Group Technology, coding and cell design

Lecture # 13-18
Group Technology/ Coding systems

2

Group Technology
Batch manufacturing is estimated to be the most

common form of production in the United States,
constituting more than 50% of total manufacturing
activity.
There is a growing need to make batch
manufacturing more efficient and productive.
In addition, there is an increasing trend toward
achieving a higher level of integration between the
design and manufacturing functions in a firm.
An approach directed at both of these objectives
is group technology (GT).
3

Group Technology
Group technology is a manufacturing philosophy

in which similar parts are identified and grouped
together to take advantage of their similarities in
design and production.
Similar parts are arranged into part families, where
each part family possesses similar design and/or
manufacturing characteristics.
For example, a plant producing 10,000 different
part numbers may be able to group the vast
majority of these parts into 30-40 distinct families.

4

Group Technology
The manufacturing efficiencies are generally

achieved by arranging the production equipment into
machine groups or cells, to facilitate work flow.
Grouping the production equipment into machine
cells, where each cell specializes in the production of
a part family, is called cellular manufacturing.

5

Group Technology
GT is most appropriately applied under the following

conditions:
The plant currently uses traditional batch
production and a process type layout and this
results in much material handling effort, high inprocess inventory, and long manufacturing lead
times.
The parts can be grouped into part families. This
is a necessary condition. Each machine cell is
designed to produce a given part family or limited
collection of part families, so it must be possible to
group parts made in the plant into families.

6

Group Technology
There are two major tasks that a company must

undertake when it implements group technology.
These two tasks represent significant obstacles to the
application of GT.
Identifying the part families. If the plant makes
10,000 different parts, reviewing all of the part
drawings and grouping the parts into families is a
substantial task that consumes a significant amount
of time.
Rearranging production machines into machine
cells. It is time consuming and costly to plan and
accomplish this rearrangement and the machines are
not producing during the changeover.

7

Group Technology - Part Families
Group

technology offers substantial benefits to
companies that have the perseverance to implement it.
The benefits include:
GT promotes standardization of tooling, fixturing and
setups.
Material handling is reduced because parts are moved
within a machine cell rather than within the entire
factory.
Process planning and production scheduling are
simplified.
Setup times are reduced, resulting in lower
manufacturing lead times.
Work-in-process is reduced.
Worker satisfaction usually improves when workers
collaborate in a GT cell.
Higher quality work is accomplished using group
technology.
8

Part Families
A part family is a collection of parts that are similar

either because of geometric shape and size or because
similar processing steps are required in their
manufacturing.
A group of parts that possess similarities in geometric
shape and size, or in the processing steps used in
their manufacture
Part families are a central feature of group technology
There are always differences among parts in a family
But the similarities are close enough that the parts
can be grouped into the same family

9

Two parts that are identical in shape and size but

quite different in manufacturing:
(a)1,000,000 units/yr, tolerance = ±0.010 inch, 1015
CR steel, nickel plate (CR = Cold Rolled )
(b)100/yr, tolerance = ±0.001 inch, 18-8 stainless
steel

10

Ten parts that are different in size and shape, but

quite similar in terms of manufacturing
All parts are machined from cylindrical stock by
turning; some parts require drilling and/or
milling

11

The biggest single obstacle in changing over to
group technology from a conventional production
shop is the problem of grouping the parts into
families.
There are three general methods for solving this
problem, which involve the analysis of much data
by properly trained personnel.

12

1) Visual inspection - using best judgment to group parts

into appropriate families, based on the parts or photos of the
parts
2) Production flow analysis - using information contained
on route sheets to classify parts
3) Parts classification and coding - identifying similarities
and differences among parts and relating them by means of
a coding scheme

13

Group Technology
1) The visual inspection method is the least

sophisticated and least expensive method. It
involves the classification of parts into families
by looking at either the physical parts or their
photographs and arranging them into groups
having similar features.
Although this method is generally considered to
be the least accurate of the three, one of the first
major success stories of GT in the United States
made the changeover using the visual inspection
method.
14

Group Technology
2) 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)

16

Group Technology
Initially, a machine—component chart must be

formed. This is an M x N matrix, where
M = number of machines
N = number of parts
x = 1 if part j has an operation on machine i; 0
otherwise.
If the machine—component chart is small, parts
with similar operations might be grouped
together by
manually sorting the rows and columns.

