Exploring Drafting, 12th Edition Page 166 (186 of 640)

166
Exploring Drafting
created in surface modeling are similar to
solid models, but the objects are not defi ned
as solid. Surface models have an outer “skin”
to represent exterior surfaces. See Figure 7-24.
While a surface model is not considered as
realistic as a solid model, the quality of the
representation is very similar. Therefore, surface
models are primarily used for presentation
purposes.
Another type of 3D modeling is parametric
modeling, discussed in the following sections.
Overview of CAD Systems
and Software
There are many CAD software programs
available for a variety of applications. The
type of program used normally depends on
the application or drawing discipline for
which it is used. CAD programs range in
capability from simple 2D drawing programs
to advanced 3D modeling and presentation
programs. While basic CAD programs require
relatively inexpensive hardware to drive the
system, higher-end programs may require
computer equipment costing several thousands
of dollars.
As discussed in Chapter 4, a basic CAD
system consists of a computer, monitor (display
device), keyboard, pointing device, and output
device. Data is stored on the hard drive of the
computer or on portable media. The primary
component of the CAD system is the software.
The software is the set of instructions that tells
the computer what to do and when to do it.
CAD software programs can be classifi ed
in several different ways. Some CAD programs
provide 2D drawing capability only. These
programs have many of the functions
previously discussed in this chapter, with
the exception of 3D-based tools. Other CAD
programs provide more advanced tools for
3D modeling purposes. These programs are
used to construct models defi ned with 3D
coordinates. Models created in this manner are
then typically used for additional purposes,
such as testing, analysis, and prototyping.
Common 3D modeling commands are
discussed in Chapter 13.
CAD programs with 3D modeling
functions are typically based on a specifi c type
of 3D modeling, such as solid modeling or
surface modeling. In solid modeling, objects
called solid models are created to represent
the entire mass of an object. See Figure 7-23.
A solid model is considered to be defi ned
from the actual material making up the object.
Solid models are used in mechanical drafting
applications because they can be analyzed for
properties such as mass and volume. Models
Model courtesy of SolidWorks Corp.
Figure 7-23. A solid model represents the entire mass
of an object and the material used in its construction.
Autodesk, Inc.
Figure 7-24. A surface model is a representation of the
outer “skin” of the various surfaces simulating the object.
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166
Exploring Drafting
created in surface modeling are similar to
solid models, but the objects are not defi ned
as solid. Surface models have an outer “skin”
to represent exterior surfaces. See Figure 7-24.
While a surface model is not considered as
realistic as a solid model, the quality of the
representation is very similar. Therefore, surface
models are primarily used for presentation
purposes.
Another type of 3D modeling is parametric
modeling, discussed in the following sections.
Overview of CAD Systems
and Software
There are many CAD software programs
available for a variety of applications. The
type of program used normally depends on
the application or drawing discipline for
which it is used. CAD programs range in
capability from simple 2D drawing programs
to advanced 3D modeling and presentation
programs. While basic CAD programs require
relatively inexpensive hardware to drive the
system, higher-end programs may require
computer equipment costing several thousands
of dollars.
As discussed in Chapter 4, a basic CAD
system consists of a computer, monitor (display
device), keyboard, pointing device, and output
device. Data is stored on the hard drive of the
computer or on portable media. The primary
component of the CAD system is the software.
The software is the set of instructions that tells
the computer what to do and when to do it.
CAD software programs can be classifi ed
in several different ways. Some CAD programs
provide 2D drawing capability only. These
programs have many of the functions
previously discussed in this chapter, with
the exception of 3D-based tools. Other CAD
programs provide more advanced tools for
3D modeling purposes. These programs are
used to construct models defi ned with 3D
coordinates. Models created in this manner are
then typically used for additional purposes,
such as testing, analysis, and prototyping.
Common 3D modeling commands are
discussed in Chapter 13.
CAD programs with 3D modeling
functions are typically based on a specifi c type
of 3D modeling, such as solid modeling or
surface modeling. In solid modeling, objects
called solid models are created to represent
the entire mass of an object. See Figure 7-23.
A solid model is considered to be defi ned
from the actual material making up the object.
Solid models are used in mechanical drafting
applications because they can be analyzed for
properties such as mass and volume. Models
Model courtesy of SolidWorks Corp.
Figure 7-23. A solid model represents the entire mass
of an object and the material used in its construction.
Autodesk, Inc.
Figure 7-24. A surface model is a representation of the
outer “skin” of the various surfaces simulating the object.
Copyright Goodheart-Willcox Co., Inc.

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Chapter 7 Computer-Aided Drafting and Design
167
a part begins by drawing a 2D profi le called a
sketch to create the most basic feature or shape
of the part, called the base feature. Additional
features can be created from sketches or other
modeling methods. Because features form the
basic building blocks of a part, this process is
referred to as feature-based modeling.
Each part is created as a separate electronic
fi le. Several completed parts can then be used to
create an assembly by assembling and mating
the parts together. An assembly is a functional
mechanism made up of multiple components
(parts). An example of an assembly is a set of
gears. An assembly is created as a separate
electronic fi le and contains references of the
individual part fi les. The part fi les are linked
to the assembly fi le so that if a part fi le is
edited, the assembly updates automatically to
refl ect the edit. The completed assembly can
be simulated to determine the effectiveness
of the design and to check for interference
between parts.
