GROUP TECHNOLOGY AND CELLULAR MANUFACTURINGMIDDLE EAST TECHNICAL UNIVERSITYMechanical Engineering Department ME 445 Integrated Manufacturing Systems

BATCH MANUFACTURING

IS A DOMINANT MANUFACTURING ACTIVITY IN THE WORLD, GENERATING A GREAT DEAL OF INDUSTRIAL OUTPUT

IT ACCOUNTS 60% - 80% OF ALL MANUFACTURING ACTIVITIES

CHARACTERISTICS OF BATCH MANUFACTURING:

High level of product varietySmall manufacturing lot size

Time onmachine5%Moving and waiting95%Cuttingless than30%Positioning, loading, gauging, idle, etc.70%

WHAT IS GROUP TECHNOLOGY?

Group technology (GT) is a philosophy that implies the notion of recognizing and exploiting similarities in three different ways:1. By performing like activities together2. By standardizing similar tasks3. By efficiently storing and retrieving information about recurring problems

Large manufacturing system can be decomposed into smaller subsystems of part families based on similarities in 1. design attributes and 2. manufacturing features

DESIGN ATTRIBUTES:part configuration (round or prismatic)dimensional envelope (length to diameter ratio)surface integrity (surface roughness, dimensional tolerances) material typeraw material state (casting, forging, bar stock, etc.)

PART MANUFACTURING FEATURES:operations and operation sequences (turning, milling, etc.)batch sizesmachine toolscutting toolswork holding devicesprocessing times

An essential aspect of the integration of CAD and CAM is the integration of information used by engineering and manufacturing and all the other departments in a firm.

Group technology emphasis on part families based on similarities in design attributes and manufacturing, therefore GT contributes to the integration of CAD and CAM.

The Basic Key Features for a Successful Group Technology Applications:

Group LayoutShort Cycle Flow ControlA Planned Machine Loading

Group LayoutIn most of todays factories it is possible to divide all the made components into families and all the machines into groups, in such a way that all the parts in each family can be completely processed in one group only. The tree main types of layout are Line LayoutGroup LayoutFunctional Layout

Line LayoutLine Layout is used at present in simple process industries, in continuous assembly, and for mass production of components required in very large quantities.

Functional LayoutIn Functional Layout, all machines of the same type are laid out together in the same section under the same foreman. Each foreman and his team of workers specialize in one process and work independently.This type of layout is based on process specialization.

Group LayoutIn Group Layout, each foreman and his team specialize in the production of one list of parts and co-operate in the completion of common task. This type of layouts based on component specialization.

The Difference between group and functional layout:

FamiliesThe word Family is used as a name for any list of similar parts. The families used with group layout are lists of parts which are similar because they are all made on the same group of machines. This type of family is called a Production Family. However, not all parts which are similar in shape will appear in the same family.

The other important features that is important choosing the families;Manufacturing tolerancesRequired quantitiesMaterialsSpecial features, which will require the use of different machines

Groups A group is a list of machines, selected for layout together in one place, because it contains all necessary facilities to complete the processing of a given family of parts. A family of parts can only be defined by relating it to a particular group of machines, and a group by relating it to a family. Groups vary greatly in type and size, widely in the number of machines and different machines types.

As group size is reduced, more types of machine will be needed in more than one group and there is an increased risk that some new machines must be purchased. Another factor in choosing the size of group is the number of people who will be employed in them.

Group technology begun by grouping parts into families, based on their attributes. There are three methods that can be used to form part families:

Manuel visual inspectionProduction flow analysisClassification and coding

Manual visual inspection involves arranging a set of parts into groups known as part families by visually inspecting the physical characteristics of the parts.

Manual visual inspectionincorrect resultshuman errordifferent judgment by different peopleinexpensive least sophisticatedgood for small companies having smaller number of parts

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)

Coding methods: 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

The variations in codes resulting from the way the symbols are assigned can be grouped into three distinct type of codes:

Monocode or hierarchical codePolycode or attributeHybrid or mixed code

MONOCODE (HIERARCHICAL CODE) This coding system was originally developed for biological classification in 18th century.

