From: Proceedings of the AI and Manufacturing Research Planning Workshop. Copyright © 1996, AAAI (www.aaai.org). All rights reserved.
Critics and Advisors: Heuristic Knowledgeand Manufacturability
J. J. Rivera
Sandia
National
Laboratories
MS0660P. O. Box 5800
Albuquerque,
NM 87185-0660
jjriver@sandia.gov
W. A. Stubblefield
Sandia National Laboratories
MS0722P. O. Box 5800
Albuquerque, NM87 ! 85-0722
wastubb@sandia.gov
A. L. Ames
Sandia
National
Laboratories
MS0660P. O. Box 5800
Albuquerque,
NM 87185-0660
alames@sandia.gov
Abstract
In recent years, muchof the progressin Computer-Aided
Manufacturing
has emphasized
the use of simulation,finiteelementanalysis, and other science-basedtechniquesto plan
and evaluate manufacturingprocesses. Theseapproaches
are all basedon the idea that wecan build sufficiently
faithful modelsof complexmanufacturing
processessuch as
machining,welding, and casting. Althoughthere has bcen
considerableprogressin this area, it continuesto suffer
fromdifficulties: the first of these is that the kindof highly
accurate modelsthat this approachrequires may’take many
person monthsto construct, and the secondis the large
amount of computing resources needed to run these
simulations.
Twodesign advisors, Near Net-Shape Advisor and
Designfor MachinabilityAdvisor, are being developedto
explorethe role of heuristic, knowledge-based
systemsfor
manufacturingprocesses, both as an alternative to more
analytical techniques, and also in support of these
techniques.Currentlythe advisorsare bothin the prototype
stage. All indications lead to the conclusion that the
advisors will be successful and lay the groundworkfor
additionalsystemssuchas these in the future.
146
AI & Manufacturing Workshop
Introduction:
A Hybrid model of Design
Artificial intelligence research views design, whether
of a computerprogram, logic circuit or mechanicaldevice,
as a process of searching through a design space for
solutions that satisfy such constraints as functionality,
reliability, size, choice of material, etc. For physical
devices, manyof these constraints reflect the desire to
minimizethe cost and difficulty of makingthe component
through a given manufacturing process. However, the
addition of manufacturahility constraints to the design
space model raise significant theoretical and practical
issues. Included are the issues as they apply to the
development of computerprograms that will assist human
designers in improving the manufacturability of their
designs.
Theoretical and practical issues are discussed in the
context of two design advisors currently being built. Near
Net-shape Advisor. helps a designer select the process
(casting or rough machining) for bringing a part to the
point that it is ready for finish machining. Design for
MachinabiliO,Advisor, checks a part design for features,
such as excessively tight tolerances, that makeit difficult
to machine. These programs are intended as design
machining,it also tends to promote"one-piece"designs
From: Proceedings of the AI and Manufacturing Research Planning Workshop. Copyright © 1996, AAAI (www.aaai.org). All rights reserved.
assistants: they will providetheir users with information
that minimizethe needfor joining. Theseconsiderations
about options andpotential problems,while leaving final
suggest that near net process selection must take place
early in the processwhenthe artifact’s basic structure is
decisions to the user. Correctness, usefulness and user
acceptance of these programsdependson an appropriate
being determined.However,other factors that influence
understandingof the waythat humansthink and act during
near net process selection cannot be knownuntil much
the design process. To improve our understanding of
later in design. For example,thin walls are difficult to
humandesign methods, the next section describes the
cast; accurate specification of wall thickness generally
cannot be madeuntil the designis fairly well advanced.
design space model.
Asa result, it is difficult to locatethis decisionin the topdownmodel:Dodesigners choosea near net process early
Models of Design
andadapt subsequentdesigndecisionsto fit that process7
Or, do they postponethe decision until later, choosinga
Acentral problemconcernsthe representation of the
near net process that can best realize a designthat was
design space and the ways a humanor machine moves
chosenfor other reasons7It is clear that both approaches
through it to solve a design problem. Top-down
can lead to a less than optimaldesign/processcombination.
approaches (figure !) emphasize a hierarchical
representationof designspace: beginningwith an abstract
specification of a designproblem,designproceedsthrough
incremental
Candidate
a series of specializations that ultimately produce a
design
__~Possible
Design
refinement
concrete artifact design. Generally, top-downapproaches
emphasize
functionalityin the initial specification, adding
manufaeturabilityconstraints as designdecisionsare made.
incremental
Design
design
Candidate
Candidate
_~Possible
Design[~
refinement
Functional I
Specification
incremental
design
refinement
i
I
Candidate
_~Possible
Design[ r.
