Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (33 page)

Another issue that may be a problem is off-screen entities. In this situation,
entities are drawn outside the borders of the drawing or of individual viewports
(if the CAD system uses viewports). These more or less invisible entities will not
appear on the hardcopy of the drawing, and if the user forgets that they put these
entities in the drawing, there may be a large amount of wasted space in the CAD
data file. CAD users may use this technique to store design information that is
relevant to the design, but is not supposed to be shown.

2-D CAD 143

Some CAD systems will have limits on the CAD data file size or at least
the number of entities that can be stored in the file. The older mainframe-based
systems were notorious for their limitations in this regard. Most of the CAD sys-
tems developed since that time are less likely to have problems.

In terms of a typical CAD data file size, it can range from 50–100 kilobytes
for detail drawings to 5–10 megabytes for a very complicated assembly drawing.
Also, note that the neutral files used for translations will usually expand the file
size by perhaps by a factor of 2 or 3. Therefore, a large drawing could wind up at
15–30 megabytes as an IGES file.

5.14 CHAPTER EXERCISES

1. Use the 2-D capability of your CAD system (not using 3-D at all, if
possible) to create a drawing of the object that was sketched in the Chapter 4
Exercises. Use the same views, scale, etc.

2. Record whether the CAD system use the “paper space” approach or
the “model space” approach.

3. Change the scale of the views so that the drawing fits on a larger paper
size.

4. Determine the formula of a line in the drawing. Use this formula to
extrapolate the X and Y values for a point on the line. Use the CAD system to
measure or analyze this location on the line. Record how closely the values agree.

5. Try using a grid snap option, if available.

6. Try using an automatic geometric relationship detection, if available.

7. Try to export the drawing in a neutral file, and then try to import the
drawing into an engineering analysis program.

8. After saving the drawing to a file, record the size of the file in bytes.
Make changes to the drawing and resave the file. Record any change in the file
size.

9. Try to create a user-defined line font (such as long-short dashes) or a
user-defined color (specifying the percentage of red, green, and blue).

5.15 CHAPTER REVIEW

1. Explain the difference between using a CAD system as “smart paper”

versus a graphics program to create a drawing that creates an image.

What are some of the advantages of the “smart paper” approach?
2. If a line is needed on a CAD drawing that models the edge of a block

that is 11.55 mm long, but the tolerance for the dimension showing the

length of the line is only to 1 decimal place, what X and Y values

144 Chapter 5

could be entered into the CAD system to create the line that preserves

the model?

3. Should the user or the CAD system round off the dimension values?

Why?

4. Are viewports and viewing angles necessary to project geometry auto-

matically between views? Why?

6

Managing Two-Dimensional CAD

6.1 INTRODUCTION

This chapter provides vital information to someone managing a 2-D CAD envi-
ronment or a CAD user interested in the administrative aspects of CAD. Depend-
ing on company size, the management function may be the responsibility of a few
users, a single person, or even an entire department. It could be controlled by the
engineering department or the IT (Information Technology) department. Regard-
less of the situation, it is probably best that those administering the CAD system
be somewhat familiar with design and engineering functions. The previous chap-
ters should provide sufficient knowledge for IT professionals.

Some of the information in this chapter would be relevant to the 3-D CAD
environment also. A later chapter covers management for 3-D CAD. Issues ad-
dressed in this chapter include plotting, drawing management, translations, cus-
tomizations, and system administration.

6.2 PLOTTING

Since the end product of the 2-D CAD system is going to be drawings, an essen-
tial aspect of managing the 2-D CAD system is going to be plotting. Plotting is

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146 Chapter 6

meant to refer to all kinds of hardcopy of the drawings. However, it is generally
called plotting since the original machines that produced all the hardcopy from
CAD systems were plotters (pen plotters, bed plotters, electrostatics, etc.). These
machines could trace out lines as a pen was mechanically moved across the pa-
per. For a long time, this would look very different from what computer printers
could do. Computer printers either printed just characters (numbers, letters, etc.),
or they produced a low resolution image of dots, and they usually only worked
with smaller sizes of paper (such as A or B size or metric A4 or A3).

Now, computer printing technologies (such as laser printers, jet printers,
etc.) use the large paper sizes, and they have resolutions high enough to look as
good as the plotters. Therefore, plotting may now be done with printers as well as
plotters. In fact, actual pen plotters
are probably only rarely used. But, one needs
to realize that the concepts and terminology from pen plotters are still relevant,
and that many printers still “emulate” the pen plotter operation.

Another important development is the use of on-line drawings and other
documentation. In this case, there is no physical hardcopy from the CAD system
directly. Instead, the electronic data of the CAD drawing is sent to a computer
system file server, (or virtual vault or cabinet). Instead of just viewing hardcopy
prints, end-users of the drawings search, view, and print via a computer system.
This is discussed in more detail later.

