UNIT 5 PLANNING AND CONTROL FOR Planning and

Control for MASS

MASS PRODUCTION Production

Objectives
After completion of this unit, you should be able to:
• understand the nature of mass / flow production, identify the situations
under which mass production is justified and appreciate both the
desirable and undesirable features of mass production
• see how assembly lines and fabrication lines are designed, get an idea of
how modular production and group technology could be used to
advantage in mass production and understand the role of automation
including robotics in mass production.

Structure
5.1 Introduction
5.2 When to Go For Mass Production
5.3 Features of a Mass Production System
5.4 Notion of Assembly Lines and Fabrication Lines
5.5 Design of an Assembly Line
5.6 Line Balancing Methods
5.7 Problems and Prospects of Mass Production
5.8 Modular Production and Group Technology
5.9 Automation and Robotics
5.10 Summary
5.11 Key Words
5.12 Self-assessment Exercises
5.13 Further Readings

5.1 INTRODUCTION
Kinds of Production Systems: flow Shops, Job Shops and Projects

As you already know, production involves the transformation of inputs (such

as men, machines, materials, money, information and energy) to desirable
outputs in the form of goods and services. It is customary to divide
production systems into three categories: the flow shop, the job shop and the
project. The flow shop exists when the same set of operations is performed in
sequence repetitively; the job shop exists where the facilities are capable of
producing many different jobs in small batches; the project is a major
undertaking that is usually done only once. It consists of many steps that
must be sequenced and coordinated.

141
Operation The flow shop employs special purpose equipment (designed specifically for
Planning and
Control the mass-scale production of a particular item or to provide a special service).
The job shop contains general-purpose equipment (each unit is capable of
doing a variety of jobs). The project, like the flow shop, requires a sequence
of operations, except that the sequence lacks repetition. Each project
operation is unique and seldom repeated. For example, the production line for
automobiles is a flow shop; the machine shop that makes hundreds of
different gears in batches of 50 at a time is a job shop; building a bridge or
launching a satellite in space is a project.

This unit is concerned with the problems of mass production encountered in

flow shops. Batch production and its problems, job shops and projects are
discussed in further units of the Block.

Nature of Mass Production

It was Henry Word who in 1913 introduced the ‘assembly line’ and the
notion of ’mass production’. It is erroneous to think that mass production
means production in millions or for the masses, though this may be an
outcome. Mass production refers to the manner in which a product is
produced. This involves the decomposition of the total task into its minutest
elements (shown usually on a precedence diagram) and the subsequent
regrouping of these elements according to the norms of production. An
assembly line consists of work stations in sequence where at each work
station the above carefully designed portion of work is done. Mass production
requires that all like parts of an assembly line be interchangeable and that all
parts be replaceable, characteristics which permit production and
maintenance of large quantities.

The assembly line is a production line where material moves continuously at

a uniform average rate through a sequence of work stations where assembly
work is performed. Typical example of these assembly lines are car
assembly, electrical appliances, TV sets, computer assemblies and toy
manufacturing and assembly. A diagrammatic sketch of a typical assembly
line is shown in Figure l. The arrangement of work along the assembly line
will vary according to the size of the product being assembled, the precedence
requirements, the available space, the work element and the nature of the work
to be performed on the job.

WSn

WSi

142 Work Station i

Material movement between work stations could be manual, as for instance Planning and
Control for MASS
when operators sitting in a row pick up the part from the output of the Production
previous operator, work on it and leave it in a bin to be picked up by the next
operator; or through the use of conveyors which carry the part at a
predetermined speed so chat there is adequate time for each work station to
complete its allocated share of work. There are various types of conveyor that
are used in assembly lines; the most widely used are belt, chain, overhead,
pneumatic and screw conveyors.

It may be of interest to note that assembly lines could have varying degrees
of automation, starting from the purely manual on the one hand to the fully
automated line on the other. However, the underlying principle of the
assembly line and mass production remains unchanged, although the labour
content may be reduced through robotization.

5.2 WHEN TO GO FOR MASS PRODUCTION

It is generally agreed that mass production is justified only when
production quantities are large and product variety small. The ideal
situation for mass production would be when large volumes of one
product (without any changes in design) are to be produced continuously
for an extended period of time. Thus the rate of consumption (or demand)
of the product as compared to the rate of production decides whether
continuous or batch production is called for Obviously, only if the rate of
demand is greater than or equal to the production rate, mass or
continuous production could be sustained. if the rate of demand is less
than the production rate, batch production with suitable inventory
buildups could be resorted to.

Apart from the above consideration, the economics of the matter would have
to be evaluated before deciding as to whether an assembly line is justified or
not. This is Illustrated by the following example.