Specification
Figure 2
~r
Component
Specification
Component
Specification[
Component
Design
Component
Design
1
Component
Specification
Component
Design
Figure1
Constructionof the manufacturablityadvisors reveals
a number of problems with the top-down model. For
example,choice of a near net process can place strong
constraints on the physical structure of an artifact: Cast
parts tend to be difficult to weld due to the tendencyof
castings to formpores andother microscopicimperfections
in the metal’s crystalline structure. However,because
casting can produce parts of greater complexity than
Incremental models, also known as bottom-up
approaches,are an alternative that, like top-downmodels,
have both positive and negative aspects. Incremental
theories proposethat designers workby constructing a
completesolution to a simplified portion of the design
problem.Theythen extendthe functionalityof this partial
designincrementally,until it satisfies the full requirement
(figure 2). Incremental design differs from top-down
approachesin that all designrefinementstake place at the
samelevel of abstraction. It can be characterized as a
processof tinkeringwith candidatedesignsin an effort to
improve their performance. Incremental models allow
both functional and manufacturabilityconstraints to be
flexibly applied throughoutthe design process. However,
practical experience indicates difficukies with this
approach.Somedifficult to machinefeatures can be fixed
throughlocal, incrementalchanges:the designercan relax
a tolerance, or changea fillet radius without requiring
major changes in a design. However,other problems
require extensive re-design. For example,excessively
deep holes maybe difficult to machineaccurately. One
Rivera
147
advisors will use rules and methodsto automatically
search solid modelsfor relevant features and analyze
their effect on manufacturability. In addition, we will
offer this knowledge to the users in the form of
hypertext documentsthat will allow them to access it
directly in the early stages of design.
Reliance on heuristics to navigate design space.
Opportunistic design oRenrequires the user to make
decisions based on inadequate knowledge. For
example, early selection of a near net process may
require a decision before the ramifications of that
process on the final design are well understood. Much
of the knowledgeused in this process is necessarily
heuristic in nature. The advisors will need to function
well in the absence of complete knowledge, either by
makingreasonable default assumptions about missing
data, using fuzzy or probabilistic
reasoning
techniques, or exploiting appropriate heuristics to
reason with partial information.
that ofa the
designer
consider
is to re-design
the
From:solution
Proceedings
AI and should
Manufacturing
Research
Planning Workshop.
Copyright © 1996, AAAI (www.aaai.org). All rights reserved.
part to eliminate the such holes entirely. The incremental
modelis limited in its ability to account for this sort of
large jump through the design space. For similar reasons,
the incremental modelcannot provide a suitable answer to
the problemof near net process selection.
These observations suggest a hybrid model of design
that exploits both hierarchical decompositionof the design
problem and the ability to move incrementally through
design space. Opportunistic design recognizes the
importance of hierarchical representation of the design
space [Guindon, 1990]. However,it also recognizes that
humansnavigate this space in very flexible ways, moving
freely betweendifferent levels of abstraction, using topdown design to capture major design features and
incremental approaches to refine and evaluate those
decisions. Humansare very good at deciding when to
abandon an approach and move back to a higher level of
abstraction to rethink decisions madeat that level.
Opportunistic design approaches encompass many
aspects of current design process, including the problems
outlined in this discussion. For example, near net process
selection can workinitially at a high level of abstraction,
choosing a basic process/shape combinationthat considers
the costs of joining and assembly, if subsequent
refinementsto this design create features that are difficult
to produce with the chosen process, the designer may
either choose to modifythe design to suit the process or
retreat to an earlier stage of design and propose a new
process/shape combination. Similarly, on finding a
difficult to machinefeature in a part, a humandesigner will
distinguish problems that can be resolved with local,
incremental changes from those requiring a more basic
rethinking of the design. Again, this requires the ability to
movefreely through the design space, in a flexible manner
that fits the opportunistic model.
.
Design
m°di~cati°ns.,f°r[ I
I[ Cda~ii~te
AI & Manufacturing Workshop
Performance
simulation,
reliability
estimates,etc.
Functional ]
requirements
Figure 3
.
148
Manufacturing
I I processmodels,cost
manuiacmraomty[
II
Writing Advisors to Support Opportunistic
Design
Adoption of the opportunistic model has several
ramifications for the creation of design assistants. In
particular, our work on design advisors has been shapedby
the following implications of the opportunistic design
model:
i. Support different levels of abstraction by a design
assistant. Althoughour advisors are intended to infer
relevant features from a CADsolid model, we must
also recognize that much of their knowledge also
needs to be available to the user early in design, when
sufficiently detailed models maynot be available.