17

Parts Classification and Coding
3) Parts Classification and Coding
This is the most time consuming of the three
methods. In parts classification and coding,
similarities among parts are identified, and
these similarities are related in a coding
system.

20

 Most classification and coding systems are one of the following:
 Systems based on part design attributes
 Systems based on part manufacturing attributes
 Systems based on both design and manufacturing attributes
 Part Design Attributes
 Major dimensions
 Basic external shape
 Basic internal shape
 Length/diameter ratio
 Material type
 Part function
 Tolerances
 Surface finish

21

Part Manufacturing Attributes
Major process
Operation sequence
Batch size
Annual production
Machine tools
Cutting tools
Material type

22

Classification and coding systems are devised to

include both a part's design attributes and its
manufacturing attributes. Reasons for using a
coding scheme include:
Design retrieval A designer faced with the task of
developing a new part can use a design retrieval
system to determine if a similar part already
exists. A simple change in an existing part would
take much less time than designing a whole new
part from scratch.

23

Automated process planning The part code for a new part

can be used to search for process plans for existing parts
with identical or similar codes.
Machine cell design The part codes can be used to design
machine cells capable of producing all members of a
particular part family, using the composite part concept.

24

Coding methods:
These 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.

25

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

26

Monocode or hierarchical code
The structure of Monocode is like a tree in

which each symbol amplifies the information
provided in the previous digit.

27


Structure of Monocode
30

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.

31

Poly Code
Chain-type structure, known as a polycode, in which

the interpretation of each symbol in the sequence is
always the same; it does not depend on the value of
preceding symbols, so 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.
32

Group Technology
Mixed (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.

36

Some of the important systems
Opitz classification system –the University of

Aachen in Germany, nonproprietary, Chain type.
Brisch System –(Brisch-Birn Inc.)
CODE (Manufacturing Data System, Inc.)
CUTPLAN (Metcut Associates)
DCLASS (Brigham Young University)
MultiClass (OIR: Organization for Industrial
Research), hierarchical or decision-tree coding
structure
Part Analog System (Lovelace, Lawrence & Co.,
Inc.)
37

Group Technology
The OPITZ classification system:
It is a mixed (hybrid) coding system
Developed by Opitz, Technical University of Aachen,

1970
It is widely used in industry
It 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
38

Group Technology
 The Opitz coding system consists of three groups of digits:

Form
code
12345

part geometry
and features
relevant to part
design

Supplementary
code
6789

information
relevant to
manufacturing
(polycode)

Secondary
code
ABCD

Production
processes and
production
sequences

39

Form code in Opitz system for rotational parts in classes 3, and 4

43

Supplemental code in Opitz system

44

The OPITZ classification system
Example: A part coded 20801
2 - Parts has L/D ratio >= 3
0 - No shape element (external shape elements)
8 - Operating thread
0 - No surface machining
1 - Part is axial

45

Example

46

Example: Given the part design shown define the

"form code" using the Opitz system
Step 1: The total length of the part is 1.75, overall
diameter 1.25, L/D = 1.4 (code 1)
Step 2: External shape - a rotational part that is
stepped on both with one thread (code 5)
Step 3: Internal shape - a through hole (code 1)
Step 4: By examining the drawing of the part (code 0)
Step 5: No auxiliary holes and gear teeth (code 0)
Code: 15100

47

Group Technology
 SELECTION OF CLASSIFICATION AND CODING SYSTEMS

For the purpose of selecting or developing your own

code, it is important to understand the attributes of
classification and coding systems.

