IMPLEMENTATION OF DFA IN CONCEPTUAL AND EMBODIMENT DESIGN USING DECISION
SUPPORT PROBLEMS
Timothy W. Simpson, Matthew D. Bauer, Janet K. Allen, and Farrokh Mistree
Systems Realization Laboratory
George W. Woodruff School of Mechanical Engineering
Georgia Institute of Technology
Atlanta, Georgia
ABSTRACT
How can Design for Assembly be implemented when the
conceptual form of a product being designed is known but the
information defining its physical form is vague and incomplete?
In this paper, we answer the preceding question from a decisionbased perspective. Specifically, we adapt Boothroyd and
Dewhurst’s Design for Assembly method for use in conceptual
and embodiment design. We achieve this adaptation through the
use of Decision Support Problems, and we illustrate our
adaptation through an example, namely, the design of an aircraft
evacuation system. Our emphasis in this paper is on presenting the
conceptual framework used to make suitable Boothroyd and
Dewhurst’s Design for Assembly to conceptual design and not the
numerical results per se.
1 OUR FRAME OF REFERENCE
It is well known that Design for Assembly (DFA) analyses such
Boothroyd and Dewhurst's DFA (Boothroyd and Dewhurst, 1989)
are applicable and useful once the design (geometric and material
features) of a component is known, i.e., late in the embodiment
design phase. But what if we want to address the issue of design
for assembly very early on a time-line, for example, when only
concepts are known? Eight governing principles provide the
foundation for Boothroyd and Dewhurst's work which is used to
facilitate assembly and reduce costs of existing products. It is
difficult, however, to apply these criteria as written during
conceptual and embodiment design when the information defining
a concept’s physical form is vague and incomplete.
Several methods have been developed for incorporating DFA in
the early stages of design. At Sony, they have developed a set of
rules for product design which includes design for assembly
(Yamigiwa, 1988) . Computer advisory systems such as those
described by (Onwubiko and Bekey, 1993; Jakiela, et al., 1985;
Mantÿlä, 1990; Kroll, et al., 1988) have been developed to aid
designers in the incorporation of DFA early in the design process.
Several CAD applications have also been developed in addition to
computer advisory systems, (Sturges and Yang, 1992; Henson, et
al., 1993) . Moreover, Sturges and Kilani, 1992) , describe the
development of an Integrated Design for Assembly Evaluation
Reasoning System which utilizes existing solid-modeling
packages to evaluate the assemblability of a design and makes
design for assembly recommendations at component, system, and
process levels.
In this paper, we develop an extension to the Boothroyd and
Dewhurst DFA approach using a decision-based perspective and
selection Decision Support Problems (DSPs) which incorporates
abstracted DFA principles for use during conceptual and
embodiment design. Review of various methods used for selection
are presented in (De Boer, 1987; De Boer, 1989; Vadde, et al.,
1992). We therefore do not review other methods in this paper.
Suffice it to say that we create and solve
• a preliminary selection DSP using abstracted DFA principles as
criteria for use during conceptual design, and
• a selection DSP for use during embodiment design again using
abstracted DFA principles for our selection criteria.
The conceptual framework of our approach is shown in Figure
1. In the left hand column of Figure 1, we show the Pahl and
Beitz, 1988, design process within which our approach is
implemented. We incorporate our abstracted DFA principles into
conceptual and embodiment design through the use of preliminary
selection and selection DSPs (Mistree, et al., 1994) . Our
approach consists of developing a design process which
incorporates product design for assembly both qualitatively1 and
1 qualitative method - an approach which provides a set of rules or
guidelines for designers to follow.
quantitatively2. By transforming Boothroyd and Dewhurst’s eight
governing DFA principles into abstracted DFA principles for use
during conceptual and embodiment design as shown in the right
hand column of Figure 1, we create a qualitative approach for
guiding designers through the product realization process. By
incorporating abstracted DFA principles in preliminary selection
and selection DSPs as shown in the middle column of Figure 1,
we provide a quantitative DFA evaluation method.
In this paper, we detail our approach in the following sequence.
In Section 2, we discuss how we extend Boothroyd and
Dewhurst’s DFA principles, and in Section 3, we present our
aircraft evacuation example and the formulation of our selection
Decision Support Problems. Finally in Section 4, we discuss the
results of our example and expound on the ramifications of
extending Boothroyd and Dewhurst’s Design for Assembly
principles. In this paper, we are concerned with the
implementation of abstracted DFA principles through the DSPs
rather than the detailed results of our example per se.
