1. Group Technology and Cellular Manufacturing3

GROUP TECHNOLOGY AND GROUP TECHNOLOGY AND CELLULAR MANUFACTURINGCELLULAR MANUFACTURING

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Manufacturing SystemsManufacturing Systems

ProjectJobbingBatchLineContinuousHybrid Manufacturing Systems (group

technology, cellular manufacturing, FMS, etc)

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WHAT IS GROUP TECHNOLOGY?

Group technology (GT) is a philosophy that implies the notion of recognizing and exploiting similarities in three different ways:

1. By performing like activities together

2. By standardizing similar tasks3. By efficiently storing and retrieving information about recurring problems

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Large manufacturing system can be decomposed into smaller subsystems of part families based on similarities in

1. design attributes and

2. manufacturing features

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DESIGN ATTRIBUTES:part configuration (round or

prismatic)dimensional envelope (length to

diameter ratio)surface integrity (surface roughness,

dimensional tolerances) material typeraw material state (casting, forging,

bar stock, etc.)

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PART MANUFACTURING FEATURES:operations and operation sequences

(turning, milling, etc.)batch sizesmachine toolscutting toolswork holding devicesprocessing times

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An essential aspect of the integration of CAD and CAM is the integration of information used by engineering and manufacturing and all the other departments in a firm.

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Group technology emphasis on part families based on similarities in design attributes and manufacturing, therefore GT contributes to the integration of CAD and CAM.

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The Basic Key Features for a Successful Group Technology Applications:

•Group Layout

•Short Cycle Flow Control

•A Planned Machine Loading

Group LayoutGroup Layout

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In most of today’s factories it is possible to divide all the made components into families and all the machines into groups, in such a way that all the parts in each family can be completely processed in one group only.

The tree main types of layout are

•Line Layout

•Group Layout

•Functional Layout

Line LayoutLine Layout

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•Line Layout is used at present in simple process industries, in continuous assembly, and for mass production of components required in very large quantities.

Functional LayoutFunctional Layout

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•In Functional Layout, all machines of the same type are laid out together in the same section under the same foreman. Each foreman and his team of workers specialize in one process and work independently.This type of layout is based on process specialization.

Group LayoutGroup Layout

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•In Group Layout, each foreman and his team specialize in the production of one list of parts and co-operate in the completion of common task. This type of layouts based on component specialization.

The Difference between group and functional layout:

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FamiliesFamilies

The word ‘Family’ is used as a name for any list of similar parts.

The families used with group layout are lists of parts which are similar because they are all made on the same group of machines.

This type of family is called a ‘Production Family’.

However, not all parts which are similar in shape will appear in the same family.

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The other important features that is important choosing the families;

Manufacturing tolerancesRequired quantitiesMaterialsSpecial features, which will require

the use of different machines

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GroupsGroups

A group is a list of machines, selected for layout together in one place, because it contains all necessary facilities to complete the processing of a given family of parts.

A family of parts can only be defined by relating it to a particular group of machines, and a group by relating it to a family.

Groups vary greatly in type and size, widely in the number of machines and different machines types. 17

As group size is reduced, more types of machine will be needed in more than one group and there is an increased risk that some new machines must be purchased. Another factor in choosing the size of group is the number of people who will be employed in them.

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Group technology begun by grouping parts into families, based on their attributes.

There are three methods that can be used to form part families:

◦Manual visual inspection◦Production flow analysis◦Classification and coding

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Manual visual inspection involves arranging a set of parts into groups known as part families by visually inspecting the physical characteristics of the parts.

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Manual visual inspection◦incorrect results◦human error◦different judgment by different people

◦inexpensive ◦least sophisticated◦good for small companies having smaller number of parts

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Production flow analysis: Parts that go through common operations are grouped into part families.

The machines used to perform these common operations may be grouped as a cell, consequently this technique can be used in facility layout (factory layout)

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Coding methods: are employed in classifying parts into part families

Coding refers to the process of assigning symbols to the parts

The symbols represent design attributes of parts or manufacturing features of part families

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The variations in codes resulting from the way the symbols are assigned can be grouped into three distinct type of codes:

◦Monocode or hierarchical code◦Polycode or attribute◦Hybrid or mixed code

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MONOCODE (HIERARCHICAL CODE)MONOCODE (HIERARCHICAL CODE)

This coding system was originally developed for biological classification in 18th century.

The structure of monocode is like a tree in which each symbol amplifies the information provided in the previous digit.

