ch6 group technology and cellular manufacturing

8/8/2019 Ch6 Group Technology and Cellular Manufacturing

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GROUP TECHNOLOGY ANDCELLULAR MANUFACTURING

MIDDLE EAST TECHNICAL UNIVERSITY

Mechanical Engineering Department

ME 445 ME 445 Integrated Manufacturing SystemsIntegrated Manufacturing Systems


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BATCH MANUFACTURING

IS A DOMINANT MANUFACTURING ACTIVITY IN THE WORLD,

GENERATING A GREAT DEAL OFINDUSTRIAL OUTPUT

IT ACCOUNTS60% - 80%

OF ALL MANUFACTURING ACTIVITIES


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CHARACTERISTICS OFBATCH MANUFACTURING:

High level of product variety Small manufacturing lot size


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Time onmachine

5%

Moving and waiting9 5%

Cuttingless than

30%

Positioning, loading, gauging,idle, etc.

70%


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WHAT IS GROUP TECHNOLOGY?

Group technology (GT) is a philosophy thatimplies the notion of recognizing and exploitingsimilarities in three different ways:

1. By performing like activities together 2 . By standardizing similar tasks3 . By efficiently storing and retrievinginformation about recurring problems


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Large manufacturing system can bedecomposed into smaller subsystemsof part families based on similarities in

1. design attr ibu t es and2 . m a nuf act u r ing fe at u r es


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DESIGN ATTRIBUTES:part configuration ( round or prismatic)dimensional envelope (l ength to diameter

ratio)surface integrity ( surface roughness,

dimensiona l to l erances)material typeraw material state ( casting, forging, bar

stock, etc.)


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PART MANUFACTURING FEATURES:

operations and operation sequences( turning, mi ll ing, etc.)batch sizes

machine toolscutting toolswork holding devices

processing times


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An essential aspect of theintegration of CAD and CAM isthe integration of information usedby engineering and manufacturingand all the other departments in afirm.


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Gro up t e ch n olo gy emphasis onpart families based on similaritiesin design attributes and

manufacturing, thereforeGT

contributes to the integration of CAD and CAM.


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The Basic Key Features for a Successful GroupTechnology Applications:

Group Layout

Short Cycle Flow Control

A Planned Machine Loading


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Group Layout

In most of todays factories it is possible todivide all the made components into familiesand all the machines into groups, in such a way

that all the parts in each family can becompletely processed in one group only.

The tree main types of layout are

Line Layout

Group Layout

Functional Layout


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Line Layout

Line Layout is used at present in simple process industries, in continuous assembly, andfor mass production of components required in

very large quantities.


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Functional LayoutIn Functional Layout, all machines of the

same type are laid out together in the samesection 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.


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Group Layout

In Group Layout, each foreman and his teamspecialize in the production of one list of partsand co-operate in the completion of common

task. This type of layouts based on componentspecialization.


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The Difference between group and functional layout:


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FamiliesThe word Fa mi ly is used as a name for

any list of similar parts. The familiesused with group layout are lists of parts

which are similar because they are allmade on the same group of machines.This type of family is called aPro du ct io n Fa mi ly. However, not allparts which are similar in shape willappear in the same family.


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The other important features that isimportant choosing the families;Manufacturing tolerances

Required quantitiesMaterialsSpecial features, which will require the

use of different machines


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Groups

A group is a list of machines, selected for layout together in one place, because itcontains all necessary facilities to complete

the processing of a given family of parts. Afamily of parts can only be defined by relatingit to a particular group of machines, and agroup by relating it to a family. Groups vary

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


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As group size is reduced, more types of machine will be needed in more than onegroup and there is an increased risk thatsome new machines must be purchased.

Another factor in choosing the size of group is the number of people who will beemployed in them.


