Introduction To DFMA: Objectives of This Course
Introduction To DFMA: Objectives of This Course
Introduction To DFMA: Objectives of This Course
What is DFMA?
The concept of DFM (Design for Manufacture) is not new, it dates back as early as 1788 when
LeBlanc, a Frenchman, devised the concept of inter-changeable parts in the manufacture of
muskets which previously were individually handmade. DFM is the practice of designing
products keeping manufacturing in mind. Design for manufacture means the design for ease
of manufacture for the collection of parts that will form the product after assembly. Similarly
DFA is called Design for Assembly. DFA is the practice of designing product with assembly in
mind. Design for assembly means the design of the product for ease of assembly. So design
for Manufacture and assembly is the combination of DFM and DFA as shown in Figure M1.1.1
Design for
Assembly (DFA)
History of DFMA
1. Eli Whitney is an inventor from America used some DFM techniques in earlier times
before the term DFM came in to existence. Whitney incorporated the concept of
interchangeable parts for manufacturing musket for U.S. government. Prior to this
innovation, each craftsman was responsible to manufacture the complete product by
using saw and files to shape each part and fit them together.
2. Henry Ford an American industrialist was renowned for his advanced and extensive use
of assembly lines. The manual assembly operation was broken down into small chunks of
repetitive work that could be carried out at high efficiency. Ford in his book My Life
and Work described about the successful model T car that includes simplicity in
operation, absolute reliability and high quality in materials that used in that model. The
concept used at that time by Ford is now referred as DFM.
3. General Electric used value analysis techniques in the late 1940s. With the help of value
analysis techniques, it is possible to find the cost of a product and obtaining the design
alternatives for the product at the lowest cost. The philosophical approach of value
analysis is through questioning and comparing the value and cost of each features and
each element of a product design.
4. The book Metal Engineering Processes edited by Roger W. Boltz is one of the books
from a series of handbook published by ASME in 1941. This book provides a series of
guidelines to designer in enhancing the manufacturability of metal components made
with a number of manufacturing processes such as casting, forging, extrusion, machining,
joining, finishing etc. Though Boltz used the word DFM, he is the first person arrange
and plan DFM methodology.
5. In the year 1960, people started to use the terms producibility and manufacturability and
about 1985, design for manufacturability and its short form DFM were widely used.
6. Geoffrey Boothroyd and A.H. Redford studied automatic assembly and provided various
design guidelines to make the assembly process easier.
7. In the recent years various trade association and vendors of parts are issuing booklets to
the product designers providing a series of guidelines and tolerance and materials
recommendations for parts. These kinds of publications have provided valuable and
authorative assistance to product designers.
Suggestions for
simplification of product
structure
Suggestions for more economic
materials and processes
Prototype
Production
Figure M1.1.2: Common steps taken in a DFMA study (Source: G. Boothroyd, P. Dewhurst and
W. Knight Product Design for Manufacture and Assembly, 2nd edition)
DFMA not only reduces the manufacturing cost of the product but it helps to reduce the
time to market and quality of the product.
DFMA provides a systematic procedure for analyzing a proposed design from the point
of view of assembly and manufacture.
Any reduction in the number of parts reduces the cost as well as the inventory.
DFMA tools encouraged the dialogue between the designer and manufacturing engineer
during the early stages of design.
7. We have been doing it for years: Sometimes industry uses the design for producibility
concept to fine-tune the design. There is a misconception that they are doing the similar
practice of DFMA.
8. It is only value analysis: The objective of DFMA and value analysis are same, however
DFMA is used at the early stages of design and can be used in every stages of design.
9. DFMA is only one among many techniques.
10. DFMA leads to products that are more difficult to service.
11. Prefer design rules: Sometimes design rules guide the designer in the wrong direction.
12. Refuse to use DFMA: Individual doesn't have the incentive to adopt the new technology
and use the tools available.
Course Overview
In this course, the following topics shall be covered through different modules.
Various types of materials, its classification, suitable materials for product design and
various methods of material selection, various mechanical properties of material (Module
I).
Various casting design, machining design, designing of formed components (Module II,
Module III, and Module IV).
Various design recommendations for cleaning, design for polishing, plating and coating,
and Heat treatment and various design recommendations (Module V).
Various design recommendation for permanent joining such as welding, soldering and
brazing (Module VI).
Various design recommendation for riveting, screw fastening etc. (Module VII).