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The following figure illustrates the structure of a monocode:

A monocode (hierarchical code) provides a large amount of information in a relatively small number of digits

useful for storage and retrieval of design-related information such as part geometry, material, size, etc.

it is difficult to capture information on manufacturing sequences in hierarchical manner, so applicability of this code in manufacturing is rather limited

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POLYCODE (ATTRIBUTE CODE):POLYCODE (ATTRIBUTE CODE): The code symbols are independent of

each other

Each digit in specific location of the code describes a unique property of the workpiece◦ it is easy to learn and useful in manufacturing

situations where the manufacturing process have to be described

◦ the length of a polycode may become excessive because of its unlimited combinational features

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MIXED CODE (HYBRID CODE):MIXED CODE (HYBRID CODE):

It is the mixture of both monocode and polycode systems. Mixed code retains the advantages of both systems. Most coding systems use this code structure.

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MIXED CODE (HYBRID CODE):MIXED CODE (HYBRID CODE):The first digit for example, might be used to

denote the type of part, such as gear. The next five position might be reserved for a short attribute code that would describe the attribute of the gear. The next digit (7th digit) might be used to designate another subgroup, such as material, followed by another attribute code that would describe the attributes.

A code created by this manner would be relatively more compact than a pure attribute code while retaining the ability to easily identify parts with specific characteristics.

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The OPITZ classification The OPITZ classification system:system:it is a mixed (hybrid) coding systemdeveloped by Opitz, Technical

University of Aachen, 1970it is widely used in industryit provides a basic framework for

understanding the classification and coding process

it can be applied to machined parts, non-machined parts (both formed and cast) and purchased parts

it considers both design and manufacturing information

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The Opitz coding system consists of The Opitz coding system consists of three groups of digits:three groups of digits:

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Form Supplementary Secondary code code code 12345 6789 ABCD

part geometry and features relevant to part design

information relevant to manufacturing(polycode)

Production processes and production sequences

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For the purpose of selecting or developing your own code, it is important to understand the attributes of classification and coding systems.

SELECTION OF CLASSIFICATION AND CODING SYSTEMS

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Some of the important classification and coding system attributes include:

1. Flexibility for various applications such as part family formation, process planning, costing, and purchasing2. Accuracy, to provide correct information on parts3. Expandability, to accommodate information on more part attributes deemed important later on4. Ease of learning5. Ease of retrieval6. Reliability and availability of software7. Suitability for specific applications


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Matching these attributes with the objectives of an organization would be helpful in selecting or developing a coding system to meet organizational needs.


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Group technology is a management strategy to help eliminate waste caused by duplication of effort.

BENEFITS OF GROUP TECHNOLOGY

BENEFITS OF GROUP TECHNOLOGYBENEFITS OF GROUP TECHNOLOGY

It affects all areas of a company, including:

engineeringequipment specification facilities planning process planningproduction control quality control tool design purchasing service

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Some of the well-known tangible and intangible benefits of implementing GT :

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1. Engineering design

• Reduction in new parts design• Reduction in the number of drawings through

standardization• Reduction of drafting effort in new shop drawings• Reduction of number of similar parts, easy retrieval

of similar functional parts, and identification of substitute parts


2. Layout planning

Reduction in production floor space required

Reduced material-handling effort

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3. Specification of equipment, tools, jigs, and fixtures

Standardization of equipmentImplementation of cellular

manufacturing systemsSignificant reduction in up-front

costs incurred in the release of new parts for manufacture

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4. Manufacturing: process planning

Reduction in setup time and production time

Alternative routing leading to improved part routing

Reduction in number of machining operations and numerical control (NC) programming time

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5. Manufacturing: production control

Reduced work-in-process inventoryEasy identification of bottlenecksImproved material flow and reduced

warehousing costsFaster response to schedule changesImproved usage of jigs, fixtures, pallets,

tools, material handling, and manufacturing equipment

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6. Manufacturing: quality control

Reduction in number of defects leading to reduced inspection effort

Reduced scrap generationBetter output qualityIncreased accountability of operators

and supervisors responsible for quality production, making it easier to implement total quality control concepts.

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7. Purchasing

Coding of purchased part leading to standardized rules for purchasing

Economies in purchasing possible because of accurate knowledge of raw material requirements

Reduced number of part and raw materials

Simplified vendor evaluation procedures leading to just-in-time purchasing

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8. Customer service

Accurate and faster cost estimates

Efficient spare parts management, leading to better customer service

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CELLULAR MANUFACTURINGCELLULAR MANUFACTURING

Cellular manufacturing is an application of group technology in manufacturing in which all or a portion of a firm’s manufacturing system has been converted into cells.

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CELLULAR MANUFACTURINGCELLULAR MANUFACTURING

A manufacturing cell is a cluster of machines or processes located in close proximity and dedicated to the manufacture of a family of parts.

The parts are similar in their processing requirements, such as operations, tolerances, and machine tool capacities

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The primary objectives in implementing a cellular manufacturing system are to reduce:

setup times (by using part family tooling and sequencing)

flow times (by reducing setup and move times and wait time for moves and using smaller batch sizes)

reduce inventories market response times

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In addition, cells represent sociological units that have more tendency to teamwork.