Once the modeling process is completed,
working drawings can be generated from
the part and assembly fi les to document the
engineering design process. Drawings are
created as separate electronic fi les. However,
the part and assembly fi les are linked to the
drawing fi les so that if the model is edited, the
corresponding drawing is updated to refl ect
the edit.
The following are typical steps used in
parametric modeling to model parts and
construct an assembly from the parts. These
steps are discussed in more detail in the
following sections.
1. Create a 2D sketch that defi nes the profi le
of the base feature of a part. Make the
sketch on the appropriate plane (front,
top, or side).
2. Add geometric constraints and dimensions
to the sketch. Fully defi ne the sketch.
3. Using the appropriate modeling
tool, create the base feature from the
completed sketch.
4. Create additional features as needed to
complete the part.
Parametric modeling programs that are used
to create solid models are most common in
mechanical engineering applications. This
chapter focuses on the common tools used in
these programs.
Parametric Modeling
Parametric modeling is an advanced form
of 3D modeling in which the model dimensions,
or parameters, are defi ned and modifi ed as
needed throughout the modeling process.
In this type of modeling, parameters control
feature sizes and locations and a change to
one parameter affects the entire model. See
Figure 7-25. Because parameters control the
size and location of features, parametric models
are sometimes referred to as “intelligent”
models.
Parametric modeling programs used in
mechanical design and engineering involve a
different workfl ow in comparison to 2D CAD
drawing programs and other types of CAD
modeling programs. In parametric modeling,
3D models called parts are created to represent
mechanical components. Each part is made
up of the various features that defi ne the part.
An example of a part is a shaft. In many cases,
Model courtesy of SolidWorks Corp.
Figure 7-25. Parametric modeling programs provide
powerful modeling tools that allow object dimensions
to be changed throughout the modeling process. Any
changes made affect the entire model.
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Chapter 7 Computer-Aided Drafting and Design
165
is not lost. It is defi nitely to your advantage
to follow your instructor’s fi le storage and
organizational techniques without variation.
Not following an accepted protocol may result
in you having to refer to the old saying—“If
you do the crime, you will pay the time!” In
other words, if you don’t consistently save
and back up your work often, there is always
a chance that you will end up doing it again.
A typical protocol for achieving safe and
reliable fi le storage may include the following:
1. Save your work every 15 minutes.
2. Save fi les to a network server rather than
a local hard drive. Save to a network
server with automatic fi le backup
capability, if possible, to ensure fi le security.
3. Back up work weekly to another location
separate from the network server, such
as one or more portable media devices.
Work can be saved to CDs, DVDs, or
fl ash drives.
This is just a sample protocol and will
probably vary based on your unique situation
and your instructor’s recommendations.
When work is completed on a project and
the fi les do not need to be used on a daily basis,
an electronic fi le archive is typically created.
An electronic archive typically consists of all
of the fi les related to a single project. This may
involve hundreds of fi les and many different fi le
formats. Some CAD programs provide special
tools for creating fi le archives. No matter what
method you use, there are several important
points to keep in mind.
Files to be archived must be saved in a
format and on electronic media that will
be compatible with computer software and
hardware of the future. Hence, the long-
term usefulness of CAD fi les is of greater
consequence than how much storage space
they require. As with fi les in a currently active
project, archived fi les must be accessible so
that they can be located, opened, and edited if
needed. The same fi le naming conventions, folder
organization practices, and backup procedures
used for active fi les should also be applied
when assembling an electronic archive.
features based on the fl oor layout. If the fl oor
plan is “referenced” by the electrical plan, it
becomes a “dependent” of the parent fi le.
When opening the parent fi le (the electrical
plan), the software must be able to locate the
reference fi le (the fl oor plan). If the fi le cannot
be located, you must supply the fi le path. This
is another reason to maintain an orderly fi le
storage system. Make sure to organize your
projects so that fi les referenced by other fi les
can be located by the software.
Saving, Backing Up, and
Archiving Files
Whether working with manually created
drawings or CAD fi les, the need for secure,
organized archives of original drawings is of
utmost importance. Storing manually created
drawings in physical fi les requires a secure
facility, adequate organizational techniques,
and plenty of space. At the same time, proper
storage of CAD drawings creates an entirely
different set of problems and challenges.
Since CAD drawings are electronic fi les,
there must be a means of storage that protects
against cybertheft, hardware failure, and fi le
damage. Preventing the loss of work is a primary
concern in every project. As most computer
users are no doubt aware, computers do
go down periodically. Hardware fails and
occasionally, power fails. Hours of work can
be lost in the blink of an eye if fi les are not
saved to hard drives and servers frequently.
It is for these reasons that most CAD fi rms
establish a protocol of fi le storage and backup
strategies. This protocol usually requires that
employees save and back up their work at
regular intervals. Generally, these strategies
are very in-depth routines. A good fi le storage
protocol is designed so that everything possible
is done to guarantee that valuable work and
hours are not lost.
Your CAD instructor will probably set up
a similar protocol for saving and backing up
fi les. Methods will vary, but fi les are typically
saved on a network server and backed up
to another location so that valuable work
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