Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (34 page)

Managing 2-D CAD 149

Support for line weights becomes more complicated when the plotting de-
vices no longer have pens. Now there are a few choices for handling line weight.
One approach is to have the CAD system actually show the line weight as part of
the image on the monitor (instead of colors or tags as mentioned earlier); then the
CAD system uses that information to create the proper bitmap data to send to the
plotting device.

Another approach uses pen mapping in the CAD system or plot manager
with a plotting device that can emulate the pens. In this case, there are no me-
chanical pens on the plotting device, but the device can behave as if they existed.
The plotting device would allow the administrator to define a certain number of
virtual pens. For each virtual pen, the device would allow the administrator to
indicate the weight of the pen. The weight would be a distance for the thickness
of the line (e.g., in inches, mm, picas, or points, etc.). For example, Pen 1 would
be 0.5 mm wide, Pen 2 would be 1.0 mm, and Pen 3 would be 1.5 mm. There are
a few issues to be addressed with this approach.

First, the file format must be able to handle this “pen” type of information.
HPGL would be an example of a format that handles pens. Secondly, the CAD
system must know which pen number is mapped to which tag in the CAD system
(this pen map would generally be established by an administrator for the system).
Now, when the CAD user creates a line and a medium weight is desired for that
line, the user uses the command that tags that line’s weight as medium. The CAD
data file saves this information. When the CAD system needs to make a hardcopy
of this CAD data file, the CAD system translates the request for medium weight
to a specific virtual pen number (that has been pre-established for this weight).
The plot data file eventually gets to the plotting device, and the request for the
specific pen number is interpreted by the device to switch to the desired line
thickness (switching virtual pens).

Lastly, the data files could actually define their own line weight infor-
mation or use its own “overstrike” algorithm (repeating the line with a slight off-
set to get thicker lines). In this case, the plotter data file or the neutral plot file
(such as CGM mentioned earlier) actually contains the information on how thick
each line in the file is expected to be. This has a few drawbacks. First of all, the
plot files will become larger since each file will contain information on line
weights that could have been stored in the plotter device. Secondly, the plotting
device will have to spend more time interpreting the data file since it must deter-
mine what is real plot data and what is line weight data.

Table 6.1 summarize issues to consider for setting up the plotter device.

6.2.3 Drawing Print Release

An important concept with respect to the hardcopy drawings is the drawing or
print release. This is simply an event which is the clear delineation of when the

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TABLE
6.1

General Plotter Setup

Issue Comments

Pen mapping If a pen plotter format (such as HPGL) is chosen for the CAD system

and plotter, then a map may be developed and saved in the system to

indicate what pen information on the device is equivalent to the entity

information in the CAD system (color, etc.).

Clipping Many plotting devices can have a maximum plot size. This would in-

volve clipping or removing unwanted data from beyond these sizes.

This is often an issue for CAD system that uses the “model space”

approach explained in Chapter 5.

Nesting More sophisticated plotting devices handle the optimization of paper

use by making various plots of various sizes fit together on the paper

size (perhaps even rotating the images). This is referred to as “nest-

ing.” Normally paper is fed from a roll.

Rotating Many plotting devices or plot managers will allow the plots to be ro-

tated. This may help to fit more plots on a given sheet or roll.
Scaling Many plotting devices or plot managers will allow the plots to be scaled.

In this case, the plots can be made larger or smaller as needed. This

scaling can often be done in the CAD system as well as the plotter

device, so administrators should be careful to avoid getting “scale of a

scale” problems.

drawing is complete enough to have those beyond the engineering department ac-
cess the drawing. Completion of the drawing would include being checked and
approved as well as having the appearance of the drawing completed. Those be-
yond the engineering department could include manufacturing, purchasing, mar-
keting, technical publications, and field support functions within the company
(with manufacturing also meaning to include suppliers or vendors). In the case of
engineering being done as a contractor
to another company or for a government
project, release could be to the end-user or customer, as well.

With paper drawings being used as the sole end-product, release of the
drawing would mean physically making the drawing available to the other
groups. The released or official drawing would be filed in an appropriate vault.
When different parties need to access the drawing, then they would request a
copy of the drawing, and then it would be sent to them according to a docu-
mented procedure.

The actual release procedure goes by many different acronyms. The term
generally used in this work for drawings is ECR (meaning Engineering Change
Record or Release). Other terms are ECO (Engineering Change Order) or ECN
(Engineering Change Notice or Number). Even though these terms indicate a
change, this process is assumed to apply to the release of brand new drawings as

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well as revisions to existing drawings (or a combination of new and revised draw-
ings). One vital characteristic of the ECR is that they would be cataloged or
tracked via a number for each release (ECR number, ECN number, etc.). This
allows records keeping of what drawings are affected by a particular release ac-
tivity. The ECR process would typically include assigning the unique ECR num-
ber (or number/letter combination), listing the drawings (by drawing or part
number) that are part of the release, having the package approved for release, re-
cording the date and time of the release, etc.

