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egekwu.331 1
ISAT 331
Module 3: GROUP TECHNOLOGY
AND
PROCESS PLANNING
egekwu.331 2
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.
egekwu.331 3
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
egekwu.331 5
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
egekwu.331 6
Process Layout – typical of most job shops
egekwu.331 7
GT layout
egekwu.331 8
WIP Distribution – Machined Part Fabrication
5% Moving and Waiting - 95%
Cutting
< 30%Position, loading,gauging, idle, etc. 70 %
egekwu.331 9
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
egekwu.331 10
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)
egekwu.331 11
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.
egekwu.331 12
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
egekwu.331 13
Design Attributes
Grouping parts into families is based on design and/or manufacturing attributes (features)
egekwu.331 14
Manufacturing Attributes
egekwu.331 15
Design and Manufacturing Attributes
egekwu.331 16
Grouping Methods – [ Bedworth Figs. 5.3
and 5.4]
egekwu.331 17
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.
egekwu.331 18
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.
egekwu.331 19
Part Families Dev’t - Fig 5.5
egekwu.331 20
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.
egekwu.331 21
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)
egekwu.331 22
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.
egekwu.331 23
Example-Hierarchical (Bedworth fig. 5.6)
egekwu.331 24
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.
egekwu.331 25
Example - Attribute Code (fig. 5.7)
How might one use attribute code for retrieving part families that require identical processing?
egekwu.331 26
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)
egekwu.331 27
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.
egekwu.331 28
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.
egekwu.331 29
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.
egekwu.331 30
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
egekwu.331 31
DCLASS Code - fig. 5.8
egekwu.331 32
DCLASS - fig. 5.14
egekwu.331 33
DCLASS - fig. 5.15
egekwu.331 34
DCLASS - fig. 5.16
egekwu.331 35
DCLASS - Tables 5.3, 5.4, and 5.5
egekwu.331 36
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 ...
egekwu.331 37
MICLASS contd. - fig. 5.17
egekwu.331 38
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.
egekwu.331 39
Other Coding Systems -----figs. 5.10 and 5.12
Examples:1. CODE eight-digit hybrid code2. OPITZ nine-digit hybrid code
egekwu.331 40
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).
egekwu.331 41
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.
egekwu.331 42
SLCA contd. (fig. 5.21)
egekwu.331 43
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
egekwu.331 44
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
egekwu.331 45
Fig 5.23 and Table 5.6
egekwu.331 46
SLCA dendrogram
egekwu.331 47
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.
egekwu.331 48
Enhancements to SLCA
Anderberg’s Algorithm:» Sij = 2a/(2a + b + c)
» this gives more weight to similar machines and thus limits or controls Chaining.
egekwu.331 49
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
egekwu.331 50
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..
egekwu.331 51
ALCA - fig. 5.25
egekwu.331 52
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.
egekwu.331 53
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.
egekwu.331 54
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.
egekwu.331 55
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.
egekwu.331 56
Tooling Costs fig. 5.28
egekwu.331 57
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.
egekwu.331 58
Types of Layout - fig. 5.20
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