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Group technology begun by grouping partsinto families, based on their attributes.There are three methods that can be usedto form part families:

Ma nue l v isu al inspe ct io n Pro du ct io n f low a n al ysis

Cla ssifi cat io n a nd co ding


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Ma nu al v isu al inspe ct io ninvolves arranging a set of partsinto groups known as part families by visually inspecting thephysical characteristics of the

parts.


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Ma nu al v isu al inspe ct io n incorrect results human error different judgment by different people inexpensive least sophisticated

good for small companies havingsmaller number of parts


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Pro du ct io n f low a n al ysis : Parts that gothrough common operations are groupedinto part families.

The machines used to perform thesecommon operations may be grouped as acell , consequently this technique can beused in facility layout ( factory l ayout )


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Co ding me tho ds : are employed in

classifying parts into part families

C oding 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 begrouped into three distinct type of codes:

Mo n oco de or h ie rarch ical co de Pol yco de or attr ibu t e H yb r id or mixed co de


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MONOCODE (HIERARCHICAL CODE)

This coding system was originallydeveloped for biological classification in18 th century.

The structure of m o n oco de is like a treein which each symbol amplifies theinformation provided in the previous digit.


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The following figure illustrates the structure

of a monocode:


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A m o n oco de (hierarchical code) providesa large amount of information in arelatively small number of digitsuseful for storage and retrieval of design-related information such as p artge o me tr y, m at e r ial , size , etc.it is difficult to capture information on

manufacturing sequences in hierarchicalmanner, so applicability of this code inmanufacturing is rather limited


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POLYCODE (ATTRIBUTE CODE):

The code symbols are independent of eachother

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

situations where the manufacturing process have tobe described

the length of a polycode may become excessivebecause of its unlimited combinational features


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Differences in information storage capacitybetween m o n oco de and p ol yco de :

A ssume that a code consists of a fivesymbo l s and that in each of the five code

fiel ds the digits 0 to 9 are used. Determinehow many mutua ll y exc l usivecharacteristics can potentia ll y be stored inthe monocode and the polycode


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N umber of characteristics may be stored in a monocode:

10 1 + 10 2 + 10 3 + 10 4 + 10 5 = 111110

N umber of characteristics may be stored in a po l ycode:

10 + 10 + 10 + 10 + 10 = 5 0


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MIXED CODE (HYBRID CODE):

It is the mixture of both m o n oco de and

p ol yco de systems. Mixed code retains theadvantages of both systems. Most codingsystems use this code structure.


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MIXED CODE (HYBRID CODE):

The first digit for example, might be used todenote the type of part , such as gear. The nextfive position might be reserved for a shortattribute code that would describe the attribute of the gear. The next digit ( 7 th digit) might be used

to designate another subgroup, such asmateria l , followed by another attribute code thatwould describe the attributes.

A code created by this manner would berelatively more compact than a pure attributecode while retaining the ability to easily identifyparts with specific characteristics.


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Th e OPITZ cla ssifi cat io n sys t em :

it is a mixed (hybrid) coding systemdeveloped by O pi t z , Technical University of

Aachen, 1 97 0

it is widely used in industryit provides a basic framework for understandingthe classification and coding processit can be applied to machined parts, non-machined parts (both formed and cast) andpurchased partsit considers both design and m a nuf act u r inginformation


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The Opitz coding system consists of three groupsof digits:

Form Supplementary Secondarycode code code12345 6789 ABCD

part geometry and

features re l evant to part design

information

re l evant tomanufacturing ( po l ycode)

Production

processes and productionsequences


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PART FAMILY FORMATION


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PART FAMILY FORMATION:One of the primary uses of coding systems is todevelop part families.

Example: Consider the family of ferrous partsformed by first three digits of Opitz form code; 1 3 1.

This implies that the attributes associated with thefamily members are length/diameter ratio in therange 0. 5 to 3 .0, all parts stepped to one end andinternal shape elements with threads.

A number of mathematical approaches have alsobeen developed to form part families usingclassification and coding system.


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For the purpose of selecting or developingyour own code, it is important to understandthe attributes of classification and codingsystems.