One effect of the release process on the hardcopies of the drawings is that a
date/time stamp may be placed on the hardcopies. This may be done manually
(with an actual stamp), or it may be automatically printed on the drawing by the
hardcopy device. In either case, the master hardcopy of the drawings would be
given a marking that indicates the official date and time that the copy was made.

When drawings are duplicated or copied for use by other departments,
there should also be consideration given to putting an “UNCONTROLLED” or
“FOR REFERENCE ONLY” stamp on the drawing (again, either manually or via
a customization to the plotting process and hardcopy device). Clearly once a pa-
per copy of the master drawing in a vault has been made, the CAD system is not
going to be able to automatically inform those receiving the copies when there is
a change to the master drawing in the vault (at least if a totally paper-driven pro-
cess is relied upon). In order to inform anyone that receives the copy that it may
or may not be the latest revision level, these other stamps are put on the drawings.
This is an important task in having a quality system comply with standards such
as ISO 9001.

This section has somewhat oversimplified the drawing release process, but
the important aspects and its relevance to the CAD system are clear (particularly
with respect to file management or data management discussed later in this
chapter). CAD users and administrators will need to work with their own com-
panies to determine the actual details of their process and how it affects the plot-
ting process.

6.2.4 “On-Line” Imaging Systems

The computer systems or networks that run the CAD system have also developed
means for digitally storing and retrieving drawings without any hardcopy at all.
This type of system can be referred to as an on-line system. In order to see the
drawings, personnel (in the engineering department or otherwise) must use a
computer as well. Of course, at that point, the person should be able to make a
hardcopy on their own printer; but, companies must be clear to consider these
hardcopies as uncontrolled.

A major distinction among the on-line systems is whether they use a native
CAD data file, a neutral CAD data file, or an image or bitmap file. If the on-line

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system uses the native CAD data file, then not only is a computer required to even
see the drawings, but one also needs to have a copy of the CAD system software,
or a special viewer program. This can be a significant problem if people outside
of the engineering department, or outside of the company, or outside of the coun-
try need to see the drawings. One advantage of this approach, though, is that the
drawings can be analyzed, red-lined, or even revised with little difficulty by the
wider audience (assuming they have the software/and have been trained to use it).

In order to still use a CAD type of data file, but make the drawings more
accessible, a neutral CAD data file could be used instead of the native file. Exam-
ples, would be DXF or IGES files. These files still contain CAD vector data (so
they can be analyzed and edited), and many different CAD systems can recognize
and view these files. Unfortunately, not all CAD systems will translate them
100% correctly, so there is a significant potential for error unless a carefully ap-
proved list of tested programs are enforced for the wider audience.

In order to break free from the CAD system (so that anyone, anywhere
could see them), many companies have taken the image file approach. This is a
very different type of file than the CAD file. As mentioned above, it is a bitmap.
This means that the program showing the drawing to the user cannot really recog-
nize any of the entities in the drawing (there are just individual pixels shown). An
image file viewer program (many of which are provided at no cost) is generally
not be able to distinguish between the number 1 in a dimension and the small
lines that might be a segment of the geometric definition of the part documented
by the drawing. Or, this program would not be able to distinguish between the
upper case letter O and the number zero. Typical file types for an image file
would be TIFF, GIF, JPEG, BMP, and PCX (with TIFF probably being the most
common). An advantage to this approach (besides being CAD system independ-
ent) is that there are many programs that catalog and manage these image files.
And, there are many other types of software (besides CAD) that can read these
image files (notably Web browsers). Therefore, by using the image file approach,
these packages would be able to easily work with the CAD images.

6.2.5 Batch Plotting

In the course of releasing packages of drawings, it may be necessary to create
hardcopies (or image files) for an entire list of drawings. This is usually referred
to as batch plotting. In many cases, this operation can be done automatically.
Usually, a company can obtain computer programs that can work from a data file
with a list of drawings (by file name, drawing number, revision level, etc.). These
programs then retrieve the images or the CAD data files and produce the hard-
copies (or image files). The list of drawings required may be generated by an
ECR process, a manufacturing job request, or in conjunction with a purchasing
request for quotation/outsourcing.

Managing 2-D CAD 153

6.3 FILE OR DATA MANAGEMENT

As many drawings are created, released, and stored in the CAD system, it be-
comes necessary to implement a file management or data management strategy
for the drawings. This strategy is meant to organize the information so that it can
be protected (from intentional or accidental destruction), controlled, and made
easily available for future revisions. Beyond the inherent benefits of organization,
a documented procedure that implements this strategy can be a requirement for
the company quality system to comply with standards such as ISO 9001.