The structure of monocode is like a tree in which each symbol amplifies the information provided in the previous digit.

The following figure illustrates the structure of a monocode:

A monocode (hierarchical code) provides a large amount of information in a relatively small number of digitsuseful 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

POLYCODE (ATTRIBUTE CODE): The code symbols are independent of each other

Each digit in specific location of the code describes a unique property of the workpieceit is easy to learn and useful in manufacturing situations where the manufacturing process have to be describedthe length of a polycode may become excessive because of its unlimited combinational features

Differences in information storage capacity between monocode and polycode:Assume that a code consists of a five symbols and that in each of the five code fields the digits 0 to 9 are used. Determine how many mutually exclusive characteristics can potentially be stored in the monocode and the polycode

Number of characteristics may be stored in a monocode:

101 + 102 + 103 + 104 + 105 =111110

Number of characteristics may be stored in a polycode:

10 + 10 + 10 + 10 + 10 = 50

MIXED CODE (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.

MIXED CODE (HYBRID CODE):The first digit for example, might be used to denote the type of part, such as gear. The next five position might be reserved for a short attribute code that would describe the attribute of the gear. The next digit (7th digit) might be used to designate another subgroup, such as material, followed by another attribute code that would describe the attributes.

A code created by this manner would be relatively more compact than a pure attribute code while retaining the ability to easily identify parts with specific characteristics.

The OPITZ classification system:it is a mixed (hybrid) coding systemdeveloped by Opitz, Technical University of Aachen, 1970it is widely used in industryit provides a basic framework for understanding the classification and coding processit can be applied to machined parts, non-machined parts (both formed and cast) and purchased partsit considers both design and manufacturing information

The Opitz coding system consists of three groups of digits:FormSupplementarySecondary codecodecode 123456789ABCDpart geometry and features relevant to part designinformation relevant to manufacturing(polycode)Production processes and production sequences

PART FAMILY FORMATION:One of the primary uses of coding systems is to develop part families.

Example: Consider the family of ferrous parts formed by first three digits of Opitz form code; 131. This implies that the attributes associated with the family members are length/diameter ratio in the range 0.5 to 3.0, all parts stepped to one end and internal shape elements with threads.

A number of mathematical approaches have also been developed to form part families using classification and coding system.

For the purpose of selecting or developing your own code, it is important to understand the attributes of classification and coding systems. SELECTION OF CLASSIFICATION AND CODING SYSTEMS

Some of the important classification and coding system attributes include:

1. Flexibility for various applications such as part family formation, process planning, costing, and purchasing2. Accuracy, to provide correct information on parts3. Expandability, to accommodate information on more part attributes deemed important later on4. Ease of learning5. Ease of retrieval6. Reliability and availability of software7. Suitability for specific applicationsSELECTION OF CLASSIFICATION AND CODING SYSTEMS

Matching these attributes with the objectives of an organization would be helpful in selecting or developing a coding system to meet organizational needs.SELECTION OF CLASSIFICATION AND CODING SYSTEMS

Group technology is a management strategy to help eliminate waste caused by duplication of effort.

BENEFITS OF GROUP TECHNOLOGY

BENEFITS OF GROUP TECHNOLOGYIt affects all areas of a company, including:

engineeringequipment specification facilities planning process planningproduction control quality control tool design purchasing service

BENEFITS OF GROUP TECHNOLOGYSome of the well-known tangible and intangible benefits of implementing GT :Engineering design

Reduction in new parts designReduction in the number of drawings through standardizationReduction of drafting effort in new shop drawingsReduction of number of similar parts, easy retrieval of similar functional parts, and identification of substitute parts

BENEFITS OF GROUP TECHNOLOGY2. Layout planning

Reduction in production floor space requiredReduced material-handling effort

BENEFITS OF GROUP TECHNOLOGY3. Specification of equipment, tools, jigs, and fixtures

Standardization of equipmentImplementation of cellular manufacturing systemsSignificant reduction in up-front costs incurred in the release of new parts for manufacture

