Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (36 page)

6.3.4 Multisheet Files

Some CAD systems allow multiple sheets of a drawing to be stored within a sin-
gle operating system file. Generally, as drawings become larger (since they are
documenting something large or complex), the designer will put the drawing on
larger and larger drawing sizes. Eventually, the largest size of drawing (E for inch
or A0 for mm for instance) is reached. At that point, designers will make use of
multiple sheets or pages to document the design under the single drawing num-
ber. Recall that Sheet is an attribute of the standard Title Block.

Using the file manager (i.e. no “metadata”) approach, one is forced to use a
file naming scheme to record the sheet number. In a database manager CAD sys-
tem, the sheet number may be a metadata attribute. If the CAD system offers this
option, the multisheet file is likely to be a better approach than a separate file for
each sheet. It can be quite cumbersome for a designer to have to open one file to
see one sheet, and then open another file for the next sheet (while trying to docu-
ment the design with all the sheets at once).

6.4 DRAWING TRANSLATIONS

Almost certainly the biggest complaint about CAD systems in general is the in-
ability to exchange drawings between different CAD systems perfectly. As men-
tioned earlier, most CAD system vendors have chosen to use a proprietary data
format for their files (primarily so they can develop their software to do whatever
they want). Therefore, the files are not directly interchangeable. Although it
would probably be a simple matter to have near perfect interchangeability for 2-
D drawings, it is hard to imagine how the software companies could actually
agree on what format to use. Instead de facto standards prevail where one vendor
becomes so prevalent the others must follow to survive. One can assume that un-
til a single vendor becomes that dominant, there will be competing formats.

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In order to deal with this file incompatibility issue, neutral files have been
developed. These neutral files may arise from a vendor that decides to make the
format public domain to some degree (such as the DXF file from AutoDesk®).
The neutral files may come from industry groups such as the National Computer
Graphics Association (i.e. the IGES file). Or, they may come from government
agencies such as National Institute for Science and Technology (NIST). These
files are like a third party for the CAD data. The originating CAD system creates
the neutral file. The neutral file is exchanged between the two computer systems.
Finally, the receiving CAD system reads or imports the neutral file.

6.4.1 Errors in Translation

In the process of using the neutral file, errors or problems are generally incurred.
If one CAD system uses viewports and the paper space technique, but the other
system does not, then the data moved between CAD systems may be stripped of
the view scale information. In this case, some of the geometry will be either
much larger or much smaller than the rest. Or, the geometry may appear to be
the correct relative size, but dimensions show incorrect values. In the latter case,
a dimension from a Front View with a scale of 0.5 that indicates a value of 100
mm may show a value of 50 mm after the view scale intelligence is lost (al-
though the line would be 50 mm long if one measured the hardcopy of the draw-
ing, one needs to look at the view scale to know that the real, physical part
would really be 100 mm long as indicated by the original CAD system’s dimen-
sion value). This is a very serious problem. Users of the 2 CAD systems simply
need to be aware of the problem and verify that the translation issues are re-
solved. Fortunately, once two companies have determined the problems and ex-
changed a few drawings successfully, then most future translations should be
acceptable (at least as long as both companies remain on the same versions of
the same two CAD systems). Figure 6.2 shows some typical results from this
translation problem.

Another typical problem for the CAD data exchange is the use of hidden or
no-show entities. Many CAD systems allow the user to hide entities (such as
lines, notes, dimensions, etc.) so that they do not appear in the hardcopy of the
drawing. The CAD system may permit groupings (symbols, clumps, blocks, etc.)
to be defined, but never used or instanced. However, this is information that is
stored in the CAD data file that can be restored or shown at a future date. One
needs to be aware that the neutral file may, or may not, transfer this data and/or
leave this information hidden. As with the viewport problem, testing must be per-
formed to determine what happens for the two CAD systems in question and deal
with the situation accordingly.

Managing 2-D CAD 159

FIGURE
6.2

Bad results of CAD data exchange with dissimilar CAD systems.

6.4.2 Vector vs. Raster Data

One concept that the CAD administrator needs to understand clearly is the differ-
ence between vector and raster data. This has a direct bearing on the feasibility of
drawing translations. Vector data in a file means that there is mathematical defini-
tion to the parts of the drawing that show the object or part. There are real X and
Y values for the geometric entities. This means that the CAD system can create
and calculate whatever it needs to function properly. Files such as DXF, DWG,
and IGES contain vector data.