Process
Evaluation
DFA Criteria
Pahl &
Beitz
Need
DSPs
Figure 4
Section 3
Clarification
Section 3.1
Conceptua
Preliminary
Selection DSP
?
Embodimen
Selection DSP
?
Figure 2
Section 2.1
Section 3.2
Detail
Boothroyd
& Dewhurst
Product
FIGURE 1. CONCEPTUAL FRAMEWORK OF OUR
APPROACH TO THE EXTENSION OF DFA
2 EXTENDING THE USE OF BOOTHROYD AND
DEWHURST’S DESIGN FOR ASSEMBLY
The overall concurrency of DFA with Pahl and Beitz’s design
process is an important step in designing for assembly, Figure 1.
Boothroyd and Dewhurst’s DFA analysis is applicable to detailed
designs and existing products, but the importance of DFA in
conceptual design cannot be neglected. In conceptual design,
information governing a product’s geometric form is unavailable
and incomplete, making the application of DFA principles a very
subjective or qualitative procedure. As we proceed through
conceptual design and into embodiment design, applying DFA
becomes increasingly more objective or quantitative as more
information becomes available. Ideally, improvements in the
design process will be realized by eliminating or minimizing the
need for secondary DFA analysis and redesign after detail design.
Improvements in products will be apparent in their decreased cost
of assembly and increased reliability. Neglecting the principles of
DFA in the conceptual design phase makes revisions unavoidable.
In embodiment design, DFA provides a means to improve the
final product’s manufacturability and assembly. Moreover,
product information, i.e., geometric form, material features, etc.,
is available in the embodiment design phase, making the
application of DFA a more objective process. As embodiment
design leads into detail design, DFA becomes an integral process
2 quantitative method - an approach which requires quantitative
information to provide an objective evaluation or comparison (Kroll,
et. al., 1988)
in designing for manufacturability. In detail design, components’
positions, types of fasteners, and number of sub-assemblies, for
example, can be optimized in accordance with DFA principles.
In conceptual design, shown in the left hand column of Figure
1, little is known about the geometry or form of solution concepts,
making the application of DFA during this phase very subjective
because we cannot readily apply Boothroyd and Dewhurst’s eight
DFA principles to a concept. In order to use DFA in conceptual
design, it is essential to understand the characteristics of a concept
which will cause later assembly and manufacturing difficulties.
With this understanding, we can map Boothroyd and Dewhurst’s
DFA principles into conceptual design as abstracted DFA
principles as depicted in Figure 2. These abstracted DFA
principles can be implemented in preliminary selection DSPs as
selection criteria as illustrated in Figure 1. We present the results
of our extension of DFA pictorially in Figure 2 and discuss below.
The DFA principles we have abstracted are intended to help
designers determine which concepts, when realized, will be
designed better for assembly. We illustrate several different
abstractions in Figure 2 as we outline below.
1. Design evolution progresses from left to right. Since
Boothroyd and Dewhurst’s DFA principles apply to existing
designs and designs in the detail design phase, they are located on
the right side of the diagram. The abstracted DFA principles we
create are located on the left hand side of the diagram since they
are applicable earlier in the design evolution.
2. As designs evolve from a concept to the final detail solution,
we assert that the abstraction of the design decreases. From the
way they are structured, Boothroyd and Dewhurst’s DFA
principles are very concrete and apply to designs which are
concrete. In order to apply the DFA principles in the early stages
in design, particularly during conceptual design, we must abstract
these DFA principles so that they can be applied to concepts
which are less concrete, i.e., more abstract, than finalized designs.
The abstraction occurs in two ways:
• the boundaries of the principles become less solid, and
• the principles themselves become less concrete.
We represent these two abstractions in Figure 2 through
amorphisizing shapes and decaying borders. The change from a
solid border to a dotted one symbolizes the abstraction of the
bounds of our abstracted DFA principles in conceptual design,
and the amorphisizing shapes symbolizes the abstraction of
Boothroyd and Dewhurst's principles. The arrows indicate our
mapping of Boothroyd and Dewhurst's principles into the
abstracted DFA principles for conceptual design.
