01-04-2011, 04:05 PM
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ABSTRACT
Traditional approach to gearing up a cellular manufacturing system (CMS) is to consider only two dimensions, viz, “machine – component”. However, when the problem is addressed in a context of “product – part – machine” dimension, it provides a different perspective to the problem. The current study is an effort in this direction. A formulation of the CMS problem to a product oriented plant as well as a solution procedure has been proposed. A measure for product ownership has been proposed. Practitioners often face the difficult choice of what machines to dedicate to the cells and what to keep centralised. The study has provided a quantitative basis for resolving this conflict. The results show that while a high product ownership can guarantee a high component ownership, the reverse does not. The results underscore the need for including the “product” dimension to the CMS design problem.
INTRODUCTION
Cellular manufacturing system (CMS) has been viewed as a system from conventional manufacturing to integrated manufacturing system and the factory of the future. CMS offers the potential to move from inflexible, repetitive batch, mass production to more flexible small-lot production at reasonable costs. Cell formation design is obviously a key issue in CMS design. In general, for a production facility with a given number of machines and part mix to be processed in the facility, there are three specific decisions in cell formation design, that are the number of manufacturing cells to be established, the machines constituting each cell and the parts assigned to each cell. Many manufacturing firms which hither to satisfy their customers while operating job shop production systems have recently had to rethink because of the superiority of group technology. Implementations CMS have some benefit that essential for competitive advantages of organization (Silviera, 1999). For medium size industry based on manufacturing, design of CMS will give the contribution to reduce source of the waste as setup times, material handling effort and WIP inventory.
The basic idea of CMS is to process a collection of similar parts (part families) on a dedicated cluster of dissimilar machines (machine groups). Forming and formation the machine groups need an approach that comprehensive because it should consider aspects of processes and layout problems. Design of CMS has some problems that are the framework of design, laying out the manufacturing cells and evaluating the layouts. The main objective of the research is introducing a suggested the framework to design the manufacturing cells under shop floor organization aspects. The aspects will be compared using multi criteria approach for evaluating existing layout and manufacturing cells proposed. The certain questions must be researched and answered are:
(1) How the framework of designing manufacturing cells?
(2) What layout objectives should be included? And, (3) How the evaluating the layout proposed using multi criteria approach?
Many cell formation methods in the literature do not consider real-life organization factors in production facility. Due to the complexity of the cell formation problems, the research is developed to include some aspects in the organization of production facility. Alternative selection in layout problem is usually included multi attribute with the result that needed multi attribute analysis. In this research, existing and proposed layouts have been compared using the multi criteria decision making approach.
DESIGNING CELLS
When designing cells, one must be certain that the prerequisites for cellular manufacturing are in place before attempting to shift to cells (or in designing new cells). The prerequisites include reliable machines, short (<10 minutes) changeover times (for cells which manufacture multiple part types), and an able work force. One can quickly see that cellular manufacturing cannot be successfully accomplished without taking into account both the system and the machine.
Each level’s operation interacts and therefore places constraints or functional requirements on the adjacent level. One must keep in mind both the big picture (the production system) as well as the smaller picture (machine or operation design) when designing a cell or an array of cells. At the system level one must take into account customer demand rate (which determines the Takt time), length of product life, and skill level of operators. At the process or machine level one must be careful to design machines which are ergonomic, easy and fast to load and unload (proper filtering), have minimal changeover times, and machine footprints (the rectangular size of the machine on the floor) which reduce operator walking distances. Only if all these issues are considered in designing cells will the full benefits of cellular manufacturing be achieved.
CELL DESIGN METHODOLOGY
When moving to cellular manufacturing, one must remember that cellular manufacturing requires quick (<10 minutes) changeovers, reliable machines and a willing, able (cross trained) workforce.