SELECTION OF CLASSIFICATION ANDCODING SYSTEMS


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Some of the important classification and coding systemattributes 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 partattributes deemed important later on4 . Ease of learning5 . Ease of retrieval6 . Reliability and availability of software7 . Suitability for specific applications

SELECTION OF CLASSIFICATION AND CODINGSYSTEMS


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Matching these attributes with theobjectives of an organization would behelpful in selecting or developing acoding system to meet organizational

needs.

SELECTION OF CLASSIFICATION AND

CODING SYSTEMS


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Group technology is a m a n a gemen tstrategy to help eliminate waste caused byduplication of effort.

BENEFITS OF GROUP TECHNOLOGY


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It affects all areas of a company, including:

engineeringequipment specificationfacilities planningprocess planningproduction controlquality controltool designpurchasingservice


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

1. Engineering design

Reduction in new parts designReduction in the number of drawings through

standardizationReduction of drafting effort in new shop drawings

Reduction of number of similar parts, easy retrievalof similar functional parts, and identification of substitute parts


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2 . Layout planning

Reduction in production floor spacerequiredReduced material-handling effort


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3 . Specification of equipment, tools, jigs, andfixtures

Standardization of equipmentImplementation of cellular manufacturingsystems

Significant reduction in up-front costsincurred in the release of new parts for manufacture


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4 . Manufacturing: process p l anning

Reduction in setup time and productiontimeAlternative routing leading to improvedpart routing

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


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5 . Manufacturing: production contro l

Reduced work-in-process inventory

Easy identification of bottlenecksImproved material flow and reducedwarehousing costsFaster response to schedule changesImproved usage of jigs, fixtures, pallets, tools,material handling, and manufacturing equipment


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6 . Manufacturing: qua l ity contro l

Reduction in number of defects leading to

reduced inspection effortReduced scrap generationBetter output qualityIncreased accountability of operators andsupervisors responsible for quality production,making it easier to implement total quality controlconcepts.


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BENEFITS OF GROUP TECHNOLOGY7

. Purchasing

Coding of purchased part leading tostandardized rules for purchasingEconomies in purchasing possiblebecause of accurate knowledge of rawmaterial requirements

Reduced number of part and raw materialsSimplified vendor evaluation proceduresleading to just-in-time purchasing


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8 . Customer service

Accurate and faster cost estimatesEfficient spare parts management, leadingto better customer service


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CELLULAR MANUFACTURING

Cellular manufacturing is an application of group technology in manufacturing inwhich all or a portion of a firmsmanufacturing system has been convertedinto cells.


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CELLULAR MANUFACTURING

A manufacturing cell is a cluster of machines or processes located in closeproximity and dedicated to themanufacture of a family of parts.

The parts are similar in their processingrequirements, such as operations ,to l erances , and machine too l capacities


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The primary objectives in implementing acellular manufacturing system are toreduce:

setup times (by using part family tooling

and sequencing)flow times (by reducing setup and movetimes and wait time for moves and usingsmaller batch sizes)reduce inventoriesmarket response times


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In addition, cells represent sociologicalunits that have more tendency toteamwork. This means that motivation for process improvements often arisesnaturally in manufacturing cells.

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


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Functional and cellular layouts of an electronics plant:


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C e ll D esign

Design of cellular manufacturing system isa complex exercise with broad implicationsfor an organization.

The c e ll design p roc ess involves issuesrelated to both system structure andsystem operation


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Structural issues include:

Selection of part families and groupingof parts into families

Selection of machine and processpopulations and grouping of these intocells

Selection of tools, fixtures, and pallets

Selection of material-handlingequipmentChoice of equipment layout


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Issues related to procedures include:

Detailed design of jobsOrganization of supervisory and supportpersonnel around the cellular structureFormulation of maintenance and inspectionpoliciesDesign of procedures for production p l anning ,

schedu l ing, contro l , and acquisition of re l ated software and hardwareModification of cost control and reward systemsOutline of procedures for interfacing with theremaining manufacturing system (in terms of work flow and information, whether computer controlled or not)


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Eval u at io n o f C e ll D esign D

ec

isio

ns

The evaluation of design decisions can be

categorized as related to either the system structureor

the system operation .


