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CHAPTER 1
INTRODUCTION
1.1 QUALITY
Quality is an important aspect of any manufacturing process. Only
high quality products can survive in the market. The customer not only wants
quality, precision and trouble free products, but also wants them at an attractive
price. The American Society for Quality Control (1983) defines quality as “The
totality of features and characteristics of a product or service that bear on its
ability to satisfy [a user’s] given needs”.
When a product consists of two or more components, then the
quality of that product depends upon the quality of assembly. Assembly in the
manufacturing process consists of putting together all the component parts and
sub assemblies of a given product, fastening, performing inspections and
functional tests, labelling, separating good assemblies from bad, and packaging
and or preparing them for final usage.
The quality of an assembly depends on the quality of the parts being
assembled. The mating parts will have many quality characteristics. The mating
part quality characteristic that contributes for the assembly decides the quality
of that assembly. The quality of the assembly depends on the resulting
clearance or interference between the mating parts. The resulting clearance or
interference is the result of variation in the mating part quality characteristics.
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Therefore, the variations in mating part quality characteristics play a major role
in the quality of assembly.
In nature, two extremely similar things are difficult to obtain. If at
all we come across exactly similar things, it must be only by chance. This fact
holds good for production process as well. No production process is good
enough to produce all items of products exactly alike. In all manufacturing
process.-manufacturing of components with zero variation is practically not
possible. All the components in a same manufacturing line will differ from one
or the other. Therefore, the variability is inevitable in any manufacturing
process.
Variation in a product’s performance is an important aspect of
product quality. Off-line quality control methods reduce performance variation
and hence the product’s lifetime cost. A quantitative measure of performance
variation is the expected value of monetary losses during the product’s life span
due to this variation.
1.2 ASSEMBLY CONCEPTS
1.2.1 Design for manufacturing and assembly
Each part or component of a product must be designed so that it not
only meets design requirements and specifications, but also can be
manufactured economically and with relative each. It would improve the
productivity and allows the manufacturer to remain competitive
(Kalpakjian.1995) .
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Design for manufacture and assembly (DFMA) is a comprehensive
approach to production of goods and integrates the design process with
materials, manufacturing methods, process planning, assembly, testing, and
quality assurance. In order to effectively implementing DFMA, the designers
must have the knowledge of characteristics such assembly variability in
machine performance, surface finish and dimensional accuracy of the work
piece, processing time, effect of process method on part quality, capabilities and
limitation of materials. Establishing a quantitative relationship is essential in
order to optimize the design for ease of manufacturing and assembly at
minimum production cost. DFMA recognizes the inherent interrelationships
between design and manufacturing. After individual parts have been
manufactured, they are assembled in to a product.
1.2.2 Assembly process
Assembly is unique in comparison with the methods of
manufacturing such assembly machining, grinding and welding in that most of
these processes involve only a few disciplines and possibly only one. Most of
these operations cannot be performed without the aid of equipment, and thus
the development of automatic methods has been necessary rather than optional.
Assembly, on the other hand, may involve in one machine all the fastening
methods, such as riveting, welding, screw driving, and adhesive application, as
well as automatic parts selection, probing, gauging, functional testing, labelling
and packaging. The state of the art in assembly operations has not reached the
level of standardization; much manual work is still being performed in this area.
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Consideration in assembly process
a) Design of the product
b) Producibility
c) Concept
The design of a part and its component will have more effect on the
parts manufacturing costs than all the equipment and processing put together.
Unfortunately, product design is usually concerned primarily with functional
performance, and the “producibility” of the product is secondary or neglected
altogether. The state of art advances, and then the management must take steps
to ensure optimum compromise between functions and manufacturing cost is
reached.
The next important consideration in assembly process is concept.
Many factors influence this consideration, such as volume of parts per time
unit, product life, frequency of design changes, available labour volume and
costs, management attitudes, and competitive pressures. The variety of
available concepts expands continuously with the state of the art. The major sub
divisions in this concept are
i. Continuous assembly (bottling, cigarette manufacturing)
li. Intermittent assembly (indexing units)
Each of these subdivisions mentioned above offers the choice of
single station, multiple rotary stations, multiple in-line machines, “power and
free" conveyor design which permits accumulation of parts with a combination
of intermittent and continuous operation.
