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Cellular Manufacturing

Adapted from:

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Introduction to Cellular Manufacturing (CM)

• Product layouts (assembly lines, mass production one a few products on the same line) is the most efficient of the basic layout options

• Many products are not made in volumes that require a product layout

• Cellular manufacturing (group technology) – forms families of products that have common production requirements

• Locate machines, people, jigs, fixtures, drawings, measuring equipment, material handling equipment together (focused factory)

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Introduction to Cellular Manufacturing

• The cellular approach is to organize the entire manufacturing process for particular or similar products into one group of team members and machines known as a "Cell".

• These "cells" are arranged in a U-shaped layout to easily facilitate a variety of operations.

• Parts or assemblies move one at a time (or in small batch sizes).

• The parts are handed off from operation to operation without opportunity to build up between operations.

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Introduction to Cellular Manufacturing

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Introduction to Cellular Manufacturing

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Introduction to Cellular Manufacturing

• Fast setup and quick changeovers are essential to Cellular Manufacturing systems since production runs are shorter.

• Setup reduction principles are used to achieve one piece flow and mixed model synchronization.

• All cells concentrate on eliminating waste.

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Empowered Employees in CM

• Goals and tracking charts are maintained and posted.

• Problems are solved through daily cell meetings and problem solving teams.

• The inventory management system is a KANBAN Demand Pull instead of a work order/kit picking system.

• Cells are responsible for planning, scheduling and expediting directly with vendors.

• They establish and maintain a KANBAN system with the vendors.

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Advanced CM

• The cell operates like an independent business with total responsibility for quality, manufacturing and delivery of the product to the customer.

• All cells have the resources within their organization to accomplish their mission.

• The requirements are known and goals are established.

• Cell members are flexible and work in teams to accomplish their goals including continuous improvement.

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Benefits of CM

• Common tooling required for many products (fewer setups)

• Tooling can be justified since many products require it (more volume when products are grouped)

• Minimized material handling

• Simple production schedule

• Short cycle time

• Low WIP

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Benefits of CM

• Cross-training – employees operate several machines

• Minimized material handling costs – since no paperwork is required and distance is small

• Employees accept more responsibility of supervision (scheduling of parts within cell, scheduling of vacation, purchasing of material, managing a budget)

• Simple flow pattern and reduced paperwork

• Buffers are small if batch size is small

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Disadvantages of CM

• Lower equipment utilization

• Increased set-up costs

• Less flexibility than functional departments

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Family Formation

• Various levels – macro and micro

• Macro – entire factories (focused factories) can specialize in a particular type of part

• Micro – families can be based on similarities in part geometry (group shafts, flat parts, gears, etc…), process requirements (castings, forgings, sheet metal parts, heat-treated parts, printed circuit boards)

• How are these groupings determined?

– Coding

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Finding Part Families

• Visual Inspection of physical parts or photographs to identify similarities.

• Coding and Classification of parts by examining design and/or manufacturing attributes.

– OPITZ System

– MICLASS System

• Here a code is assigned to specific features of the part.

– Is the part cylindrical or prismatic ?

– Does it have threads?

– Does it have through slots?

– Does it require heat treatment?

• This requires a large initial time investment in coding and classifying all parts.

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Finding Part Families

• Production Flow Analysis : Since the parts in a part family have similar manufacturing processes, it is possible to identify similar parts by studying the route sheets.

• Parts with similar routes can be grouped into families.

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Group Analysis

• To create part families and machine groups a part-machine matrix is created.

• This is a 0-1 matrix in which a one signifies that a machine is required for a given part.

• While creating this matrix the machine refers to a "type" of machine.

• Thus, if there are 5 identical CNC lathes we will create one row in the matrix for these lathes.

• Also, the number of times a part visits a machine is not considered at this stage

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Group Analysis

• Once a the part-machine matrix is created, it is customary to remove approximately 10% of the most heavily used machines.

• Several copies of these machines are likely to be available and thus it is always possible to split these machines between different groups later.

• The remaining matrix is then inspected for part families.

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Group Analysis

• To identify the part-families the rows and columns are interchanged such that a block-diagonal structure is obtained. There are several algorithms that can be used to do this. A simple algorithm for this problem can be described as follows:

– Pick any row and draw a horizontal line through it.

– For each 1 in the row that has been crossed once draw a vertical line through the corresponding column.

– Pick each new column identified in the previous step. For each 1 in the column that has been crossed once draw a horizontal line through the row.

– Repeat this process until there are no singly-crossed 1s in the matrix.

– Remove the rows and columns that have been crossed to form a part family-machine group.

– Continue for the rest of the matrix

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Group Analysis

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Coding

• GT coding and classification schemes attempt to capture design and manufacturing attributes such as the main shape, size, features of the product, production quantity, and material.