Example 5.1

As a manager of a plant you have to determine whether you should purchase

a component part or make it in the plant. You can purchase the item at Rs. 10
per piece. With an investment equivalent to an annual fixed cost of Rs 20,000
and a variable cost of Rs. 2.50 per piece an assembly line can be setup to
manufacture the part. A third option open to you is to make the part at
individual stations with an annual fixed cost of Rs. 10,000 and a variable cost
of Rs. 5 a piece. Assuming that the annual demand is expected to bt around
3500 units which alternative would you suggest?

Solution

The choice from the three alternatives (purchase, produce at individual

stations, or employ an assembly line for production) is simplified by plotting
143
Operation a cost vs quantity chart for these options. If Q is the quantity purchased or
Planning and
Control produced then total cost equals:

Rs. (IOQ), if the part is purchased; Rs. (10,000 + 5Q), if the part is made at
individual stations; and

Rs. (20,000 + 2.5Q) if the part is made on an assembly line. These cost
functions are plotted in Figure II and the break-even points at quantity levels
of 2000 and 4000 reveal the following decision rules:

For annual requirements in the range 0—2000, it is cheapest to buy,

For annual requirements' in the range 2000- 4000, it is cheapest to produce

on individual stations, and only for annual requirements of 4000 or more, is
an a assembly line justified.

Thus for an annual requirement of 3500, you should not recommend the
installation of an assembly line.

Fig II. Cost of Three Alternatives

5.3 FEATURES OF A MASS PRODUCTION

SYSTEM
A mass production system operating as a continuous floor line exhibits
certain desirable and undesirable features. These are summarised below:

Advantages

1) A smooth flow of material from one work station to the next in a logical
order. Although straight line flow is common, other patterns of flow

144
 exhibited in Figure Ill are also employed when constraints on space or Planning and
Control for MASS
movement so indicate. Production

Figure III: Kinds of Flow Patterns

We have already exhibited the following common flow patterns in Unit 4 of

Block I

a) I-flow. (b) L-flow. (c)U-flow. (d) S-flow. (e) O-flow.

2) Since the work from one process is fed directly into the next, small in
process inventories result.

3) Total production time per unit is short.

4) Since the work stations are located so as to minimise distances between

consecutive operations, material handling is reduced.

5) Little skill is usually required by operators at the production line; hence

training is simple, short and inexpensive.

6) simple production planning and control systems are possible.

7) less space is occupied by work in transit and for temporary

storage.

145
Operation Disadvantages
Planning and
Control
1) A breakdown of one machine may lead to a complete stoppage of the
line that follows the machine. Hence maintenance and repair is a
challenging job.
2) Since the product dictates the layout, changes in product design may
require major changes in the layout. This is often expressed by saying
that assembly lines are inflexible.
3) The pace of production is determined by the ‘slowest’ or ‘bottleneck’
machine. Line balancing proves to be a major problem with mass
manufacture on assembly lines.
4) Supervision is general rather than specialized, as the supervisor of a line
is looking after diverse machines on a line
5) Generally high investments are required owing to the specialized nature
of the machines and their possible duplication on the line.

5.4 NOTION OF ASSEMBLY LINES AND

FABRICATION LINES
lt is useful to consider two types of line balancing problems:

i) assembly line balancing, and

ii) fabrication line balancing.

The distinction refers to the type of operation taking place on the line to be
balanced. The term ‘assembly line’ indicates a production line made up of
purely assembly operations. The assembly operation under consideration
involves the arrival of individual component parts at the work place and the
departure of these parts fastened together in the form of an assembly or sub-
assembly.

The term ‘fabrication line’, on the other hand, implies a production line made
up of operations that form or change the physical, or sometimes, chemical
characteristics of the product involved. Machining or heat treatment would
fall into operations of this type.

Although there are similarities between the problem of assembly line

balancing and that of fabrication line balancing, the problem of balancing a
fabrication line or machine line is somewhat more difficult than the assembly
line balancing problem. It is not so easy to divide operations into relatively
small elements for regrouping. The precedence restrictions are usually tighter
in the fabrication line. An assembly operator may easily shift from one
assembly job to another, but a machine tool may not be utilised for a variety
of jobs without expensive changes in setup and tools.

Some methods by which the balance of fabrication operation times can be

achieved are as follows:
146
1) changing machine speeds Planning and
Control for MASS
2) using slower machines on overtime Production
3) providing a buffer of semi-finished parts at appropriate places
4) using mechanical device for diverting parts
5) methods improvement.

