Cellular Manufacturing

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Cellular Manufacturing

• Chris Hicks
• Chris.Hicks_at_newcastle.ac.uk http//www.staff.ncl.a
  c.uk/chris.hicks

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References

• Apple J.M. (1977) Plant Layout and Material
  Handling, Wiley, New York.
• Askin G.G Standridge C.R. (1993) Modelling and
  Analysis of Manufacturing Systems, John Wiley
• ISBN 0-471-57369-8
• Black J.T. (1991) The Design of a Factory with a
  Future, McGraw-Hill, New York, ISBN
  0-07-005550-5

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References (cont.)

• Burbidge J.L. (1978)
• Principles of Production Control
• MacDonald and Evans, England
• ISBN 0-7121-1676
• Gallagher C.C. and Knight W.A. (1986)
• Group Technology Production Methods in
  Manufacture
• E. Horwood, England ISBN 0-471-08755-6
• Hyde W.F. (1981)
• Data Analysis for Database Design
• Marcel Dekker Inc
• ISBN 8247-1407-0

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Manufacturing Layout

• Process (functional) layout, like resources
  placed together.
• Group (cellular) layout, resources to produce
  like products placed together.

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Scientific Management

• F.W.Taylor 1907
• Division of labour - functional specialism
• Separation of doing and thinking
• Workers should have exact instructions
• Working methods should be standardised
• Specialisation led to functional layouts

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Process Layout

• Like machines placed together
• Labour demarcation / common skills
• Robust wrt machine breakdown
• Common jigs / fixtures etc.
• Sometimes high utilisation
• Components travel large distances
• High work in progress
• Long lead times
• Poor throughput efficiency
• Often hard to control

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Group Technology (Cellular Manufacturing)

• Group Technology is a manufacturing philosophy
  with far reaching implications.
• The basic concept is to identify and bring
  together similar parts and processes to take
  advantage of all the similarities which exist
  during all stages of design and manufacture.
• A cellular manufacturing system is a
  manufacturing system based upon groups of
  processes, people and machines to produce a
  specific family of products with similar
  manufacturing characteristics (Apple 1977).

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Cellular Manufacturing

• Can be viewed as an attempt to obtain the
  advantages of flow line systems in previously
  process based, job shop environments.
• First developed in the Soviet Union in 1930s by
  Mitrofanov.
• Early examples referred to as Group Technology.
• Promoted by government in 1960s, but very little
  take up.
• In 1978, Burbidge asked What happened to Group
  Technology?
• Involves the standardisation of design and
  process plans.

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Group (Cellular) Layout

• Product focused layout.
• Components travel small distances.
• Prospect of low work in progress.
• Prospect of shorter lead times.
• Reduced set-up times.
• Design - variety reduction, increased
  standardisation, easier drawing retrieval.
• Control simplified and easier to delegate.
• Local storage of tooling.

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Group (Cellular) Layout

• Flexible labour required.
• Sometimes lower resource utilisation due to
  resource duplication.
• Organisation should be focused upon the group
  e.g. planning, control, labour reporting,
  accounting, performance incentives etc.
• Often implemented as a component of JIT with team
  working, SPC, Quality, TPM etc.
• Worker empowerment is important - need people to
  be dedicated to team success. Cell members should
  assist decision making.

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Characteristics of Successful Groups
Characteristic Description Team Specified team
of workers Products Specified set of products
no others Facilities Dedicated machines /
equipment Group layout Dedicated
space Target Common group goal for
period Independence Groups can reach goals
independently Size Typically 6-15 workers
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Adapted from Black (1991)
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Implementation of Cellular Manufacturing

• Grouping - identifying which machines to put into
  each cell.
• Cell / layout design - identifying where to put
  to place machines.
• Justification
• Human issues

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Types of Problem

• Brown field problem - existing layout, transport,
  building and infrastructure should be taken into
  account.
• Green field problem - designers are free to
  select processes, machines, transport, layout,
  building and infrastructure.
• Brown field problems are more constrained, whilst
  green field problems offer more design choice.

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Grouping Methods

• Eyeballing
• Classification of parts
• Product Flow Analysis
• Cluster Analysis
• Matrix methods (e.g. King 1980)
• Similarity Coefficient methods
• Layout generation without grouping
• Beware
• Different methods can give different answers
• There may not be clear clusters
• Cellular manufacturing not always appropriate

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Classification of Parts

• Based upon coding.
• Many schemes available.
• Basic idea is to classify according to geometry,
  similar shapes require similar processes.
• Grouping codes together is synonymous with
  grouping together like parts.
• Very prevalent in 1960s and 70s.
• Many schemes aimed at particular sectors.

