SME DFM Tech paper

1. Implementation of the Concept "Design
for Manufacturing" in Mechanical
Engineering Education
authors
R.D. WEILL M.P. WEISS
Professor Professor
Israel Institute of Technology, Israel Institute of Technology,
Technion Technion
Haifa, Israel Haifa, Israel
abstract
After reviewing the main meanings of the concept "Design for Manufacturing"
(DFM) and its impact on mechanical design, the paper analysis some practical
examples of application of DFM, in particular in the field of dimensioning and
tolerancing. It evaluates then briefly details some of the advanced computerized
technologies intended to improve DFM. Finally, it considers the implications of
DFM in relation to sound mechanical engineering education.
conference
INTERNATIONAL CONFERENCE ON EDUCATION IN MANUFACTURING
March 13-15, 1996
San Diego, California
terms
Design
Manufacturing
CAD/CAM
Society of Manufacturing Engineers
One SME Drive · P.O. Box 930 · Dearborn, Ml 48121
Phone (313) 271-1500
ER96-204

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3. ER96-204
INTRODUCTION - THE MEANING OF "DESIGN FOR PRODUCTION"
It is commonly admitted that 80% of the cost of a product is determined in its
design phase. This means that the majority of the decisions concerning manufacturing,
assembling and inspection of the product, have already been taken before and that the
manufacturing department has limited possibilities to improve the manufacture of the
parts.
Obviously, this situation is not satisfactory when the design personnel has not
the required qualification for manufacturing planning. Many efforts have therefore been
spent, and specially in the recent years, to try to improve this situation by "Designing
for Manufacturing", or "DFM". However, much more efforts have to be spent to really
introduce the philosophy of "Design for Manufacturing" in design departments. It
appeared therefore necessary to implement already in the curriculum of the education
of manufacturing engineers the concept "Design for Manufacturing" by mixing design
education and manufacturing education at the very early stages of training. It is clear
also that a good manufacturing engineer has to be trained as well in design techniques
as in manufacturing techniques, because he has also to design many kinds of tooling
and fixtures, and, in many cases, he will be in design activities during a part of his
career.
For the reasons just mentioned, the authors of the paper thought that the
concept "Design for Manufacturing" (DFM) is appropriate to be analysed during this
conference on "Manufacturing Engineering Education". The efforts made in the
Technion to implement it in the curriculum of Manufacturing Engineering will be
described to illustrate a special approach.
Because the expression "Design for Manufacturing" can have different
meanings, it seemed useful to analyze briefly the design process in relation with
production requirements. Certainly, in the first phases of designing, the emphasis has
to be on the functionality of the product. In a more advanced phase, the functionality is
translated in functional dimensions and tolerances which have to be respected
imperatively. In the last phase, functional dimensioning and tolerancing are
reconsidered and adapted to manufacturing capabilities with a view to economic
manufacturing.
Obviously, the DFM concept concerns essentially the last phase. Here the
designer has to consider capabilities and costs of the different manufacturing
processes in relation with the nature of the design. The functioning of the product
depending on its assembling, it is important for the interchangeability of the parts to

4. ensure a given accuracy in the processing operations. The design should also consider
the feasibility and cost of the inspection stage.
In conclusion, although the design drawing has first of all to define a product
adapted to its function, it should consider also manufacturing requirements, allowing
large tolerances and a maximum freedom for the production planner to find the most
performant and economic conditions for manufacturing. To illustrate these general
principles, the following sections will describe practical means to evaluate process
capabilities and their costs at the design stage. Emphasis will be put on the methods
used for dimensioning and tolerancing according to the DFM concept. In addition, the
new advanced technologies (CAD/CAM, Artificial Intelligence, Rapid Prototyping, etc.),
will be briefly reviewed in relation with their contribution to the DFM principle. In a last
section, the application of DFM in manufacturing engineering education at the
Technion is described.
SOME EXAMPLES OF THE INTRODUCTION OF THE DFM PRINCIPLE IN DESIGN
ACTIVITIES
Process capabilities and process costing evaluations
When a designer is looking for
convenient manufacturing processes
to produce the parts, he would like to
have access to fast information
sources giving the capabilities and
the cost of the different technologies.
Unfortunately, such documentation is
not commonly available and, in the
case of costing, is not very reliable.
General information about
characteristics of processes such as
quantity, weight, size, form, nature of
material, accuracy, rough blank
preparation, etc. can be found in
textbooks or manuals (e.g. in G.
Halevi and R.D. Weill, HAL 95). But,
precise information such as the
relation between surface finish and
cost is not commonly available
(Figure 1)