This means that motivation for process improvements often arises naturally in manufacturing cells.

Manufacturing cells are natural candidates for just-in-time (JIT) implementation.

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Cell DesignCell Design

Design of cellular manufacturing system is a complex exercise with broad implications for an organization.

The cell design process involves issues related to both system structure and system operation

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Structural issues include:

Selection of part families and grouping of parts into families

Selection of machine and process populations and grouping of these into cells

Selection of tools, fixtures, and pallets

Selection of material-handling equipment

Choice of equipment layout56

Issues related to procedures include:

Detailed design of jobsOrganization of supervisory and support

personnel around the cellular structureFormulation of maintenance and inspection

policiesDesign of procedures for production

planning, scheduling, control, and acquisition of related software and hardware

Modification of cost control and reward systems

Outline of procedures for interfacing with the remaining manufacturing system (in terms of work flow and information, whether computer controlled or not)

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Evaluation of Cell Design Evaluation of Cell Design DecisionsDecisions

The evaluation of design decisions can be categorized as related to either

the system structure or

the system operation.

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Typical considerations related to the system structure include:

Equipment and tooling investment (low)

Equipment relocation cost (low)Material-handling costs (low)Floor space requirements (low)Extent to which parts are completed

in a cell (high)Flexibility (high)

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CELL FORMATION CELL FORMATION APPROACHESAPPROACHES

Machine - Component Group Analysis:

Machine - Component Group Analysis is based on production flow analysis

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Production flow analysisProduction flow analysis involves four involves four stages:stages:

Stage 1: Machine classification.

Machines are classified on the basis of operations that can be performed on them. A machine type number is assigned to machines capable of performing similar operations.

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Stage 2: Checking parts list and production route information.

For each part, information on the operations to be undertaken and the machines required to perform each of these operations is checked thoroughly.

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Stage 3: Factory flow analysis.

This involves a micro-level examination of flow of components through machines. This, in turn, allows the problem to be decomposed into a number of machine-component groups.

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Stage 4: Machine-component group analysis.

An intuitive manual method is suggested to manipulate the matrix to form cells. However, as the problem size becomes large, the manual approach does not work. Therefore, there is a need to develop analytical approaches to handle large problems systematically.

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EXAMPLE:EXAMPLE:

Consider a problem of 4 machines and 6 parts. Try to group them.

Machines 1 2 3 4 5 6

M1 1 1 1

M2 1 1 1

M3 1 1 1

M4 1 1 1

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Components

Machines 2 4 6 1 3 5

M1 1 1 1

M2 1 1 1

M3 1 1 1

M4 1 1 1

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Components

Rank Order Clustering Algorithm:Rank Order Clustering Algorithm:

Rank Order Clustering Algorithm is a simple algorithm used to form machine-part groups.

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Step 1:Assign binary weight and calculate a decimal weight for each row and column

Step 2: Rank the rows in order of decreasing decimal weight values.

Step 3: Repeat steps 1 and 2 for each column.

Step 4: Continue preceding steps until there is no change in the position of each element in the row and the column.

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EXAMPLE:EXAMPLE:Consider a problem of 5 machines and 10 parts. Try Consider a problem of 5 machines and 10 parts. Try to group them by usingto group them by using Rank Order Clustering Rank Order Clustering Algorithm.Algorithm.

Machines 1 2 3 4 5 6 7 8 9 10

M1 1 1 1 1 1 1 1 1 1

M2 1 1 1 1 1

M3 1 1 1 1

M4 1 1 1 1 1 1

M5 1 1 1 1 1 1 1 1

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Components

Table 1

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

M1 1 1 1 1 1 1 1 1 1 1007

M2 1 1 1 1 1 451

M3 1 1 1 1 568

M4 1 1 1 1 1 1 455

M5 1 1 1 1 1 1 1 1 1020

29 28 27 26 25 24 23 22 21 20

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Binary weight

Components

Table 2

Binaryweight

Machines 1 2 3 4 5 6 7 8 9 10

24 M5 1 1 1 1 1 1 1 1

23 M1 1 1 1 1 1 1 1 1 1

22 M3 1 1 1 1

21 M4 1 1 1 1 1 1

20 M2 1 1 1 1 1Decimal

equivalent 28 27 27 27 28 20 28 26 11 11

29 28 27 26 25 24 23 22 21 20

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Binary weight

Components

Table 3

Binaryweight

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

24 M5 1 1 1 1 1 1 1 1 1020

23 M1 1 1 1 1 1 1 1 1 1 1019

22 M3 1 1 1 1 900

21 M4 1 1 1 1 1 1 123

20 M2 1 1 1 1 1 115Decimal

equivalent 28 28 28 27 27 27 26 20 11 11

29 28 27 26 25 24 23 22 21 20

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Binary weight

Components

Table 4

Similarity Coefficient-Based Similarity Coefficient-Based ApproachesApproaches

In similarity coefficient methods, the basis is to define a measure of similarity between machines, tools, design features, and so forth and then use it to form part families and machine groups.