5.5 DESIGN OF AN ASSEMBLY LINE

The Broad Objective in Design

As you have just seen the two most important manufacturing developments,
which led to progressive assembly are the concept of interchangeable parts
and the concept of the division of labour. These permit the progressive
assembly of the product, as it is transported past relatively fixed assembly
stations, by a material handling device such as a conveyor. The work
elements, which have been established through the division of labour
principle. are assigned to the work stations so that all stations have nearly an
equal amount of work to do. Each worker, at his or her station, is assigned
certain of the work elements. The worker performs them repeatedly on each
production unit as it passes the station.

The assembly line balancing problem is generally one of minimizing the total
amount idle time or equivalentely minimizing the number of operators to do
a given amount of work at a given assembly line speed. This is also known as
minimizing the balance delay. ‘Balance delay’ is defined as the amount of
idle time for the entire assembly line as a fraction of the total working
time resulting from unequal task time assigned to the various stations.

Killbridge and Wester after studying the variation in idle times at stations
caused by different assembly line balances concluded that high balance delay
for an assembly line system for a specific product is caused by

i) wide range of work element times

ii) a large amount of inflexible line mechanisation and
iii) indiscriminate choice of cycle times.

However, as we shall see, the cycle time is often dictated by a specific

desired production rate, which may not lead to a low balance delay.

Division of Work into Parts: The Precedence Diagram

The total job to be done or the ‘assembly’ is divided into work elements. A
diagram that describes the ordering in which work elements should be
performed is called a ‘precedence diagram’. Figure IV shows the
precedence diagram for an assembly with 12 work elements. Note that tasks
2 and 4 cannot begin until task 1 is completed.

Moreover, there is no restriction on whether task 2 is done first or task 4.

These two tasks are un-related meaning thereby that they may be done in 147
Operation parallel or even with partial overlap. A task may well have more than one
Planning and
Control immediate predecessor. For example, in the precedence diagram of Figure IV
task 12 has 3 immediate predecessors and cannot begin until all the three
work elements 8, 9 and 11 are completed.

Duration
i

For example means element 3 and duration 6

Figure IV: Precedence Diagram for an Assembly with 12 Elements

The ordering dictated by the precedence diagram may be the result of

technological restrictions on the process or constraints imposed by layout,
safety or convenience. The precedence diagram forms the basis for the
grouping of work elements into work stations.

Grouping of Task for Work Stations end Efficiency Criteria

Depending on the desired production rate of the line, the cycle time (CT) or the
time between the completion of two successive assemblies can be
determined. This determines the conveyor speed in the assembly line or the
time allocated to each operator to complete his share of work in a manual
line.

The individual work elements or tasks are then grouped into work stations
such that

i) the station time (ST), which is the sum of the times of work elements
performed at that station and should not exceed the cycle time, CT.
ii) the precedence restrictions implied by the precedence diagram are not
violated.
There are many possible ways to group these tasks keeping the above
restrictions in mind and we often use criteria like line efficiency, balance delay
and smoothness index to measure how good or bad a particular grouping is.
These criteria are explained below:

1) Line efficiency (LE): This is the ratio of total station time to the product of
the cycle time and the number of work stations. We can express this as
K
STi
LE i 1
100%
148 (K)(CT)
where Planning and
Control for MASS
S Ti = station time of station i Production

K = total number of work stations

CT = cycle time.

2) Balance delay (BD). This is a measure of the line inefficiency and is the
total idle time of all stations as a percentage of total available working
time of all stations.

Thus
K
(K)(CT) STi
BD i 1
100%
(K)(CT)

Balance delay is thus (100—LE) as a percentage.

3) Smoothness index (SI): This is an index to indicate the relative

smoothness of a given assembly line balance. A smoothness index of 0
(zero) indicates a perfect balance. This can be expressed as:

where

STmax = maximum station time

STi = station time of station i

K = total number of work stations.

It may be noted that in designing an assembly line, the number of work

stations, K. cannot exceed the total number of work elements N (in fact K is
an integer such that (1 < K < N ). Also the cycle time is greater than or equal
to the maximum time of any work element and less than the total of all work
element times, that is

where
Ti is the time for work element i
N is the total number of work elements, Tmax is the maximum work element
time and CT is the cycle time.

There is yet no satisfactory methodology which guarantees an optimal

solution for all assembly line balancing problems. The emphasis has been on 149
Operation the use of heuristic procedures that can obtain a fairly good balance for the
Planning and
Control problem. For reviews of procedures available for assembly line balancing
refer to Buffa and Kilbridge and Wester Two commonly used methods for
obtaining a good balance for an assembly line balancing problem are
presented in the next section.