3. By integrating Boothroyd and Dewhurst's eight principles, we
create the following abstracted DFA principles for use in
conceptual design.
• Relax Constraints - A solution concept which has fewer
constraints and tight fittings may be easier to assembly.
• Proper Material Selection - Some materials are easier to use in
certain applications and some materials are better suited for
manufacturing and precision equipment. Choosing the right
material will help simplify the assembly process.
• Minimize Solution Complexity - We feel that a more complex
solution concept requires more parts and components and thus
is more difficult and costly to assemble.
• Minimize Relative Motion of Parts - If there is no need for
relative motion between two parts, then there is no functional
need to make two parts, unless they must be made of different
materials. By minimizing necessary relative motion and
number of different materials, we can decrease the number of
parts hence decreasing the complexity of the solution and
thereby facilitate assembly.
• Maximize Solution Symmetry - A more symmetric concept will
lend itself to having more components/parts which are the
same, thus minimizing the number of parts which can be
assembled incorrectly and reorientations needed during
assembly.
We assert that by incorporating DFA considerations into
conceptual design, designers will be able to produce significantly
better products in terms of design for assembly. By accomplishing
this, we assert that the cost of the product will be reduced two
fold. First, the product will be easier to assemble, and second, the
product will require minimal redesign.
concept which is more symmetric is likely to have fewer
dissimilar parts and more parts which are similar in geometry
and function. This will help minimize solution complexity. The
same holds true in reverse; if a concept is less symmetric, then
the solution is likely to be more complex.
Minimize Solution
Complexity
Maximize Solution
Symmetry
Relax Constraints
Minimize
Relative
Motion
Acessible and
Visible
Eliminate
Adjustments
Ease of
Handling
Proper Material
Selection
Prevent
Improper
Installation
Minimize Solution
Complexity
Reduce Parts
Minimize
Relative
Motion
Relax Constraints
Self-Aligning
and Locating
Minimize
Reorientations
Maximize
Solution
Symmetry
Maximize
Symmetry
DESIGN EVOLUTION
INCREASING ABSTRACTION
FIGURE 2. MAPPING DFA INTO CONCEPTUAL DESIGN
Many abstracted DFA principles interact each other, Figure 3.
These interactions stem from the translation of Boothroyd and
Dewhurst’s principles into more abstract DFA principles. The
overlap occurs because the translated DFA principles are more
abstract than Boothroyd and Dewhurst’s eight principles.
The interactions illustrated in Figure 3 are:
• Constraints inversely affect complexity and vice versa.
Complex concepts often require smaller tolerances, and
concepts which need smaller tolerances can be more complex
when realized.
• Constraints inversely affect relative motion and vice versa.
Systems requiring relative motion of parts often require them to
be very accurately fitted together, increasing the need for tight
tolerances. Similarly, if large tolerances are used, relative
motion between parts becomes less possible.
• Complexity inversely affects relative motion and vice versa. A
concept which has more relative motion between its parts is
likely to be more complex than a concept which does not.
Similarly, a concept which has many parts moving relative to
one another is likely to be more complex than a concept which
does not have many moving parts.
• Symmetry inversely affects complexity and vice versa. A
FIGURE 3. INTERACTIONS BETWEEN ABSTRACTED
DFA PRINCIPLES
While we recognize the inherent interactions which exist
among our abstracted DFA principles, it is perferable to have the
abstracted DFA principles uncoupled for use in the preliminary
selection DSPs. However, we are more concerned with the
implementation of DFA in conceptual design than in the
abstracted DFA principles themselves.
During embodiment design, the use of DFA has the greatest
impact on the product's final design and layout. In embodiment
design, as shown in the left hand column of Figure 1, we begin to
create preliminary form and layout designs for our solution
concepts. By having abstracted DFA principles which apply
during embodiment design, we can select designs which require
minimal redesign for assembly in the final product design.
Since both layout and form are created during embodiment
design, all eight of Boothroyd and Dewhurst's DFA principles are
applicable to some extent. However, we must translate each
principle into a slightly more abstract form in order to apply it
more easily in embodiment design and to utilize it as a selection
criterion in a selection DSP as indicated in the middle column of
Figure 1. As depicted in Figure 4, this abstraction is done on a
one-to-one basis as we map Boothroyd and Dewhurst's eight
principles directly into eight abstracted DFA principles for
embodiment design. The abstracted DFA principles are illustrated
in Figure 4 and explained as follows.