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Single-Linkage Cluster Analysis (SLCA):

It is a hierarchical machine grouping method known as single-linkage cluster analysis using similarity coefficients between machines.

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The procedure is to construct a tree called a dendrogram.

The similarity coefficient between two machines is defined as the ratio of the number of parts visiting both machines and the number of parts visiting one of the two machines:

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S =

+ Z - Xij

k=1

N

jk ijk

X

Y

ijk

ik

k

N

( )1

where: Xijk = operation on part k performed both on machine i and j,Yik = operation on part k performed on machine i,Zjk = operation on part k performed on machine j.

SLCA ALGORITHMS SLCA ALGORITHMS

It helps in constructing dendrograms.

A dendrogram is a pictorial representation of bonds of similarity between machines as measured by the similarity coefficients.

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The steps of algorithm are as follows:

Step 1: Compute similarity coefficients for all possible pairs of

machines,Step 2: Select the two most similar machines

to form the first machine cell,Step 3: Lower the similarity level (threshold)

and form new machine cells by including all the machines with similarity coefficients not less than the threshold value,

Step 4: Continue step 3 until all machines are grouped into a single cell.

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EXAMPLE:EXAMPLE:Consider the matrix of 5 machines and 10 components Consider the matrix of 5 machines and 10 components given below.given below.

Machines 1 2 3 4 5 6 7 8 9 10

M1 1 1 1 1 1 1 1 1 1

M2 1 1 1 1 1

M3 1 1 1 1

M4 1 1 1 1 1 1

M5 1 1 1 1 1 1 1 1

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Components

Develop a denrogram and discuss the resulting cell structures.

Step 1:Step 1: Determine similarity coefficients between all Determine similarity coefficients between all pairs of machines.pairs of machines.

Machinepairs

M1M2

M1M3

M1M4

M1M5

M2M3

M2M4

M2M5

M3M4

M3M5

M4M5

SC 0.55 0.30 0.67 0.70 0.00 0.83 0.30 0.00 0.50 0.40

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SC = 59 +5-5

= 0.556 12

Similarity coefficients of machine pairs

Step 2: Select machines M2 and M4, having the highest similarity coefficients of 0.83 to form the first cell.

Step 3: The next lower coefficient of similarity is between machines M1 and M5. Use these machines to form the second cell.

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Step 4:Step 4: The next lower coefficient of similarity is The next lower coefficient of similarity is now 0.67 between machines M1 and M4. At this now 0.67 between machines M1 and M4. At this threshold value machines M1, M2, M4, and M5 will threshold value machines M1, M2, M4, and M5 will form one machine group. The other possible groups form one machine group. The other possible groups will be evaluated by the same way.will be evaluated by the same way.

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0.00

0.50

0.670.70

0.83

M4 M1M2 M3M5

Dendrogram

EXCEPTIONAL PARTS & BOTTLNECK EXCEPTIONAL PARTS & BOTTLNECK MACHINES:MACHINES:

One of the important goal in cell design is to create mutually independent machine cells. However, it may not always be economical or practical to achieve this goal.

In practice, therefore, some parts need to be processed in more than one cell. These are known as exceptional parts and the machines processing them are known as bottleneck machines.

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The problem of exceptional elements can possibly be eliminated by:

Generating alternative process plans

Duplication of machinesSubcontracting these operations

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EVALUATION OF CELL DESIGN:EVALUATION OF CELL DESIGN:

In design of cells, there will be more than one alternative solution. The objective is to find the best alternative.

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Assume we have the following alternative Assume we have the following alternative

cell configuration:cell configuration:

Similaritycoefficient

Number of cells formed

Cell configuration

1.00 5 (M1), (M2), (M3), (M4), (M5)

0.83 4 (M2, M4), (M5), (M1), (M3)

0.70 3 (M2, M4), (M1, M5), (M3)

0.67 2 (M1, M2, M4, M5), (M3)

0.50 1 (M1, M2, M3, M4, M5)

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The criteria is to minimize the distance that the parts should travel during the processes; in other words, to minimize the material handling costs of intercell (between cells) and intracell (within cell) movements of the parts.

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The following factors affect the cost of intercell and intracell movements of parts.

1. The layout of machines in a group2. The layout of machine groups3. The sequences of parts through

machines and machine groups

The total distances moved by a component visiting a number of machines in a cell has to be determined.

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1. Group Technology and Cellular Manufacturing3

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