PRODUCTION PLANNING PROBLEMS IN CELLULAR MANUFACTURE

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1 PRODUCTION PLANNING PROBLEMS IN CELLULAR MANUFACTURE J. Riezebos Assistant professor Production systems design, University of Groningen, P.O.Box 800, 9700 AV Groningen, The Netherlands, tel/fax /3850. ABSTRACT This paper analyses what kind of coordination between cells is necessary when using cellular manufacture compared to a production situation that is functionally organized. Three levels of coordination are distinguished: internal, horizontal and vertical. Two types of relations between units are considered: sequential and lateral. It is shown that there is a change in coordination requirement if cellular manufacture is used in stead of functional manufacture. This is illustrated with some cases in which the production planning problems of firms that use cells in the production of parts are analyzed. INTRODUCTION In metalware fabrication many firms have changed to cellular manufacture without fundamentally changing their production planning and control systems. Often they face problems with the support given by these (MRP) systems. This paper analyses the kind of coordination between and within cells that is necessary in cellular manufacture compared to functional organized manufacture and presents the results of 5 short case studies performed in small batch mechanical parts producing firms that use cellular manufacture. In the literature on the comparision of cellular and functional layout a distinction is made between cellular systems that use mainly the flow line type of cell and systems that use hybrid cells with a functional layout. The firms we have studied use the latter type. COORDITION WITH SEQUENTIAL AND LATERAL RELATIONS The concept of a relative autonomous production unit is used in determining the coordination requirements in cellular and functional manufacture. In both situations a kind of autonomous unit is used: in cellular manufacture it is a cell in which the same kind of products are made (using different operations) and in functional production it is a production department in which the same kind of operations are performed (on different products). To compare the coordination requirement in functional and cellular manufacture three levels of coordination are distinguished (see figure 1): internal coordination (within the unit), horizontal coordination between units and vertical coordination with other organizational areas (for example R&D, purchasing, engineering, maintenance). Fig. 1 Three coord ination levels Differences in vertical coordination requirements are generally caused by differences in the degree staff functions are decentralized to the units /1/. In general there is a stronger tendency to take decentralizing support functions in consideration in cellular manufacture, especially if the cells are designed according to sociotechnical principles. In /2/ it is stated that the focus on improving the skills of workers by decentralizing support functions is an important difference between production islands in Germany and cells in the USA. So a change to cellular manufacture can result in a change in vertical coordination requirements. In the rest of this paper the focus is on internal and horizontal coordination. 1

2 To study the difference in internal and horizontal coordination requirements two types of relations are considered, sequential and lateral relations:! Sequential relations are caused by the product flow as specified in the process plan.! Lateral relations are caused by production characteristics. They exist because of (a) pool of shared resources (e.g. tools, fixtures, transportation equipment, people, knowledge) resulting in potential conflicts if more units wants to use one of these resources in the same (b) period; available flexibility between units that can be used in planning the system, e.g. - existing alternatives in processing route resulting in the need to make a choice when and where to process the operation(s) of a specific product (allocation decision); - pooled dependence between cells that produce for the same assembly by using information on the delivery dates other cells can guarantee: if one of these cells has a machine breakdown it won t be necessary to give priority to a component for the same end product in the current cell. If one of these relations exists between two units it causes a horizontal coordination requirement. If such a relation exists within a unit it causes an internal coordination requirement. Due to a sequential relation a horizontal coordination requirement between two units results if a unit delivers a product to another unit. Due to a lateral relation a horizontal coordination requirement between two units results if it is possible that these two units share tools, equipment or people. An internal coordination requirement within a unit results for example if a workpiece flows from one machine to another in the same unit (sequential relation) or if an operator can setup one machine while another machine he is serving is processing a job (lateral relation). FUNCTIOL MANUFACTURE In functional manufacture there are many sequential relations between units, as each production department basically can perform only one type of operation. All products that need different operations have to visit different departments. The horizontal coordination requirement therefore encloses the control of the flow of products between these departments in the factory. Within the units there are few sequential relations between the machines, as all machines basically can perform the same type of operation. The sequential relations that still exist within a production department can e.g. be caused by a required reclamping or refixturing of the workpiece or by the restricted number of tool slots in a machine. Internal coordination of the sequential relations within a production department can be restricted to these few specific products. Lateral relations within a production department are generally present. Machines often can make use of the same tool sets, machine operators are able to operate more machines in the same department, because the same type of processing is applied. The existence of lateral relations between departments is less obvious. Workers normally cannot be interchanged between departments. Some routing flexibility can exist in the sequence in which the departments are visited to make a product, or in the type of processing used to perform an operation, but this flexibility is rarely used. Contrary, the use of shared transportation equipment for the handling of material between the departments is common. CELLULAR MANUFACTURE Because of the combination of different machine tools within a cell, products that require processing on more machines often can stay within the cell. Therefore sequential relations within a cell exist. In literature it is assumed that this intracell coordination of sequential relations easily is accomplished /3/. 2