Lastly, the course is completed with an introduction to CAD, various types of geometric
model, different types of features, procedure for feature extraction from part and
assembly model (Module VIII).
What is Manufacturing?
The English word manufacture is several centuries old. The term manufacture comes from two
Latin words, manus (hand) and factus (make). As per oxford English dictionary manufacture refers
to make or produce goods in large quantities, using machinery.
In assembly operation two or more components are joined to create a new entity. The new entity
is called assembly, subassembly based on its state in the product. If the entity is an intermediate
state of the product, it is called subassembly. Some other terms are also referred based on the
joining process. The assembly created by welding operation is called weld met.
.
Figure M1.2.2: Definition of manufacturing in terms of economic value
Operation
An operation is a distinct action performed to produce a desired result. Operations can be
1) Materials handling and transporting
2) Processing
3) Packaging
4) Inspecting and testing
5) Storing
Treatments
Treatments operate continually on the work piece. These modify the product in process without
tool contact. Some examples include heat treating, curing, galvanizing, plating, finishing,
(chemical) cleaning and painting. These are often done in large tanks or furnaces or rooms, away
from workers as they can be harmful.
Rapid prototyping
Heat treating
Other
The complexity of the manufacturing system design where choices of system design trade off with
parts variety.
Summary
Manufacturing becomes successful by understanding how the system works, how goods are
controlled, the decision making at the correct level. Engineers must possess a broad fundamental
knowledge of design, metallurgy, processing, economics, accounting and human relations.
Classifying materials
In manufacturing of a product, a raw material is converted into a finished product. There are various
types of classifications available in the literature. Materials come under three basic categories:
metals, ceramics and polymers. A mixture of these fundamental types forms a composite.
Two classification schemes are shown below:
Type 1 classification
Engineering materials can be classified into six broad families as shown in Figure M1.3.1.
Metals
Polymers
Elastomers
Ceramics
Glasses
Hybrid composite materials
Type 2 classification
In this type of classification, engineering material can be classified into two categories: Metals and
non-metals as shown in Figure M1.3.2. Again non-metals are classified into organic & inorganic
as shown in Figure M1.3.3 & Figure M1.3.4 respectively. Metals can be classified into two
categories: ferrous and non-ferrous metals. Ferrous metals contain iron in it. Pure iron has limited
use but when alloyed with carbon it has a great commercial value. Some of the common alloys of
iron are steel and cast iron which contain different percentages of carbon in it. Steel contains 0.02%
to 2.11% of carbon and rest iron, manganese, chromium, nickel, and molybdenum in it. Cast iron
contains 2% to 4% of carbon in it and the rest are iron & silicon.
Non-Ferrous metals contain other metallic elements other than iron in it. They include metals
aluminum, copper, gold etc.
Ceramics are compounds. These compounds contain a metallic and a non-metallic part. The
non-metals can be oxygen, nitrogen and carbon. Examples of ceramics include carbides, clay, silica,
alumina etc.
Polymers are compounds which consist of repeating units in them called as mers. Mers
share electrons to form very large molecules - usually of carbon and some other elements like
oxygen, hydrogen, nitrogen, chlorine etc. Polymers are further classified into thermosetting,
thermoplastics and elastomers. Some of the common polymers are polythene, PVC, etc.
Composites consist of two or more phases of materials. The phases are processed separately
and then bonded together to achieve properties superior to the constituents. Some of the materials
used in the phases are wood or fiber etc. which are a homogenous mass bonded together with epoxy.
Some of the common applications of composites are aircraft, tennis rackets, car bodies, etc.
Selection by analysis
Selection by synthesis
Selection by similarity
Selection by inspiration
Figure M1.3.5: Graphical representation of the connection among the elements for material
selection.
1. Selection by analysis
The inputs to this method are the technical requirements. The analysis proceeds in four steps:
Translation of requirements: It is often expressed initially in non-technical terms into statement
of objectives and constraints the design must meet.
Analysis of the component for which a material is sought, identifying performance matrices and
expressing these as equations that measure performance.
Identification from these equations of the material properties that determine performance.
Figure M1.3.6: Selection by analysis requires the selection of materials from a database
of materials. (Source: Materials and Design by Mike Ashby and Kara Johnson, 2nd Ed)
Screening of a database of materials and their properties, eliminating those that fail to meet
the constraints, and ranking those that remain by their ability to maximize the performance
matrices.
