Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (55 page)

The CAD database software should provide for the revision level for all the
3-D items (parts, assemblies, BOMs, etc.). In other words, each released item in
the vault should have a discernable revision level. As a model is checked out,
changed, and then checked back in, the CAD system should make allowance for
the new revision level to be indicated. Of course, revision level often should be
tied to the Engineering Change Order or ECO or release process, and this process
may be controlled by software other than the CAD system. Therefore, it may be
unpractical to have the CAD data management automatically set the revision
level (such as incrementing the level by 1 number or letter each time a change is
made). Also, 3-D models may have to go through an approval process within the
engineering department before it is released, and this approval cycle may gener-
ate changes and new database models (i.e. checked out, changed, and checked
in again).

11.3.4 Product Data Management

Particularly since 3-D CAD systems can create and maintain Bills of Material, 3-
D CAD systems can provide a level of product data management. Product data
management can be thought of as a total system which allows a manufacturer to
completely specify its product information. Despite the BOM capability for as-
semblies in a typical 3-D CAD system, the product data management activity is
often managed by a separate class of software called PDM software. It is essen-
tially a level above the CAD system (although it may appear to be merely a fea-
ture of the CAD system).

PDM would go beyond the traditional CAD system to include such items
as specifications (essentially documents that cover departments like marketing,
materials, production planning, purchasing, etc.), technical publications (docu-
ments such as field manuals or parts lists), configuration management (different
options or special features of the product that can be purchased), etc.

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The PDM system could also dictate control over design processes. For in-
stance, the PDM system may control what designers are allowed to work on
which project. It may control which users have to approve changes to the design
(the types of users are often referred to as “roles”). It may control how data is
controlled and/or released to manufacturing (often referred to as “states” of the
data). The installation, use, and management of PDM software is beyond the
scope of this work, but CAD users (and 3-D CAD users in particular) should be
aware of this class of software and its capabilities.

11.4 FILE ORGANIZATION

There are many types of operating system files that would be used in the context
of a 3-D CAD system. A number of them have been mentioned already (PRT,
ASM, DWG). Some other possible types of files would be files for Bills of Mate-
rial and database files for the metadata. Most of these files (except perhaps for the
Bill of Material file) would be in binary format. Table 11.1 lists information
about some of these files.

A somewhat unique file type for 3-D CAD systems is the model file. The
model file is often a sort of personal database of CAD data. Depending on the
user or the context, a model file may contain part data, assembly data, drawing
data, analysis data or whatever a user is working on at the moment. The model
file allows this dispersed design data to be brought together in a single operating
system file (as opposed to working with dozens, hundreds, or even thousands of
separate files that all share the same owner and privileges). As personal data,
though, it is usually considered temporary, and finished work is generally filed
back into the vault as the separate PRT, ASM, etc. files.

It is important that all these types of files be organized and carefully man-
aged on the computer system or network. As mentioned previously, there may be
some files that need to be protected from accidental deletion or corruption (such
as released drawings, models, and BOMs). These files would probably be kept in
a folder or directory or file system that has restricted access. Other files, such as
the model file, are less critical. These files may not require the same level of re-
stricted access.

Due to the widespread use of computer networks, there are some obvious
approaches to organizing the 3-D CAD databased on the network architecture.
For instance, model files would typically be kept on an individual’s workstation.
Critical or shared data such as the metadata databases obviously would be best
kept on file servers. However, since the amount of 3-D CAD data for a single
assembly model might be 100s of megabytes, and all this data might need to be
copied from the file server to the workstation at one time, the performance of the

Managing 3-D CAD 275

TABLE
11.1

Typical Operating System Files for 3-D CAD Systems

Item

Typical file

extension Description

Part models PRT Typically each 3-D part model eventually is stored as a

single operating system file. This PRT file is not

standardized however. Each CAD system uses its own

proprietary format. It contains all the data for 2-D

sketch planes, 3-D surfaces and edges, features, part

history, etc.

Assembly
ASM Typically each level of assembly (assemblies as well as

models
subassemblies) is stored as a single operating system

file. Again, this ASM file is not standardized for all

CAD systems. It would contain information such as

the part instances used in the assembly, the 3-D loca-

tion of each instance, data about each instance such as

whether it is translucent, pointers to parent and/or

child relationships, etc.

Drawings DWG Drawings are generally independent operating system

files as well. Unlike 2-D CAD, though, this file would

contain pointers between various geometric 2-D enti-

ties (like lines and arcs) and 3-D entities (edges)

found in the associative 3-D model (contained in a

PRT or ASM file).