Raster data can be thought of as a bitmap. It is just a set of bits that indicate
the color (or just the black/white) of dots on a monitor or a paper copy. There is
no distinction between dark dots that are part of a dimension or note or the dots
that are part of a line segment for a geometric entity (such as a line or circle).
There are no X- and Y-values in the raster data that says where a line starts and
where it ends. It should be clear that it is basically impossible to use this type of
data to exchange CAD data between CAD systems. There are raster-to-vector
conversion programs, but they are of extremely limited value. They may find that

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certain dots form a line, but they will not be able to recognize the difference be-
tween a line segment that is geometric, and a line that is part of a note or dimen-
sion (at least not virtually every time). Furthermore, the X- and Y-values that
would be extrapolated by a raster-to-vector program are going to be of limited
precision; they must follow the pattern of dots, but a line segment may not
mathematically end at a point of a dot. Files such as TIFF, GIF, JPEG, PCX, and
BMP contain raster data, and they would not be appropriate for direct CAD data
translations.

6.5 TEXT FONTS

Another unique problem for CAD data administration is the fonts used for text
(or character data). This is not to be confused with line fonts that are used for
geometric entities in the drawing (things like solid, hidden, phantom, center line
fonts). For common computer software such as word processing, text has fonts
that indicate the appearance of the letters, numbers, etc. Examples are Courier,
System, and Arial, etc. However, these fonts are generally shown as bitmap
data. It does not lend itself to the kind of data that CAD data files use (i.e., vec-
tor data).

Therefore, CAD systems generally have their own fonts defined for the text
based on a stroke table. This is a set of instructions that can create the various
letters, numbers, and symbols that would be shown on a drawing. For instance,
making a number 1 would involve creating 3 lines (a bottom line, the tall vertical
line, and the small line at an angle at the top). Clearly this means text shown on a
drawing is somewhat primitive, but generally that is desirable. The text should be
as crisp, clear, and simple as possible. There is no real advantage to the “prettier”
styles available in word processing. Figure 6.3 shows some sample text based on

FIGURE
6.3

Typical text fonts for CAD.

Managing 2-D CAD 161

a few standard stroke tables. Some CAD systems will also allow the customiza-
tion of these stroke tables to create new letter styles.

6.6 CAD SYSTEM CUSTOMIZATION/

AUTOMATION

An important advantage to using a CAD system is the ability to customize and
automate processes. The amount of customization and automation varies widely
between CAD systems, but the need for this capability also varies widely (usually
depending on the size and scope of the company using the CAD system). Exam-
ples of customization would be setting system-wide defaults, defining plotting
and printing functions, templates, and user interface modifications. Examples of
automation would be macros, application programming interfaces (API’s), ac-
cessing system commands and utilities, or combinations of these.

6.6.1 Defaults

System-wide defaults allow an administrator to define the company-standards for
the drawings. For instance, if a company wants all drawings to use a text height
of 5 mm (so that all notes are easily readable), then the CAD system often allows
this to be done. This information is usually stored in a file on the operating sys-
tem, and the parameters in this file are usually changed through a graphical user
interface program. This type of file is often referred to as a config file. Depending
on the operating system, this file may then be secured for access by approved
administrators only. This allows the company to enforce the standard text height.
Keep in mind that there may be hundreds of parameters that may be customized
in this manner. For text height alone, character data may appear in Title Blocks,
notes, dimensions, GD&T symbols, decals, balloons, etc. For each of these po-
tential character fields, one may need to set standards for text font, height, slant,
gap, line spacing, etc. For many companies, the settings for the default CAD sys-
tem will be adequate.

Other config files may be used to customize plotting. Considering the po-
tential for complexity in the plotting process (as discussed earlier), most CAD
systems have to have the capability to set defaults or standard settings for such
options as paper size, plotter and printer queues, multiple copies, color or black-
and-white, orientation (such as landscape and portrait), plotting to file, etc. As
with the system-wide config file, there will often be a graphical user interface
utility to set and manipulate this information.

6.6.2 Templates

Another typical customization option in CAD systems is the template file. This is
a standardized CAD drawing that has been saved on the system using desired de-

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faults, setting, configuration, etc. On some systems this is referred to as the proto-
type drawing. When the user creates a brand new CAD drawing, this file is used
as a starting point. Besides being useful for customization settings, this may also
be useful for standardizing the Title Block, Revision Block, etc. Since the tem-
plate drawing should be an actual file on the operating system, it should again be
possible to control access to the file so that only appropriate users or administra-
tors are able to change it.

6.6.3 User Interface Alterations

Some CAD systems have the ability to augment or modify the standard user in-
terface. For instance, the graphics and/or text for icons on the CAD system’s icon
panel or menu bar can be changed. This would allow a CAD administrator to
restrict access to certain functions or commands. Or, if a company wanted to have
a standard new command added to the CAD system, this option would allow for
creation of new icons on the icon panel. Usually this capability is used in con-
junction with the customized automation functions. In other words, new icons are
added to the interface, and these new icons run new programs that are developed
by users and/or administrators of the CAD system.

6.6.4 Macro Programs