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1.2.3 Assembly systems
(i) Manual assembly lines
(ii) Automated assembly
Manual assembly lines, or, more generally, manual flow lines, are
used in high-production situations where the work to be performed can be
divided into small tools (called work elements) and the tasks assigned to the
workstations on the line. One of the key advantages of using manual assembly
line is specialization of labour. By giving each worker a limited set of tasks to
do repeatedly, the worker becomes a specialist in those tasks and is able to
perform them more quickly and more consistently.
Automated assembly refers to the use of mechanized and automated
devices to perform the various functions in an assembly line or cell. Much
progress has been made in the technology of assembly automation in recent
years.
Today there are number of equipment builders, and some captive in-
house organizations, capable of planning and developing a complete automated
manufacturing systems by any one of the known concepts and utilizing most of
the established techniques. It can effectively minimize the overall production
cost. Some of this progress has been motivated by advances in the field of
robotics.
Parts may be assembled by manual (hand) or by automatic equipment
(robots), the choice depends on factors such as complexities of the product, the
number of parts to be assembled, the protection required to prevent damage (or)
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scratching of finished surfaces of the parts, and the relative costs of labour and
machinery required for automated assembly. An analysis of the design should
first be made with regard to the appropriate and economical method of
assembly.
1.2.4 Selective assembly
In interchangeability, the parts randomly selected will fit properly
with any randomly selected mating component. In some cases this random
assembly or full interchangeability is not found to be achieved. For e.g., fit a
part at its low limit is assembled with a mating part at its high limit, the fit so
obtained may not fully satisfy the functional requirements of the assembly. Also
machine capabilities are sometimes not sufficient to satisfy the needs of random
assembly. Complete interchangeability in those cases, however, is obtained by
selective assembly.
Normally in selective assembly, the components are put into groups
according to size and then assembled with mating components also classified
according to size in the same number of groups. Corresponding groups are then
expected to assemble and function properly.
1.2.5 Interchangeable systems
The object of all modern methods of manufacturing is to produce
parts of absolute accuracy. But it is not always possible, particularly in mass
production, to keep the exact measurement. Given sufficient time, any operator
could work to and maintain the sizes to within a close degree of accuracy, but
there would still be small variations. It is known that if the deviations are within
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certain limits, all parts of equivalent size will equally fit for operating in
machines and mechanisms. Certain deviations are, therefore, recognized and
allowed to ensure interchangeability of mating parts, coupled with the designed
degree of tightness or looseness on assembly. When a system of this kind has
been worked out, so that one component will assemble correctly with another
mating component, both being chosen at random, the system is called an
interchangeable system, sometimes called a limit system or a system of limits
and fits.
Interchangeability of their parts is, therefore, a major pre-requisite for
economic production, operation and maintenance of machinery, mechanisms,
and instruments. It is by interchangeable spare parts that various machines,
machine tools, tractors, motor cars, air planes, and many others, can be
dismantled for replacement of work parts in service conditions, in the field, and
also in many local work shops with least possible loss of time. If
interchangeability is not achieved, selective assembly will be required, that is
each part must be selected to fit its mating part.
1.3 INTERCHANGEABLE MANUFACTURING
1.3.1 Interchangeability
The term interchangeability, as used here, refers to absolute
interchangeability. In this sense, interchangeable parts are parts that are so
made that they can be assembled interchangeably after final inspection without
machining or fitting, and any possible combination of these parts will assemble,
interchange, and function properly. To insure this end, the extreme limits
permitted must be constantly checked against each other.
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Complete interchangeability means that all parts will assemble even
though all the parts are at their extreme limits. There should be no rejected
assemblies because of improper tolerances, and the assembly time will be
reduced to a minimum. Where service wear is not a factor, interchangeability in
the field is also attained. Where functional specifications of the product allow
reasonable latitude on fits, complete interchangeability can often be achieved at
little or no additional cost provided that the tolerances of the parts are correctly
specified
1.3.2 Interchangeable Manufacturing
Interchangeable manufacturing consists of machining the component
parts of a given mechanism in a manufacturing department with in such limits
that they may be assembled in the assembling department without detriment to
the functioning and without machining. The advantages of such a method of
manufacture are self-evident, and need not be dwelt upon further. It is obvious
that with proper equipment and control, the component parts of a mechanism
can thus be manufactured in large quantities at a low direct labor cost.
