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ISAT 331

Module 3: GROUP TECHNOLOGY

AND

PROCESS PLANNING

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GROUP TECHNOLOGY – [Chapter 5 of Bedworth]

Introduction of GT Development of Part Families Coding and Classification-basis for GT

» coding schemes» examples of coding systems

Cellular Manufacturing Economic Considerations - production

planning, tool analysis.

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Definition of GT

GT is an engineering and manufacturing philosophy that groups parts together based on their similarities in order to achieve economies of scale in a small-scale environment.» economies of scale is associated with

large-scale production» economies of scope is also realized

egekwu.331 4Production Quantity

Pro

duct

Var

iety

100 10,000 1 M

Product Variety vs Production Quantity

Hard

Soft

Low High

Job Shop

Mass Production

Mid VarietyMid Production(Most Difficult)

Changeover (set up)Time

MH automated

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Mass Production

Production Quantity

Pro

duct

Var

iety

100 10,000 1 M

Types of Production Plant (facilities) and Layout

Hard

Soft

Low High

•Fixed Position (Large)•Process

•Product (Flow line)•Process (Quantity)

•Process (Batch)•Cellular (GT families)•FMS (GT families- automated MH)

Job Shop

Mid VarietyMid Production

(Apply GT)

Efficiency

Flexibility

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Process Layout – typical of most job shops

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GT layout

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WIP Distribution – Machined Part Fabrication

5% Moving and Waiting - 95%

Cutting

< 30%Position, loading,gauging, idle, etc. 70 %

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Characteristics of Job Shops - (operations

scheduling)

low-volume production, lot sizes small machining centers organized by

manufacturing function high labor content in product costs general-purpose machinery significant changeover time little automation of material handling large variety of products

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Need for GT

need to improve productivity in a job shop or batch production.

approx. 75% of all manufactured parts in the US are made on a small lot basis.

need for design retrieval and cell mfg. grouping similar parts should improve

design and manufacturing efforts - HOW?? - (design, mfg., and tool engineers)

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Importance of GT to CAD/CAM Integration

GT facilitates structuring and archiving of product data e.g. design and manufacturing attributes.

provides common language for users. facilitates integration of different part-

related information. GT is key to CAPP-computer aided

process planning.

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Geometric Characteristics Production Process Characteristics

Attributes

Grouping into Part Families

GT Classification

SIZE SHAPENO. of OperationsSequence

Type of Operation

ProcessCondition

ToolingType

HoldingMethod

ProcessMethod

based on

based on

based on

Successful groupingis key to GT Implementation.

determined by

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Design Attributes

Grouping parts into families is based on design and/or manufacturing attributes (features)

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Manufacturing Attributes

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Design and Manufacturing Attributes

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Grouping Methods – [ Bedworth Figs. 5.3

and 5.4]

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Methods for Developing Part Families

Three Basic General Methods (A) Manual Visual Search

– low reliability, not used in formal GT application– different knowledge of processes result in

different groupings– differences in identification of important

attributes– groupings differ ‘cause different tool/machine

combination can be used in fabricating a part.

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Part Families Development contd.

(B) Production Flow Analysis (PFA)– analyzes sequence of operation for part

fabrication (Route Sheet- show transparency).– parts that go through similar operations are

grouped together– machines used for the operations are also

grouped together– mach./component chart is formed and sorted -

clustering techniques are often used– depends on accurate routing sheets.

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Part Families Dev’t - Fig 5.5

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Part Families Dev’t contd.

(C) Classification and Coding– coding involves the assignment of

representative symbols to a part– symbols relate to different part attributes– coding system is unique to a company– expensive but payback is high because it forms

basis for design info. retrieval & cell production– for robustness, design and mfg. attributes are

coded - E.G. shape, material, size; and tolerances, processes, tool requirement, etc.

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Classification and Coding contd.

There are many coding systems for GT application - no single system is universally accepted

3 basic types of systems are:» hierarchical (monocode)» attribute (polycode, a chain code, discrete

code or fixed-digit code)» hybrid (mixed)

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Hierarchical Code

characters in a code are dependent on the meaning of previous characters

characters “amplify” the information of the previous character

adequate for capturing design specific information (shape, material, size, etc.)

not robust enough for analyzing process-related information.

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Example-Hierarchical (Bedworth fig. 5.6)

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Attribute Code

Characters are independent of others in the code

each part attribute is assigned a specific position in the code

preferred by manufacturing - easy to identify parts that require similar processes

disadvantage - code could be very long.

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Example - Attribute Code (fig. 5.7)

How might one use attribute code for retrieving part families that require identical processing?

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Hybrid Code

combines the benefits of an attribute code (ease of identifying specific part features) and the need for a compact code (data base space and management)

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Selecting a Coding System - Factors to Consider

> 100 coding systems to choose from A) Objective - user needs (engineering,

manufacturing or both)» Engineering Objectives - retrieval, part

information, mfg capability and producibility analysis.

» Mfg. Objectives - info. for part families, process plan retrieval, machine groupings.

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Selecting Coding System contd.

B) Robustness - able to handle current and future parts.