The solution is shown by the white circle that satisfies all the constraints as shown in Figure
M1.3.6.
Figure M1.3.7: Selection by synthesis. (Source: Materials and Design by Mike Ashby and Kara
Johnson, 2nd Ed)
2. Selection by synthesis
This process is experimental and depends on experience of the designer. The inputs here can include
the design requirements expressed as features showing intentions, aesthetics and perceptions.
Basically the solution will depend on previously solved problems that have some features common
with the problem at hand. While this may be seen as a drawback since the method uses past
experience, it encourages a kind of cross pollination where developments in one field can be
adapted for use in another. This methodology is called technology coupling.
Circle colored
in orange represents a synthesis of solution from the three colored circles as shown in Figure
M1.3.7.
3. Selection by similarity
A substitute material may be sought when the existing material is no longer available or fails to
meet a design requirement. In such cases an established material can be used instead of the existing
one, simply because it may have the right mix of attributes and may be meeting the design
requirements.
A common approach is "capture-edit-search" wherein we first capture the attribute profile of the
incumbent, then editing or relaxing the design constraints on non-critical attributes to include a
wide range of materials and finally searching for the substitutes that meet the constraints.
Find substitutes by matching attributes profiles. Colored circles have attributes that match as nearly as
possible attributes of the white circle (the incumbent material) as shown in Figure M1.3.8.
Figure M1.3.8: Selection by similarity. (Source: Materials and Design by Mike Ashby and
Kara Johnson, 2nd Ed)
Figure M1.3.9: Selection by inspiration (Source: Materials and Design by Mike Ashby and
Kara Johnson, 2nd Ed)
4. Selection by inspiration
Designers usually get their ideas from other designers, colleagues and from their environments.
And many ideas are triggered by accident, perhaps by some chance encounter with someone or
some situation. The encounter thus becomes inspiring and provokes creative thinking. Such
encounters can include interaction with materials, with products or by browsing books. Circles
represent solutions inspired by a random walk through a collection or catalog of materials and
products as shown in Figure M1.3.9.
The behavior of material is mainly determined by various mechanical properties of the material
when subjected to different loading conditions. Such properties mainly include Youngs
modulus, various types of strength of the material, hardness, ductility etc. and are found to be
very important both for design & manufacturing viewpoint. The design engineer should also
consider the manufacturing constraints during the design of a part.
Three basic types of stresses which are produced when a material is subjected to various loading
conditions are
a) Tensile Stress
b) Compressive Stress
c) Shear stress
Tensile properties
Tensile strength is defined as the ability of a material to support axial load without rupture and is
determined through the tensile test. When equal and opposite forces are applied simultaneously
at both the ends that pulls the material, it tries to elongate it and the diameter reduces. The test
specimen and general setup has been shown in the Figure M1.4.1.
(a)
(b)
Figure M1.4.1: (a) Test specimen (b) Setup for the test
Due to the stretching of the specimen in tensile test, the initial test specimen length L0 is
increased to L and area A0 is reduced to A. The tensile testis carried out at a constant cross head
speed and extensometers of required gauge length are used to capture the elongation depending
on the requirement. In this process the material first elongates, then necking occurs & the
fracture is produced. The necking phenomenon is observed mostly in tensile test and it also
mainly depends upon the material that isused for test. If the material is brittle there is no chance
of necking. From the measured data from the tensile test, the stress- strain curve is plotted.
Mainly there are two type of stress-strain curves which are described below:
1) Engineering stress-strain
2) True stress strain
Engineering stress-strain
Engineering stress-strain is mainly illustrated by taking original cross section and original length.
The stress-strain diagram for a metal is shown in Figure M1.4.2.
F
A
e=
LL
L
The amount of strain that the material sustain before failure is an important property in
mechanical engineering, which is used specially in manufacturing. This property is called
ductility and is measured in terms of elongation or area reduction.
EL =
L L
L
AR =
A A
A
True Stress-strain
In the computation of engineering stress, the original cross sectional area has been used.
However, during the process of loading the area reduces. In the computation of true stress the
actual or instantaneous area is used. As the length increases, the cross sectional area decreases.
Hence the calculated stress value will be higher. The instantaneous load divided by instantaneous
cross-sectional area is called true stress.