BENEFITS OF GROUP TECHNOLOGY4. Manufacturing: process planning

Reduction in setup time and production timeAlternative routing leading to improved part routingReduction in number of machining operations and numerical control (NC) programming time

BENEFITS OF GROUP TECHNOLOGY5. Manufacturing: production control

Reduced work-in-process inventoryEasy identification of bottlenecksImproved material flow and reduced warehousing costsFaster response to schedule changesImproved usage of jigs, fixtures, pallets, tools, material handling, and manufacturing equipment

BENEFITS OF GROUP TECHNOLOGY6. Manufacturing: quality control

Reduction in number of defects leading to reduced inspection effortReduced scrap generationBetter output qualityIncreased accountability of operators and supervisors responsible for quality production, making it easier to implement total quality control concepts.

BENEFITS OF GROUP TECHNOLOGY7. Purchasing

Coding of purchased part leading to standardized rules for purchasingEconomies in purchasing possible because of accurate knowledge of raw material requirementsReduced number of part and raw materialsSimplified vendor evaluation procedures leading to just-in-time purchasing

BENEFITS OF GROUP TECHNOLOGY8. Customer service

Accurate and faster cost estimatesEfficient spare parts management, leading to better customer service

CELLULAR MANUFACTURINGCellular manufacturing is an application of group technology in manufacturing in which all or a portion of a firms manufacturing system has been converted into cells.

CELLULAR MANUFACTURINGA 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

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

In addition, cells represent sociological units that have more tendency to teamwork. This means that motivation for process improvements often arises naturally in manufacturing cells.

Manufacturing cells are natural candidates for just-in-time (JIT) implementation.

Functional and cellular layouts of an electronics plant:

Cell DesignDesign 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

Structural issues include:

Selection of part families and grouping of parts into families Selection of machine and process populations and grouping of these into cells Selection of tools, fixtures, and pallets Selection of material-handling equipmentChoice of equipment layout

Issues related to procedures include:

Detailed design of jobsOrganization of supervisory and support personnel around the cellular structureFormulation of maintenance and inspection policiesDesign of procedures for production planning, scheduling, control, and acquisition of related software and hardwareModification of cost control and reward systemsOutline of procedures for interfacing with the remaining manufacturing system (in terms of work flow and information, whether computer controlled or not)

Evaluation of Cell Design DecisionsThe evaluation of design decisions can be categorized as related to either the system structure or the system operation.

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)

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)

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.

System cost and performance are affected by every decision related to system structure and system operation.

It is necessary to evaluate each important design parameter and relate its performance to pre-established criteria. For example, structural variables such as number of machines must be balanced against operational variables such as machine utilization and throughput time using analytical and simulation approaches.

CELL FORMATION APPROACHES Machine - Component Group Analysis:

Machine - Component Group Analysis is based on production flow analysis

Production flow analysis involves four stages: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.

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.

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.

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.

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

Components

Rank Order Clustering Algorithm:Rank Order Clustering Algorithm is a simple algorithm used to form machine-part groups.

Step 1:Assign binary weight and calculate a decimal weight for each row and column using the following formulas:

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.

EXAMPLE:Consider a problem of 5 machines and 10 parts. Try to group them by using Rank Order Clustering Algorithm.ComponentsTable 1

Binary weightComponentsTable 2

Binary weightComponentsTable 3

Binary weightComponentsTable 4

Similarity Coefficient-Based ApproachesIn similarity coefficient methods, the basis is to define a measure of similarity between machines, tools, design features, and so forth and then use it to form part families and machine groups.

Single-Linkage Cluster Analysis (SLCA):

It is a hierarchical machine grouping method known as single-linkage cluster analysis using similarity coefficients between machines.The procedure is to construct a tree called a dendrogram.

The similarity coefficient between two machines is defined as the ratio of the number of parts visiting both machines and the number of parts visiting one of the two machines:where:Xijk = operation on part k performed both on machine i and j,Yik = operation on part k performed on machine i,Zjk = operation on part k performed on machine j.