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Coding issues

• Part / component population
• inclusive should cover all parts.
• flexible should deal with future parts /
  modifications.
• should discriminate between parts with different
  values for key attributes.
• Code detail - too much and the code becomes
  cumbersome - too little and it becomes useless.
• Code structure - hierarchical (monocode), chain
  (polycode) or hybrid.
• Digital representation - numeric, alphabetical,
  combined.

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Product Flow Analysis

• Developed by Jack Burbidge (1979).
• Uses process routings.
• Components with similar routings identified.
• Three stages
• Factory flow analysis.
• Group analysis
• Line analysis
• (See Askin and Standridge p177-179)

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Factory Flow Analysis

• Link together processes (e.g. machining, welding,
  pressing) and subprocesses (turning, milling,
  boring) used by a significant number of parts.
• Large departments are formed by combining all
  related processes.
• These are essentially independent plants that
  manufacture dissimilar products.

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Group Analysis

• Breaks down departments into smaller units that
  are easier to administer and control.
• The objective is to assign machines to groups so
  as to minimise the amount of material flow
  between the groups.
• Small inexpensive machines are ignored, since
  they can be replicated if necessary.

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Group Analysis

• Construct a list of parts that require each
  machine. The machine with fewest part types is
  the key machine.
• A subgroup is formed from all the parts that need
  this machine plus all the other machines required
  to make the parts.
• A check is then made to see if the subgroup can
  be subdivided.
• If any machine is used by just one part it can be
  termed exceptional and may be removed.

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Group Analysis

• Subgroups with the greatest number of common
  machine types may be combined to get groups of
  the desired size.
• The combination rule reduces the number of extra
  machines required and makes it easier to balance
  machine loads.
• Each group must be assigned sufficient machines
  and staff to produce its assigned parts.

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Process Plan Example
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Applying Grouping

• Steps
• 1. Identify a key machine. Either E or F.
• Create a subgroup to D,E and F.
• 2. Check for subgroup division. All parts visit F
  and so subgroup cannot be subdivided. Only part 7
  visits machine D so it is exceptional and is
  removed.
• 1. Identify an new key machine for remaining 6
  parts. A is the new key machine with subgroup
  A,B,C producing parts 1,2 3.
• 2. Subgroup division - C only used for part 3,
  therefore exceptional and can be removed.

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Applying Grouping

• 1. Identify next key machine. Only parts 4,5, 6
  remain as well as machines C and D.
• 2. All parts use all machines - no subdivision
  possible.
• 3. Cell designer can now recombine the three
  subgroups into a set of workable groups of
  desired size.
• 4. The final solution must provide adequate
  machine resources in each group for the assigned
  parts. If exceptional parts exist, or if groups
  are not self contained, then plans must be made
  for transport.

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Rank Order Clustering
1. Evaluate binary value of each row. 2. Swap
rows over to get them in rank order.
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Rank Order Clustering
Next apply same method to the columns
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Rank Order Clustering
Next swap over columns to get in rank order.
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Rank Order Clustering
ROC has got a solution close to a block diagonal
structure. The process can be repeated
iteratively until a stable solution is found.
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Similarity Coefficients

• Consider a pair of machines I,j,
• ni number of parts visiting machine i
• nj number of parts visiting machine j
• nij number of parts visiting i and j.
• Define similarity coefficient as
• sij max(nij/ni,nij/nj)
• Values near 1 denote high levels of interaction.
• Values near 0 denote little or no interaction.

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Similarity Coefficients
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Clustering

• We start with 6 clusters, one for each machine.
• With a threshold of T 1 machines A and B can be
  grouped. Likewise E and F.
• There are several methods for updating similarity
  coefficients between newly formed clusters and
  existing clusters.
• The single linkage approach uses the maximum Sij
  for any machine i in the first cluster and any
  machine j in the second cluster. Therefore any
  single pair of machines can cause groups to be
  combined

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Updating Similarity Coefficients (Using Single
Linkage)
Next consider the highest value of T possible.
This gives the cluster CD at T 0.75. The
coefficients then need to be updated again.
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Dendogram
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Variety Reduction

• Basic principle always use common designs and
  components wherever possible.
• Modular design.
• Standardisation.
• Redundant features.
• Can base upon geometric series.
• Imperial / metric series.
• Reduced estimated work planning.
• Simplified stock control.
• Less problems with spares.

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Variety Reduction

• May use slightly more expensive parts than
  necessary.
• Increases the volume of production of items.
• Reduced planning / jigs and fixtures etc.
• Reduced lead times.

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Product Family Analysis

• There are a number of different ways of
  identifying part families. The following factors
  should always be considered
• How wide is the range of components?
• How static is workload?
• What changes are anticipated?
• Is Group Technology aimed purely at manufacturing
  or is standardisation and modularisation of
  design a major issue?