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Typical considerations related to thesystem 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 acell (high)Flexibility (high)

l i f ll d i


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Evaluations of cell system design areincomplete unless they relate to the

operation of the system .

A few typical performance variablesrelated to system operation are:

Equipment utilization (high)Work-in-process inventory (low)

Queue lengths at each workstation (short)Job throughput time (short)Job lateness (low)


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A major problem throughout the celldesign process is the necessity of trading off against each other objectivesrelated to structural parameters and

performance variables .


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For example, higher machine utilizationcan be achieved if several cells route their parts through the same machine. Thedrawbacks are increased queuing andcontrol problems .


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System cost and performance are affected byevery decision related to sys t em s tr u ct u r e and

system

ope

ratio

n .

It is necessary to evaluate each importantdesign parameter and relate its performance topre-established criteria.

For example, structural variables such asnumber of machines must be balanced againstoperational variables such as machineutilization and throughput time usinganalytical and simulation approaches.


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CELL FORMATION APPROACHES

Mach ine - Co mp o nen t Gro up A n al ysis :

Machine - Component Group A na l ysis isbased on production flow analysis


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Production f l ow ana l ysis involves four stages:

Stage 1: M achine classification .

Machines are classified on the basis of operations that can be performed on them.

A machine type number is assigned tomachines capable of performing similar

operations.


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Stage 2 : C hecking parts list and production route information .

For each part, information on theoperations to be undertaken and themachines required to perform each of

these operations is checked thoroughly.


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Stage 3 : F actory flow analysis .

This involves a micro-level examination of flow of components through machines.

This, in turn, allows the problem to bedecomposed into a number of machine-component groups.


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Stage 4 : M achine-component group

analysis .

An intuitive manual method is suggested

to manipulate the matrix to form cells.However, as the problem size becomeslarge, the manual approach does not work.Therefore, there is a need to developanalytical approaches to handle largeproblems systematically.


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EXAMPLE:

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

Machines 1 2 3 4 5 6

M 1 1 1 1

M 2 1 1 1

M 3 1 1 1

M 4 1 1 1

Components


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Machines 2 4 6 1 3 5

M 1 1 1 1

M 2 1 1 1

M 3 1 1 1

M 4 1 1 1

Components


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Ra nk Or de r Cl us t e r ing Al g or ith m :

R ank Order C lustering Algorithm is a

simple algorithm used to form machine- part groups.

Step 1: Assign binary weight and


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Step 1: Assign binary weight andcalculate a decimal weight for each row andcolumn using the following formulas:

Decimal we

Decimal we b pj n p

ight for row i = b

ight for column j =

ip m-p p= 1

m

p= 1

n

2

2


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

Step 3 : Repeat steps 1 and 2 for eachcolumn.

Step 4 : Continue preceding steps untilthere is no change in the position of eachelement in the row and the column.


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EXAMPLE:Consider a problem of 5 machines and 10 parts. Try to

group them by using R ank Order C l ustering A l gorithm.

Machines 1 2 3 4 5 6 7 8 9 10

M 1 1 1 1 1 1 1 1 1 1M 2 1 1 1 1 1

M 3 1 1 1 1

M4

1 1 1 1 1 1M 5 1 1 1 1 1 1 1 1

Components

Table 1


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Machines 1 2 3 4 5 6 7 8 9 10 D ecimalequivalent