In interchangeable manufacturing, the minimum clearances should be
as small as the assembling of the parts and their proper operation under service
conditions will allow. The maximum clearances should be as great as the
functioning of the mechanism permits. The difference between the maximum
and minimum clearances establishes the sum of the tolerances on the
companion surfaces.
In practice, which is correct for selective assembly, of making
tolerances represents the normal variation of the manufacturing on an
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interchangeable basis. If such a practice adds nothing to the expense of
production, there is no harm in employing it; but too often it imposes,
unnecessary refinement in manufacture, as in almost every case, the closer the
tolerances the more exacting and expensive will be the manufacturing
processes. With selective assembly manufacturing, on the other hand the closer
the tolerances, the fewer the subdivisions in the size that will be required, and
the smaller the stock of parts it is necessary to carry. This introduces a factor in
selective assembly manufacturing which is not present in interchangeable
manufacturing. The economical balance between the increased cost of
manufacturing to closer tolerances and the decreased cost of investment
represented by a smaller stock of different sized parts establishes the proper
course to follow when manufacturing on the basis of selective assembly.
Ultimate economy here as elsewhere, is the main end sought.
1.3.3 Interchangeable assembly
Mating parts constitute a pair of assembly called male and female
parts. We shall take a hole (female) and a shaft (male) assembly for analysis.
The process capability of shaft is 6gs and the process capability of hole is 6oh.
In interchangeable manufacturing, the mating parts are manufactured
and assembled at random. The maximum clearance (Cmax) in the assembly is
the difference between the maximum dimension of the hole (5hmax) and the
minimum dimension of the shaft (§smm). The minimum clearance (Cmin) is the
difference between the minimum dimension of the hole (5hmin) and maximum
dimension of the shaft (5smax). This is clearly shown in figure 1.1.
The clearance variation in assembly (8cr) is the difference between the
maximum clearance and minimum clearance (8tT = Cmax - Cmin). The clearance
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Hole tolerance
MaximumClearance
Figure 1.1 Interchangeable assembly
variation (§cr) cannot be less than the sum of hole tolerance (6ah) and shaft
tolerance (6os). From this it is clear that the variation in clearance (5cr) depends
on the hole tolerance and shaft tolerance. If clearance variation (Scr) is to be
minimized, it may require a better process or a better machine to reduce the
manufacturing tolerance (process capability). This may require a high initial
investment, which may not be possible under economical considerations. In
some high precision assemblies it may not be possible to have a closer
tolerances with certain limits. In such cases, it is possible only by selective
assembly.
1.4 SELECTIVE ASSEMBLY
Selective assembly is the method of obtaining high precision
assemblies from relatively low precision components. The problem of
producing mating parts to have specified clearance while assembling pose a
great challenge to the engineers. Mating parts constitute a pair of assembly
called male and female parts like in (1) fuel injection pump plunger and
cylinder, (2) connecting rod split bearings, (3) piston and piston bearings and
generally all pin and bush assemblies.
It is sometimes found that it is not economic to manufacture parts to
the required high degree of accuracy for their correct functioning. Instead, they
are made in an economic manner, measured to the required high accuracy and
graded or sorted into groups each of which contains such parts of the same size
to within close limits. They are then assembled with mating parts which have
been similarly graded, i.e., one or more of the components concerned are first
manufactured to larger tolerances than the accuracy demanded by
interchangeability; the parts are then measured, graded into groups according to
size; and finally corresponding groups are assembled together.
A relatively smaller clearance variation may be achieved than in
interchangeable assembly, with the components manufactured with wider
tolerance. In selective assembly, the mating parts are partitioned to form
groups with smaller tolerance and then the corresponding groups are assembled
interchangeably.
1.4.1 Manufacturing for selective assembly
Selective assembly refers to a method of manufacturing similar in
many of its details to interchangeable manufacturing, in which component parts
are sorted and mated according to size and assembled or interchanged with little
or no machining. Companion parts made to the extreme limits are not supposed
to interchange. For instance, a maximum male component will not assemble
with a minimum female part. However, the maximum male and female, or the
minimum male and female must interchange.
?