5.6 LINE BALANCING METHODS

Kilbridge and Wester Method

In this procedure proposed by Kilbridge and Wester numbers are assigned

to each operation describing how many predecessors it has. Operations
with the lowest predecessor number are assigned first to the work
stations. The procedure consists of the following steps:

1) Construct the precedence diagram for the work elements. In the

precedence diagram, list in column I all work elements that need not
follow others. In column II, list work elements that must follow those in
column 1. Continue to the other columns in the same way. By so
constructing the columns the elements within a column can be assigned
to work stations in any order provided all the elements of the previous
column have been assigned.
2) Select a feasible cycle time, CT. By a feasible time we mean one for
which
N
Tmax CT Ti
i 1

3) Assign work elements to the station such that the sum of elemental times
does not exceed the cycle time CT. This assignment proceeds from
column 1 to II and so on, breaking intra column ties using the criterion of
minimum number of predecessors.
4) Delete the assigned elements from the total number of work elements
and repeat step 3.
5) If the station time exceeds the cycle time CT due to the inclusion of a
certain work element, this work element should be assigned to the next
station.
6) Repeat steps 3 to 5 until all elements are assigned to work stations.

Example 7.2: Design an assembly line for a cycle time of 10 minutes for the
following 12 elements. Use Kilbridge- Wester Method.

Elements 1 2 3 4 5 6 7 8 9 10 11 12
Immediate
1 2 1 4 3,5 6 7 6 6 10
predecessors

Duration (in
5 3 4 3 6 5 2 6 1 4 4 7
minutes)
150
Solution: First of all the precedence diagram is completed as shown below in Planning and
Control for MASS
fig V using the above data. Grouping is done preliminarily as shown: Production

Figure V: Grouping of work elements Into Columns For Killbridge Westrer Method

Table 1
Station Element (in min) Station sum (in min) Idle time (in min)
I 1 5 5
2 3
II 6 4
4 3
3 4
III 10 0
5 6
IV 6 5 5 5
V 7 2
9 1 7 3
10 4
8 6
VI 10 0
11 4
VII 12 7 7 3
Total = 50 minutes
Assignment of work elements to stations (Wester and Kilbridge Method)

We shall try yet another grouping as shown in the Fig. VI below reducing the
number of work options from 7 to 6 now.

Fig. VI ( Grouping (Re) of Work Elements.

151
Operation I II III IV V VI
Planning and
Control
1 2 4 5 3 6 7 9 10 8 11 12

Fig. VI (a) Grouping of Work Elements.

Table 2

Station Element (in min) Station sum (in min) Idle time (in min)

1 5
I
2 3 8 10 - 8 = 2

4 3
II
5 6 9 1

3 4
III
6 5 9 1

7 2
9 1 7 3
10 4

8 6
V
11 4 10 0

VI 12 7 7 3

Total = 50 minutes

From the above results, we see that there has been an improvement in the line
efficiency from 71.43% to 83.33% (an improvement of 11.90%) and also the
values of balance delay and smoothness inulex have gone down considerably
which is a positive sign for line balancing. You may still try out yet another
combination to improve upon the line efficiency of the above work stations.

It is interesting to note here that if the cycle time is reduced from 10 minutes
to 9 minutes and regroupings are further attempted with suitable
combinations, the line efficiency looks up to 92.6% figure. Regroupings are
shown below for a cycle time of 9 minutes. Cycle time = 9 min.

152
 Table 3 Planning and
Control for MASS
Production
Station Element (in min) Station sum (in min) Idle time

I 1 5 8 1
2 3

II 4 3 9 0
5 6

III 3 4 9 0
6 5

IV 7 2 8 1
8 6

V 10 4 8 1
11 4

VI 9 1 8 1
12 7

Total = 50 minutes

(Improvement)

By reducing the duration of cycle time, the line efficiency can be further
increased.

Helgeson and Birnie Method (Ranked Positional Weight Technique)

This method proposed by Helgeson and Birnie is also known as the ranked
positional weight technique. It consists of the following steps:

1) Develop the precedence diagram in the usual manner.

2) Determine the positional weight for each work element (a positional
weight of an operation corresponds to the time to the longest path from
the beginning of the operation through the remainder of the network).

3) Rank the work element based on the positional weight in step The
work element with the highest positional weight is ranked first.
4) Proceed to assign work elements to the work stations where elements of
the highest positional weight and rank are assigned first.
5) If at any work station additional time remains after assignment of an
operation, assign the next succeeding ranked operation to the work
station, as long as the operation does not violate the precedence
relationships and the station time does not exceed the cycle time.
6) Repeat steps 4 and 5 until all elements are assigned to the work stations.
153
Operation
Planning and Example Let us take up the illustration of balancing of the same
Control
assembly line by Helgeson and Birnie Method considered previously by the
Kilbridge-Wester method. For the precedence diagram shown in Figure IV,
and a desired cycle time of 10, we first construct the table of positional
weights of all elements as shown. For example, the positional weight of
operation 6 equals the maximum of

, since there are 3 paths

from the concerned

operation to the end of the network. Following the above steps we

obtain the assignments of work elements shown in Table 4.

