Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (56 page)

The creation of the instructions for the machine tools is driven by the ge-
ometry created in the CAD system. Obviously, the tool must follow the contours
and features of the part in order to create the part. This is done by creating tool
paths that not only follow the direct surface of the part, but also by cutting away
unneeded material in the raw material (that may not be directly on the part). The
creation and management of the geometry of these tool paths is the primary func-
tion of the CAM software. Depending on the factors such as the hardness of the
material being cut and the surface finish desired, the tool path information may
become a very large amount of data. Although the tool path data can be done
with just 2-D geometry (X- and Y-values) for flat-plate types of parts (often using
machines called 2-axis mills), in many cases this is done with the 3-D part geom-
etry (using X- , Y- , and Z-values with machines called 3-axis mills). Depending
on the complexity of the geometry, the 3-D part geometry may require 4- or even

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5-axis mills; in this case, there are X, Y, Z coordinates for the tool path and some
angles for rotating the part or jigs that are holding the part.

The tool path data is kept in data files. The data is often in a type of lan-
guage or format that is understood by CAM systems (for example, one is called
APT). These files are somewhat readable; one can edit the commands for the ma-
chine and the geometric tool path data. However, these types of language files are
not what is actually sent to the machine tool. Instead, the machine tools have
electronic control units that receive a low-level format or data stream. This data
stream is created by software called a postprocessor. The postprocessor can be
thought of as being like a driver for a specific type of hardware connected to a
computer system.

The integration of CAD and CAM would be specialized for each company.
It needs to take into consideration the particular CAD system, the CAM system,
the number and type of machine tools employed, and how these systems are to be
connected. Unlike the CAD/CAE integration, there may be little need to have
data transferred from the CAM system back to the CAD system. However, the
CAM information is going to be more dependent on the overall engineering and
manufacturing processes and management. When 3-D parts are revised in the
CAD system, the revision control needs to extend to the CAM system. The
changes to the parts need to be appropriately reflected in the tool paths that create
the part. Thus, the CAD to CAM integration may need to be tighter or more care-
fully controlled from a data management perspective.

11.6 TRANSLATIONS

As with drawings, there is a fairly common need to send and receive 3-D models
between companies using different CAD systems. In the context of 3-D CAD,
this need is usually driven by the actual design process. For instance, as designers
are creating detailed 3-D models of their assembly, they may need to see 3-D
models of components that are purchased from a supplier or vendor. Of course,
the designers could create 3-D models for those purchased components, but obvi-
ously time and effort would be saved if 3-D models could be received from the
vendors instead (reading in or importing a file). When the designer is finished,
that design can then become a product that is bought by yet another company.
That company may want to see the product as a 3-D model also, so there would
be a need to send out a 3-D model as well (writing out or exporting a file).

There are a number of types of 3-D translations (unlike 2-D translations). It
is important to understand how 3-D modeling works to some extent (as explained
in previous chapters) in order to understand the different types. Some types are
very simple and produce only information that can be viewed (can not be
changed); other types would be more functional.

Managing 3-D CAD 279

TABLE
11.2

3-D Translation File Types

Translation or

neutral file Description

IGES surfaces Surface data. ASCII format. The most typical 3-D

IGES type of file.

IGES wireframe X,Y,Z wireframe. ASCII format.

STEP Usually part models, part instances, and assembly

structure. ASCII format.

STL A tesselated file developed for stereo lithography or

rapid prototyping.

VRML A tesselated file developed for web pages. The file

can be enhanced with tags for annotation, menus,

surface appearances, etc.

VDA A format based on German automobile manufacturers

(Verband der Automobilindustrie)

The simplest sort of translation would be the tessellated file. This is a for-
mat that takes each surface of the 3-D model and creates small triangles, tessella-
tions, or polygons to approximate the surfaces. This file can be created for
individual part models, entire assembly models, or part instances. When the tes-
sellated file is imported, the 3-D model should look like the 3-D model. However,
there is no surface or analytical data (such as NURBS definitions), and there is no
part history (unless the receiving CAD system infers or extrapolates this some-
how). File types that are tesselated include VRML (Virtual Reality Markup Lan-
guage) and STL (Stereo Lithography). The VRML file is used with web browser
plug-ins to view and interact with a 3-D model in the context of websites. The
STL file is used by machines that create physical 3-D models based on the 3-D
part models (so called Rapid Prototyping machines or 3-D Printers). The VRML
and STL file may be used as a neutral file to translate between 3-D CAD systems
as well.

The next type of file would be a surface data file. This is a file that converts
the 3-D part model into just surfaces. The file is basically a list of the information
needed to create the surface model (i.e. using NURBS information). In this case,
the surfaces are not approximated (as in the tessellated file). This imported model
should look better, but the surfaces are often not stitched, so the part model will
not have a bounded volume. Also, depending on the basic part tolerance of the
two dissimilar CAD systems, there may be “untrimmed” surfaces. Of course,
since there are real surfaces, it may be possible to manually stitch and/or trim the
surfaces as needed. The surface data file would have no part history.

