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[Type text] [Type text] PEC/ME/IV YR/ 7TH SEM/CIM/KRP

ME6703 COMPUTER INTEGRATED MANUFACTURING SYSTEMS

UNIT III - CELLULAR MANUFACTURING Group Technology(GT), Part Families – Parts Classification and coding – Simple Problems in Opitz Part

Coding system – Production flow Analysis – Cellular Manufacturing – Composite part concept – Machine

cell design and layout – Quantitative analysis in Cellular Manufacturing – Rank Order Clustering Method -

Arranging Machines in a GT cell – Hollier Method – Simple Problems.

TEXT BOOK:

1. Mikell.P.Groover “Automation, Production Systems and Computer Integrated Manufacturing”, Prentice Hall of India, 2008. 2. Radhakrishnan P, Subramanyan S.and Raju V., “CAD/CAM/CIM”, 2nd Edition, New Age International (P) Ltd, New Delhi, 2000.

REFERENCES:

1. Kant Vajpayee S, “Principles of Computer Integrated Manufacturing”, Prentice Hall India, 2003.

2. Gideon Halevi and Roland Weill, “Principles of Process Planning – A Logical Approach” Chapman & Hall, London, 1995.

3. Rao. P, N Tewari &T.K. Kundra, “Computer Aided Manufacturing”, Tata McGraw Hill Publishing Company, 2000.

4.V.Jayakumar, “Computer Integrated Manufacturing”, Lakshmi Publications.

UNIT III CELLULAR MANUFACTURING

What is Group Technology (GT)?

GT is a theory of management based on the principle that similar things should be done similarly

GT is the realization that many problems are similar, and that by grouping similar problems, a single

solution can be found to a set of problems thus saving time and effort

GT is a manufacturing philosophy in which similar parts are identified and grouped together to take

advantage of their similarities in design and production

Where to implement GT?

Plants using traditional batch production and process type layout

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If the parts can be grouped into part families

How to implement GT?

Identify part families

Rearrange production machines into machine cells

Types of Layout

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 three main types of layout are:

• Line (product) Layout

• Functional Layout

• Group Layout

Line (product) Layout

• It involves the arrangements of machines in one line, depending on the sequence of operations. In

product layout, if there is a more than one line of production, there are as many lines of machines.

• 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 Layout

• 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.

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Group Layout

• 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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Process flow before the group technology

Process flow after the group technology

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Identifying Part Families

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

in 1. Design attributes and

2. Manufacturing features

1. Design Attributes:

part configuration (round or prismatic)

dimensional envelope (length to diameter ratio)

surface integrity (surface roughness, dimensional tolerances)

material type

raw material state (casting, forging, bar stock, etc.)

2. Part Manufacturing Features:

Operations and operation sequences (turning, milling, etc.)

batch sizes

machine tools

cutting tools

work holding devices

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processing times

Group technology emphasis on part families based on similarities in design attributes and manufacturing,

therefore GT contributes to the integration of CAD and CAM.

Part Families

A part family is a collection parts having similar design characteristics or manufacturing processes

Three methods for identifying parts families

• Visual inspection

• Classification and coding

• Production flow analysis

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Forming Part Families –

1. Visual Inspection Method

incorrect results

human error

different judgment by different people

inexpensive

least sophisticated

good for small companies having smaller number of parts

Forming Part Families – 2. Classification and Coding

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( Eg. OPITZ coding system)

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Reasons for using a coding Scheme :

Selection of a coding system:

Coding systems:

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i)OPITZ classification system

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Form Code (digits 1 – 5)

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Example: 1. Optiz part coding System

• Given the rotational part design below, determine the form code in the Optiz parts classification

and coding system.

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Solution

• Length-to-diameter ratio: L/D = 1.5 Digit 1 = 1

• External shape: both ends stepped with screw thread on one end Digit 2 = 5

• Internal shape: part contains a through hole Digit 3 = 1

• Plane surface machining: none Digit 4 = 0

• Auxiliary holes, gear teeth, etc.: none Digit 5 = 0

The form code in the Optiz system is : 15100

Example: 2. Optiz part coding System

Part class:

Rotational part, L/D = 9.9/4.8 ≈ 2.0 based on the pitch circle diameter of the gear, so the first digit = 1

External shape:

The part is stepped on one side with a functional groove, so the second digit is 3

Internal shape:

The third digit is 1 because of the through hole

Plain surface machining:

The fourth digit is 0 because there is no plain surface machining

Auxiliary holes and gear teeth:

The fifth digit is 6 because there are spur gear teeth on the part

The form code in the Optiz system is : 13106

3.

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L / D = 4.2 / 35 = 0.12

4.

5.