Consequently, we are providing the users with
multiple representations of relevant knowledge: The
~
Support for multiple views and representations of the
design. The strength of opportunistic design results
from human’sability to combineeasily multiple points
of view in refining the design; this is difficult to
capture in an artificially intelligent program,given the
current state of the art. For example, analysis of the
manufacturability of a feature requires a different
representation of that feature than does an analysis of
its functionality. Wehave approachedthis problemby
recognizing that our advisors are specialists; they are
intended to be part of a much larger design
environment called the Product Realization
Environment.This environmentwill incorporate tools
for analyzing different aspects of design including
both functionality and manufacturability.
These
advisors, as well as future manufacturabUityadvisors,
will ignore functionality while other tools will
emphasizefunctionality over manufacturability (figure
3). This approach will rely on the human user to
select the appropriate tool for the current stage of
design and integrate the findings of the different tools.
important indices.
Generally, performance is the key
From: Proceedings of the AI and Manufacturing Research Planning Workshop.
© 1996,
(www.aaai.org).
All rights
reserved.
index.Copyright
Each of
the AAAI
hypotheses
will be
evaluated
by
The remainder of this paper will present the advisors in
more detail and discuss the specific ways in which they
address these issues.
Near Net-Shape
Advisor
Although
mostof the componentsbeing built here at
Sandia National Laboratories and by industry require
extensive finish machining, it is not always obvious that
the part should he machinedto a rough shape from stock
metal. This decision is madeearly during the design phase
and affects the manufacturing costs and schedule. Because
of the design engineer’s lack of understanding of
manufacturing procedures and constraints, costs can be
excessive when manufacturing the component per the
design. Early in the design phase, the Near Net-Shape
Advisor will assist the design engineer with design tradeoff decisions for casting versus machiningas the near netshape manufacturing process. With the aid of the Near
Net-Shape Advisor, the design engineer will invest more
time during the conceptual phase of the design identifying
machiningand casting details.
The advisor will consider such factors as the estimated
cost of different manufacturingprocesses, the availability
of materials that meet the product requirements (strength,
ductility,
and conductivity) and the quantity of the
component to be produced. A hypothesis for a component
is created based on the part design, the material, and the
manufacturing process by the advisor.
Various
assumptions will be generated through a selection of
equivalent materials that will meet the product
requirements input by the user. These are evaluated by the
NearNet-Shape Advisor. Straightforward criteria exist for
an assumption which definitely matches a machining or
casting manufacturing process, however most of the
hypothesesdo not fit into either of these categories. Each
hypothesis will be evaluated to suggest the most
economicalprocess for achieving near net-shape.
A matrix is being generated with 3 indexes of
performance: cost, performance, and schedule. Modelsof
this information are available in literature [Dallas 1976]
and through trade associations. Casting and machining
experts at Sandia are providing other supporting domain
knowledge. From this matrix, a complex weighted sum
equation will be created with the user selecting the
applying the weighted sum equation. Establishing the cost
of a component,based on complexity and delta volume, is
very difficult. Complexityof a componentis determined
by its features. In the literature, a common
definition of
feature is a set of adjacent faces boundinga depression.
Such a def’mition is adequate for machiningapplications
where the most commonfeatures are depressions caused
by material removal (holes, pockets, and slots). It
inadequate for applications where other kinds of features
(symmetries and orthogonal directions) are important.
Features are defined as physical design characteristics and
geometric patterns in a part [Ames1988]. These features
are considered manufacturingfeatures which are recurring
shapes with some fixed engineering significance.
A
feature is a parametricshapeassociated with such attributes
as its intrinsic geometricparameters(length, width, depth,
position, orientation, geometric tolerances, material
properties, and references to other features) [Mantyla, Nau,
&Shah, 1996]. Ames’swork on feature recognition will
be extended and accessed by the Near Net-Shape Advisor.
The feature recognizer will identify whether the
component can be machined from standard stock.
Geometricfeatures such as pockets, slots, and undercuts
are identified by the recognizer. These features are
indicators as to the complexity of the componentand the
identification of them will be necessary to recommenda
near net-shape process.