6.2.1 The Plotting Process

Figure 6.1 shows an overall plotting process. This process needs to coordinate the
actions of the CAD system (the application program), the computer’s operating
system software, the plotting device itself, and perhaps a computer network. With
so many functions to coordinate, setting up a plotting system can be somewhat
challenging. Each of the parts of the process must be properly configured.

The first step is solely a function of the CAD system. This program con-
tains the actual data or CAD drawing, so obviously it must start the process. The
demand for the hardcopy may be driven by the user, or it might be generated by
an automatic system that creates and/or updates drawings. The CAD system may
also have access to a number of different print queues (a special computer pro-
gram and resources that is always running in the background waiting and control-
ling the sending of data). So, at some point, the user may need to not only select
the drawing to be plotted, but also indicate which queue is going to be used. Of-
ten CAD users have access to smaller hardcopy systems for check prints (used by
people verifying that drawings are acceptable), and then they would also have
access to a larger system plotter or plot room system for full size prints.

Once the request for the specific hardcopy has been initiated, the CAD sys-
tem and/or operating system must then create data that can be used by the spe-
cific plotter device. The CAD system’s proprietary data file (such as a DWG file)

Managing 2-D CAD 147

FIGURE
6.1

Schematic for plotting process.

is not appropriate for the plotter since the plotters are sold to companies with
many different CAD systems. In order to enable the plotters to work with various
systems, printers have their own language or data they are able to receive. There-
fore, at some point in the process, the CAD data must be converted to the
printer’s language.

In Figure 6.1, there are few paths shown for getting this plotter data to the
plotting device (or actually its queue). In the simpler path, the user’s computer
(which has the CAD system running on it) has a plotter attached directly to it. In
this case, the CAD system would create the exact data instructions to get the
drawing produced on the plotter. This process is often done via a special, device-
specific programming called a driver. In this simple setup, getting the correct
driver is often the biggest problem. Some of the formats that plotting devices use
would be PostScript®, HPGL, or PCL. So, these systems would need a PostScript
driver, an HPGL driver, etc. In this arrangement, the data is probably going to a
queue that is running on the same computer as the CAD system. This queue will
allow multiple drawings to be sent to the plotting device, and it allows the user to
continue work with the CAD system or other activities while the copies are being
printed. Figure 6.1 also shows the CAD system on a workstation sending the in-
structions to a network printer.

The more complicated path shown in Figure 6.1 uses a sort of off-line ap-
proach. In this case, the CAD system is not communicating directly with the plot-
ter. Instead, some sort of independent plotter file is created by the CAD system

148 Chapter 6

and stored on a disk drive somewhere. This could be a proprietary plot file for the
CAD system; this could be a “neutral” file of some kind (such as a CGM file);
this could also be a plotter device file (such as an HPGL file) that contains the
specific instructions to create the hardcopy. In this case, the file is going to be
used by a program that is independent of the CAD system. Although this plot
manager program may be written and supported by the CAD system vendor, it
could also be a third party program that accepts the independent files and man-
ages where, when, and how they are plotted.

The plot manager program now becomes the program that will need the
proper driver and access to the proper print queues. The plot manager may also
be a program that operates across an entire network, and thus can print to plotter
devices located throughout a building, a campus, or even across an entire enter-
prise with offices across the country.

6.2.2 Plotter Setup

Assuming that a process is in place to send the plot data from the CAD system to
an actual device, the next issue to consider is the setup of the plotter device. This
procedure has to be coordinated with the CAD system’s or the plot manager pro-
gram’s requirements. Indeed, plotting devices should not be obtained until one
has checked that the device is compatible with the particular CAD system. For
instance, if a CAD system only generates HPGL data for plotting, then the device
is going to need to support HPGL (or the plot manager program is going to have
to do a translation). Once the device is obtained, then the device will probably
have to be “set” to accept HPGL. Fortunately, many plotter devices now have au-
tomatic format detection where the device looks at the data stream coming to it,
and it then automatically changes to accept that format as needed.

Another important setup issue to consider, which is probably unique to
CAD systems, is line weight. Line weight is a term that refers to how
“heavy/thick” or “light/thin” the lines are on the drawing. Different parts of the
drawing traditionally are made with lines that are given standardized weights
(refer to Section 4.10). This helps the reader of the drawing more easily visual-
ize the object in the drawing documents (e.g. visible edges of the part are
heavier; hidden edges are lighter). When the plotting device was a pen plotter
that used mechanical pens moving across the paper, it was possible to have the
plotter select different pens that produced the needed line weight. Of course, the
user must have somehow added the information in the CAD system to indicate
which lines are heavy, medium, or light weight. This might be shown as differ-
ent colors in the CAD drawing by selecting all the appropriate entities and en-
tering a command that the CAD system would save as a tag in the data file for
those entities.