Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (52 page)

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would be used to create a physics-based approach to having the part instances
move. As the constraints remove degrees of freedom, however, the system will
have to be carefully underconstrained (to just allow the motions that are correct).
Then, forcing functions must be supplied to fully specify the behavior of the sys-
tem (to control those remaining degrees of freedom). These functions may be
gravitational, or they may be motion functions (such as rotate a gear at 1 RPM for
1 minute). In any case, this mechanism model will need to be solved for a certain
number of intervals (such as 1 second), and if it solves successfully at each inter-
val, then the system should be able to show an animation of the system through
the simulation. This sort of model can be more difficult to create, but it can han-
dle more sophisticated motions.

10.5.2 Part-to-Part Associativity

Another advanced feature of assembly modeling is part-to-part associativity.
Usually part models are self-contained with respect to their geometric informa-
tion. That is, the surfaces of the parts are totally controlled by the dimensions,
geometric constraints, and part history of that individual part model. In the part-
to-part associativity capability, one part model in an assembly model actually
controls the surfaces in another part model.

In Figure 10.9, a hose is shown in relation to a ramp. But in Figure 10.10,
the ramp has raised and the hose has actually changed shape. There is no such
thing as flexible surfaces for the part model of the hose; and there is only one
hose part model in the assembly. So in this example, the hose part model has sim-
ply updated based on geometry that was driven by the assembly model. The con-
trol points of the 3-D spline that the hose was swept along are constrained to be
driven by the position of the ramp. First, the ramp moved, then the control points
moved, then the part model changed based on the new control point positions.
This is an example of part-to-part associativity.

This is a rather complicated issue from a data management point of view.
The part model is mostly self-contained, but some of the part model data is really
driven by the assembly model. This can be a valuable capability. Another com-
mon application of this technique is when parts are made by a mold (such as plas-
tic injection molding). The mold cavity really needs to be driven by the part, or
vice versa.

10.5.3 Bill of Material Creation

The last advanced capability for assembly models presented here is the creation
of Bills of Material (BOMs). There are more advanced capabilities for assembly
models that could be discussed, but this is one that is most commonly used. As
mentioned at other times in this work, the BOM is a note or document that indi-
cates what items are necessary to assemble or create a particular level of a prod-

Assembly Modeling 259

FIGURE
10.9

The hose part model is shown in its normal state.

uct structure. A BOM may be for a low-level assembly such as the pedals and
crank for the crank subassembly shown in Figure 10.6. Or, there could be a
much larger BOM for the top-level assembly such as the entire bicycle shown in
Figure 10.1.

The BOM is a list of items that is usually shown in some sort of logical
arrangement. They may be arranged by part number or other identifier; they may
be arranged by the assembly and subassemblies; they may be arranged by mate-
rial type, etc. Often this BOM information is included on 2-D drawings that are
created for assemblies to show what items need to come together to form what is
shown in the drawing. Figure 10.11 shows a sample BOM. Note that it has a col-
umn for QTY or quantity. This makes it easy to understand how many different
or unique items go together to form the assembly, and how many of them are
required.

The BOM should look pretty familiar. It is basically the same as the assem-
bly structure discussed all along for assembly models. It shows the same hierar-
chy or parent/child relationships of the part instances and the subassemblies (the
levels of the assembly are shown indented in Figure 10.11). The only real differ-
ence is that the all the separate instances of the same part are not listed; instead,
they are gathered up and their quantity indicated. The BOM in Figure 10.11 is

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FIGURE
10.10

The hose part model has updated based on the raised ramp.

also an example of an explosion. It shows all the levels with all their constituents
from the top level (the BIKE_ASSY) down through each subassembly. In fact,
there could be a separate BOM for each of the subassemblies, as well.

The assembly structure information can be used to create a BOM based on
the assembly model. This BOM data could then be passed on to the systems that
track all the parts for a product or the entire company. Note that there are some
extra columns in the listing of Figure 10.11. There is a column for REV (or revi-
sion), and one for MATERIAL. These are common attributes that a BOM system
will need. It is important to know what revision of the parts or assemblies are
being built, and it is important to know what materials are used for these items.
Some 3-D CAD systems will allow these attributes to be stored with the part and
assembly models, and then they can be included in the BOM data.

One problem with having the CAD system create the BOMs is that there
can be data management problems between the CAD system and the BOM sys-
tem. Usually, the manufacturing or shop of the company will not use the 3-D
CAD system, so the BOM directly contained in the CAD system would not be
available to them. Instead, there may be a separate system that needs to integrate
with the CAD system. There is often little difficulty in doing this in the direction
from the CAD system to the main BOM system. Unfortunately, the manufactur-
ing system may also need to drive changes back to the CAD system BOM (such

Assembly Modeling 261

FIGURE
10.11

An example of a BOM.

as a changing a component from manufactured in-house to out-sourced). This
process can be much more difficult to handle in an automated fashion.

