). For

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the CAD systems that use viewports and view scale, then the user needs to be
more careful in interpreting the geometric data from the drawing.

A few CAD systems actually provide techniques to “overlay” multiple
CAD drawings so that they can all be seen at the same time. This capability is
quite valuable in the context of the 2-D design methodology. Now the top level
designer’s drawing (that shows the full product) can be seen on top of other de-
signer’s layouts. In this case, it can be quite easy to make sure that the detail
drawings will correctly document the needed parts and items. These CAD sys-
tems will typically also use the viewport and projections techniques mentioned
earlier so that the drawing’s definition in the Front View, Top View, Right View,
etc. can all be made consistent, and hopefully, the final design will have fewer
interference problems (where parts do not work or fit properly or be easily as-
sembled). Although this overlay technique is pretty powerful, it is somewhat ir-
relevant given that the 3-D design methods provide all these benefits and many
more. In a sense, the overlay 2-D method with projections is a “poor man’s 3-D
design.” But, given that 3-D design has become relatively affordable, it makes
little sense not to just use 3-D design methods instead.

5.12 TWO-DIMENSIONAL CAE BENEFITS

Most, if not all, analytical functions performed by engineers have been affected
by the use of computer software. These types of computer programs are usually
classified as CAE or Computer Aided Engineering. Probably the most common
of these programs is called FEA or Finite Element Analysis. These programs at-
tempt to predict the behavior of solid materials (such as metals) under various
loads or forces. Another common type of program is called CFD or Computa-
tional Fluid Dynamics. These programs attempt to predict the behavior of fluids.
In each of these cases, the CAE software needs to use a fairly large amount of
geometric information about the components being analyzed.

Assuming that a 2-D CAD system has been used properly (i.e. following
the “smart paper” approach), the CAD drawing can be useful for engineering
analysis. If the geometric entities are scaled and drawn accurately, then the data
about these geometric entities probably can be transferred to the CAE program
directly. Usually this involves the use of a neutral file format such as IGES. The
desired result is to have lines or line segments from the CAD program to be rec-
ognized as lines or line segments in the CAE program. And, when 2 line seg-
ments meet at a corner, for instance, the line segments must both really end at the
intersection. Although a small gap between these lines would still look correct
and would have no detrimental effect on the designer’s drawing, such a gap could
cause difficulties in the CAE program. Many of these geometric difficulties stem
from the generation of something in the CAE program called a mesh. A mesh is a
grid or framework of points within the confines of the part’s geometry that con-

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trols where calculations can be performed (refer to Figure 5.11). The creation of
meshes often involves the application of automated geometric calculations, and
issues like lines in a corner not touching will often cause errors.

Another aspect of engineering analysis with 2-D CAD data will involve the
2-D geometric properties. One obvious calculation would be area. If a part of a
drawing has geometric entities that form a closed section, then the CAD system
should be able to calculate the area of that section. Another common requirement
for calculation in a CAD system is section properties such as inertia or moment
of inertia. The inertia gives an indication of a cross section’s ability to resist being
deformed based on a specified axis; it weights the distribution of the area with
respect to an axis. This calculation is also useful for finding a CG or Center of
Gravity (generally these are misnomers for a mathematical centroid or center of
area, but the relation to a real center of gravity is too strong and so the term CG is
used anyway). The CG would be a point at the geometric center of any arbitrary
shape. Both the inertia and the CG are very often applied in formulae for analyz-
ing mechanical parts and assemblies.

5.13 CAD DATA FILES

Most of the issues covered in this chapter are primarily for the CAD user. Most of
the items in the next chapter are most relevant to the CAD administrator or man-
ager. But, one issue that is important to both groups is how the CAD data is actu-
ally stored on the computer system or network. This is done with something
called a file or a CAD data file. The files will be somewhat automatically man-
aged by the computer system, and the CAD system may hide some aspects of the
file management, but it is still essential that CAD users understand as much as
possible about the files to make optimal use of the system.

5.13.1 File Types and Names

Files are often identified by their extension or type. Most users would recognize
that a file that ended with DOC would be a word processing document file, and a
file that ends with DBF would likely be a database file. The same applies to 2-D
CAD, but the file extensions may or may not be associated with a specific CAD
system vendor. The most common extension would be DWG. This extension is
used by a number of CAD systems. On a PC-based or Windows system, the
DWG file would be in upper case. On a unix system, the file extension would
often be lower case (such as dwg). When looking at the computer system, then,
files will appear with names such as Bracket.DWG or bracket.dwg.

