3 group technology / cellular manufacturing (inselfertigung)
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3 Group Technology / Cellular Manufacturing
(Inselfertigung)
Layout & Design Chapter 3 / 2(c) Prof. Richard F. Hartl
Group Technology (GT)
Observation already in 1920ies:product-oriented departments to manufacture standardized products in machine companies lead to reduced transportation
Can be considered the start of Group Technology (GT):Parts with similar features are manufactured together with standardized processes small "focused factories" are created as independent operating units within large facilities.
More generally, GT can be considered a “theory of management” based on the principle "similar things should be done similarly“
"things" .. product design, process planning, fabrication, assembly, and production control (here); but also other activities, including administrative functions.
Layout & Design Chapter 3 / 3(c) Prof. Richard F. Hartl
When to use GT?
See also Chapter 1 (Figure 1.5) Pure item flow lines are possible, if volumes are very
large. If volumes are very small, and parts are very different, a
functional layout (job shop) is usually appropriate In the intermediate case of medium-variety, medium-
volume environments, group configuration is most appropriate
Layout & Design Chapter 3 / 4(c) Prof. Richard F. Hartl
Cellular Manufacturing
Principle of GT: divide the manufacturing facility into small groups or cells of machines cellular manufacturing
Each cell is dedicated to a specified family of part types (or few “similar” families). Preferably, all parts are completed within one cell
Typically, it consists of a small group of machines, tools, and handling equipment
Layout & Design Chapter 3 / 5(c) Prof. Richard F. Hartl
Different Versions of GT
The idea of GT can also be used to build larger groups, such as for instance, a department, possibly composed of several automated cells or several manned machines of various types.
GT flow line classical GT cell GT center
Layout & Design Chapter 3 / 6(c) Prof. Richard F. Hartl
GT flow line
All parts assigned to a group follow the same machine sequence and require relatively proportional time requirements on each machine.
Automated transfer mechanisms may be possible. mixed-model assembly line (Chapter 4)
(Askin & Standridge, 1993, p. 167).
fräsen(aus)bohrendrehenschleifenbohren
Layout & Design Chapter 3 / 7(c) Prof. Richard F. Hartl
classical GT cell
Allows parts to move from any machine to any other machine. Flow is not unidirectional.
Since machines are located in close proximity short and fast transfer is possible.
(Askin & Standridge, 1993, p. 167).
Layout & Design Chapter 3 / 8(c) Prof. Richard F. Hartl
GT center
Machines located as in a process (job shops) But each machine is dedicated to producing only certain
Part families only the tooling and control advantages of GT; increased material handling is necessary
When large machines have already been located and cannot be moved, or
When product mix and part families are dynamic would require frequent relayout of GT cell
(Askin & Standridge, 1993, p. 167).
Layout & Design Chapter 3 / 9(c) Prof. Richard F. Hartl
Typical Manufacturing Cell (1)
Often u-shaped for short transport
Even if process layout not possible
Often typical material flow
Layout & Design Chapter 3 / 10(c) Prof. Richard F. Hartl
Typical Manufacturing Cell (2)
Example with 3 workers
Also u-shaped
Layout & Design Chapter 3 / 11(c) Prof. Richard F. Hartl
Advantages of GT Cell
Short transportation and handling (usually within cell) Short setup times because often same tools and fixtures can be
used (products are similar) High flexibility (quick reaction on changes) Investment cost low (no advanced technology necessary) Clear arrangement, few tools/machines easy to control High motivation and satisfaction of workers
(identification with “their" products) Small lot sizes possible short flow times
Layout & Design Chapter 3 / 12(c) Prof. Richard F. Hartl
How to Build Groups/Cells
Basic Idea: Typical Part Families
Items that look alike Items that are made with the same equipment
Layout & Design Chapter 3 / 13(c) Prof. Richard F. Hartl
Items That Look Alike
Most products that look similar are manufactured using similar production techniques (if similar material)
Parts are grouped because they have similar geometry (about the same size and shape) they should represent a part family,
e.g. cog wheels (gear wheels)of similar size and material
Layout & Design Chapter 3 / 14(c) Prof. Richard F. Hartl
Items That Are Made with Same Equipment
Layout & Design Chapter 3 / 15(c) Prof. Richard F. Hartl
How to Build Groups/Cells
Visual inspection “Items that look alike” may use photos or part prints utilizes subjective judgment (experience)
Classification & coding by examination of design & production data (same equipment) most common in industry time consuming & complicated
Layout & Design Chapter 3 / 16(c) Prof. Richard F. Hartl
Codes
The code should be sufficiently flexible to handle future as well as current parts
The scope of part types must be known (e.g. parts rotational, prismatic, sheet metal, etc.?)
