Manufacturing Systems II Chris Hicks Chris.Hicks@newcastle.ac.uk http://www.staff.ncl.ac.uk/chris.hicks
Topics • Group Technology (Cellular Manufacture) • Inventory Management • Material Requirements Planning • Just-in-Time Manufacture
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
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
Manufacturing Layout • Process (functional) layout, like resources placed together. • Group (cellular) layout, resources to produce like products placed together.
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
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
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).
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.
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.
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.
Characteristics of Successful Groups Characteristic Description Team Specified team ofworkers 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
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
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.
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
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.
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.
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)
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.
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.
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.
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.
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.
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.
Rank Order Clustering 1. Evaluate binary value of each row. 2. Swap rows over to get them in rank order.
Rank Order Clustering Next apply same method to the columns
Rank Order Clustering Next swap over columns to get in rank order.
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.
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. Similarity Coefficients
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
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.
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. 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. Variety Reduction
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? Product Family Analysis
Manufacturing Layout • Concerned with the relative location of major physical manufacturing resources. • A resource may be a machine, department, assembly line etc. • A block plan can be produced that shows the relative positioning of resources. • Evaluation criteria are required such as minimising transport costs, distance travelled etc.
Approaches • Many methods are based upon a static deterministic modelling approach. • Dynamic effects may be “guessed” by trying out a variety of scenarios. • Dynamic and stochastic effects may be evaluated by simulation. • A premium may be placed upon favourable attributes • Flexibility dealing with changes in design, demand etc. • Modularity the ability to change the system by adding or removing component parts to meet major changes in demand. • Reliability • Maintainability
Spine Layout • Spine is central core for traffic. • Secondary aisles for traffic into departments • Each department has input /output storage areas along the spine. • Point of use storage reduces material flow.
Loop Structure Circular Structure
Layout Configurations • Eli Goldratt I, V, U, W • I layouts have linear flow with no direction changes, empty pallets may go in reverse direction. • V and U lines have more direction changes but may help with empty pallets. • Rectilinear layouts may restrict operators from working multiple machines. • Circular layouts may enable operators to work multiple machines.