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Selection of Classification and Coding Systems

Identified objectives, reasons and users

Robustness and Expandability

Automation and Efficiency

Simplicity and User Interface

Cost

ii) MICLASS

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iii) DCLASS coding system:

3. Forming Part Families – Classification and Coding: Production Flow Analysis (PFA)

PFA is a method for identifying part families and associated machine groupings based on the required

manufacturing processes needed for each part.

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BENEFITS OF GROUP TECHNOLOGY

It affects all areas of a company, including:

engineering

equipment specification

facilities layout

process planning

production control

quality control

tool design

purchasing

service

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

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

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

• Standardization of equipment

• Implementation of cellular manufacturing systems

• Significant reduction in up-front costs incurred in the release of new parts for manufacture

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

5. Manufacturing: production control

• Reduced work-in-process inventory

• Easy identification of bottlenecks

• Improved material flow and reduced warehousing costs

• Faster response to schedule changes

• Improved usage of jigs, fixtures, pallets, tools, material handling, and manufacturing equipment

6. Manufacturing: quality control

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• Reduction in number of defects leading to reduced inspection effort

• Reduced scrap generation

• Better output quality

• Increased accountability of operators and supervisors responsible for quality production, making it

easier to implement total quality control concepts.

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

8. Customer service

• Accurate and faster cost estimates

• Efficient spare parts management, leading to better customer service

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Cellular 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

A manufacturing cell: is a cluster of machines or processes located in close proximity and dedicated to the

manufacturing of a family of parts

Why cellular manufacturing?

Reduce setup times: „By using part family tooling and sequencing

Reduce flow times:„By reducing setup and move times and wait time for moves and using smaller batch

sizes

Reduce inventories

Reduce lead time

Design

Machine cell Design : Types :-

Type - 1

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Type 2

Type 3

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Type 4

Best Machine Arrangement

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• 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 (Structural Issues)

and system operation (Procedures Issues)

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 layout

Procedural Issues include:

• Detailed design of jobs

• Organization of supervisory and support personnel

• Formulation of maintenance and inspection policies

• Design of procedures for production planning, scheduling, control, and acquisition of related

software and hardware

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• Modification of cost control and reward systems

Cell Formation Approaches

1. Machine - Component Group Analysis:

Machine - Component Group Analysis is based on production flow analysis

Production flow analysis involves four 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.

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. „

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.

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.

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

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Rank Order Clustering Algorithm

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

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

following formulas:

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.

n

1=p

m

1=p

p-m

ip

2 = jcolumn for weight

2b = i rowfor weight

pn

pjbDecimal

Decimal

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Exceptional parts and Bottleneck machines:

• Not always to get neat cells

• Some parts are processed in more than one cell

o Exceptional parts

o Machine processing them are called bottleneck machines

Solutions for overcoming this problem ?

• Duplicate machines

• Alternate process plans

• Subcontract these operations

Problem1:-

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Problem 2:-.

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Problem 3:-.

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Problem 4:-. Solve the following part – machine incidence matrix by rank order clustering method.

**

Parts 1 2 3 4 5 6 7 8 9 10 11 12

A x x x x x

B x x x x

C x x x

D x x x x x

E x x x

F x x x

G x x x x

H x x x

Machines

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Parts 1 2 4 8 10 3 6 9 5 7 11 12

A x x x x x

D x x x x x

F x x x

C x x x

G x x x x

B x x x x

E x x x

H x x x

Machines

Machine cell Part family

1 2 4 8 10 A D F

3 6 9 C G

5 7 11 12 B E H

Hollier Method 1.

The first method uses the sums of flow "From" and "To" each machine in the cell. The method can be outlined

as follows:

1. Develop the From—To chart from part routing data. The data contained in the chart indicates number of

part moves between the machines for workstations)

2. Determine the "From” and "To" sums for each machine. This is accomplished by summing all of the

"From" trips and "To" trips for each machine (or operation ).The "From" sum for a machine is determined by

adding the entries in the corresponding row and the "To" sum is found by adding the entries in the

corresponding column.

3. Assign machines to the cell based on minimum "From" or To sums. The machine having the smallest

sum is selected. If the minimum value is a "To" sum, then the machine is placed at the beginning of the

sequence. If the minimum value is a “From” sum, then the machine is placed at the end of the sequence. Tie breaker rules:

(a) If a tie occurs between minimum. "To" sums or minimum "From" sums, then the machine with the

minimum “From/To” ratio is selected.

(b) If both "To" and "From" sums are equal for a selected machine, it is passed over and the machine with the

next lowest sum is selected.

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(c) If a minimum "To" sum is equal to a minimum "From" sum, then both machines are selected and placed at

the beginning and the end of the sequence respectively.

Reformat the From-To chart : After each machine has been selected, restructure the From-To chart by eliminating the

row and column corresponding to the selected machine and recalculate the "From" and "To" sums. Repeat steps 3 and 4

until all machines have been assigned.