The code must discriminate between parts with different values for key attributes (material, tolerances, required machines, etc.)
Layout & Design Chapter 3 / 17(c) Prof. Richard F. Hartl
Codes
Many coding systems have been developed None is universally applicable Most implementations require some customization
Functional classification coding based on part design attributes coding based on part manufacturing attributes coding based on combination of design & manuf. attributes
Structural classification Hierarchical Structure Chain Type Structure Hybrid structure (combination)
Layout & Design Chapter 3 / 18(c) Prof. Richard F. Hartl
Hierarchical Code
Meaning of a digit depends on values of preceding digits. The value of 3 in the third place may indicate
the existence of internal threads in a rotational part: "1232" a smooth internal feature: “2132"
Hierarchical codes are efficient:they only consider relevant information at each digit
But they are difficult to learn and remember because of the large number of conditional inferences.
Layout & Design Chapter 3 / 19(c) Prof. Richard F. Hartl
Chain Code
Each value for each digit of the code has a consistent meaning. The value 3 in the third place has the same meaning for all parts.
Easier to learn but less efficient (longer for same info) Certain digits may be meaningless for some/many parts.
Layout & Design Chapter 3 / 20(c) Prof. Richard F. Hartl
Hybrid Code
Both hierarchical and chain codes have advantages, many commercial codes are hybrid (combination of both)
Some section of the code is a chain code and then several hierarchical digits further detail the specified characteristics.
Several such sections may exist. One example of a hybrid code is Opitz
Layout & Design Chapter 3 / 21(c) Prof. Richard F. Hartl
Optiz Classification System
Three sections
Form Code:5 digitsdescribes the primary design attributes, e.g. shape
Supplementary Code:4 digitsmanuf. attributes. e.g.dimensions, material, accuracy, starting work piece shape
Secondary Code:company specific, e.g. type and sequence of prod. operations
12345 6789 ABCD
Layout & Design Chapter 3 / 22(c) Prof. Richard F. Hartl
Optiz Classification System
Layout & Design Chapter 3 / 23(c) Prof. Richard F. Hartl
Optiz in More Detail2 2 4 0 0
Layout & Design Chapter 3 / 25(c) Prof. Richard F. Hartl
Production Flow Analysis (PFA)
Basic idea: Items that are made with the same processes / the same
equipment These parts are assembled into a part family Can be grouped into a cell to minimize material handling
requirements.
Layout & Design Chapter 3 / 26(c) Prof. Richard F. Hartl
How to Build Groups/Cells using PFA
Many clustering methods have been developed Can be classified into:
Part family grouping: Form part families and then group machines into cells
Machine grouping: Form machine cells based upon similarities in part routing and then allocate parts to cells
Machine-part grouping: Form part families and machine cells simultaneously.
Layout & Design Chapter 3 / 27(c) Prof. Richard F. Hartl
Machine-Part Grouping: Obtain Block Diagonal Structure
Construct matrix of machine usage by parts sort rows (machines) and columns (parts) so that a
block-diagonal shape is obtained
Then it is easy to build groups: Group 1: parts {13, 2, 8, 6, 11 }, machines {B, D} Group 2: parts { 5, 1, 10, 7, 4, 3}, machines {A, H, I, E} Group 3: parts { 15, 9, 12, 14}, machines {C, G, F}
Layout & Design Chapter 3 / 28(c) Prof. Richard F. Hartl
King’s Algorithm (Rank Order Clustering) Binary Ordering
