13.

Assembly Modeling And Assembling Parts

Components can consist of existing parts and subassemblies, or components can be created directly within
assembly mode. Placing existing components to form an assembly is referred to as bottom-up assembly
design, while creating components within assembly mode is referred to as top-down assembly design.

Parts in assembly mode maintain their associativity with their separate part files. Within part mode, if a
dimension value is modified, the part instance in assembly mode is modified. Correspondingly, if an
instance of a part is modified in assembly mode, the component in part mode is modified. In addition,
when a part is created within assembly mode using top-down assembly design, a new pan file is created
that can be modified separately within part mode. When a component is placed into an assembly, the
component’s separate part or assembly file is placed into memory, and it remains there until the parent
assembly is erased from memory.

13.1. CREATING ASSEMBLY AND ASSEMBLY MODELING

During assembly design, parts for an assembly are created using normal part modeling tools and
techniques (Figure 13.1). As with any parametric model, the intent of the design needs to be considered. In
addition, how components of an assembly are parametrically linked is also an important consideration.
MAK 112E-4 Computer Aided Technical Drawing

Figure 13.1. Assembly drawing of a system

Dr C Erdem IMRAK @ 2002

You are familiar with most of the techniques used to create solid models with computer tools. Most engi -
neered systems, however, do not consist of a single part, but comprise multiple parts that work together
and form the system or assembly. A car engine is composed of several subsystems, each of which serves a
distinct purpose in keeping the engine working properly. Some of these subsystems are the cooling
system, the exhaust system, the fuel and combustion system, the drive train, and so on. Each subsystem is
composed of other subsystems or pans. For example, the fuel and combustion system contains many
single parts (the fuel line, the fuel filters, etc.), as well as other subsystems (e.g., the fuel pump).

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 13.2. ASSEMBLY HIERARCHY

It is easier to work with assembly models if they are organized in a logical manner. An assembly can
usually be thought of as composed of several smaller subassemblies, each of which in turn may consist of
other subassemblies or individual components. The organization or structure of a system is sometimes
referred to as its hierarchy. The hierarchy can be thought of as similar to an inverted family tree. Another
way of thinking of an assembly hierarchy is that it is analogous to a corporate structure. The president of
the company is at the top of the hierarchy and has several vice presidents reporting to her or him at the
next level. Each vice president, in turn, is responsible for several managers, and so on until you reach the
lowest level in the company hierarchy, where the laborers who work for the firm are located.

A simple assembly is shown in Figure 13.2, along with an accompanying hierarchy. The assembly
consists of two subassemblies, each of which in turn consists of two separate components — a gear and a
shaft.

Figure 13.2. Simple Assembly and Hierarchy

Organizing an assembly in this way will enable you work more efficiently with it. The technique is similar
to the way you organize your files on a computer, establishing folders to group files together in your
workspace. One of the advantages in creating an assembly hierarchy is that subassemblies can be dealt
with as a whole rather than as separate components. For example, one subassembly can be moved as a
single unit within the system, rather than moving each of the individual components of the system
separately, just like the way you can move around entire folders and all of the files in them on your
computer workstation.

13.2.1. Nonhierarchical Organization

Most 2-D CAD systems support the concept of layers. Layering is a facility that allows the various
graphics elements of a drawing to be grouped together in the database. This facility is used most often to
control what is seen and/or editable on the screen and what is printed or plotted. Layering in most systems
is nonhierarchical; that is, no one layer has precedence over another.

13.2.2. Hierarchical Organization

Often, groups of objects must be brought into assemblies. Such assemblies usually reflect the grouping of
elements in the final product. For example, solids representing nuts and bolts might be grouped together in
a hardware assembly. The most natural way to create these assemblies uses a hierarchical approach in

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 which the real relationships of parts in an assembly are reflected. In the example, the hardware assembly
would be a subassembly of the final product. A hierarchical structure allows parts to be grouped together
logically into subassemblies

13.2.3. Networked Hierarchical Organization

The use of standardized parts to improve manufacturing efficiency is a technique that goes back to the
19th century. To assist this manufacturing goal, 3-D modeling systems must share common parts, both
within an assembly and with other assemblies. This sharing across assemblies consti tutes a networked
hierarchy in which parts exist in several hierarchical trees. The basic geome tries of standard parts are
stored in a central database and are never changed. When a variation on a standard part is needed, the
geometry is copied, and the copy is modified. Standard components are stored in central locations where
all members of the design team can use them.

13.4. ASSEMBLY CONSTRAINTS

The first thing you will likely want to do after setting up your system hierarchy is to orient the instances so
that they are properly located in space relative to one another. When creating pans, you used cons traints to
establish geometric and dimensional relationships between 2-D entities. Thus, you constrained two lines to
be parallel or perpendicular to one another, or you constrained the diameter of a circle to be a specific size
and its center to be located given distances from lines on the drawing or from edges on an object. In
assembly modeling, you can apply constraints between two 3-D parts so that the pans will maintain
dimensional or geometric relationships with respect to one another within the asse mbly.

One of the more useful assembly constraints that can be applied is to define different lines on two
different parts to be coincident with one another. This is especially useful in dealing with cylindrical shafts
that fit within a cylindrical hole in another part. Figure 13.3 shows the coincident-lines constraint between
the centerline of Shaft2 and the centerline of Gear2.