Table 4 (Helgeson and Birnie Method)

Element Positional weight 1 (PW) Element in

descending
order
Path 1 – 2 – 3 – 6 – 7 – 8 – 12 = 5 + 3 + 4 + 5 + 2 + 6 + 7 = 32
Path 1 – 4 – 5 – 6 – 10 – 11 – 12 = 5 + 3 + 6 + 5 + 4 + 4 + 7 = 34 Max of
1 Path 1 – 2 – 3 – 6 – 10 – 11 – 12 = 5 + 3 + 4 + 5 + 4 + 4 + 7 = 32 these =
Path 1 – 4 – 5 – 6 – 7 – 8 – 12 = 5 + 3 + 6 + 5 + 2 + 6 + 7 = 34 34
Path 1 – 2 – 3 – 6 – 9 – 12 = 5 + 3 + 4 + 5 + 1 + 7 = 25
Path 1 – 4 – 5 – 6 – 9 – 12 = 5 + 3 + 6 + 5 + 1 + 7 = 27
2 27 (3) = 27
3 24 (5)=24
4 29 (2)=29
5 26 (4) = 26
6 20 (6) = 20
7 15 (7) = 15
8 13 (9) = 13
9 8 (11) = 8
10 15 (8) = 15
11 11 (10) = 11
12 7 (12) = 7

Table 5: Assignment of week elements to stations helgeson & Birnie method

Station Element i Positional Ti Station sum Idle

wt time
I 1 34(1) 5 8 2
4 29(2) 3
II 2 27(3) 3 9 1
5 26(4) 6
III 3 24(5) 4 9 1
6 20(6) 5
IV 7 15(7) 2 6 4
10 15 (8) 4
154
 V 8 13 (9) 6 10 0 Planning and
Control for MASS
11 11 (10) 4 Production
VI 9 8 (11) 1 8 2
12 7 (12) 7
50 minutes
= Total

Solution.

Smoothness index
You may try the same with a cycle time of 9 minutes.

Example Design an assembly line for a cycle time of 10 minutes for

the following 10 work elements.
Element 1 2 3 4 5 6 7 8 9 10
Immediate predecessor - 1 1 2,3 4 4 6 5 7,8 9
Duration (in min) 5 10 5 2 7 5 10 2 5 7

Use (a) Wester and Kilbridge Method.

(b) Helgeson and Birnie Method.

Calculate the line efficiency, balance delay and smoothness index in both cases.

Solution.

7
10
2 10 6 5
2 5 7
5
1 4 9 10
5 7
3 5
2
8

Fig. VII Precedence Diagram

155
Operation Wester and Kilbridge Methed (Cycle le time = 10 min)
Planning and
Control
Station i Idle time
I (1) + (3) = 5 + 5 = 10 0
II (2) = 10 0
III (4) + (6) = 2 + 5 = 7 3
IV (5) = 7 3
V (7) = 10 0
VI (8) + (9) = 2 + 5 = 7 3
VII (10) = 7 3
Total = 58

Line efficiency

Balance delay

Smoothness Index
Helgeson and Birnie Method i Positional Weight

1
(longest time of path)

9
10

Table 6
Positional Station
Station ldle time
weight sum
I (1) (44) 5 5 10 – 5 = 5
II (2) 39 10 10 0
III (3) + (4) 34, 29 5+2 7 3
IV (6) 27 5 5 5
V (7) 22 10 10 0

156
VI (5) + (18) 21, 14 7+2 9 1 Planning and
Control for MASS
VII (9) 12 5 5 5 Production

VIII (10) 7 7 7 3
58
minutes
Total

58
Line efficiency 100 72.5%
8 10

Halunce Delay = 100 – 72.50 = 27.5%

Smoothness Index

From the above example, for the same set of elements etc., Wester and
Kilbridge Method groups more eftieiently than Helgeson and Birnie Method.

Linear Programming Method of Line Balancing. Assume that a job is

broken down into 6 elemental tasks and the total duration of all such tasks is
30 minutes. The cycle time. i.e.. the length of time available to the workpiece
is on each work station is 10 minutes Thus the minimum number of stations
required are 30/10 = 3 and the maximum number of work stations may be 6
that is equal to the number of tasks involved.
5
3

4 3

1 5

5 7
6
4 6
2

Fig. VIII Precedence Diagram

The problem now reduces, to find out the exact number of work stations
needed and which tasks will be assigned to which station. Figure VlII shows
the precedence diagram.