How to obtain block-diagonal shape? Example: 5 machines; 6 parts: Interpret rows and columns as binary numbers
Sort rows w.r.t. decreasing binary numbers
Sort columns w.r.t. decreasing binary numbers
part
machine 1 2 3 4 5 6
A - 1 - 1 - -
B 1 - 1 - 1 1
C - 1 1 1 - 1
D 1 - - - 1 1
E - - - 1 1 -
Layout & Design Chapter 3 / 29(c) Prof. Richard F. Hartl
Binary Ordering
Sort rows w.r.t. decreasing binary numbers
New ordering of machines: B – D – C – A - E
part value
machine 1 2 3 4 5 6
A - 1 - 1 - -
B 1 - 1 - 1 1
C - 1 1 1 - 1
D 1 - - - 1 1
E - - - 1 1 -
25
3224 16
23 8
22 4
21 2
20 1
0101002 = 22 + 24 = 20
20 + 21 + 25 = 35
20 + 21 + 23 + 25 = 43
20 + 22 + 23 + 24 = 29
21 + 22 = 6
Layout & Design Chapter 3 / 30(c) Prof. Richard F. Hartl
Binary Ordering
part
machine 1 2 3 4 5 6 value
B 1 - 1 - 1 1 43
D 1 - - - 1 1 35
C - 1 1 1 - 1 29
A - 1 - 1 - - 20
E - - - 1 1 - 6
value
20 = 1
21 = 2
22 = 4
23 = 8
24 = 1623
+ 2
4 =
24
22 +
24
= 2
0
21 +
22
= 6
20 +23 +
24 =25
20 +21 +
22 =7
22 +23 +
24 =28
Sort columns w.r.t. decreasing binary numbers
New ordering of parts:
6-5-1-3-4-2
Layout & Design Chapter 3 / 31(c) Prof. Richard F. Hartl
Result of Binary Ordering
part
machine 6 5 1 3 4 2
B 1 1 1 1 - -
D 1 1 1 - - -
C 1 - - 1 1 1
A - - - - 1 1
E - 1 - - 1 -
value 28 25 24 20 7 6
2 groups: Group 1: parts {6, 5, 1 },
machines {B, D} Group 2: parts { 3, 4, 2},
machines {C, A, E}
Parts 1, 4, and 2 can be produced in one cell
No complete block-diagonal structure
Remaining items: 6, 5, and 3 produced in both cells Or machines B, C, and E have to be duplicated
Layout & Design Chapter 3 / 32(c) Prof. Richard F. Hartl
Repeated Binary Ordering
Binary Ordering is a simple heuristic no guarantee that „optimal“ ordering is obtained
Sometimes a better better block-diagonal structure is obtained by repeatingthe Binary Ordering until there is no change anymore
Layout & Design Chapter 3 / 33(c) Prof. Richard F. Hartl
Example Binary Ordering
(contd.) After sorting of rows and
columns:
part
machine 6 5 1 3 4 2 value
B 1 1 1 1 - - 60
D 1 1 1 - - - 56
C 1 - - 1 1 1 39
A - - - - 1 1 3
E - 1 - - 1 - 18
value 28 25 24 20 7 6part
machine 6 5 1 3 4 2 value
B 1 1 1 1 - - 60
D 1 1 1 - - - 56
C 1 - - 1 1 1 39
E - 1 - - 1 - 3
A - - - - 1 1 18
value 28 26 24 20 7 5
No change of groups in this example
Layout & Design Chapter 3 / 34(c) Prof. Richard F. Hartl
Single-Pass Heuristic Considering Capacities (Askin and Standridge)
extension of simple rule with binary sorting: All parts must be processed in one cell (machines must
be duplicated, if off-diagonal elements in matrix) All machines have capacities (normalized to be 1) Constraints on number of identical machines in a group Constraints on total number of machines in a group
Layout & Design Chapter 3 / 35(c) Prof. Richard F. Hartl
Example Single-Pass Heuristic (Askin and Standridge)
7 parts, 6 machines Given matrix of processing times (incl. set up times) for
typical lot size of parts on machines Entries in matrix not just 0/1 for used/not used) All times as percentage of total machine capacity
At most 4 machines in a group
Not mot than one copy of each machine in each group
Layout & Design Chapter 3 / 36(c) Prof. Richard F. Hartl
Example Single-Pass Heuristic (contd.)
part
machine 1 2 3 4 5 6 7 sum min. # machines
A 0.3 - - - 0.6 - - 0.9
B - 0.3 - 0.3 - - 0.1 0.7
C 0.4 - - 0.5 - 0.3 - 1.2
D 0.2 - 0.4 - 0.3 - 0.5 1.4
E - 0.4 - - - 0.5 - 0.9
F - 0.2 0.3 0.4 - - 0.2 1.1
1
1
2
2
1
2
= 9 machines
Layout & Design Chapter 3 / 37(c) Prof. Richard F. Hartl
Example Single-Pass Heuristic (contd.)