• Reduce Part Count - Decreasing the number of parts in a design
will reduce material costs and reduce assembly time.
• Eliminate Adjustments - Designs which require fewer adjustments increase the assemblability of a design.
• Self-Aligning and Self-Locating Parts - Parts and subassemblies which are self-aligning and self-locating will reduce
assembly time and assembly costs.
• Adequate Access and Visibility - Designing parts and subassemblies which are easy to access and easy to see will ensure
better design for assembly.
• Ease of Handling of Parts - Parts that are easier to move and
manipulate ease assembly.
• Minimize Reorientations - Minimizing the number of necessary
reorientations of parts or sub-assemblies in a design will reduce
assembly time and ensure the final product is more suited for
assembly.
• Prevent Improper Installation - Keeping a design simple and
avoiding complex fittings and connections will facilitate a final
product’s assembly. In addition, creating obstructions which
prevent improper installation and otherwise anticipating and
avoiding characteristics of a design which will make
installation difficult and will facilitate assembly.
• Maximize Symmetry - Maximizing symmetry or emphasizing
the asymmetry will facilitate final product assembly.
Abstracted DFA
Principles for
Conceptual Design
Relax
Constraints
Proper Material
Selection
Minimize
Solution
Complexity
Minimize
Relative
Motion
Abstracted DFA
Principles for
Embodiment Design
Acessible and
Visible
Boothroyd & Dewhurst’s
DFA Principles for
Existing and Detail Design
Eliminate
Adjustments
Ease of
Handling
Prevent
Improper
Installation
Reduce Parts
Self-Aligning
and Locating
Minimize
Reorient.
Maximize
Solution
Symmetry
Acessible &
Visible
Eliminate
Adjust.
Ease of
Handling
Prevent
Improper
Install.
Reduce
Parts
Self Align.
& Locating
polygons in the DFA column of Figure 1 have been embodied in
the form of abstracted DFA principles illustrated in Figure 4. It is
these DFA principles that we will incorporate in our preliminary
selection and selection DSPs for the original design of an aircraft
evacuation system. We use the abstracted DFA principles shown
in the left column of Figure 4 as criteria in a preliminary selection
DSP during conceptual design. The criteria are not absolute, most
of them require a relative basis for comparison. Therefore, we feel
that we can incorporate these criteria very nicely into a
preliminary selection DSP, middle column of Figure 1.
Preliminary selection involves the selection of the most-likely-tosucceed concepts for further development (Mistree, et al., 1994) .
The word formulation of the Decision Support Problem
representing preliminary selection DSP is given in Table 1.
TABLE 1. WORD FORMULATION OF PRELIMINARY
SELECTION DSP (MISTREE, ET AL., 1994)
Given
A set of concepts.
Identify
The principal criteria influencing selection. The
relative importance of the criteria.
Capture
Experience-based knowledge about the concepts
with respect to a datum and the established
criteria.
Rank
The concepts in order of preference based on
multiple criteria and their relative importance.
Minimize
Reorient.
Maximize
Symmetry
Maximize
Symmetry
DESIGN EVOLUTION
INCREASING ABSTRACTION
FIGURE 4. ABSTRACTION OF BOOTHROYD AND
DEWHURST’S DFA PRINCIPLES FOR CONCEPTUAL
AND EMBODIMENT DESIGN
Similarly to our approach in conceptual design, the abstracted
DFA principles for embodiment design are developed for use as
attributes in a selection DSP. Since physical form and quantitative
information becomes more available in embodiment design, the
application of the abstracted DFA principles in selection DSPs is
more objective.
3 AN AIRCRAFT EVACUATION SYSTEM DESIGN
To demonstrate the efficacy of incorporating DFA in the early
stages of design, we offer an example, the design of an aircraft
evacuation system. Assume that our goal is the original design of
an aircraft evacuation system which is versatile and can be used
on a variety of passenger aircraft. In keeping with the Pahl and
Beitz approach, our design work begins with the clarification of
the task and concludes upon completion of embodiment design.
We use the example to illustrate a quantitative approach for
incorporating DFA considerations into conceptual and embodiment design. We do not discuss the actual design of the aircraft
evacuation system in detail but concentrate on the incorporation
of the abstracted DFA principles into preliminary selection and
selection DSPs in conceptual and embodiment design.