Another problem is that there may be part models in the assembly model
(and thus in the assembly structure) that are needed from the point of view of
design, but they are not really part of the final product. For instance, a simple
surface model may be created for the seat of the bicycle, but there may actually
be many parts that are in the seat. In this case, the BOM from the assembly model
will only indicate a single seat, when, in fact, it should indicate all the parts
within the seat. Either the seat is going to have to be completely modeled, or a
means of editing the BOM from the CAD assembly model will be needed. For
another example, a purchased component (such as a chain for the bicycle) may
only be shown as a single item in the manufacturing BOM system (to indicate

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“just buy one chain”). But, to show the location and movement of the chain, a
designer may model it as a number of small links put together (as a real chain
would be). In this case, the BOM from the CAD system would have too many
items, and again, the model needs to be changed or the BOM data from the CAD
system needs to be edited. One option in the CAD system may be to exclude
items from the BOM data created.

More information on data management issues and BOMs are presented in
the next chapter.

10.6 CHAPTER EXERCISES

1. Create an assembly model that includes part models of different densi-
ties. Have the 3-D CAD system calculate the total weight and center of mass for
the assembly model. Record whether changes to part models instanced into that
assembly model are immediately reflected in the weight and/or center of mass for
the assembly model.

2. List 5 different assembly constraints and the number of degrees of
freedom they remove.

3. Record whether your CAD system has the capability for animations.

4. Record whether your CAD system has the capability for part-to-part
associativity.

5. Record whether your CAD system has the capability for Bill of Mate-
rial generation.

10.7 CHAPTER REVIEW

1. What are the 3 essential characteristics of a proper assembly model?
2. Explain some of the advantages of the use of vectors in the control of

parts in an assembly model.

3. Why are some degrees of freedom not removed from an assembly

model that is going to analyzed as a mechanism (using a kinematic

analysis)?

11

Managing Three-Dimensional CAD

11.1 INTRODUCTION

This chapter presents information on managing or administering a 3-D CAD sys-
tem. As with the 2-D CAD system, it is best if the person performing the admin-
istration tasks has some knowledge about CAD and the tasks designers and
engineers are trying to accomplish with the system. However, since 3-D CAD is
more demanding than 2-D CAD, it will be more difficult for administrators to
become completely fluent in the use of the 3-D CAD system. Hopefully though,
the previous chapters on various types of 3-D modeling will at least provide some
level of understanding. Administrators should do their best to familiarize them-
selves with the information in these chapters.

In addition to being more demanding on designers or users, 3-D CAD also
tends to be more demanding on administrators. The 3-D CAD system is able to
solve many more kinds of problems and organize more data for various parts of
an organization. As such, there are then more administration and management
tasks. It turns out that the 3-D CAD system can contain and manage more than
just design documentation (such as drawings). It can contain types of information
such as product structures, standard parts libraries, analysis data, Bills of Mate-
rial, tooling models, and manufacturing or processing data.

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It is critical that an appropriate level of resources be allocated to carefully
organize and maintain a 3-D CAD system. This may mean that all users perform
some basic administration functions. Or, this may mean that more personnel are
dedicated to full-time administration. These needs are probably greatest during
the transition from 2-D to 3-D systems. After this transition is complete and all
the data management procedures are in place, the demands for administration
should be diminished. Regardless, it is essential that the 3-D CAD system be
carefully managed; otherwise, a large amount of data will be unorganized and/or
lost; this situation should be avoided. Consultants could be retained to assist with
creating the proper level of data organization.

11.2 DATABASE MANAGEMENT

(THE “ADMIN” PERSPECTIVE)

Probably the most important issue for administration of a 3-D CAD system is the
potential for expanded database management activities. 3-D CAD systems can
produce a very large amount of data in a short amount of time. 3-D CAD systems
also often bring together data from a wide range of activities that may have been
previously segregated (so-called islands of automation). The 3-D CAD system
database manager could be responsible for tracking 3-D models, drawings, Bills
of Material, manufacturing or NC data,
and analysis data. These various items
may have relationships between them so that relational database concepts are
needed, as well. These various items may also require revision control where var-
ious obsoleted records need to be retained for future reference by engineering and
long-term product support functions (perhaps up to 30 years into the future de-
pending on the longevity of the company’s product).

This section discusses some of the background for understanding why the
3-D CAD database needs to operate in certain ways. This is intended to assist the
administrator of the system. However, it is important for administrators to regu-
larly consult with the users of the system to make sure that their needs are being
met by the database system. A later section of this chapter presents data manage-
ment (the user’s view of the data).