I. Begin with a finished product that will be sold as is to customers. For the final product: translate demand from customers into a Takt time for each individual part in the final product. For takt time use 7 hours 40 minutes = 27,600 available seconds per shift (or adjust depending on the labor contract). Then assess how long the product will be in operation, and the likelihood of design changes, and their impact on the process, in terms of fixturing, machining, etc.
II. Break out parts according to size and weight. Those parts that are too large (require two hands), too heavy (>10lbs) or too small (parts which can be grabbed between two fingers may present handling difficulties) are candidates for automated material handling systems, i.e. transfer lines.
III. For remaining parts, obtain estimates of machining/assembly times for each operation, including machine time and manual time. Machining times may come from past experience, or a material removal data handbook . Assembly times may come from a Boothroyd and Dewhurst estimate, or from timed samples. 2 ft / sec should be used for walking speed to estimate operator travel times between machines.
IV. Survey existing equipment and assess capacity by comparing the required processing time for each operation with the takt time. If designing or buying new equipment, buy machines with enough capacity such that predicted customer demand is 85% of capacity (the cycle time of every machine should be less than 85% of the Takt time) using bottleneck or theory of constraints analysis. Thus, the cell will not be running at 100% of capacity (see Black [1991] on designing cells to run at less than 100% capacity.) to stay with customer demand, and will be able to increase production should customer demand increase.
LITERATURE REVIEW
CMS can be defined as an application of group technology (GT) which involves grouping machines or processes on the basis of parts or the part families they process (Groover, 2000). Cell Formation (CF) is the first phase of CM, and it deals with the identification of the part family or families and associated machine groups that constitute each cell. The complete review of techniques dedicated to CF has been presented in a paper by Mansouri, et. al. (1998). The second phase of CM consists of the system design of each of the previously identified cells. Typical decisions in this phase include equipment layout; selection/design of tooling and fixtures; design of material handling equipment; determination of the number of machine operators; assignment of the operators to the machines or workstations; specification of the capacity of buffers between workstations; and the formulation of machine-setup policy in a workstation (Singh, 1996). Other factors that have to do with operation and control of the cell are to be included in this phase since they have proven to be an important influence on the performance of a manufacturing system. It is not possible to delineate a strict sequence of decisions to be made in connection with cell design. One can, however, say that structure oriented decision and to proceed procedure-oriented ones. Furthermore, the system structure and the procedures can be changed as experience is derived during the operation of the cell system over time. Within the group of structural decisions, identification of part families and machine groups takes on particular significance, since most subsequent decisions depend on these choices.
The facility layout problem deals with finding the most effective physical arrangement of facilities, personnel, and any resources required to facilitate the production of goods or services. It has attracted the attention of many researchers because of its practical utility and interdisciplinary importance. Historically, two basic approaches have most commonly been used to generate desirable layouts: a qualitative one and a quantitative one. These approaches are usually used one at a time when solving a facility layout problem. With qualitative approaches, layout designer provide subjective evaluation of desired closeness between departments. Then, overall subjective closeness ratings between various departments are maximized. These subjective closeness ratings can be used: absolutely necessary, essentially important, important, ordinary, unimportant and undesirable, to indicate the respective degrees of necessity that to given departments be located close together. Quantitative approaches involve primarily the minimization of material handling costs between various departments. Many researchers have questioned the appropriateness of selecting a single criterion objective to solve the facility layout problems because qualitative and quantitative approaches each have advantages and disadvantages. The major limitations on quantitative approaches are that they consider only relationship that can be quantified and to not consider any qualitative factors. The shortcoming of qualitative approaches is their strong assumption that all qualitative factors can be aggregated into one criterion. In real life, the facility layout problem must consider quantitative and qualitative criteria and this falls into the category of the multi objective facility layout problem. Many models and solution approaches have been developed to deal with the problem of manufacturing cell design/formation since 1970s. The design of manufacturing cells with respect to multiple criteria has been an attractive research topic since 1990. This section presents