48

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.

49

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.

50


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

51

Functional and Cellular layouts of an electronics plant:

52

Cell 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.

54

Evaluation of Cell Design Decisions
The evaluation of design decisions can be categorized

as related to either
the system structure
or
the system operation.

55

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)

56

The system operation
Evaluations of cell system design are incomplete

unless they relate to the operation of the system.
A few typical performance variables related 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)

57

Cell Design
A major problem throughout the cell design

process is the necessity of trading off against each
other objectives related to structural parameters
and performance variables.
For example, higher machine utilization can be
achieved if several cells route their parts through
the same machine. The drawbacks are increased
queuing and control problems.

58

Cell Design
System cost and performance are affected by every

decision related to system structure and system
operation.

59

CELL FORMATION APPROACHES
Machine - Component Group Analysis:
Machine - Component Group Analysis is based

on production flow analysis
Production flow analysis involves four stages:

60

Production flow analysis
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.

61

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.

62

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.

63

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.

64

EXAMPLE:
Consider a problem of 4 machines and 6 parts. Try to
group them.
Components
Machine
s

1

2

3

4

5

6

M1

1

1

1

M2

1

1

1

M3

1

1

1

M4

1

1

1

65

Components
Machine
s

2

4

6

1

3

5

M1

1

1

1

M2

1

1

1

M3

1

1

1

M4

1

1

1

66

Quantitative Analysis in Cellular Manufacturing
Rank Order Clustering Algorithm:
Rank Order Clustering Algorithm is a simple

algorithm used to form machine-part groups.

67

Rank Order Clustering Algorithm
 Step 1: Assign binary weight and calculate a decimal weight for

each row and column using the following formulas:

m

Decimal weight for row

i = ∑ b ip 2 m-p
p =1

n

Decimal weight for column j = ∑ b pj 2 n − p
p =1



Where “i” is row no.; “j” is column number; m is number of
columns; n is number of rows; p is the component/part row or
column number
68

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.

69

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

1

2

3

4

5

M1

1

1

1

1

1

1

1

1

M2
M3

1

M4
M5

1
1

1
1

1

9

10

1

1

1
1

1

1

1

1

1

8

1

1

7

1
1

6

1
1

1

1

1
70

Binary weight
29

28

27

26

25

24

23

22

21

20

7

8

9

10

Decimal
equivalent

1

1

1

1

1007

1

1

451

Components
Machines

1

2

3

4

5

M1

1

1

1

1

1

1

1

1

M2
M3

1

M4
M5

1
1

1

1
1

1

1

1

1

1

6

568
1

1

1

1

1

1

1

455
1020
71

Binary weight
29 28 27

26 25 24

23 22

21 20

9

10

Components
Binary
weight

Machines

1

2

3

4

5

6

7

8

24

M5

1

1

1

1

1

1

1

1

23

M1

1

1

1

1

1

1

1

1

1

22

M3

1

21

M4

1

1

1

1

1

1

20

M2

1

1

1

1

1

Decimal
equivalent

1

1

1

28 27 27 27 28 20 28 26 11 11

72

Binary weight
29

28

27

26

25

24

23

22

21

20

9

Components
Binary
weight

Machines

1

5

7

2

3

4

8

6

24

M5

1

1

1

1

1

1

1

1

23

M1

1

1

1

1

1

1

1

22

M3

1

1

1

21

M4

1

1

1

20

M2

1

1

1

Decimal
equivalent

10

1020
1

1

1
1

Decimal
equivalent

1019
900

1

1

123

1

1

115

28 28 28 27 27 27 26 20 11 11

73

Solve using Rank Order Clustering Technique
Components
Machines

1

M1

2

3

1

6

7

8

9

1
1

1

M3
1

M4

1

1

1
1

1

1

M6
M7

5

1

M2

M5

4

1
1

1
1

1
74

Group Technology, coding and cell design

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