3 Sequential relations between cells can be distinguished in relations due to the segmentation of the main product flow and relations due to more or less temporary intercell movements that deviate from the normal flow. The percentage orders that needs intercell movements between processing cells generally is small compared with the total number of orders processed, making it possible to coordinate the flow of these orders using another coordination mechanism. Lateral relations within a cell are generally restricted to the presence of multi-functional operators, as the shared use of other resources (tools, fixtures) by different machines is negligible, except the use of product specific pallets. Contrary, lateral relations between cells are generally present. They can be caused by shared use of specific resources (e.g. multi-functional operators), alternative process routings for specific products if some cells are not completely disjoint, which happens quite often due to the allocation of similar machines to different cells, and the use of pooled dependence between cells. LITERATURE ON RELATIONS BETWEEN CELLS Burbidge /3/ describes different types of sequential relations between cells using the notion of processing stages in production, also used in describing the period batch control (PBC) planning system. A prefabrication stage (material production), fabrication stage (component processing), finishing stage (painting) and assembly stage are considered. Burbidge states that reducing the number of stages has a significant effect on the investments in stock. As far as we see, this effect is mainly due to the way PBC operates, e.g. giving each processing stage the same internal leadtime. So the definition of the processing stages depends on the possibility for a cell to make the necessary production within one period. In one of the cases presented in his paper the definition of processing stages depended on the detail scheduling policy used. Burbidge considers sequential relations and distinguishes between cross flow and back flow relations. We interprete a cross flow as a flow of material between cells at the same processing stage and a back flow as a flow of material from a cell to a preceeding cell in the processing stage sequence, opposite to the normal flow of material following the direction of the processing stages. Burbidge states that if cross flow relations are allowed between cells, throughput times and stocks will be increased, quality control will be more difficult, handling costs will be increased and it will be impossible to hold the cell foreman responsible for quality, cost and completion by due date. However, there can be several reasons to accept cross flow relations or even back flow relations between cells, at least temporary. Burbidge mentions three reasons: support a quick change to group technology, design modifications and introduction of new products. Less acceptable according to him are capacity related reasons, e.g. intermediate operations performed by another cell or a subcontracter in stead of investing in the necessary machines. Rolstadås /4/ notes that due to practical adaptions there are often preparational or supplementary operations that will be performed outside the cell. There are three classes of parts: those completely manufactured in one cell, those needing operations outside the cell on single machines and those needing to be processed in another cell. The existence of the latter two classes result in sequential relations, which affects the planning of a cellular manufacturing system. Dale and Russell /5/ report on a redesign of a machine shop resulting in simple cells and complex cells. The simple cells are placed in line such that only simple delivery relations between these cel remain. They are controlled using a simple production control system (e.g. PBC) oriented towards obtaining the benefits of group technology like short throughput times, high volume flexibility and low work in progress. The complex cells are designed such that overlap of work from one cell to another is possible, In this way queues of parts are balanced and fluctuations in market demand can be met. So the complex cells have a high mix flexibilty due to the usage of the lateral relations with other cells in planning the production. In this way short throughput times can be guaranteed while producing with an acceptable utilization rate during the year. 3

4 Willey and Ang /6/ showed that changes in component mix and volume can result in imbalance in workloads between and within cells. In situations where production cells are not completely disjoint these problems can be mitigated by transferring workloads between cells, so lateral relations between cells are used in the control of the production. They tested several heuristics and distinguished between the procedures applied to allocate jobs to cells, to select a job from the machine queue and when to transfer the intercell workload. The results of their simultation experiments show that the decision when and to which alternate machine centre workloads are being transferred can have significant influence on shop performance. From this literature survey we conclude that there are several good reasons to accept sequential relations between cells that deviate from the normal flow of material and that in some cases it can be worthwile to consider usage of existing lateral relations between cells in the planning procedures to improve shop performance. SEQUENTIAL AND LATERAL RELATIONS BETWEEN CELLS IN PRACTICE All 5 Dutch mechanical parts producing firms we have studied use cellular manufacture for their small batch production of parts. In 3 firms the parts are used in the assembly of complex customer specific machines. Parts production is usually done in hybrid cells, e.g. using a functional layout within the cells. As can be seen in table 1 all firms have a central prefabrication cell. The operations that are performed are for example sawing and lasercutting. Often these operations are combined with material handling activities for other cells. The operations that are performed in the fabrication cells can be divided in machining operations like turning, milling, grinding, etc. and sheet metal and welding operations like bowing, cutting, welding, etc. These two different types of operations are sometimes combined in a cell, but most firms only combine operations of the same type in a cell. Generally there is one special cell that operates as a service cell, making tools, prototypes and other specialities. This restcell can be used as overflow for the fabrication cells if they have capacity problems, but in general this is not done. Cases I II III IV V production situation make/engineer to order make to order assemble to order make/engineer to order make to order end product complex machines complex machines complex machines parts parts assortiment parts parts? 6000 parts/year parts 6000 parts/year Number of Cells!prefabrication !machining !sheet metal !combined sheet metal / machining !finishing !assembly Type rest cell tool fabrication tool fabrication tools + specials tools+prototypes Table 1: Characteristics of the firms 4