Three types of constraints equations namely cycle time constraint, step or

task constraint and precedence constraints, will be formulated to solve the
problem like an assignment model.
157
Operation
Planning and
5.7 PROBLEMS AND PROSPECTS OF MASS
Control PRODUCTION-VARIABLE WORK
ELEMENT TIMES.
In both the line balancing methods discussed in the previous section, it was
assumed that the work element times are constant. In practice these times
may be varying randomly owing to factors like human variability, fatigue or
carelessness on the operator's part. Even in case of machine operations, the
set up or positioning time of the part or components could lead to random
variations in the individual work clement times. Since the assembly line is
balanced for a given fixed set of work element times the effect of these
variabilities are two-fold.

i) greater idle time at some work stations, and

ii) the reduction of the average production rate of the line.

In designing lines for random work element times with given means and
variance, some modification of the deterministic line balancing method is
adopted utilising the additional criterion that the probability of the station
time exceeding the cycle time should be kept as low as possible. Some
methods of probabilistic assembly line balancing are discussed by Elsayed
and Boucher

Breakdowns at Work Stations

The mass production system consists of a number of stages in series at which
some operations are being performed. A failure or a breakdown of one stage
or work station will result in failure of the entire production system until
repair is completed.
The result would be decreased production rate. This problem is handled in
practice by providing
i) efficient maintenance service so that the broken down units are repaired
and put into service as soon as possible.
ii) buffer storage of semi-finished goods between each pair of stages, so that
the entire line does not stop due to the failure of one or more units.

The question of how much buffer storage to allocate between stages is of

great practical importance — a higher buffer stock means greater tied up
capital but a lower risk of runout and subsequent line stoppage due to
breakdowns.

The decision to estimate the size of the buffer can be governed by one or
more of the following criteria which consider as to what is the buffer size
that:

1) maximises the production rate of the system

2) minimises the total production cost

158
3) maximises the availability of the production system. Planning and
Control for MASS
Production
The problem could in general be viewed as a multi-stage queuing system
(Fig. IX) see for instance Elsayed and Boucher.

Figure IX: An Assembly Line with Buffers as a Multi-stage Questioning System

Multi-product Line

One of the major disadvantages with assembly lines is their relative

inflexibility. A line is usually designed for one product and changes in design
of the product are often difficult to accommodate on the line, unless suitable
adjustments are made at work stations. But when similar products, in which a
large percentage of the tasks are common, have to be manufactured, the
possibility of the same production line for the products can be explored.
Since tasks are fixed within stations, once balanced, it should be apparent that
station times and efficiencies will vary with the products being produced. A
great variety in these efficiencies might dictate that separate lines be utilised.
In case a multi-product line is to be designed a common procedence diagram
must be developed.

Fig. X Precedence Diagrams for a Two Product Line (a) Product 1, (b) Product 2, (c)
Combined. 159
Operation For instance precedence diagrams for a two product case are shown in Fig. X.
Planning and
Control For a cycle time of 10 the optimum solution is shown in Fig. Xl . Notice that
the line efficiencies are 73% and 100% for product 1 and 2 respectively. A
computer assisted approach for multi product, Stochastic Line Balancing is
described by Bedworth and Bailey.

Loss of efficiency over single product line but gain in equipment

effectiveness is the trade off that must be evaluated in the mixed product line
balance.

Fig. XI Work Station Assignment for the Two Product Assembly Line.

5.8 MODULAR PRODUCTION AND GROUP

TECHNOLO G Y
One criticism of manually operated assembly lines has been that they
reduce a man to a mere cog in a machine. Surely you can imagine the
boredom. monotony and fatigue of a man who spends all his time
tightening the same bolt on a part in an assembly line. It has been found
valuable to enlarge the scope of work of the worker so that he assembles
a complete ‘module’, which in turn may be used on an assembly line to
assemble a product or a number of different products sharing that
particular module. This job enrichment results in greater job satisfaction
160 for the operator by reducing the monotony of the job and giving the
operator a sense of accomplishment for assembling a complete module. Planning and
Control for MASS
In modular production we tend to specialise in the production of Production
particular parts or activities that can then be included as components of
more than one product or service. The reason for wanting to achieve such
commonality is that one part or operation, if used in several products or
services, can accumulate sufficient demand volume to warrant investment
in a flow shop.