I
Material ]
made of ]
Near Net
Advisor ]
~Near
Net
produce1 Workpiece[
[ Process
I
Rough 1
Machining
Casting
Figure 4
Near Net-Shape Advisor knowledgebase has an object
oriented architecture (figure 4). The near net advisor class
will link to an instance of a near net-shape process and
provide methodsfor querying the feature knowledgebase.
This knowledgebase provides the interface to the feature
recognition software to find instances of a feature as well
as querying the user for additional information about the
Rivera
149
feature such as dimensions and tolerances.
Rough
machining and casting subclasses exist of the near netshape process class. This set of subclasses can be
expanded to accommodateother near net-shape processes.
Each of these subclasses have their ownset of rules which
incorporate the weighted sum equation for estimating the
suitability of manufacturing process. A score is given to
each of the hypotheses. This assessment is shown to the
user. An explanation of each assessment is available for
the user which will list the advantages and disadvantages
of each rough shape process.
such as wall thickness and the flow of material within the
boundaries when casting [Ames, 1991]. The Design for
Machinability Advisor will also access the feature
recognizer referred to previously, althoughthe features that
it identifies will be different. First, the feature recognizer
will classify the component
as a rotational part, sheet metal
part, extrusion, or prismatic part. Theseclassifications are
based on shape defining features such as outer and inner
diameters, sheets, bends, and extrusion sides. Based on
this information, the recognizer will determine if the
componentwill be machinedon a lathe or mill, extruded,
or formed from sheet metal. Second, the recognizer will
then recognize shape modifying features (profiled holes,
holes, slots, curved faces, and flats). This informationwill
be incorporated by the Design for Machinability Advisor to
performan analysis of the design.
From: Proceedings of the AI and Manufacturing Research Planning Workshop. Copyright © 1996, AAAI (www.aaai.org). All rights reserved.
Design for Maehinability
Advisor
A "spell checker" for product designs is the best
description of the Design for Machinabilio, Advisor. Much
of the work in evaluating the machinability of designs
focuses on such issues as the evaluation of tool paths to
determine if all machinedareas can be reached. There are,
however, manyother potential problems in a design that
can complicate manufacturing, often unnecessarily. The
advisor will bring manufacturing technology and
knowledge closer to the process of component design.
Cost and scheduling of the machining of a componentare
related to the numberof setups required and the specialty
of the manufacturing tools. To help the design engineer
control both the costs and time of producing a component,
the Design for Machinability Advisor will identify
manufacturing features which require special tooling and
increase setup and machining time. Impact of certain
features is often not knownby the engineer and the advisor
will suggest to the engineer design trade-offs to reduce
manufacturing time and cost. For example, many
designers will specify hole sizes that do not correspond to
standard drill bit sizes, it will be simple for the design
advisor to detect such holes and suggest that they be
changed to a standard size if the specialty size is not
required. Manyother commonmachining problems are a
result of excessive tolerancing of the component. Often
these tolerances are not a requirement for functionality of
the component. By examining tolerances and looking for
situations wherethey axe unnecessarily tight or extremely
difficult to achieve, the Design for Machinability Advisor
can suggest to the design engineer design changes to
eliminate multiple machining operations. Such factors as
geometry, materials, and functionality are considered by
the advisor whenevaluating parts.
Geometry of a component is described as a set of
features. Features are application specific. Machiningis
concerned with the volumes of material that must be
removedto create the part from stock and with finding a
means of accessing material removal regions with a tool.
In contrast, castability analysis is concernedwith features
IS0
AI & Manufacturing Workshop
MaChd’vnabo~lity~
Operation
~uses
Feature
III -Ill ........ °lII ........°l
Tool ]
(Subcl~sesof availabletools)
Figure 5
An object oriented architecture also exists for the
Design for Machinability Advisor (figure 5). Modularity
is a key factor of the advisor. Eachset of rules associated
with a features attribute is a module.A control structure
applies rules for each feature attribute in turn. The
machinability advisor class provides methodsfor querying
the feature knowledgebase for componentfeatures, the
Near Net-Shape Advisor for information, and the user for
functionality
and material requirements. Rules for
detecting feature interaction problems and excessive
numbersof operations or tool changes are included in the
machinability advisor class. This class provides access to
the operation and feature plan classes. Operation applies
rules for detecting problemsin applying an operation to a
feature. It has a set of subclasses, which are machining
[Mantyla, Nan, & Shah, 1996]. Information must flow
From: Proceedings of the AI and Manufacturing Research Planning Workshop. Copyright © 1996, AAAI (www.aaai.org). All rights reserved.
