Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (57 page)

282 Chapter 11

11.8 MODEL DOCUMENTATION

The only real opportunity for a more advanced use of 3-D models as the master
source of design data (as opposed to drawings) is to replace drawings completely.
This is problematic, but it can be done in some circumstances. For a large manu-
facturer, there would be thousands or even millions of drawings spread across
their own virtual vaults and then through all the vaults of companies that are sup-
pliers or vendors for the manufacturer. As long as the systems are needed to sup-
port this legacy 2-D information, or if just one critical supplier or customer
doesn’t “do 3-D,” it becomes difficult to switch to an all 3-D documentation sys-
tem. On the other hand, if a company creates only new documentation and it can
force all suppliers to use the same 3-D CAD system, then doing 3-D model anno-
tation or documentation becomes more realistic.

Of course, relying on 3-D models for design data really implies that there is
no need for hardcopy at all. If the 3-D models are annotated to provide all the
notes/information that drawings used to provide, then it would make sense to just
have everyone receiving the models just view the 3-D models with a computer
system (for instance using a VRML-enabled web browser). This then means that
everyone viewing the models needs to ensure that the 3-D models are up-to-date
and valid. This can also be a problem for large companies and their vast supply
chains. It is probably simpler to just determine the latest revision level of a draw-
ing, and then check this against the Title Block of the drawing.

Hopefully these issues for 3-D documentation will be resolved over time
and drawings will no longer be the master source of design data. 3-D models are
far more intelligent and useful to all those who need the design information (they
can be measured, clipped, and provide data such as volume and weight). The
American Society of Mechanical Engineers (ASME) has drafted standards
(Y14.41) to cover 3-D model annotation. Hopefully this can form the foundation
for the wider adoption of 3-D model documentation.

11.9 CHAPTER EXERCISES

1. Study a company’s process for releasing 3-D models or the data related
to 3-D models. Audit the company’s actual use of the system and record whether
it complies with the standard process.

2. Determine the file extensions for the various items in the 3-D CAD
system. Record the average file size for these various types of files.

3. Determine how data from the CAD system is “backed up.” Record
whether a model that is deleted during a given week can be restored the following
week. If not, why not?

Managing 3-D CAD 283

4. Attempt to translate a 3-D part model into a neutral file format (such as
IGES or STEP), and then re-import the file back into the CAD system. Record
what, if any, problems appear.

11.10 CHAPTER REVIEW

1. Explain some of the benefits for using a CAD database to manage 3-D

CAD data instead of just the operating system file names/directories.
2. Explain some differences between the plotting of drawings and the

printing of 3-D models. Include how these hard copies are used differ-

ently and how the data from the CAD system usually differs.
3. Explain why an STL or VRML file would generally not be appropriate

for import into a CAD system for changes to the part or assembly

model.

Glossary

ACIS
A kernel for 3-D modeling, a set or library of routines that CAD system
vendors can buy for use in their system. These routines would provide the very
basic and core algorithms that create 3-D geometry. If two CAD systems both use
ACIS, then they should be much more able to exchange 3-D data. It has been
developed by Spatial Corp. which is owned by Dassault Systems. Another com-
peting part modeling kernel is Parasolid.

ADAMS®
A mechanism design CAE program (kinematics and dynamics) that
is often integrated into 3-D CAD systems.

Add
A 3-D part operation where different surfaces are brought together. Add-
ing implies that the CAD system does not determine the intersection of surfaces
such that a new bounded volume is created (i.e. adding avoids how surfaces
would be stitched up).

AIX
IBM’s version of the unix operating system.

Alphanumeric Data
Data that is made up of text such as numbers and letters.
This is to distinguish it from purely numerical data. Computer programs deal
with text data quite differently than with numerical data. Alphanumeric data
might be considered Character Data, but Character Data really implies that non-
printable codes such as LF (Line Feed) or ESC (Escape) could also be part of
the data.

Anti-aliasing
A process of making sharp geometric entities (such as lines) ap-
pear more smooth on the computer monitor. Since monitors have a finite number

285

286 Glossary

of pixels, a single line at an angle, or a circle, winds up with some level of
jaggedness (“jaggies”). Anti-aliasing is a technique of using subtle color changes
along the entities to make the entities less jagged.

Aperture Card
A mainframe punch card that includes a photographic image
of a drawing. The image can be created by a computer controlled device attached
to a CAD system. This Computer Output Microfilm (COM) technology is proba-
bly archaic. It has generally been replaced with on-line imaging systems.
Assembly
A collection of parts or details. An assembly is usually documented
in an assembly drawing. This type of drawing shows how the parts are related to
each other. The parts may be welded together (perhaps then called a weldment
drawing), they may be fastened together (as with bolts), or they may just be as-
sembled together as a working mechanism. An assembly drawing may show a
Parts List or Bill of Material, as well.

Assembly Model
A model in a 3-D CAD system for an assembly. The Assem-
bly Model contains or references Part Models as needed. Each Part Model that is
included in the Assembly Model should be instanced. This means that there can
be more than one copy of a Part Model, but all these copies are based on or con-
trolled by the one Part Model (see also Assembly Structure).