Model files MF Typically this is a user’s temporary file for work in pro-

cess. It may contain part data, assembly data, analysis

data, or drawing data. Model files prevent having to

make many copies of individual operating system

files that are all owned by the same user.

Metadata DBF Typically this is standard database program files or ta-

bles. They would control or track the location and dis-

position of the various other operating system files.

They would also contain data attributes and relational

data about the various items.

Bill of
DAT, TXT,
These files are basically reports for the ASM or DBF

material
BOM
files. They would be related to the 3-D assembly

structure data in some way, but they may also contain

non–CAD data (such as material specification or ven-

dor data). The data for Bills of Material may also be

contained in the metadata data bases themselves (in

which case they are not separate operating system

files).

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network must be designed carefully. In fact, there will likely be multiple users
trying to obtain that much data simultaneously.

11.5 INTEGRATION

Another issue to consider for managing the 3-D CAD system is integration. Inte-
gration in the context of this work is combining separate computer software sys-
tems to share data and work together in some fashion. Although this integration
of software systems may seem superfluous (since there are such obvious benefits
to their combination), one needs to realize that the separate systems were each
developed independently over the years, and each system was intended to auto-
mate specific design and/or engineering activities. Furthermore, there is a great
deal of complexity and expertise involved with each of these engineering activi-
ties, and no single software vendor has really managed to achieve a high level of
capability for each of these activities within the same software package. Thus, the
different systems have remained independent, and CAD system managers must
be prepared to deal with integration.

Naturally, this software integration has been dependent on the development
of system connectivity. At one time, each of the different software systems would
run on separate computer hardware. However, the adoption of Local Area Net-
works (LANs), Wide Area Networks (WANs), and the Internet has in many cases
made the integration process more practical.

11.5.1 CAD to CAE Integration

Probably the most important and direct integrations are between CAD and CAE
(Computer-Aided Engineering), and between CAD and CAM (Computer-Aided
Manufacturing). Indeed, these three are often combined as CAD/CAM/CAE. Al-
though it is somewhat of a dangerous generalization, one can say that designers
are concerned with the creation and management of the precise configuration of a
product, while engineers are concerned with the behavior and performance of the
product by analyzing it in the context of physical laws and approximating calcu-
lation techniques. This being said, the CAD system would generally be the sys-
tem for designers, and CAE software would be for engineers. CAE software can
include such software systems as FEA (Finite Element Analysis for analyzing
solid products under forces or loads), CFD (Computational Fluid Dynamics for
analyzing fluid motion and behavior), simulations (analyzing dynamic events
such as mechanisms, vibrations, and impacts), and perhaps E-CAD (emulating
electronic systems behavior). Also, most engineering departments would have
various in-house programs for doing engineering analysis within the specific
context of a company’s expertise and products.

Managing 3-D CAD 277

Clearly there should be a close link between the CAD and CAE systems.
The CAE system is going to be used to analyze and/or simulate the product being
designed, and the design is encapsulated in the part and assembly models that are
in the 3-D CAD system. Therefore, the engineers are going to need to get the
models for analysis. In some cases, the actual PRT and ASM files would be used
within the CAE software (particularly if the same vendor supplies the CAD and
CAE software). In other cases, there would need to be some sort of translation
between the systems based on neutral files such as IGES or STEP (refer to the
Section 11.6 on translations); however, it is usually best to avoid the translation
step if possible. Of course, the product being designed in the CAD system may
need to be changed to reflect the results and/or optimization of performance from
the CAE system. So there may also be a need for the data to be transferred from
the CAE system back to the CAD system (using the native files or the neutral
files). The network and/or systems architecture needs to reflect the need to trans-
fer this data.

11.5.2 CAD to CAM Integration

The other very common integration is between the CAD and CAM software. The
CAM software is responsible for the automation of the manufacturing of prod-
ucts. It is usually used within the context of a Manufacturing Engineering depart-
ment. The CAM software will usually be closely tied to the Numerically
Controlled (NC) equipment used by a company. NC machines are machines or
tools that operate on parts automatically. The movement of a lathe, mill, press,
robot welder, etc. is controlled by electronic systems instead of by human opera-
tors. At one time these machines worked from instructions that were read from a
paper tape with holes punched into it, but most of this equipment has been re-
placed with a direct computer network type of connection. This direct arrange-
ment may be referred to as Computer Numerical Control (CNC) or DNC.