5 Our main objective was to describe the relations that the cells in the parts producing processing stage have with other cells. First we will present the sequential relations we have found and second the lateral relations between these cells. We have made use of 5 different types of sequential relations, as illustrated in figure 2. In this figure boxes represent cells, arrows flow of material, closed/open triangles permanent respectively temporary inventory places and the difference between the organization and its environment is shown using dotted lines. The first relation describes the mechanism that is used to control the delivery of prefabricated materials. Relation 2 describes the existence of cross flow Fig. 2 Sequential relations between cells relations between the fabrication cells. The way this type of relation is used (incidentally or structurally) is noted. Relation 3 describes the existence of operations that are performed outside the main fabrication cell. The operation can be performed by a subcontracter but a quick service within the shop is also possible. In this way the differences in usage of externally performed operations as temporary overflow are shown. Relation 4 describes the existence of delivery relations to a cell that can process one arrival at a time, for example a painting department. The type of coordination for this cell is described as well as the instruments used to avoid long delays in delivery of the product. Finally relation 5 describes the existence of assembly operations and waiting times. These operations can be decoupled from the delivery of material using safety stock, safety leadtime, but coordination can also be done using detail planning or flexibility in the sequence modules of the end product are assembled. The relations found in the 5 firms we have studied are summarized in table 2. Sequential relation I II III IV V 1 push/pull max leadtime push 1 day pull 4 days push no maximum push no maximum pull 2 days 2 usage incidentally incide ntally (e.g. if machine breaks down) structural in main flow and parts production structural 10% of the orders structural 10-20% of the orders 3 usage subcontracting usage intermediate internal flows only finishing operations (anodising) structural intensively subcontracting to gain capacity only finishing operations in main process flow for heating operation incidentally for capacity reasons mostly finishing operations (30% orders) 2 quick service departments, used by all cells 4 delivered by priority planning capacity mgmt al cells FIFO overcapacity and use of overtime sheet metal cells FIFO + informal overcapacity and subcontracting one cell overcapacity and flexible operators all cells FIFO/EDD weekly load profile used in regulating flow all cells use of overtime, temporary hiring workers and subcontracting 5 planning of assembly buffer policy Table 2: fixed detailed planning safety leadtime Sequential relations flexible planning of modules safety leadtime planned start date safety stock planned start date safety stock flexible start, no detailed planning 5

6 Based on this table we can conclude that there are firms with a rather complex structure, e.g. many relations between the cells, even cross flow relations, subcontracting intermediate operations to gain capacity, assembly waiting times and finishing operations that need to be coordinated. If these firms use a simple planning system like PBC the complex control problems that will arise probably cannot be solved with the aid of the planning system. Burbidge has recognised this and proposes to simplify the sequential relations, as mentioned in the former section. The question is if that is really the best way to organize all these firms. In our opinion the planning system has to give adequate support for the type of planning problems that do arise in practice. The types of lateral relations that were studied in the analysed cases are presented in figure 3. In this figure boxes represent cells, horizontal arrows flow of materal, dotted arrows choice in the direction of the flow and ellipses pools of resources. (The dotted box in B3 is an assembly cell. It has no lateral relation with one of the former cells, only sequential relations.) In figure 3 we make the distinction between relations of type A and B. Lateral relations of type A are relations between cells due to the common use of shared resources. These resources are usually available in a central pool. If this type of relation exists some type of coordination Fig. 3 Lateral relations between cells mechanism will be necessary to coordinate the usage of this resource. The lateral relations of type B can be used to improve the performance of the shop. A distinction is made between the use of work order release choice by a planning department or the cell foremen (so before release to a specific cell) and the use of alternative routes when the work already is released to a cell and one or more operations are temporary performed in another cell. The last relation describes the existence of pooled dependence, e.g. dependeny between cells due to the fact they produce all for the same assembly. If one of these cells will not be able to deliver all parts on time, a choice has to be made what end product will be delayed in the assembly cell. If the other delivering cells are informed and involved in this decision, the pooled dependence is used, resulting in a more reliable production system. In table 3 the results of the cases are summarized. 6