Group technology provides another aspect of the same basic idea. It

refers to specialization in families of similar parts. Hence components
requiring primarily turning operations, such as shafts, are collected in one
group, while components requiring surface grinding and drilling
operations, such as plates are assigned to a different group. These groups
become the basis on which a traditional process plant layout can be
reorganised into a group technology plant layout in which machines are
arranged in such a way that each machine is assigned to the production of
only one group of parts. Group technology typically affects only
component manufacture, not the assembly stage of production.

For illustrative example of assembly lines using modular production and

group technology refer to Salvendy.

5.9 AUTOMATION AND ROBOTICS

Mass production has been assisted to a large extent by automation and
robotics in the recent past. Automation refers generally to the bringing
together of three basic building blocks: machine tools, material handling and
controls. Often a considerable amount of time is spent to load, machine and
unload work and to convey it between the single operation machines. This
restriction has been partly relieved by the development of the multiple
spindle machine. With this machine. a single motor driving several spindles
through a gear train allows multiple operations to be performed by one
machine. Machining time cycles does not change, but more machining
operations can be performed within each cycle, and several machining
operations can be performed on one machine by a single operator.

Automatic work piece indexing and transfer of work pieces from station
to station has made it possible for one operator to control the work
performed at several machining stations.Also the operator is able to load
and unload at the load station while machining is going on.

Another trend with automation has been the use of industrial robots to
perform some of the functions that were earlier done by manual
operators.

An ‘industrial robot’, as defined by the Robot Institute of America, is a

programmable, multi-function manipulator designed to move material,
parts, tools or specialized devices through variable programmed motions
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Operation for the performance of a variety of tasks. What separates an industrial
Planning and
Control robot from other types of automation is the fact that it can be reprogrammed
for different applications; hence a robot falls under the heading of flexible
automation’, as opposed to 'hard’ or dedicated automation.

Industrial robots cosist of three basic components:

1) The manipulator (or arm), which is, a series of mechanical linkages and
joints capable of movement in various directions to perform the work
task.

2) The controller, which actually directs the movements and operations

performed by the manipulator. The controller may be an integral part of
the manipulator or may be housed in a separate cabinet

3) The power source, which provides energy to the actuators on the arm.
The power source may be electrical, hydraulic, or pneumatic.

Major reasons for use of robots in industry are increased productivity,

adaptability, safely, ease of training, return on investment and greater
reliability. Robots are currently in operation in welding and assembly,
drilling and routing, inspection, material handling, machine loading, die
casting and a variety of other applications.

5.10 SUMMARY
In this unit we have presented the concept of mass production which
essentially involves the assembly of identical (or inter-changeable) parts of
components into the final product in stages at various work stations. The
relative advantages and disadvantages of mass or flow production arc discussed
and conditions favouring the installation of such a system are identified.

How to design an assembly line starting from the work breakdown structure to
the final grouping of tasks at work stations is also discussed using two
commonly used procedures—the Kilbridge-Wester heuristic approach and
the Helgeson-Birnie approach. Various problems with assembly lines
including variable work element times, breakdowns at work stations and
multi-product line are discussed.

The concepts of modular production and group technology has also been
touched to indicate how flexibility can be introduced in mass or flow
manufacture. Finally, the role of automation and the use of industrial robots
in mass production has been discussed.

5.11 KEY WOBDS

Assembly line: A sequence of work stations where parts or components of a
product are progressively worked on to produce the finished product.

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Balance delay: The total idle time of all stations as a percentage of total Planning and
Control for MASS
available working time of all stations in an assembly line. Production

Cycle time: The time after which s finished product comes off the assembly
line. It would equal the time of the bottleneck operation or the maximum
station time.

Fabrication line: A production line made up of machining or other

operations rather than assembly of components or parts.

Flow shop: A manufacturing system in which machines and other facilities

are arranged on the basis of product flow (generally used for large production
volumes and less product variety).

Group technology: A manufacturing philosophy in which similar parts are

identified and grouped together to take advantage of their similarities in
design and manufacture.

Job shop: A manufacturing system in which similar machines and

equipment art clubbed in departments and each job handled takes its own route.
(Generally used for low production volumes with great product variety.)

Line balancing: The problem of assigning tasks to work stations in an

assembly line in a way that the task times for all stations are equalized as far
as possible.

Line efficiency. The ratio of the actual working time at all stations of an
assembly line to the total allocated time at all stations.

Mass production: A manufacturing system based on interchangeable parts and

the concept of the division of labour to produce generally large quantities of a
product through successive operations/assemblies carried out at a sequence of
work stations in an assembly line.

Modular production: The principle employed in modular production

is to design, develop and produce the minimum number of parts or
operations (called ‘modules”) that can be combined in the maximum
number of ways to offer the greatest number of products or services.

Precedence diagram: A diagram showing the elemental tasks and the order
in which they may be performed. This specifies the technological and other
restrictions that must be respected while designing an assembly line.