a review on the main features of the models developed in this field. Baker, R.P and Maropoulos, P.G. (2000) develop a framework to design the manufacturing cell with 0-1 matrix incidence as input to forming machine groups. In this paper, they have been consider only minimizing exceptional elements. Although they consider cells capability however alternatives of grouping is not generate. The main limitation in this research is not considered other performance measure such as the grouping efficacy function, cells independence and others. Chan and Abhary. (1996) has been too where alternatives is generated with using two different techniques. Machine grouping is formed with single objective. Nevertheless, the approach to select layout alternatives using multi criteria decision making. They focused for automated cellular manufacturing system. Onwubolu (1998) have been develop an approach to cluster machines and components with objective minimizing the voids of block diagonal. In spite of, they give proposing grouping performance measure to evaluate the clustering results. In the same way, Hadiguna and Setiawan (2003) and also Singgih and Hadiguna (2003) have been applied hierarchical clustering in three techniques to forming machine cells. They develop a framework to laying out cells in two stages that are first, machine-component grouping and second, to laying out intra cell and inter cells. The approach advantage is applying AHP to evaluating the existing layout compared with proposed cells. The limitation is not considered some criteria that is usually used to cluster machine and component in form 0-1 matrix incidence. Akturk and Balkose (1996), by means of a coding scheme which includes both. Design and manufacturing attributes of parts, calculate the similarity and dissimilarity of parts and makes use of them in a six objective model. The objectives are concerned with minimizing: the dissimilarities based on the operation sequences, the total machine investment cost, the sum of the workload variability in each cell, the work load variability of different cells, and the number of skipping which refers to the number of machines a part skips in its operation sequence. Method suggested is a multi objective cluster analysis heuristic to deal with these objectives simultaneously. AHP is employed to determine priority of the objectives in order to unify them. The research is concerned to group part and machines without arrangement of shop floor.
HOW TO INCORPORATE CELLULAR MANUFACTURING
The implementation process of shedding the traditional manufacturing processes and embracing the drastically different cellular manufacturing techniques can be a daunting task. Management must deal with many issues including: cell design and set up, team design and placement, employee training, teamwork training, as well as other company functional issues. A project team should be put together that consists of management and production employees to handle these changes.
Cell Design and Setup should be executed to facilitate the movement of the product through its production cycle and should also be able to produce other similar products as well. The cells are arranged in a manner that minimizes material movement and are generally set up in a “U” shaped configuration.
Team Design and Placement is a crucial part of the process. Employees must work together in cell teams and are led by a team leader. This team leader becomes a source of support for the cell and is oftentimes responsible for the overall quality of the product that leaves his/her cell.
Employee Training must also accompany the change to cellular manufacturing. In cellular manufacturing workers generally operate more that one machine within a cell which requires additional training for each employee creating a more highly skilled workforce. This cross-training allows one employee to become proficient with his/her machines and while also creating the ability to operating other machines within the cell when such needs arise.
Teamwork Training should generate camaraderie within each cell and stimulate group related troubleshooting. Employees within each team are empowered to employ ideas or processes that would allow continuous improvement within the cell, thus reducing lead times, removing waste and improving the overall quality of the product.
Other issues that must be addressed include changes in purchasing, production planning and control, and cost accounting practices. Arranging people and equipment into cells help companies meet two goals of lean manufacturing: one-piece flow and high variety production. These concepts dramatically change the amount of inventories needed over a certain period of time.
• One-piece flow is driven by the needs of the customer and exists when products move through a process one unit at a time thus eliminating batch processing. The goals of once-piece flow are to produce one unit at a time continuously without unplanned interruptions and without lengthy queue times.
• High-variety production is also driven by the needs of the customer who expect customization as well as specific quantities delivered at specific times. Cellular manufacturing provides companies the flexibility to give customers the variety they demand by grouping similar products into families that can be processed within the same cell and in the same sequence. This eliminates the need to produce products in large lots by significantly shortening the time required for changeover between products.