1.4.2 Interchangeable manufacturing Vs Selective assembly
Selective assembly manufacturing (Earle Buckingham, 1941) is a
method of manufacturing which is similar in many of its details to
interchangeable manufacturing. In the selective assembly, the component parts
are sorted and mated according to size, and assembled or interchanged with
little or no machining. Because of their similarity, the two methods are often
confused, and this has led to misapprehensions in regard to the principles of
interchangeable manufacturing. The chief purpose of manufacturing, by
selective assembly is the production of large quantities of duplicate parts as
economically as possible, within such limits that they may be assembled
without further machining.
The general principles of design are identical for manufacturing on
an interchangeable basis and on a selective assembly basis. The functional
design must first be made and tested, then the manufacturing design developed.
This modifies the inventive design so that the product may be manufactured on
a large scale in an economic manner.
1.4.3 Clearances and tolerances in selective assembly manufacturing
The matter of clearances and tolerances is quite different when
manufacturing on an interchangeable basis from when manufacturing on the
basis of selective assembly. In interchangeable manufacturing, the minimum
clearances should be as small as the assembling of the parts and their proper
operation under service conditions will allow. The maximum clearances should
be as great as the functioning of the mechanism permits. The difference
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between the maximum and minimum clearances establishes the sum of the
tolerances on the companion surfaces.
The practice, which is correct for selective assembly of making
tolerances represent the normal variation of the manufacturing on an
interchangeable basis. If such a practice adds nothing to the expense of
production, there is no harm in employing it; but too often it imposes,
unnecessary refinement in manufacture, as in almost every case, the closer the
tolerances the more exacting and expensive will be the manufacturing
processes. With selective assembly manufacturing, on the other hand the closer
the tolerances, the fewer the subdivisions in the size that will be required, and
the smaller the stock of parts it is necessary to carry. This introduces a factor in
selective assembly manufacturing which is not present in interchangeable
manufacturing. The economical balance between the increased cost of
manufacturing to closer tolerances and the decreased cost of investment
represented by a smaller stock of different sized parts establishes the proper
course to follow when manufacturing on the basis of selective assembly.
Ultimate economy here as elsewhere, is the main end sought (Earle
Buckingham, 1941).
1.5 CONCLUSION
When a product consists of two or more components, then the quality
of that product depends upon the quality of assembly. The quality of an
assembly depends on the quality of the parts being assembled. Therefore the
variations in the dimensional distribution of the mating parts quality
characteristics play a major role in the quality of assembly. In interchangeable
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assembly this results in higher variation in assembly clearance. In selective
assembly this results in a greater number of surplus parts.
In selective assembly the mating part dimensional distributions are
not similar. So when the mating parts are partitioned to form selective groups,
the number of parts in the corresponding groups are not similar and will result
in surplus parts. In the methods so far suggested by the earlier researchers, the
group tolerance for selective assembly was not designed to meet the clearance
specifications. In the following chapters new methods are proposed to design
the group tolerances to meet the clearance specifications and to minimize the
surplus parts.
Chapter 2 gives the basic concepts for analysing the variation in the
quality characteristics of a product. Off-line quality control, tolerance design,
process capability analysis are the concepts discussed.
The literature review for selective assembly is given in chapter 3. The
selective assembly literature is slight (Allen Pugh, 1986). The work done by
earlier researchers in this area is classified in four groups. From the literature
review it is clear that the clearance specification was not considered so far for
designing the group tolerance in selective assembly.
In chapter 4. a new grouping method has been proposed. The group
tolerance of selective assembly is designed based on the clearance
specifications. If the mating parts are partitioned for selective assembly based
on this method, the resulting assembly will meet the clearance specifications
and surplus parts will also be minimum. The method is applied for a case
example and validated with the data obtained from MATLAB software.
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Chapter 5 gives a different method for minimizing the surplus parts
in selective assembly. The mating part with smaller standard deviation is
manufactured with shift in manufacturing mean and the resulting standard
deviation of the part population is almost same as that of the other mating part.
So the surplus parts are minimized. The method of designing the manufacturing
mean for the shift and the method of calculating the number of parts to be
manufactured in the corresponding mean is also given.
In chapter 6 the new grouping method proposed in chapter 4 is
applied with some modifications for a complex assembly - a ball bearing -
consists of three mating parts, inner race, ball and outer race. The method is
analyzed with a case example and validated with the data obtained from
MATLAB software.
The conclusion is given in chapter 7 and the scope for further work
is given in chapter 8.
The data obtained from MATLAB software for case analysis and the
results are given in the appendices.