• A large number of GT coding schemes have been developed for discrete machined parts including MICLASS, Opitz and DCLASS

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Coding

• Code should contain information about:

• Part or assembly itself

• Manufacturing process (manufacturing engineering, industrial engineering, tool engineering, scheduling, line supervision, quality assurance, etc…)

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Coding Requirements

• Precise nonambiguous meaning, no double or triple definitions for the same phrase

• Tightly structured and concise

• Easy to use

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Coding Options

• Codes can be chain or hierarchical

• Chain – each digit’s specific location is fixed for a particular meaning

• Chain Example –

– First digit is reserved for the product type

– Second digit for material

– Digits 3-6 for part geometry

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Coding Options

• Chain Advantage – easy to learn

• Chain Disadvantage – requires more digits making it difficult to handle manually and with low power computers (not as big a problem today as the price of computers has dropped)

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Coding Options

• Hierarchical code – each code character depends on the preceding one – a tree type structure

• Advantages – code can be sort since many branches can be eliminated

• Disadvantages – difficult to learn

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Code Generation

• CM codes are typically generated manually or interactively by answering a series of questions and applying appropriate coding rules.

• However, this is a slow and inconsistent procedure which inhibited the widespread use of CM.

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Opitz Coding Scheme

• Shah and Bhatnagar developed an automated CM coding system based on the Opitz coding scheme for machined parts.

• The system assigns pre-defined taxonomy codes for each feature of its feature-based CAD system.

• The generic information captured by the taxonomy codes is used to determine individual feature characteristics and the relationships between features and the entire parts.

• The CM code generator uses the resulting feature information and Opitz coding rules to generate the CM codes.

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Using Codes

• Comparing the CM codes of two products is a quick and efficient method for estimating product similarity in selected attributes.

• CM codes can be used to search a database of products and retrieve the designs and process plans of those products which are similar to a given design

• To generate new process plans automatically using a knowledge-based system

• To assess manufacturability of a product design

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Cell Layout

• Usually U, L or circular shaped

• Minimizes transportation distance for operators (human or robotic)

• Encourages multiple machines per operator

• Most machines are automatic or semiautomatic, resulting in considerable idle time

• In a job shop (functional layout) there is one operator for each machine

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Supply Push

• Input availability triggers production or work

• Emphasis on “keeping busy” to maximize resource utilization as long as there is work to be done

• Will synchronize supply with demand at each stage if:

– If all information (about product recipe, processing lead times, and part inventories) is accurate

– If forecasts of finished goods are correct

– If there is no variability in processing

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Demand Pull

• Output need triggers production

• Each station produces only on demand from its customer station

• Each station signals demand by picking up a part from its input buffer

• The supplier station produces a new unit as a replacement in the buffer

• Toyota formalized demand pull with cards called kanbans

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Kanbans

• Kanbans are attached to output flow units in the buffer between customer and supplier processes

• Each card lists the following information:

– Customer process

– Supplier process

– Parts description

– Production quantity

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Kanbans

• As the customer withdraws output flow units from the buffer, the attached kanban goes back to the supplier

• It signals an authorization for the supplier to produce the listed quantity to be replaced in the buffer

• Upon producing the required quantity, the supplier returns the output with an attached kanban to the buffer

• Kanbans control buffer inventory and provide information and discipline to the supplier as to when and how much to produce

• In the case of a process that handles multiple products, each supplier station must also know what to produce

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Problem 2 - Test 2 Summer 2001

A B C D E F

1 1 0 0 1 0 1

2 1 1 0 0 1 0

3 1 0 1 0 0 1

4 0 1 0 0 1 0

5 0 0 1 1 0 0

6 0 0 0 0 0 0

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Problem 2 - Test 2 Summer 2001

A B C D E F

1 1 0 0 1 0 1

2 1 1 0 0 1 0

3 1 0 1 0 0 1

4 0 1 0 0 1 0

5 0 0 1 1 0 0

6 0 0 0 0 0 0

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Problem 2 - Test 2 Summer 2001

A B C D E F

1 1 0 0 1 0 1

2 1 1 0 0 1 0

3 1 0 1 0 0 1

4 0 1 0 0 1 0

5 0 0 1 1 0 0

6 0 0 0 0 0 0

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Problem 2 - Test 2 Summer 2001

A B C D E F

1 1 0 0 1 0 1

2 1 1 0 0 1 0

3 1 0 1 0 0 1

4 0 1 0 0 1 0

5 0 0 1 1 0 0

6 0 0 0 0 0 0

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Problem 2 - Test 2 Summer 2001

A B C D E F

1 1 0 0 1 0 1

2 1 1 0 0 1 0

3 1 0 1 0 0 1

4 0 1 0 0 1 0

5 0 0 1 1 0 0

6 0 0 0 0 0 0

Thus all parts require all machines and only cell is formed