Example 1: Group Technology machine Sequence using Holier Method 1 Suppose that four machines 1, 2, 3,

and 4 have been identified as belonging in a GT machine cell. an analysis of 50 parts processed on these

machines has been summarized in the From-To chart of Table 2. Additional information is that 50 parts enter

the machine grouping at machine 3, 20 parts leave after processing at machine 1 and 30 parts leave after

machine 4. Determine a logical machine arrangement using Hollier Method 1.

Solution: Summing the From trips and To trips for each machine yields the "From" and "To" sums in Table

2(a). The minimum sum value is the "To" sum for machine 3. Machine 3 is therefore placed at the beginning of

the sequence. Eliminating the row and column corresponding to machine 3 yields the revised From To chart in

Table 3 (b).

Table: 1 From-to chart for example 2

Figure: 2.a From and to some example 2:first iteration

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Figure: 2.b From and to some example 2:second iteration with machine 3removed

Figure: 2.c From and to some example 2: third iteration with machine 2 removed

The minimum sum in this chart is the "To" sum corresponding to machine 2, which is placed at the front of the

sequence, immediately following machine 3. Eliminating machine 2 produces the revised From-To chart in

Table 2 (c). The minimum sum in this chart is the "To" sum for machine 1. Machine 1 is placed after machine 2

and finally machine 4 is placed at the end of the sequence.

Thus, the resulting machine sequence is 3 —> 2 —> 1 --> 4

Hollier Method 2. This approach is based on the use of From/To ratios formed by summing the total flow from

and to each machine in the cell. The method can be reduced to three steps:

1. Develop the From—To chart. This is the same step as in Hollier Method 1.

2. Determine the From/To ratio for each machine. This is accomplished by summing up all of the "From"

trips and "To" trips for each machine (or operation). The "From" sum for a machine is determined by adding the

entries in the corresponding row and the "To" sum is determined by adding the entries in the corresponding

column. For each machine, the From/To ratio is calculated by taking the "From" sum for each machine and

dividing by the respective "To" sum.

3. Arrange machines in order of decreasing From/To ratio. Machines with a high From/To ratio distribute

work to many machines in the cell but receive work from few machines. Conversely machines with a low

From/To ratio receive more work than they distribute. Therefore, machines are arranged in order of descending

From/Ip ratio. That is, machines with high ratios are placed at the beginning of the work flow and machines

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with low ratios are placed at the end of the work flow. In case of a tie, the machine with the higher "From"

value is placed ahead of the machine with a lower value.

Example 1: Group Technology Machine Sequence using Holler Method 2

Solve Example using Hollier Method 2.

Solution: Table 2 (a), containing the "From" and "To" sums, is repeated in Table (2) along with the From/To

ratios given in the last column on the right. Arranging the machines in order of descending From/To ratio, the

machines in the cell should be sequenced as follows:

3 —> 2 —> 1 --> 4

Table: 2 From To Sums And From/To Ratios For Examples

Figure: Flow Diagram For Machine Cell on example

It is helpful to use one of the available graphical techniques, such as the flow diagram to conceptualize the work

flow in the cell. The work flow is mostly in line; however, there is some back flow of parts that must be

considered in the design of any material handling system that might be used in the cell. A powered conveyor

would be appropriate for the forward flow between machines, with manual handling for the back flow.

For example for data in figure , Hollier Methods 1 and 2 are provide the same solution. This is not always the

case. The relative performance of the two methods depends on the given problem. In some problems Method 1

will outperform Method 2 and in other problems the opposite will happen. In many problems the two methods

yield identical solutions, as in Examples 1 and 2.

Two performance measures can be defined to compare solutions to the machine sequencing problem:

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• Percentage of in-sequence moves and

• Percentage of backtracking moves.

The percentage of in sequence moves is computed by adding all the values representing in sequence

moves and dividing by the total number of moves.

The percentage of back tracking moves is determined by summing all of the values representing back

tracking moves and dividing by the total number of moves.

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Cellular layout advantages

• Reduced material handling and transit time

• Reduced setup time

• Reduced work-in-process inventory

• Better use of human resources

• Better scheduling, easier to control and automate

• Less floor space required

• Reduced direct labor

• Heightened sense of employee participation

• Increased use of equipment & machinery

• Reduced investment on machinery & equipment

Cellular layout disadvantages

• Sometimes cells may not be formed because of inadequate part families

• Some cells may have a high volume of production and others very low. This results in poorly balanced cells

• When volume of production changes, number of workers are adjusted and workers are reassigned to various cells.

To cope with this type of reassignments, workers must be multi-skilled and cross-trained

• Sometimes, machines are duplicated in different cells. This increases capital investment

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