M 1 1 1 1 1 1 1 1 1 1 1007M 2 1 1 1 1 1 451

M 3 1 1 1 1 568

M 4 1 1 1 1 1 1 455

M 5 1 1 1 1 1 1 1 1 1020

2 9 2 8 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0

Binary weight

Components

Table 2


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Binaryweight

Machines 1 2 3 4 5 6 7 8 9 10

2 4 M 5 1 1 1 1 1 1 1 12 3 M 1 1 1 1 1 1 1 1 1 12 2 M 3 1 1 1 1

21

M 4 1 1 1 1 1 12 0 M 2 1 1 1 1 1

D ecimalequivalent 28 27 27 27 28 20 28 26 11 11

2 9 2 8 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0

Binary weight

Components

Table 3


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Binaryweight

Machines 1 5 7 2 3 4 8 6 9 10 D ecimalequivalent

2 4 M 5 1 1 1 1 1 1 1 1 10202 3 M 1 1 1 1 1 1 1 1 1 1 10192 2 M 3 1 1 1 1 9002 1 M 4 1 1 1 1 1 1 1232 0

M2

1 1 1 1 1 115D ecimal

equivalent 28 28 28 27 27 27 26 20 11 11

2 9 2 8 2 7 2 6 2 5 2 4 2 3 2 2 2 1 2 0

Binary weight

Components

Table 4


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S imi lar it y Co effi c ien t- Ba sed

A pp roach es

In similarity coefficient methods , the

basis is to define a measure of similaritybetween machines, tools, design features,and so forth and then use it to form partfamilies and machine groups.


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S ing le -L ink a ge Cl us t e r A n al ysis

( SLC A ):

It is a hierarchical machine grouping

method known as single-linkage cluster analysis using similarity coefficientsbetween machines.

The procedure is to construct a treecalled a dendrogram .


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

S =+ Z - X

ijk=1

N

jk ijk

X

Y

ijk

ik

k

N

!

( )1

where: X ijk = operation on part k performed both on machine i and j ,Y ik = operation on part k performed on machine i ,Z jk = operation on part k performed on machine j.


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SLCA ALGORITHMS

It helps in constructing dendrograms .

A dendrogram is a pictorialrepresentation of bonds of similaritybetween machines as measured by thesimilarity coefficients.


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The steps of algorithm are as follows:

Step 1: Compute similarity coefficients for allpossible pairs of machines,

Step 2 : Select the two most similar machines to

form the first machine cell,Step 3 : Lower the similarity level (threshold) andform new machine cells by including all themachines with similarity coefficients not less

than the threshold value,Step 4 : Continue step 3 until all machines are

grouped into a single cell.

EXAMPLE:


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

Machines 1 2 3 4 5 6 7 8 9 10

M 1 1 1 1 1 1 1 1 1 1

M 2 1 1 1 1 1M 3 1 1 1 1

M 4 1 1 1 1 1 1

M 5 1 1 1 1 1 1 1 1

Components

Develop a denrogram and discuss the resulting cell structures.


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Step 1: Determine similarity coefficients between all pairsof machines.

C 59 + 5 - 5

0.556 12

Machine pairs

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

Similarity coefficients of machine pairs


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Step 2 : Select machines M 2 and M 4 , havingthe highest similarity coefficients of 0.83 to form the first cell.

Step 3 : The next lower coefficient of similarity is between machines M1 andM5 . Use these machines to form the

second cell.

Step 4 : The next lower coefficient of similarity is now 0. 67


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between machines M1 and M 4 . At this threshold valuemachines M1, M 2 , M4 , and M 5 will form one machine

group. The other possible groups will be evaluated by thesame way.

0.00

0.50

0.670.70

0.83

M4 M1M2 M3M5

Dendrogram

EXCEPTIONAL PARTS & BOTTLNECK


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EXCEPTIONAL PARTS & BOTTLNECKMACHINES:

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

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


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The problem of exceptional elements canpossibly be eliminated by:

Generating a l ternative process p l ansDup l ication of machinesSubcontracting these operations


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EVALUATION OF CELL DESIGN:

In design of cells, there will be more than

one alternative solution. The objective is tofind the best alternative.