C) Expandability - ease of expansion. D) Differentiation - balance both

similarities and differences in parts. E) Automation - ascertain degree of

automation of coding, data base retrieval and analysis functions.

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Selecting Coding System contd.

F) Efficiency - number of digits required to code a part.

D) Cost - initial, maintenance and modification costs.

H) Simplicity - ease of use.

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DCLASS Coding System

is an 8-digits system that is partitioned into 5 code segments

based on some basic premises (5 total)» completely characterize parts on the basis

of: 1. basic shape 2. features 3. size

4. precision and 5. material type, form, and condition.

1 2 41B 3 A 1BASIC SHAPE Form Size Precision Materials

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DCLASS Code - fig. 5.8

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DCLASS - fig. 5.14

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DCLASS - fig. 5.15

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DCLASS - fig. 5.16

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DCLASS - Tables 5.3, 5.4, and 5.5

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Coding Systems contd.

MICLASS Coding System:» MICLASS = Metal Institute Classification

System» consists of two major sections (segments)» first segment is mandatory-total of 12 digits » first 4 digits describe main shape and their

elements» second 4 digits describe dimensions ...

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MICLASS contd. - fig. 5.17

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MICLASS contd.

second segment is optional can contain up to 18 characters;

reserved for company specific info. typical info include: vendors, lot sizes,

costs, producibility tips MICLASS uses an interactive computer

program for coding and classifying info. in data base - see fig. 5.19.

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Other Coding Systems -----figs. 5.10 and 5.12

Examples:1. CODE eight-digit hybrid code2. OPITZ nine-digit hybrid code

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Clustering Techniques: Single-Linkage Clustering Algorithm (SLCA)

algorithm utilizes similarity coefficient to group parts requiring similar process

similarity coefficient is calculated for each pair of machines to ascertain:» how alike the 2 machines are based on number of

parts that “visit” both machines and» number of parts that “visit” each machine only

(and doesn’t visit the second machine).

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SLCA - Similarity Coeff.

Sij = a/(a+b+c)

» where,» s = similarity coefficient between mach i and j» a = # of parts common to both machines» b = # of parts that visit only machine i» c = # of parts that visit only machine j

Determine similarity coefficient between machines A and D - fig. 5.21.

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SLCA contd. (fig. 5.21)

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SLCA Steps

calculate pair-wise similarity coefficient for each machine - coefficients will form a symmetric matrix

identify largest coefficient. - the associated machines form initial cluster

identify largest remaining coefficient - associated machines are also grouped

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SLCA contd.

repeat steps 2 and 3 above until all machines are clustered into one group - or until a threshold is reached.» threshold level is used to control number of

clusters formed. see fig. 5.23, Table 5.6 and fig. 5.24

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Fig 5.23 and Table 5.6

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SLCA dendrogram

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Advantages/Disadvantages of SLCA

An Advantage of SLCA is that it provides a powerful systematic way of grouping machines for GT mfg.

Disadvantages:» no clear direction on how to achieve ideal machine-

groups. To decide, one need info. on a) no. of inter-group/intra-group movements b) machine utilization c) planning and control and d) bottleneck machines.

» Chaining can occur – page 209 of Bedworth.

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Enhancements to SLCA

Anderberg’s Algorithm:» Sij = 2a/(2a + b + c)

» this gives more weight to similar machines and thus limits or controls Chaining.

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Average - Linkage Clustering Algorithm

(ALCA)

Sij= sij / (Ni x Nj)

» where:

» sij = sum of similarity coefficient between all machines of the two groups

» Ni, Nj is no. of machines in group i and j, respectively.

Example: machines A and B belong to group i and machines C, D, and E to group j; Calculate SAB, CDE

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ALCA Steps

calculate pair-wise similarity coefficient for all machines

locate largest coefficient - the 2 machines form initial cluster

calculate average similarity coefficient between new cell and remaining cells - revise similarity matrix

repeat steps 2 and 3 above. Examples..

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ALCA - fig. 5.25

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Facility Design with GT

facility layout is critical to many manufacturing performance measures

3 major types of machine (process) layout - line, functional, group/cell layout» Discuss in terms of: work balancing,

operational costs, material handling, setup, throughput, production control - capacity planning, job scheduling.

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GT and Mfg Cells

used to show logical implementation steps for GT

benefits include: » reduction in number of perishable tools» lower setup times» lower tooling costs - tools can be “kited”» improvement in efficiency of new

equipment.

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Economic Modeling of GT

components of product mfg cost» direct material » direct labor» overhead (materials and labor)

involves minimization of total production costs over a planning horizon (ISAT 330)» subject to constraints of labor, equipment,

demand, etc.

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Group Tooling Economic Analysis

there is a marked improvement in group tooling cost (total and unit cost) over conventional tooling cost.

see fig. 5.28.

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Tooling Costs fig. 5.28

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Typical Savings Realized from Successful GT

program-p. 221

Benefits to Design function Benefits to mfg benefits to Management see advantages and disadvantages on

pages 226 – 227 of Bedworth.» under utilization of some machines in a

group - plant-wide benefit vs. sub-optimization of individual machines.

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Types of Layout - fig. 5.20