We combine the abstracted DFA principles in one picture,
Figure 4, which spans the entire design time-line. Referring to
Figure 1, we can see that our physical representation of the dashed
TABLE 2. SOLUTION CONCEPT ACRONYMS
NDIS
NDRR
NDIC
ND3S
NDSC
FDIS
FDRR
FDIC
FD3S
FDSC
EDIS
EDRR
EDIC
ED3S
EDSC
- Normal door, inflatable slide
- Normal door, rigid ramp
- Normal door, padded cushion
- Normal door, 360° slide
- Normal door, Inflatable slide/cushion
- Fold open door, inflatable slide
- Fold open door, rigid ramp
- Fold open door, padded cushion
- Fold open door, 360° slide
- Fold open door, Inflatable slide/cushion
- Explosive bolt door, inflatable slide
- Explosive bolt door, rigid ramp
- Explosive bolt door, padded cushion
- Explosive bolt door, 360° slide
- Explosive bolt door, Inflatable slide/cushion
The method of Pugh (Pugh, 1981) forms the basis of the
algorithm developed for solving the preliminary selection DSP
(Mistree, et al., 1988; Kuppuraju, et al., 1985) . The solution steps
are as follows.
Step 1
Describe the concepts and provide acronyms
Step 2
Describe each generalized criteria, provide acronyms
and weighting constants for the specific criteria.
Step 3
Choose a datum with which all other concepts will be
compared.
Step 4
Compare the concepts rating concepts as either better
than (+1), the same as (0), or worse than (-1) the datum
Step 5
Evaluate the merit function for each concept within
each generalized criterion.
Step 6
Include interactions between generalized criteria.
Step 7
Post-solution analysis: determine the most-likely-to
succeed concepts.
Step 1 Describe the concepts and provide acronyms. We present
the results of the first two steps of our design process, i.e.,
clarification of the task and conceptual design, Table 2 and Figure
5.
Explosive Door (ED)
Normal Door (ND)
TABLE 3. DFA PRINCIPLES FOR CONCEPTUAL DESIGN
Abstracted DFA Principles for
Conceptual Design
• Minimize sol'n complexity
• Relax tolerances
• Use materials which simplify
manufacturing & assembly
• Minimize relative motion of
parts
• Maximize concept symmetry
•
•
•
•
•
Abstracted DFA Criteria for
Conceptual Design Selection
Solution complexity
Required tolerance characteristics
Required material characteristics
Amount of relative motion of
parts
Concept symmetry
TABLE 4. PRELIMINARY SELECTION CRITERIA
General Criteria
Assemblage
Fold Open Door (FD)
Rigid Ramp (RR)
Safety
Performance
Economics
360° Inflatable Slide (3S)
Specific Criteria
Solution Complexity
Required Tolerance
Req. Material Characteristics
Amt. of Relative Motion
Solution. Symmetry
Hazardous to People
Partial Deploy. Safety
Amt. of Impact
Reliability
Flow Rate
Deployment Rate
Aircraft Compatibility
Cost
Technology
Inflatable Cushion (IC)
TABLE 5. PRELIMINARY SELECTION WEIGHTINGS
Inflatable Slide (IS)
General Criteria
Weighting Scenario
Assemblage
Safety
Performance
Economics
0.3
0.22
0.28
0.2
Inflatable Slide Cushion (SC)
FIGURE 5. SOLUTION CONCEPTS
Step 2 Describe each generalized criterion, provide acronyms
We outline our abstracted DFA principles and preliminary
selection criteria for the conceptual design of our aircraft evacuation system in Table 3. The left hand column contains the
abstracted DFA principles from Figure 4 and the right hand
column contains the corresponding criteria. The remaining
selection criteria for the preliminary selection DSP are listed in
Table 4. The Assemblage criteria are the result of mapping the
principles for manual assembly into conceptual design as detailed
in Figure 4 and Table 3. The Safety, Performance, and Economics
criteria in Table 4 reflect customer requirements.
We recommend varying the weights for the preliminary
selection DSP to identify concepts which are consistent
performers. However, as we are concerned with selecting
concepts based on their design for assemblability, we use the
weighting scenario listed in Table 5 when performing preliminary
selection. The weights were determined using a prioritization
matrix (Allard, et al., 1994) .
