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.