At least 9 machines are needed Not more than 4 machines in a group at least 9/4 = 2,25 groups,
i.e. at least 3 groups
Step 1: acquire block diagonal structure e.g. using binary sorting
Step 2: build groups
Layout & Design Chapter 3 / 38(c) Prof. Richard F. Hartl
Example - Step1: Binary Sorting
For binary sorting treat all entries as 1s. Result is
part
machine 1 5 7 3 4 6 2
D 0.2 0.3 0.5 0.4 - - -
C 0.4 - - - 0.5 0.3 -
A 0.3 0.6 - - - - -
F - - 0.2 0.3 0.4 - 0.2
B - - 0.1 - 0.3 - 0.3
E - - - - - 0.5 0.4solution
Layout & Design Chapter 3 / 39(c) Prof. Richard F. Hartl
Step 2: Build Groups
Assign parts to groups (in sorting order)
Necessary machines are also included in group
Add parts to group until either the capacity of some machine would be exceeded, or the maximum number of machines would be exceeded
Layout & Design Chapter 3 / 40(c) Prof. Richard F. Hartl
Example – Step2
Iterationpart
chosengroup
assigned machines
remaining capacity
1 1
2 5
3 7
4 3
5 4
6 6
7 2
1
1
2
2
2
3
3
D, C, A
C, E, F, B
D, C, A
C, E
D, F, B
D, F, B
D, F, B, C
D (0,8), C (0,6), A (0,7)
D (0,5), C (0,6), A (0,1)
D (0,5), F (0,8), B (0,9)
D (0,1), F (0,5), B (0,9)
D (0,1), F (0,1), B (0,6), C (0,5)
C (0,7), E (0,5)
C (0,7), E (0,1), F (0,8), B (0,7)
table
Layout & Design Chapter 3 / 41(c) Prof. Richard F. Hartl
Results of Example
Machines used: One machine each of types: A, E Two machines of types: B, D, F Three machines of type: C
Single-pass heuristic of Askin und Standridge is a simple heuristic not necessarily optimal solution (min possible number of machines)
Compare result with theoretical min number of machines
Layout & Design Chapter 3 / 42(c) Prof. Richard F. Hartl
Results of Example
part
machine 1 2 3 4 5 6 7 summin.
#heuristic
A 0.3 - - - 0.6 - - 0.9 1 1
B - 0.3 - 0.3 - - 0.1 0.7 1 2
C 0.4 - - 0.5 - 0.3 - 1.2 2 3
D 0.2 - 0.4 - 0.3 - 0.5 1.4 2 2
E - 0.4 - - - 0.5 - 0.9 1 1
F - 0.2 0.3 0.4 - - 0.2 1.1 2 2
Maybe reduction possible?!
Layout & Design Chapter 3 / 43(c) Prof. Richard F. Hartl
LP for min Number of Machines
Minimize total (or weighted) number of machines used when the number of groups is given
Previous example: At least 9 machines necessary Every group has at most M = 4 machines at least 3 groups (try 3)
Layout & Design Chapter 3 / 44(c) Prof. Richard F. Hartl
Given Data
ajk ... capacity of machine k needed for part j
i I ... groups (cells)
j J ... parts
k K ... machines
M ... maximum number of machines per group
Layout & Design Chapter 3 / 45(c) Prof. Richard F. Hartl
Decision Variables
1, if part j is assigned to group i
0, otherwise
1, if machine of type k is assigned to group i
0, otherwise
ijx =
iky =
Layout & Design Chapter 3 / 46(c) Prof. Richard F. Hartl
LP
objective:
constraints:
each part must be assigned to one group
respect capacity of machine k in group i
not more than M machines in group i
binary variables
binary variables
min Kk
ikIi
y
Ii
ijx 1 Jj
Jj
ikijjk yxa KkIi ,
Kk
ik My Ii
1,0ijx JjIi ,
1,0iky KkIi ,
Layout & Design Chapter 3 / 47(c) Prof. Richard F. Hartl
Solution of LP
group parts machines remaining capacity
1 2, 4, 6 B, C, E, F B (0.4), C (0.2), E (0.1), F (0.4)
2 1, 5 A, C, D A (0.1), C (0.6), D (0.5)
3 3, 7 B, D, F B (0.9), D (0.1), F (0.5)
Optimal solution with 10 machines Theoretical minimum number was 9 machines
(not reached because of constraints) Single pass heuristic used 11 machines
Layout & Design Chapter 3 / 48(c) Prof. Richard F. Hartl
Other Approaches for Clustering
Constructive algorithms for sorting: E.g. „direct clustering“ instead of binary sorting
Use similarity coefficients for clustering Askin Standridge § 6.4.4
Group analysis after binary ordering Askin Standridge § 6.4.1
Layout & Design Chapter 3 / 49(c) Prof. Richard F. Hartl
Clustering using Similarity Coefficients
Defineni ... Number of parts visiting machine inij ... Number of parts visiting machines i and j
Similarity coefficient between machines i and j
Proportion of parts visting machine i that also visit machine j
ji
ij
j
ij
i
ijij nn
n
n
n
n
ns
,min,max
Layout & Design Chapter 3 / 50(c) Prof. Richard F. Hartl
Example for Similarity Coefficients
Machine-part matrix
Layout & Design Chapter 3 / 51(c) Prof. Richard F. Hartl
Group analysis after binary ordering
Layout & Design Chapter 3 / 52(c) Prof. Richard F. Hartl
Example