7 Lateral relation I II III IV V A tools human resources processing tools are central stored, mainly duplicated explicit pool for specific cells temporary reallocation possible processing tools are decentral stored and mainly duplicated. In assembly cells shared resources restrict planning no explicit pool temporary reallocation possible shared processing tools central stored and mainly duplicated. transport within the cell with shared resource explicit pool makes temporary reallocation not necessary processing tools central stored, no registration of usage. Many tools are duplicated, but too expensive no explicit pool, flexibility incidentally used shared processing tools (6000) central stored, other decentral stored. no registration of usage no explicit pool, flexibility incidentally used B1 possible to use use of flexibility small percentage of orders small percentage of orders small percentage of orders sometimes used large number of orders only incidentally large number of orders used in planning B2 possible to use use of flexibility possible but not preferred temporary bottleneck operation controlled by rewriting the NCprogram within 15 minutes possible mainly in machining cells used for capacity reasons sometimes incidentally by using the remaining cell often not enough, as rewriting NC programs takes 2 days, done by engineering often conventional operations are planned in this way. Rewriting NC programs is not done B3 possible to use use of flexibility cells that deliver to finishing cell not enough used cells that deliver to assembly not enough used cells that deliver to welding cell cells that deliver to welding cell cells that deliver to welding cell Table 3: Lateral relations We conclude that most of the firms that have shared resources try to duplicate as much as economically can be justified. Firms that only produce parts have more problems to justify investing in more tools compared with the firms that assemble complex machines. The latter firms also put more effort in creating explicit pools of human resources, so making use of the available flexibility. The use of alternative routes and work order release choices depends mainly on the type of products (complexity of processing, use of conventional machines or CNC machines, etc) and the time needed for rewriting NC programs (if needed). Pooled dependence is not enough used in the firms studied, although there are situations where it can be used. COPING WITH COORDITION REQUIREMENTS Using the concepts that are developed we conclude that there is a change in horizontal coordination requirement from coordinating primarily sequential relations between functional departments to coordinating both sequential and lateral relations between cells. According to the internal coordination within cells in cellular manufacture, there is greater need to coordinate sequential relations as in functional organized production departments. Many firms that use functional manufacture cope with their horizontal coordination requirements by using production planning and control systems with long time buckets for each operation. In this way the different production departments are decoupled and a high balanced load of those departments is obtained. The problems that result are well known: high work in progress inventories and long throughput times. 7

8 The cellular manufacturing firms in our case studies also cope with their coordination requirements by buying themself out of trouble. They choose to duplicate shared resources, especially tools, to loosen the existing lateral relations. The available flexibility in the release of an order to different cells is not implemented in the planning procedures these firms use. The lateral relations that need to be coordinated (shared resources) are in most cases duplicated if it concerns tools and decentrally tuned if it concerns human resources. The lateral relations between cells that make coordination possible are not enough used. They are neither supported by the (mostly MRP type) production planning systems in use by these firms. CONCLUSION The coordination requirement within and between relative autonomous units is different for firms using cellular or functional manufacture. Between cells in cellular manufacture the focus should be more on coordinating the remaining sequential relations as well as the lateral relations, as an alternative for investing in resources. Use of the available flexibility in the system should be supported by the planning systems they use. REFERENCES 1 Slomp, J, Molleman, E, Gaalman, GJC, Production and operations management aspects of cellular manufacturing - a survey of users, in: Pappas, IA, Tatsiopoulos, IP, eds, Advances in production management systems, Amsterdam, Elsevier, IFIP, 1993; Harvey, N, Socio-technical organization of cell manufacturing and production islands in the metal manufacturing industry in Germany and the USA, International Journal of Production Research, 1994, Vol. 32, No. 11, Burbidge, JL, Group Technology (GT): Where do we go from here?, in: Pappas, IA, Tatsiopoulos, IP, eds, Advances in production management systems, Amsterdam, Elsevier, IFIP, 1993; Rolstadås, A., Production planning in a cellular manufacturing environment, Computers in industry, Vol. 8, pp , North Holland, 1987, ISSN Dale, B.G., Russell, D., Production control systems for small group production, Omega The Int J of Mgmt Sci, Vol. 11, No. 2, pp , 1983, ISSN Willey, P.C.T., Ang, C.L., Computer simulation of the effects of inter-cell workload transfer on the performance of GT systems, Machine tool design & Research conference, Vol 21, pp ,