operations, which can be expanded. Operation links to an
from the customer to engineering to manufacturing as
instance of tool, whichdeterminesthe availability and cost
accurately and quickly as possible. Application of
of a tool, wear, breakage, and suitability for material.
heuristic, knowledge-basedsystems can aid in the flow of
Methodsfor determining plausible sequences of operations
information. These two advisors can be expanded to
for a feature and rules for detecting undesirable
incorporate improved, new and existing processes.
interactions betweenfeature operations are in the feature
Additional near net-shape processes, such as forging and
plan class. As various rules are fired, feedbackis provide
power metallurgy, can be added to the Near Net-Shape
to the user when problems occur with a machining
Advisor to expand the functionality and encompass all
operation. Suggestions are madeto the user and a detailed
areas of achieving rough shape. Expansion of the
discussion of the problemis available for the user.
machinability advisor would incorporate other machining
processes, geometric tolerance analysis, and extend the
feature recognizer to deal with wall thickness.
Current Status and the Future
With the knowledge gained from the development of
these two advisors and the expansion of the feature
Currently both of the advisors are in the prototype
recognizer, other advisors can be readily created to analyze
stage. By the end of Septemberthe advisors shall be fully
other manufacturing methods, analysis, reliability, and
functional and provide the foundation for the development
assembly. Analysis advisors will help designers identify
of complete systems. The investment of time in using both
features that are detrimental to the durability of the system.
of these advisors will pay-off during the detail design
Reliability advisors will help designers identify features
phase and during manufacturing with a reduction in rethat are difficult to maintain and processes that need to be
design cost because the machiningversus casting decision
improved. Assemblyadvisors will step through a complex
was correct and the manufacturingfeatures are moreeasily
tolerance analysis to determinethe ability to be assembled
predicable. Design of the advisors has supported the needs
of a system. Information can be passed quickly and
of opportunistic design in a numberof ways.
accurately from one advisor to another and to the user.
Althoughbeing developed separately, the two advisors
Mathematical models, CADmodels, and knowledge-based
will be compatible and will eventually interface with each
models can be shared between manufacturing process
other. Each of the advisors is being designed to be
advisors. These advisors will play important roles in whatmodular so additional heuristic knowledgecan be easily
if scenarios as design time is shortened and simulation
added to expandthe application of the advisors into other
becomesincreasingly important. Also, the application of
manufacturing processes. Developmentof these advisors
the heuristic, knowledge-basedadvisors will insure that
has benefited from extensive software reuse. Not only do
design decisions are as accurate as possible to avoid costly
they share several modules between them, but they also
redesign and meet or exceed the customer’s requirements
incorporate code from Sandia’s SmartWeld project
and expectations.
(discussed elsewhere in this proceeding). Support
design engineers, casting experts, machining experts and
References
other manufacturingexperts is the key to the success of the
advisors as the project develops.
Success of the project requires more than the
Guindon, R. 1990, Designing the Design Process:
development of two manufacturability
advisors.
Exploiting Opportunistic Thoughts, Human-Computer
Developmentof the prototype systems allows the project
Interaction, 5:305-344.
team to evaluate numerous design decisions of the
approach, the knowledge-hases, and the user interfaces.
Ames, A. L. 1988, Automated Generation of Uniform
The lessons learned from the development of these two
Group Technology Part Codes From Solid Model Data, In
advisors will be immense. The real success will be the
Proceedings of the 1988 ASME
International Computersin
application of these lessons to the design and improvement
Engineering Conference and Exhibition, ASME.
of current advisors and future advisors.
Application of heuristic, knowledge-basedsystems to
Mantyla, M., Nan, D., and Shah, J. 1996, Challenges in
manufacturing processes does not stop with these two
Feature-Based Manufacturing Research, Communications
advisors. Manufacturers are experiencing a competitive
oftheACM,February 1996, Vol. 39, No. 2, p 77 -85.
environment in which products are more complex, delivery
times are shorter, and product life is shorter. Quality,
Dallas , D. 1976, Tool and Manufacturing Engineers
durability, maintainability, safety, and environmental
Handbook, NewYork: McGraw-Hill.
performance are becoming more and more important
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lSl
Ames,
A. L. 1991,
Production
Ready Feature
From:
Proceedings
of the AI
and Manufacturing
ResearchRecognition
Planning Workshop. Copyright © 1996, AAAI (www.aaai.org). All rights reserved.
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Proceedings of the 1991 ACMSymposium on Solid
Modeling Foundations and CAD/CAM
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AI & Manufacturing Workshop