Step 3 Choose a datum We choose the Normal Door Inflating
Slide (NDIS), Exploding Bolt Door 3-D Slide (ED3S), and
Folding Door Inflatable Slide/Cushion Hybrid (FDSC) to serve as
datums. We have chosen NDIS because it constitutes current
practice, the ED3S because it is the most controversial of our
concepts, and the FDSC because it has potential to be the mostlikely-to succeed.
Step 7 Determine the most-likely-to succeed concepts The results
of preliminary selection are shown in Figure 6. The comparisons
of each concept against each datum (Step 4), the calculation of
merit functions for each concept (Step 5), and the creation of
different weighting scenarios (Step 6) have been omitted due to
space constraints but can be found in (Allard, et al., 1994) . As
shown in Figure 6, the NDIS, NDIC, and NDSC (Normal Door
with Inflatable Slide, Inflatable Cushion, and Inflatable
Slide/Cushion, respectively) concepts consistently rank higher and
therefore are determined to be the most-likely-to succeed concepts
for further development.
Use of the preliminary selection DSP enables us to narrow the
large list of concepts (15) to a few most-likely-to succeed
concepts (3). We now proceed to develop more quantitative
information about each concept through further engineering
analysis. Once sufficient information is generated, a selection
DSP can be formulated and solved to help the designer determine
the best concept for detail development. In the process of ana-
lyzing the concepts, we eliminated the inflatable cushion concept
since it is unsafe and does not meet performance requirements.
The completion of embodiment design consists of determining the
best concept by formulation and solution of a selection DSP,
Figure 1.
1
NDIS as datum
EDSC as datum
FDSC as datum
0.9
0.8
0.7
0.6
0.5
engineering analysis and develop additional quantitative data for
each concept for use in the selection DSP. Applying the selection
DSP Criteria to the Partially Inflated Slide Concept:
• Number of Parts: 10
• Number of Adjustments: 11 (based on one possible assembly
sequence)
• Self Aligning and Locating Parts: 1 (the valve stem)
• Visibility: 100%
• Ease of Handling: 4 of 10 parts bad (the long side pieces are
awkward to handle)
• Installation Difficulties: 0%
• Symmetry: 9 out of 10 (all parts but valve stem symmetric)
• Number of Reorientations: 0 (all activities top down and in one
workspace)
0.4
0.3
0.2
0.1
L
A
A
0
Concept Acronyms
Top View
t2
t2
TABLE 6. WORD FORMULATION OF SELECTION DSP
(MISTREE, ET AL., 1994)
Given
A set of concept variants.
Identify
The principal attributes influencing the selection.
The relative importance of the attributes.
Rate
The alternatives with respect to each attribute.
Rank
The concept variants in order of preference based
on attributes and their relative importance.
The steps for solving the selection DSP are as follows.
Step 1
Describe the alternatives and provide acronyms
Step 2
Describe each attribute, specify the relative importance
of the attributes and provide acronyms.
Step 3
Specify scales, rate the alternatives with respect to each
attribute.
Step 4
Normalize ratings.
Step 5
Evaluate the merit function for each alternative.
Step 6
Post-solution sensitivity analysis.
Further details on formulating and solving selection DSPs is
provided in (Mistree, et al., 1988) .
Step 1 Describe alternatives and provide acronyms The
selection DSP is to determine the ordinal ranking of the two slide
concepts using nine selection criteria. We perform further
wi
Section AA
A
Top View
wo
t1
FIGURE 6. PRELIMINARY SELECTION RESULTS
After further development of the two concepts, a selection DSP
is formulated for the analysis, Table 6. As discussed in (Mistree,
et al., 1994) , the selection DSP facilitates the ranking of
alternatives based on multiple attributes of varying importance.
Use of the selection DSP helps the designer rate the alternatives,
allowing not only the rank of the alternatives to be determined but
also by how much one alternative is preferred to another.