Figure 13.3. Coincident-Lines Constraint

When a component is placed into an assembly using the Assemble option, it can be f ully constrained to
existing components and features. This type of assembly is referred to as a parametric assembly. CAD
software provides a variety of constraint types for the placement of components (Figure 13.4). The
following is a description of each:

Mate . The Mate constraint type is used to place two surfaces coplanar. Any datum plane, part plane, or
planar surface can be used. As shown in Figures 13.4, selected surface faces are placed along a common

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 plane but do not actually have to touch. When a datum plane is selected, since a datum plane has no
defined thickness, CAD software provides the option of selecting either the positive side or negative side
of the datum plane.

Mate Offset. The Mate Offset constraint type is similar to the Mate constrai nt type. Unlike the Mate type,
the Mate Offset constraint places a user-specified offset distance between selected features.

Align. The align constraint type is used to place two surfaces coplanar and facing in the same direction.
Like the mate constraint, the surfaces do not have to touch. In addition, the align constraint type is used to
align axes, edges, curves and points.

Align Offset. The align offset constraint option is used to align two surfaces with a user -specified offset
distance between each surface. The value can be either positive or negative.

Orient. Like the align constraint type, the orient constraint type orients two surfaces facing in the same
direction. Unlike align, the two selected surfaces are not coplanar and no offset distance is specified.

Insert. The insert constraint type makes the axes of two revolved features coincident. The user is required
to select the surface of each feature.

Tangent. The tangent constraint type makes a cylindrical surface tangent to another surface. Th e user is
required to select the surface of each feature.

Figure 13.4 Assembly constraints

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 13.5. CONFIGURATIONS

Once you have established a hierarchy for your system, you can move the parts around within the system
to create various configurations for the assembly. A configuration is merely a “snapshot” of your system
with the parts located at specific orientations in space. From one configuration to the next, the parts and
the hierarchy for the system have not changed; however, the orientations of the parts relative to one
another have changed. For example, a car consists of many parts assembled into a system. If you open the
car door, all of the same parts are still in the assembly, only its configuration has changed: The door is in
an open position relative to the body of the car. If you open the hood and leave the door open, this is
another configuration for the system. If you open all the remaining doors and roll down the windows, you
will have yet another configuration for the system. In this way, you can see that numerous configurations
are associated with any given assembly. Figure 13.5 shows three different configurations of the gear
assembly of Figure 13.3.

Figure 13.5. Three Configurations of Gear Assembly

13.5.1 Exploded Configurations

For most mechanical systems, an assembly drawing is necessary to put the system together. You have
probably seen such a drawing if you have ever put together a furniture kit or a model. Once you have
assembled your system, you can create an exploded configuration that will essentially be an assembly
drawing for the system. Figure 13.6 shows an exploded configuration for the gear assembly of Figure
13.3. Note that this configuration shows how all the parts in the system will be put together or assembled.

Figure 13.6. Exploded Configuration

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 13.5.2. Animations

Some software packages allow you to animate your system by displaying slightly different configurations
in rapid succession. If you imagine rotating Gear 1 about Gear 2 in the gear assembly, the first
configuration to be displayed could be the initial one. Subsequent configurations can show Gear I rotated
about Gear 2 in increments of 10 degrees. In this way, there would be a total of 36 differen t configurations
showing Gear 1 rotating in steps about Gear 2. If these configurations are displayed in rapid succession,
the effect is the same as that seen with a “flip book” used to animate cartoon characters.

13.6. ASSEMBLY STRATEGY

Once again, the most important thing that you can do to successfully model an assembly is plan ahead.
Before you sit down to create an assembly, you should plan the system hierarchy on paper. Objects that
are logically grouped together (such as a bolt and the corresponding nut and washer) should be part of the
same subassembly. Objects moved together as a unit should be grouped together in one subassembly. In
short, in assembling a system, you should proceed as follows:

1. Create all required parts in the Modeler. If there is more than one copy of a given object in the
assembly, you do not have to create more than one copy of the part in the Modeler.
2. Switch to the Assembly and establish your system hierarchy. Create subassemblies as necessary
and logical for the assembly you are working with.
3. Orient the instances in the system relative to one another, applying assembly constraints as
desired. If you use assembly constraints, remember to ground one instance so that it will remain
stationary and the other instance will move relative to it.
4. Create the desired configurations for the assembly by orienting the instances and saving
intermediate configurations as appropriate. Create sequences for animation from these
configurations if that is desired.
5. Perform the necessary interference and clearance checks for the system. If parts are interfering
that shouldn’t be, reorient one or the other. You may have to go back into the Modeler to change
the size of some features in order to remove unwanted interference from your system. Since
CADD software has bi-directional associativity, if you change the part geometry, the instances
associated with it in the assembly will also change.
6. Obtain a bill of materials for the assembly if that is desired.

References

1. D.S. Kelley, Pro/Engineer Instructor, McGraw-Hill, 2001

2. R.W.Lueptow, M.T.Snyder, J.Steger, Graphics Concepts with Pro/Engineer, Prentice Hall,
2001
3. G.R.Bertoline, et.al., Technical Graphics Communication, WCB McGraw-Hill, 1997
4. J.Rooney, P.Steadman, Principles of Computer-aided Design, UCL Press, 1997
5. F.E. Giesecke, et.al., Engineering Graphics, Prentice Hall, 2000.
6. F.E. Giesecke, et.al., Modern Graphics Communication, Prentice Hall, 2001.
7. N.N. I-DEAS Master Series Student Guide, SDRC, 1998.
8. R.Rizza, Getting Started with Pro/Engineer, Prentice Hall, 2002.
9. S.A.Sorby, Solid Modeling With I-DEAS, Prentice Hall, 2000.
10. S.J. Ethier, C.A.Ethier, Instant AutoCAD Mechanical Desktop 5.0, Prentice Hall, 2002.

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