=
F
A
A L =AL
=
F
F A
L
LL +L
=
=s =s
= s (1 + e)
A A
A
L
L
Similarly true strain offers a more accurate calculation of the instantaneous elongation per unit
length of the material.The true stress is generally increased rapidly than engineering stress once
the strain increases and the accordingly, the cross sectional of the specimen decreases.
dL
L
LL +L
= ln
= ln
= ln(1 + e)
L
L
L
The behavior of nearly all type of solid material are described by three types of stress-strain
relationship diagram as shown in Figure M1.4.3.
A) Perfectly elastic
The behavior of the material is absolutely defined by its stiffness. Such material directly
fractures without yielding when it reaches ultimate strength material. These materials are called
brittle materials. Examples of brittle material are ceramics, cast iron, etc. These materials are not
suitable for forming operation, where permanent plastic deformation is required to get the final
product.
B) Elastic and perfectly plastic
For this type of material, when the stress level reaches the yield point plastic deformation begins
at the same stress level. Metals behave in this mode, when they are heated to high temperatures.
This type of behavior occurs mainly at higher temperature doesnt strain harden rather it
recrystallize during deformation.
C) Elastic and strain hardening
This kind of material obeys Hookes law in the elastic region and begins to flow at its yield
point. During cold working, most of the ductile material behave in this manner.
Figure M1.4.3: The stress-strain relationship diagram for a) perfectly elastic b) elastic &
perfectly plastic c) elastic & strain hardening.
Compression Properties
Compressive test is performed to determine the compressive strength of the material. The
material is applied equal and opposite compressive load. Engineering stress is defined as
s=
F
A
Shear properties
Shear stress involves application of load parallel to the surface of material in opposite direction
as shown in Figure M1.4.5.
The shear stress is defined as
=
F
A
(a)
(b)
Hardness
Hardness is a measure of how resistant solid matter is to various kinds of permanent shape
change when a force is applied.
Vickers hardness test: It is easier to use in comparison to other hardness tests since the required
calculations are independent of the size of the indenter, and the indenter can be used for all
materials irrespective of hardness. The unit of hardness given by the test is known as the Vickers
Pyramid Number (HV).
In Vickers hardness test the surface is subjected to a standard pressure for a standard length of
time by means of a pyramid-shaped diamond. The diagonal of the resulting indention is
measured under a microscope and the Vickers Hardness value is read from a conversion table.
The Vickers number (HV) is calculated as:
HV = 1.854(F/D2)
Where F=the applied load,kgf
D= the area of the indentation, mm2
Brinell hardness test:It is widely used for testing metals and non-metals of low to medium
hardness. A ball shaped indenter made of cemented carbide is used for harder material in this
test.
Knoop Hardness Test: It is used for generally small & thin specimen. A pyramid-shaped
diamond indenter is used whose length-to-width ratio of about 7:1.
Rockwell Hardness Test: It is used for variety of material like carbide, ceramic, tool steel etc.
where a cone-shaped indenter, with diameter 3.2 mm is forced into the specimen using a minor
load of 10 kg & then a major load of 150 kg is applied, helping the indenter to penetrate into the
specimen a certain distance beyond its initial position. This extra penetration distanced is
converted into a Rockwell hardness.
There is a good correlation between hardness & strength for most metals as hardness is usually
based on resistance to indentation, which is a form of compression.
Brinell hardness (HB) shows a close correlation with the ultimate tensile strength (TS) for steel
is given below:
TS = 3.45 (HB)
6) Anodizing is a kind of
a) Surface treatment operation
b) Machining operation
c) Heat treatment operation
d) None of the above
6) A ceramic is a
a) mixture
b) admixture
c) compound
d) colloidal solution
7) Polypropylene is a
a) Thermoplastic polymer
b) Elastomer
c) Thermoset polymer
d) Both a and c
d) compressive stress
3) If stress values were measured during a tensile test, which of the following would have the
higher value: a) engineering stress or (b) true stress?
2) The plastic region of the stressstrain curve for a metal is characterized by a proportional
relationship between stress and strain: (a) true or (b) false?
4) The stress at which the extension of the material takes place more quickly as compared to
the increase in load, is called
a) Ultimate point b) yield point
c) Elastic limit
d) breaking point
Answers:
Introduction to manufacturing process
(1). d
(2). b
(3). b
(4). a
(5). d
(6). A
(2). b
(3). b
(4). b
(5). c