L
A
D
FIGURE 7. PARTIALLY
INFLATED SLIDE
CONCEPT
wo
t1
wi
D
Section AA
FIGURE 8. FULLY
INFLATED SLIDE
CONCEPT
The selection DSP Criteria for the Fully Inflated Slide are:
• Number of Parts: 7
• Number of Adjustments: 9 (based on one possible assembly
sequence)
• Self Aligning and Locating Parts: 1 (the valve stem)
• Visibility: 5 of 7 parts easily visible (two mesh strips obscured)
• Ease of Handling: 4 of 7 parts bad (the long topside, underside,
and meshes are awkward to handle)
• Installation Difficulties: 3 of 7 parts bad (the two meshes and
the last endcap)
• Symmetry: 4 out of 7 (Only endcaps and meshes symmetric)
• Number of Reorientations: 1 (must remove assembly from
mandrel during process)
Steps 2 and 3 Describe each attribute, provide acronyms,
specify scales Using the abstracted DFA principles from the
middle column of Figure 4, eight of our selection criteria are DFA
oriented while the last criterion, material volume, is chosen to
quantify the weight of the slides. The selection criteria for use in
evaluation are given in Table 7. The different scales used in
selection DSPs are described in (Bascaran, et al., 1989; Mistree,
et al., 1988) . Explanation of the selection criteria listed and the
corresponding scales is as follows.
• Number of Parts - We use a ratio scale with low preference to
rate designs based on the number of parts or sub-assemblies
they contain.
• Number of Adjustments - We use a ratio scale with a low
preference to rate designs based on the number of adjustments
that a design will require.
• Self-Aligning and Self-Locating - We use an interval scale with
a high preference for designs with high percentages of selfaligning and self-locating parts or sub-assemblies.
• Accessible and Visible - We use an interval scale with a high
preference to rate designs based on the percentage of parts or
sub-assemblies which are accessible and visible.
• Ease of Handling - We use an interval scale with a preference
for designs with high percentages of easy to handle parts.
• Number of Reorientations - We use a ratio scale with low
preference to rate designs based on the number of reorientations
of parts or sub-assemblies that a design will require.
• Installation Problems - We use an interval scale with a high
preference to rate designs based on the percentage of parts or
sub-assemblies without anticipated installation problems.
• Symmetry - We use an interval scale with a high preference to
rate designs based on percentage of symmetric parts.
In the selection DSP, each criterion is weighted by its relative
importance, Table 8. Assuming that both concepts perform
equally, then we want DFA considerations and material volume to
be our only selection criteria. Therefore, we weight each of our
eight DFA criteria equally and place additional emphasis on the
material volume which characterizes the cost of our slide. Then,
the concept variants are evaluated in terms of the nine selection
criteria using the information in Figures 8 and 9 utilizing the
selection DSP in Table 9.
TABLE 7. DFA SELECTION CRITERIA
Criteria
Acronym
Scale
High/
Low
Pref.
High
Value
Low
Value
PART
Ratio
Low
100
0
ADJT
Ratio
Low
100
0
Number of Parts
Number of
Adjustments
Self-Aligning
and Locating
Accessible and
Visible
Ease of
Handling
Number of
Reorientations
Installation
Problems
Symmetry
SAAL
Interval
High
1
0
ACVS
Interval
High
1
0
HAND
Interval
High
1
0
REOR
Ratio
Low
100
0
INST
Interval
High
1
0
Interval
High
1
0
SYM
M
TABLE 8. WEIGHTING FACTORS FOR SELECTION DSP
Criteria
# Parts
# Adjustments
Align/Locate
Access/Visible
Ease of Handling
# Reorientations
Inst. Problems
Symmetry
Volume
Attributes
PART
ADJT
SAAL
ACVS
HAND
REOR
INST
SYMM
VOLM
Weights
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
We show in bold print the differences in DFA considerations
for our two designs in Table 9. Based on the desired preferences,
Table 7, the fully inflated slide has fewer parts/sub-assemblies
(PART), more self-aligning parts (SAAL), and fewer adjustments
(ADJT) than the partially inflated slide; however, the remaining
design for assembly considerations favor the partially inflated
slide. The partially inflated slide will require fewer reorientations
(REOR), has more parts which easy to handle (HAND), has more
symmetric parts (SYMM), fewer installation problems (INST)
and allows for easy access and visibility (ACVS) during
assembly. Based on the weighting factors in Table 8, we would
expect the partially inflated slide to be better than the fully
inflated slide in terms of DFA considerations, and the solution to
the selection DSP, shown in Table 10, verifies this.