Production system: An arrangement by which inputs (like men, machines,

material, money, information and energy) are transformed into useful goods
or services.

Project: A typical production system where production is infrequent (often,

only once) characterised by a number of related jobs to be done with
precedence restrictions.
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Operation Smoothness index: ‘The square root of the sum of squares of idle time at all
Planning and
Control work stations in an assembly line. This is an index to indicate the relative
smoothness of a given assembly line balance. The lesser the better.

Station Time: ‘The sum of the element times of all tasks allotted to a work
station in an assembly line.

Work element: The smallest portion of work identified during the work
breakdown analysis of a job. It is uneconomical or technologically absurd to
further subdivide the work elements, in designing an assembly line.

Work station: A place or stage in an assembly line where designated

work (a combination of work elements) is performed on the part or
components of the product.

5.12 SELF-ASSESSMENT EXERCISES

1) What are the key elements of mass manufacture?
2) Draw a precedence diagram for changing a car tyre. Discuss the way in
which this job could be done with a flow shop configuration. Suggest a
possible division of labour that would produce a reasonable line balance.
3) Design an assembly line for a cycle time of 10 minutes for the following
10 work elements

Elements 1 2 3 4 5 6 7 8 9 10

Immediate - 1 1 (2,3) 4 4 6 5 (7,8) 9

Predecessors

Duration in 5 10 5 2 7 5 10 2 5 7
minutes

Use

a) Kilbridge and Wester method

b) Helgeson and Birnie method.

Calculate the line efficiency, balance delay and smoothness index in both the
cases.

Ans (a) 82.85%, 17.15%,6 (b) 72.50%, 27.5%, 9.69

4) Repeat problem 3 for a cycle time of 12 minutes.

5) Why do we use buffers between stations in assembly lines? What would

be the implications of

a) too large a buffer?

b) too small a buffer?
164
Suggest a method by which the optimal buffer quantities could he found Planning and
Control for MASS
out. Production

6) An assembly line is relatively inflexible. Explain. how by using the

notion of modular production or group technology, flexibility can be
attained in a flow shop configuration.

7) A toy manufacturer intends to make 10.000 pieces per year in the 2000
hours of regular time each year. He has identified 16 work elements with
the following precedence restrictions and durations:

Table 7

Element Immediate Standard time

predecessor (hrs/piece)
1 — 0.14
2 1 0.01
3 2, 15 0.13
4 3 0.12
5 4 0.01
6 5, 12 0.10
7 6, 16 0.07
8 1 0.06
9 8 0.17
10 9 0.17
11 10, 14 0.20
12 11 0.17
13 8 0.03
14 13 0.09
15 1 0.20
16 5 0.05
a) Draw a precedence diagram for the assembly of toys.
b) Design an assembly line suitable for the toy manufacturer.
c) Compute the line efficiency, balance delay and smoothness index for
your design in (b) above.

165
Operation
Planning and
Control

II
I

X 0.20

Fig. 2.33.

Table 8. Precedence Diagram (grouping of elements)

Station Element (i) T_(i) (hrs)Station Idle time (hr)

sum (hrs)
I (1) 0.14 0.14 0.20-0.14=0.06
II (2)+(3) 0.01+0.13 0.14 0.06
111 (4)+(5)+(16) 0.12+0.01+0.05 0.18 0.02
IV (6)+(7) 0.10+0.07 0.17 0.03
V (8)+(13)+ (14) 0.06+0.03+0.09 0.18 0.02
VI (9) 0.17 0.17 0.03
VII (10) 0.17 0.17 0.03
VIII (11) 0.20 0.20 0
IX (12) 0.17 0.17 0.03
X (15) 0.20 0.20 0

166
5.13 FURTHER READINGS Planning and
Control for MASS
Production
Bedwort h, D.D. and 1. E. Bailey. Integraled Production Control Svslerns,
John Wiley: New York.
Buffa, E.S. Operation.t Management, Problems and Models, John Wiley:
New York.
Elmaghraby, S. E. The design of Production Systems, Reinhold Publishing
Corporat ion: New York.
Elsayed, E. A., and T.O. Boucher. Analysis and ControI of Production
.Systems, Prentice Hall: Englewood Cliffs.
Helgeson, W.B. and D.P. Birnie. "Assembly line Balancing Using the
Ranked Positional Weight Technique", Journal of Industrial Engineering.
Vol. 12, No. 6., (pp. 394-398).
Kilbridge, M.D. and K. Wester . "A Heuristic Method of Assembly Line
Balancing", Journal of Industrial Engineering. Vol. 1 2, No 4(1961:292-299

167