5. A.W.J. Chisholm (CHI73) has made a good analysis of the problem and has
pointed to the difficulty in costing information because of the different accounting
policies in industry. A good contribution to enrich the available information has been
given by CIRP (International Institution for Production Engineering Research) in
publishing case studies on the relation between function and surface finish (CIRP
Annals, 1986, Vol. 35/2) and should be followed by other similar actions.
In principle, functional dimensions and tolerances defined by design
requirements, should be directly transferred to manufacturing dimensions and
tolerances. The dimensions manufactured as direct dimensions have the largest
tolerances and are optimal for the economy in production.
However, for reasons of commodity or to avoid resetting, it is often decided to
transfer dimensions and tolerances to manufacturing conditions, and, consequently
tolerances have to be reduced. Unfortunately, because of the transfer of tolerances,
some parts, rejected by inspection in manufacturing, are nevertheless acceptable
functionally. It is therefore necessary to make a final functional inspection to regain the
correct parts, or to use statistical methods with a small risk of accepting bad parts.
A good example of application of
the principle of DFM to dimensioning and
tolerancing is given by L.E. Farmer et al.
(FAR 91) for a gear pump assembly
(figure 2). In this case, the fits between
the shafts (item 2 and 4) and the bearing
holes in the pump body (item 8) and in
the cover (item 5), as well as with the
gears (item 1), are preferably taken from
the ISO (International Standards
Organisation) standard system. This
choice guarantees the function as well
as the manufacturability. For the
assembling of the cover (item 5) with the
body (item 8) and with the shafts (item 2
and 4), an interesting solution has been
imagined which guarantees the function
(fitting), the manufacturing (boring holes)
and the inspection (checking of the
position of the holes). It consists in
locating the shaft bearing holes (3,4 and
5,6) and the 6 securing screws (item 15)
by using dowel pin holes (item 7) for manufacturing and inspection. The final checking
is carried out by a functional gauge which is designed by application of he Maximum
Material Condition (MMC) allowing enlargement of tolerances and use of simple

6. gauges. In the Technion, emphasis has been put on the introduction of advanced
courses on computerised dimensioning and tolerancing.
Advanced technologies intended to improve DFM
The advent of the computer has generated many new technologies intended to
improve the liaison between design and manufacturing.
Powerful CAD/CAM systems are now on the market for good geometrical
representations of mechanical parts. However, concerning design functions such as
dimensioning, their capabilities are still limited. More routine activities, such as
dimensioning of standard structures, simple tolerance chains, retrieval of standard
parts, of preferred limits and fits, of preferred sizes of process capabilities of costing for
alternatives, etc. can be easily implemented in CAD/CAM systems. More advanced
CAD/CAM programs "DFM" oriented, have been proposed recently by B. Charles, A.
Clement, et al. (CHA 89), in a tentative to integrate the assembly function and its
control by soft gauges.
In the field of tolerance transfer between design and manufacturing,
computerised modules exist now for linear dimensional chains (see FAI 86) and
extensions to 2D and 3D chains are in development.
In the field of process planning (Computer Aided Process Planning or CAPP)
which is the key link between design and manufacturing, progress has been recently
achieved, mainly by A.I. (Artificial Intelligence) applications. Development of more
simplified CAPP systems for design personnel would be welcome.
In the field of inspection, a breakthrough has occurred with the introduction of
the C.M.M.'s (Computerised Measuring Machines) which are able to check any directly
functional dimension and tolerance. The techniques of punctual probing used for the
evaluations in the C.M.M. raise however new problems related to the reliability of the
algorithms for geometrical error evaluations.
Finally, a completely new domain, called Fast Prototyping, based on
stereolithography, has recently penetrated manufacturing design. Its advantage is to
realise automatically the liaison design/manufacturing.
The introduction of the advanced techniques, relating design and manufacture,
is an integral part of the engineering education in the Technion.
IMPLEMENTATION OF DFM AT TECHNION
The prerequisite to be able to achieve ease of manufacturing in design is to be
familiar with the modern manufacturing tools and processes. One can implement