Assume we have the following alternative


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Assume we have the following alternativecell configuration:

Similaritycoefficient

Number of cellsformed

ell 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 thatthe parts should travel during theprocesses; in other words, to minimize thematerial handling costs of in t e rc e ll

(between cells) and in trac e ll (within cell)movements of the parts.


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The following factors affect the cost of in t e rc e ll and in trac e ll movements of parts.

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

and machine groups

The total distances moved by a componentvisiting a number of machines in a cell hasto be determined.


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Assumptions:

1. In the absence of the real data on thesequences in which the components visitthe machines, it is assumed that the

machines are laid out in a randommanner.2 . There is one unit distance between each

machine in a group of N machines.3 . A part has to visit two machines in a

group of N machines.

E t d di t f t ight li l t N +1


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Expected distance for a straight-line layout: N +13

Expected distance for a rectanglelayout of M rows of L machines:

M + L2

Expected distance for a square layout: 2 N

3

The total distance moved in j th cellfor the i th configuration: k i

m

d iwhere:

d ij = expected distance moved between two machinesfor i th configuration in j th ce ll

k ij = number of moves between two machines by a ll the parts for i th configuration in j th ce ll

The total cost of i t ll l and i t ll l movements


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The total cost of interce ll ul ar and intrace ll ul ar movements(TCi) for the ith configuration:

where:C 1 = cost of an intercell movement

C 2 =

cost per unit distance of an intracell movementN i = number of intercell movements for ith configuration

T = +i 1 2 N d k i ij ij j

m


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EXAMPLE:

Consider the following cell configuration.

Machines1 5 2 3 4 7 8 9 10 6

M 1 1 1 1 1 1 1 1 1 1

M 5 1 1 1 1 1 1 1 1

M 2 1 1 1 1 1

M 4 1 1 1 1 1 1M 3 1 1 1 1

Components

Consider 3 -cell case:


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Consider cell case:

Expected movement distance,

in cell (M1, M 5 ) =

in cell (M2 , M4 ) =

in cell (M3 ) = 0

2 1

31!

2 13 1!

The number of moves passing through two


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The number of moves passing through twomachines by all the parts,

in cell (M1, M 5 ) = 7in cell (M2 , M4 ) = 5in cell (M3 ) = 0

The total distance for all intercell moves for 3 -cell configuration:

1 x 7 + 1x 5 + 0 = 12

The number of intercell moves in 3 -cellconfiguration is 10.


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Assume:

C1=

$2

.00 ( cost of intercell unit movement)C2 = $1.00 ( cost of intracell unit movement)

The total cost of in t e rc e ll and in trac e llmovements in 3 -cell configuration:

2 .00 $ x 10 + 1.00 $ x 1 2 = 32 .00 $

The summary of cost calculation for all possible cellconfiguration is given in the following table:


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configuration is given in the following table:C ell

configuration

Number of

intercellmoves

Total distance of

intracellmoves

Total cost of

intercell andintracellmoves

5-cells (M1) , (M2) ,(M3) , (M4) ,(M5)

22 0 2 x 22 +1 x 0 = 44

4-cells (M2 , M4) , (M5) ,(M1) , (M3)

18 5 2 x 18 +1 x 5 = 41

3-cells (M2 , M4) , (M1 ,M5) , (M3)

10 12 2 x 10 +1 x 12 = 32

2-cells (M1 , M2 , M4,M5) , (M3) 4 30 2 x 4 +1 x 30 = 38

1-cells (M1 . M2 , M3,M4, M5)

0 44 2 x 0 +1x 44 = 44


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A survey of 53 show that the use of GT andcellular manufacturing in US industries has metwith success. The benefits reported from thesestudies include:

Reduction in throughput time by 4 6%Reduction in work-in-process inventory by 41 %Reduction in material handling by 39%Reduction in setup time by 32%Improvement in quality by 29 . 6%

ch6 group technology and cellular manufacturing

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