TABLE 9. ATTRIBUTE RATINGS FOR SELECTION DSP
Criteria
# Parts
# Adjustments
Align/Locate
Access/Visible
Ease of Handling
# Reorientations
Inst. Problems
Symmetry
Volume
Selection
Criteria
PART
ADJT
SAAL
ACVS
HAND
REOR
INST
SYMM
VOLM
Partially
Inflated Slide
Fully Inflated
Slide
10
11
0.10
1.00
0.60
0.00
1.00
0.90
0.038
7
9
0.11
0.71
0.43
1.00
0.57
0.43
0.050
Step 5 Evaluate the merit function for each alternative. The
selection DSP is solved and the merit values are computed for
each concept variant, Table 10. From this analysis, we validate
that the partially inflated slide concept is better than the fully
inflated slide concept. Using a selection DSP on two alternatives
is overkill. The selection DSP is designed to be used for several
concepts, when it is difficult to rank order them and choose the
best one. We chose to go ahead with the selection DSP, however,
to verify that the partially inflatable slide is better than the fully
inflatable slide .
TABLE 10. MERIT FUNCTIONS FROM SELECTION DSP
Concepts
Partially Inflated
Slide
Fully Inflated
Slide
Acronym
Merit
Values
Overall
Rank
PISL
0.838
1
FISL
0.707
2
4 CLOSURE
We recognize that there are many ways to incorporate DFA
considerations into a design process. In this paper, we create
Decision Support Problems with DFA principles as selection
criteria to incorporate DFA into conceptual and embodiment
design. By translating Boothroyd and Dewhurst's eight governing
DFA principles backwards along the design evolution timeline at
levels of increasing abstraction to create general DFA principles
for use during conceptual and embodiment design, we create a
qualitative approach for guiding designers through the product
realization process, minimizing the need for redesign for ease of
assembly. By incorporating DFA considerations in preliminary
selection and selection DSPs, we provide a quantitative evaluation
method for fostering the development of designs with favorable
assembly characteristics.
Use of the preliminary selection DSP in conceptual design
narrowed our concepts from 15 to 3, and use of the selection DSP
in embodiment design narrowed our concept variants from 2 to 1
based on design for assembly considerations. The improved
conceptual and embodiment design for an aircraft evacuation
system means not only will manufacturers save money through
reduced assembly costs, but human life will be improved. As the
reliability of the aircraft evacuation system increases, fewer
people will have to suffer.
We have tried to abstract DFA principles which are domain
independent. We have applied these DFA criteria to the design of
an aircraft evacuation system, but recognize that they could be
expanded, pending the examination of other designs and more
rigorous examples. As stated earlier our intent is to illustrate our
application of DFA in conceptual and embodiment design through
Decision Support Problems and provide a new starting point for
applying DFA earlier in the design process to minimize later
redesign for assembly.
As it exists, Boothroyd and Dewhurst’s systematic approach is
applicable to detailed designs or existing products, when
information defining its physical form and shape is known and
available. Our extension of DFA into conceptual and embodiment
design is driven by the current formulation of DFA for redesign or
use in detail design. We assert that the incorporation of DFA in
conceptual and embodiment design will have the same effect as
using DFA late in the design process or during product redesign.
Moreover, we assert that incorporating DFA throughout the
design process minimizes the amount of redesign necessary and
produces better conceptual designs for development in
embodiment design. This follows from the premise that earlier
incorporation allows selection of conceptual designs that truly
embody the principles of DFA thereby realizing the benefits of
DFA analysis while reducing time to market and minimizing the
secondary cost of redesign for assembly.
Application of abstracted DFA principles during conceptual
and embodiment design through Decision Support Problems will
facilitate assembly, reduce assembly cost, and minimize the need
for redesign for assembly of a final product. By raising design for
assembly issues in the early stages of design through the
implementation of abstracted DFA principles in selection DSPs,
we have not only created a design process which affects how
engineers design but also what they design.
ACKNOWLEDGMENTS
Drs. J.A. Shupe and Sudhir Hublikar of B.F. Goodrich posed
this problem. Financial support from NSF Grant DDM 93-96052
is gratefully acknowledged. We appreciate the support received
from J. Peplinski and P. Koch, and the guidance of Dr. D. Rosen.
We acknowledge the participation of C. Allard, J. Elliot, and M.
Marston in the development of this approach. M. Bauer and T.
Simpson are funded by The G.W. Woodruff School of
Mechanical Engineering, Georgia Tech.
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