7. effectively only well-known techniques, the details of which have been used by the
designer, or demonstrated to him, before.
Therefore, a prerequisite to advanced design courses must be study and
practice of up-to-date manufacturing processes. Such study must include forming and
machining techniques of a variety of materials, the precision with which the different
parts can be machined or measured, assembly techniques, and many tools of
manufacturing engineering, usually categorized as production engineer's tools. Many of
these tools are computerized, and transfer the design into machine codes smoothly and
efficiently, with moderate involvement of the designer.
In modem design study (as in the Technion) the details of conceptual design are
being taught the first time, very early in the curriculum, long before studying
manufacturing processes. It is being taught and practiced again, as a full design cycle,
in the advanced stage of studying, so that at graduation, each student has had
experience of designing at least one project with a full design cycle, including the
manufacturing aspects. To be able to do that, the curriculum has to include a detailed
course on manufacturing technology, including hands-on experience in designing and
performing manufacturing steps, in different techniques, early in the second year of
study. When the students will reach the stage of designing complete projects, they will
have already learned DFM.
As any design education, so DFM, can be taught only by doing, i.e. by designing
the detailed manufacturing process of simple and more complicated parts, and
afterwards getting feed-back by seeing them actually being manufactured. The many
available computer programs, that simulate the machining process, and show how the
part looks like in the different manufacturing stages, and in some degree the fast
prototyping techniques that are being developed, can be used instead of actual
machining, but this substitute is good only to a certain degree, and the real machining
has to be used, at least partially. The computer simulations are efficient tools for people
who know the manufacturing techniques well, not for those who are learning those
techniques.
In several larger industrial companies, the university DFM education is not being
considered as enough for efficient novel design. One of the authors, that worked for a
major aerospace R&D company (The RAFAEL Missile Div.) as a manager of a design
department used to send his new engineers to work in the main company's workshop,
in manufacturing process design, for five to six weeks. Only then would the new recruit
start work at his/her assigned task. The results were always good, and the projects
designed by engineers that had the short workshop training, were better "designed for
manufacturer, than work by other engineers. In one special case, the new candidate
had to spend six full months working for the workshop, getting much wider
manufacturing practice. In retrospect, it came out that this time was being well spent,
and this individual became one of the most efficient design engineers.

8. Finally, the new concept of Concurrent Engineering (CE) can help in educating
engineers for DFM. In CE, one has to take into consideration all the important issues,
including DFM, in the earliest stages of design, namely in the conceptual design. In this
stage one has to evaluate the different proposed solutions, produce new versions and
choose the best one, so that when reaching the details design stage, manufacturing will
be efficient and economical.
CONCLUSION
Industry would like to receive from the design department a product which is
right the first time it is manufactured. To this end, the concept of "Design for
Manufacturing" (DFM) has been coined and many approaches have been suggested
for its application. It seemed therefore timely to introduce its philosophy in the
education curriculum of future manufacturing engineers. The Technion for its part, is
striving to implement DFM by giving a parallel education in design and manufacturing
based on extensive project work, hands-on sessions in processing, and many computer
based courses on technologies combining design and manufacturing. The success of
this approach is based on qualified teachers in both domains.
BIBLIOGRAPHY
CHA 89 B. Charles, A. Clement, et al., Toward a Computer Aided Functional
Tolerancing Model, 1st CIRP Seminar on Computer Aided Tolerancing.
December 11-12,1989, Jerusalem, Israel
CHI 73 A,W.J. Chisholm, Design for Economic Manufacture, Annals of CIRP vol.
22p, 1973.
FAI 86 D. Fainguelernt, R. Weill and P. Bourdet, 1986, Computer Aide Tolerancing
and Dimensioning in Process Planning, Annals of CIRP, Vol. 35/1.
FAR 91 L.E. Farmer, C.A. Gladman, K.H. Edensor, The scope of tolerancing
problems in engineering, CIRP Seminar on Computer Aided Tolerancing,
Penn State University, May 16-17, 1991.
HAL 95 G. Halevi, R.D. Weill, 1995, Principles of Process Planning, A Logical
Approach, Chapman and Hall, 1995.