Assembly Structure
In a 3-D CAD Assembly Model, the list of the parts and
relationships that are contained within the Assembly Model. Each instance of
each Part Model would be a separate item in the structure. An Assembly Struc-
ture can be used to automatically generate a Parts List or Bill of Material.
ASCII Code
A set of binary codes (1s and 0s) determined by the American
Standard Code for Information Exchange that are equated to letters, numbers,
punctuation, control codes, etc. They form the basis for the actual binary data on
most CAD systems. Basically, only mainframe computers use a different code
system.

ASCII File
A file that uses the ASCII Codes, but it is also generally assumed to
be a file that has a structure that allows the contents to be seen with a file editor or
text editor program. Basically, all the information in this type of file is plain to
see. This is in contrast to the Binary File. ASCII files may have file extensions of
TXT, DAT, ASC, PRN.

AutoCAD®
A CAD program from AutoDesk, Inc. It essentially was the de
facto standard for PC-DOS computers. The DXF neutral file for CAD came from
this system.

Axis Plane
See Coordinate System.

Backlighting (“backfaces”)
A feature of a 3-D graphics system to make both
sides of a surface bright or shaded. In normal 3-D part modeling or solid model-
ing, one side of a surface is facing outside the part and thus is shaded according
to the light sources available; the other side of the surface is facing inside and
would be dark. Backlighting forces both sides of the surface to be shaded. This is
helpful in Surface Modeling tasks (where part models aren’t always solid).

Glossary 287

Balloons
A number in a circle with a leader line (a line with an arrowhead)
pointing to an item in an assembly drawing. The item numbers generally corre-
spond to the list of parts that are documented in the assembly’s Parts List or Bill
of Material.

Bandwidth
Generally used in conjunction with computer networks to indicate
the raw information carrying capacity of the network. Insufficient bandwidth can
lead to a bottleneck where CAD workstations are trying to send data, but the net-
work is unable to efficiently respond.

BASIC
A computer programming language. As the name implies it is a basic
language that is intended to be used by personnel with a minimum of training and
computer systems background making it a very widely used language. In general,
BASIC programs are interpretive. This means that the computer needs to digest
or decode each line of the program as it is read, then convert this to Machine
Code types of instructions before performing the next line of programming. This
is the least efficient approach in comparison to the use of a Compiler. During the
history of the PC platform, there were compiled versions of BASIC; and with the
advent of the Windows operating system, GUI-based versions of BASIC were
created that are event driven and making use of Dynamic Link Libraries (DLLs)
(which somewhat blurs whether it is interpretive or compiled). A BASIC file
would typically have a file extension of BAS.

Bills of Material (BOMs)
Lists of data about how products are assembled or
grouped. The Bill of Material includes Part List data (i.e. what parts are included
in the assembly); but, it is also implies some nongeometric information such as
the material the parts are made from, whether the items are purchased or manu-
factured in-house, etc. Bills of Material record the quantity of items; that is, if a
specific part (identified by a specific Part Number) is used 5 times in a level of
assembly, then this item is listed once in the bill, but it has a quantity of 5. Bills of
Material usually have parent/child relationships where one level of assembly can
include other levels of assembly. A top or final level of assembly shows the last
items that are brought together to form the actual shipped product. Bills of Mate-
rial can appear as notes on a CAD drawing; they can also be controlled and gen-
erated from a separate computer software system such as BOM, MRP, or ERP
software.

Binary
First, refers to a number system with the base of 2. The only two possi-
bilities in this number system are 0 and 1. The 0s are 1s are an abstraction of the
behavior of electronic devices that process information based on a high voltage or
a low voltage. Second, can also refer to an executable file on a unix system. Sys-
tem-wide programs for a unix system would typically be stored in a directory
such as /bin or /usr/bin where bin stood for binary.

Binary File
A file that presumably uses ASCII codes, but the file does not in-
clude the codes necessary for the file to be directly edited (see ASCII File). In
this case, the data is just a long stream of bytes (or 1s and 0s) that must be inter-

288 Glossary

preted by a program that knows the structure of the data. Binary Files are not
universally transportable because different computer systems order the bits in a
byte in different directions (big-endian versus little-endian).

BIOS or Basic Input/Output System, (a PC-type of system)
A special inte-
grated circuit device (or chip) and its associated programming that stores perti-
nent setup information for the configuration of the system. This is information
that allows a specific computer system to be unique. For instance, the BIOS setup
can indicate what types of peripherals are attached to the system. For example,
some computers may have 8 gigabyte disk drives, while others may have 16 gi-
gabyte disk drives. Through a special program that is setup by the computer man-
ufacturer, this information can be modified and stored on the BIOS chip.
Bit
A binary digit. It is a single value in a number system with the base of 2.
The only two possibilities in this number system are 0 and 1; therefore bits are
either 0 or 1. Dealing with numbers and data via individual bits is usually too
cumbersome; this type of data is usually converted to Hexadecimal (base 16).
Bitmap
A set of data that indicates the color of individual Pixels on the com-
puter monitor. A graphical image can be captured in a Bitmap, but it is not able to
be modified in the sense that geometric entities (such as lines and arcs) can be
individually discerned by computer software (although object recognition or
OCR software can be useful in attempting to do this somewhat). This is in con-
trast to the Vector Graphics or Vector Data that is made up of information on a
geometric entity basis. Types of files that are based on Bit Maps include JPEG,
PCX, and TIFF files.