It is important to know the way the computer system stores these names,
since there may be restrictions or limitations on the names. Often the file exten-
sion is limited to 3 characters, so when creating a CAD data file for a CAD draw-

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A sample 2-D mesh.

5.11

IGURE

F

2-D CAD 141

ing, generally always limit the file extension to 3 characters (the CAD system
may automatically apply the extension). Another issue is that the period (.) is of-
ten used to separate the main file name from the file extension; therefore, the pe-
riod should be avoided as part of the drawing file name. Depending on the system
used, the delimiter for directories would often be restricted in the CAD data file
naming. For the “Windows” systems, this would be the backslash (\); for the
Unix systems, this would be the forward slash (/).

In terms of the main file name, it can be descriptive (such as bracket.dwg),
but usually companies have a part numbering system that manages all the product
data for the company. This is often a proper choice for the CAD data file name. A
company may produce many drawings that would say “BRACKET” in the Title
Block, but there will only be one drawing that would use a given drawing number
or part number. Of course, considering the 2-D design methodology presented in
the previous chapter, there were some types of drawings in the hierarchy that
were not usually released (such as layouts); they may not have part numbers.
These nonreleased drawings often can benefit from descriptive file names (such
as engine_trans_layout.dwg), instead of the part number file names.

Some more sophisticated CAD systems allow the user to create “normal”
names and numbers for drawings (such as “ENGINE TRANSMISSION LAY-
OUT 44202”) independently of the CAD data file name. Systems like this would
probably allow the use of things like the period or the slash in the drawing name
and/or number. This metadata-based type of CAD system uses a special database
or lookup table to cross-reference the real names to the data file names.

5.13.2 File Format

Most, if not all, the CAD file types are considered binary data files. This means
that one can not directly view and manipulate the data in the file. Files which one
can directly view are known as ASCII files. Some CAD systems will have the
ability to read and write both formats.

One implication of the file format (both its structure and specific data) is
that you usually have to read, write, and view the file with the exact CAD sys-
tem that created it. This is a serious limitation in general, but this proprietary
data file approach does allow the CAD system vendor to accelerate performance
of the CAD system. The CAD data file is custom-made for the particular CAD
program.

However, care has to be taken when copying or moving the binary data files
from one type of system to another. Not all computer systems use the same byte
order (the 1s and 0s in the binary file go in different directions). In this case, the
CAD system may be forced to use an ASCII format of the CAD data file instead
(the ASCII file does not have this byte order problem). This is also why “neutral
files” are almost always in ASCII format.

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Another issue for file format is translating CAD data files from one CAD
system to another. Getting CAD drawings from one CAD system to another is
known as a translation. The translators usually use a neutral file. Usually, the
sending CAD system creates the neutral file from their proprietary format. Then
the neutral file is copied or moved to the other CAD system. Finally, the neutral
file is translated into the receiving CAD system. The most common neutral files
are called DXF and IGES. Refer to the next chapter on 2-D CAD management
for more information on drawing translations.

5.13.3 File Size

The size of CAD data files can be an important issue. Although most computer
systems have a great deal of available disk space at a relatively low cost, it is
generally important not to waste the space. Also, most company’s CAD systems
use a file server. The file server’s disk space is going to be a resource that needs
to be carefully monitored and managed, and it also should not be wasted.

Probably the biggest impact on the size of the CAD data file is the file for-
mat (as discussed above). The binary file format is much more compact than the
ASCII file. Therefore, the binary file format should be used as much as possible
(unless the drawings need to be quite portable between systems).

The next biggest impact on the size of the CAD data file is likely to be the
number of entities in the CAD drawing. This may generally be related to the
complexity or busyness of the drawing. A drawing with many views of a compli-
cated or intricate object is going to take many more lines, arcs, splines, etc. to
describe, and the CAD data file is going to be larger accordingly. However, there
are some issues that can aggravate this situation. For instance, if a drawing has
free-form geometry (such as wires, hoses, wavy curves), and the curves are actu-
ally being stored in the CAD data file as 100s or 1000s of tiny line segments, then
the file will be unnecessarily large. This situation of having many tiny lines is
usually not done by the CAD user directly; instead, a drawing may be translated
from another CAD system through a neutral file, and that neutral file does not
support entities such as splines. The neutral file breaks the curves down into the
many small line segments. Although the CAD drawing looks correct, it is actu-
ally using a large amount of disk space.