SLCA ALGORITHMS It helps in constructing dendrograms.

A dendrogram is a pictorial representation of bonds of similarity between machines as measured by the similarity coefficients.

The steps of algorithm are as follows:

Step 1: Compute similarity coefficients for all possible pairs of machines,Step 2: Select the two most similar machines to form the first machine cell,Step 3: Lower the similarity level (threshold) and form new machine cells by including all the machines with similarity coefficients not less than the threshold value,Step 4: Continue step 3 until all machines are grouped into a single cell.

EXAMPLE:Consider the matrix of 5 machines and 10 components given below.ComponentsDevelop a denrogram and discuss the resulting cell structures.

Step 1: Determine similarity coefficients between all pairs of machines.

Similarity coefficients of machine pairs

Step 2: Select machines M2 and M4, having the highest similarity coefficients of 0.83 to form the first cell.

Step 3: The next lower coefficient of similarity is between machines M1 and M5. Use these machines to form the second cell.

Step 4: The next lower coefficient of similarity is now 0.67 between machines M1 and M4. At this threshold value machines M1, M2, M4, and M5 will form one machine group. The other possible groups will be evaluated by the same way.

EXCEPTIONAL PARTS & BOTTLNECK MACHINES:One of the important goal in cell design is to create mutually independent machine cells. However, it may not always be economical or practical to achieve this goal.

In practice, therefore, some parts need to be processed in more than one cell. These are known as exceptional parts and the machines processing them are known as bottleneck machines.

The problem of exceptional elements can possibly be eliminated by:

Generating alternative process plansDuplication of machinesSubcontracting these operations

EVALUATION OF CELL DESIGN:In design of cells, there will be more than one alternative solution. The objective is to find the best alternative.

Assume we have the following alternative cell configuration:

The criteria is to minimize the distance that the parts should travel during the processes; in other words, to minimize the material handling costs of intercell (between cells) and intracell (within cell) movements of the parts.

The following factors affect the cost of intercell and intracell movements of parts.

The layout of machines in a groupThe layout of machine groupsThe sequences of parts through machines and machine groups

The total distances moved by a component visiting a number of machines in a cell has to be determined.

Assumptions:In the absence of the real data on the sequences in which the components visit the machines, it is assumed that the machines are laid out in a random manner.There is one unit distance between each machine in a group of N machines.A part has to visit two machines in a group of N machines.

Expected distance for a straight-line layout:Expected distance for a rectangle layout of M rows of L machines:Expected distance for a square layout:The total distance moved in jth cell for the ith configuration:where:dij = expected distance moved between two machines for ith configuration in jth cellkij = number of moves between two machines by all the parts for ith configuration in jth cell

The total cost of intercellular and intracellular movements (TCi) for the ith configuration:where:C1 = cost of an intercell movementC2 = cost per unit distance of an intracell movementNi = number of intercell movements for ith configuration

EXAMPLE:Consider the following cell configuration.Components

Consider 3-cell case:Expected movement distance,

in cell (M1, M5) = in cell (M2, M4) = in cell (M3) = 0

The number of moves passing through two machines by all the parts,

in cell (M1, M5) = 7in cell (M2, M4) = 5in cell (M3) = 0 The total distance for all intercell moves for 3-cell configuration:

1 x 7 + 1x 5 + 0 = 12

The number of intercell moves in 3-cell configuration is 10.

Assume:C1 = $2.00 (cost of intercell unit movement)C2 = $1.00 (cost of intracell unit movement)

The total cost of intercell and intracell movements in 3-cell configuration:

2.00 $ x 10 + 1.00 $ x 12 = 32.00 $

The summary of cost calculation for all possible cell configuration is given in the following table:

A survey of 53 show that the use of GT and cellular manufacturing in US industries has met with success. The benefits reported from these studies include:

Reduction in throughput time by 46%Reduction in work-in-process inventory by 41%Reduction in material handling by 39%Reduction in setup time by 32%Improvement in quality by 29.6%