The most common example of a surface data file is an IGES file. The IGES
file is a format that has many capabilities and has been in use for many years. It

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can be used for 2-D data and drawings, and it can be used for more complicated
3-D models. However, in most cases, when a 3-D IGES file is transferred be-
tween CAD systems, it contains just surface data. One can more specifically refer
to this file as an IGES Surfaces file. A common use of the IGES Surfaces file is to
send part data from a CAD system to a CAE system. The surface data file often
contains all the data needed by the CAE type of software.

The next type of file would be a product definition file. This is a translation
format that tries to preserve as much information as possible between the CAD
systems; this can include nongeometric information such as attributes about what
material or processing is used for a part model. Since the manufacturing of com-
ponents is so often tied to assemblies (and their BOMs), the product definition
type of file should support the exporting and importing of assembly models.

The basic file of this type is the STEP (Standard for the Exchange of Prod-
uct data) file. The STEP file is governed by a standards body (such as NIST or
ISO). The STEP file may also be related to data systems called schemas that can
provide added levels of intelligence to the translation. Two common application
protocols for the STEP file are AP203 (Configuration of 3D design) and AP214
(Automotive design processes). If an assembly model is translated with a STEP
file, then instances should be preserved (i.e. there is a master part model and it
can be copied a number of times for each use of the part in the context of the
assembly).

Beyond these three types of translations, there are ways to transfer 3-D
CAD data between systems that are more direct. For instance, some CAD system
vendors supply software that can create part files in the exact format of another
CAD system. In this case, there is no neutral file such as IGES or STEP. Also,
CAD systems are often based on kernel software libraries that provide the most
basic 3-D data calculation methods. Two common modeling kernels are ACIS®
and Parasolid®. If two CAD systems share the same kernel, then more exact part
models may be exchanged. These files are much more likely to be modifiable
after the translation.

11.7 PRINTING

A significant concern for the management of 2-D CAD systems is plotting
(systems and procedures for making hardcopies of drawings). This is important
since 2-D CAD is really part of a paper driven system, and these copies of the
drawings (whether paper or electronic image data such as TIFF files) are the mas-
ter source of the design and engineering data. There is no real direct equivalent
with respect to 3-D CAD (hardcopy of 3-D models is more of an attempt to make
realistic pictures).

Of course, 3-D part and assembly models are used to help create drawings,
but then the standard 2-D CAD plotting system can still be used for these draw-

Managing 3-D CAD 281

ings. The 3-D models really just created the geometry in the drawings, so the 2-D
data from these models should still be acceptable to the plotting system devel-
oped for the 2-D CAD system. So, in the context of 3-D models, making a hard-
copy is really just standard printing of what is seen on the computer monitor, not
plotting in the special sense for drawings.

Even when a company’s design data is still based on drawings, making
hardcopies of 3-D models is still desirable. In this situation, hard copies of the 3-
D models would be used to help show the state of the design in a general sense.
Indeed, they can be most effective in demonstrating a design’s appearance or
functionality to those less fluent in reading drawings or blueprints. The 3-D
model hard copy clearly looks much more realistic than a drawing. These hard-
copies can also be valuable for presentations, brochures, and advertisements.

Probably the most common approach to getting hard copy of the 3-D mod-
els is the screen dump. In this case, the data shown on the screen is sent to the
printer (instead of creating some sort of industry standard plot or picture file).
Graphics adapters that run 3-D CAD systems should have a sufficiently high res-
olution so that simply translating the data in the graphics memory to a hardcopy
format creates an acceptable result on the printer. Of course, this means that the
hard copy can be no better than what is shown on the computer monitor. To im-
prove on the screen’s resolution (which is particularly important if the hardcopy
is going to be blown up larger than the screen image size), then some sort of plot
file is needed. Unlike 2-D plotting, the 3-D plot data is going to be bitmap-type of
data where all the surfaces of the model are shaded to create specific colors at a
large number of specific X,Y locations.

A typical format for encapsulating the screen dump data would be XWD
(for Xwindows Dump). Once the data is in this format, then it can be translated to
a format or language that is acceptable for the printer (such as Color Postscript).
Another possible file (that also can be used without a screen dump) is CGM
(Computer Graphics Metafile). Again, it is important that the file format supports
bitmap type of data, and since the resolution is usually high (on the order of 1
megapixel), these files can become rather large (many megabytes).

Regardless of the format of the data, one needs to carefully consider how
well the printer supports colors. The 3-D models shown in the CAD system gen-
erally use realistic shading on a surface-by-surface basis (although they rarely
show effects such as shadows). This shading typically requires support for true
color, 24 bit, or 16 million colors. So
for a hardcopy to
look acceptable, the
printer needs to support this many colors as well. In addition to the accurate sup-
port of a large color palette at high resolution, the printer also needs to translate
colors from the monitor to the hardcopy accurately. When a red shade is shown
on the monitor, then it should not become an orange or pink shade in the hard-
copy. Often, there are adjustments that can be made on the printer to improve this
translation or correlation of colors.