MRP ve Jit üretim yöntemi sistemlerinin incelenmesi ve karşılaştırılması

Abstract

Günümüz imalat sistemlerinde, daha verimli imalat yapabilmek için çok çeşitli yöntemler kullanılmaktadır. Hücresel imalat ile üretim, bu yöntemlerden günümüzde en popüler olanıdır. Tabii ki yalnızca hücresel imalat, tek başına imalattaki önemli sorunların üstesinden gelemez. İmalat çevresinin düzenlenmesi, hücresel imalattan önce başarılması gereken safhalardandır. Hücresel imalat başarılınca da tek başına yeterli değildir. Hücresel imalatın koordinasyonu için, imalat yönetimi sistemlerinin kullanılması gerekmektedir. Bu çalışmanın esas amacı, imalatın efektif yapılabilmesi için en önemli şartlardan biri olan, imalatın yönetilmesi probleminin, çözümünü araştırmaktır. Şirketler için asıl önemli olan, imalat maiyetlerini azaltmak ve müşteriye teslimin güvenilirliğini arttırmaktır. Doğru imalat yönetimi sistemleri seçildiğinde, bir isteği sağlamak için diğerinden fedakarlık etmek gerekmediği, iki talebin birden karşılanabildiği, hem daha ucuza, hem daha efektif bir üretim yapılabildiği, hem de teslimat terminlerine uyulabildiği görülmektedir. Ancak farklı tipte farklı yöntemlerle çalışan imalat yönetimi sistemleri vardır. Bu çalışma, kurulması çok fazla para ve zamana mal olan bu imalat yönetimi sistemlerinden, en çok hangisinin, şirketlerin yapısına uyduğunu belirlemeye çalışmaktadır. Farklı imalat yönetimi sistemleri, farklı şartlarda olumlu sonuçlar vermektedirler. Bu sebeple, üretim yönetimi sistemlerinin yapısı, felsefesi incelenmiş, hangi şartlar altında başardı olabilecekleri araştırılmıştır. Bu araştırma simülasyon yardımıyla desteklenmiştir. Farklı şartlar, deneyler ile oluşturulmuş, gerçeğe yakın kurulan iki sistemin bu şartlara tepkileri ölçülmeye çalışılmıştır. İmalat sistemi için seçilen performans kriterlerine göre dört farklı deney uygulanmış, alınan sonuçlar grafiklerle yansıtılmaya çalışılmış, sonuçlara hipotez testi uygulanarak anlamlılıkları test edilmiştir. Sonuç olarak MRP ve JIT sistemlerinin hangi şartlarda, seçilen kriterler doğrultusunda diğerine üstün geldiği açıklığa kavuşturulmuştur.

Nowadays, companies are feeing hard competition of manufacturers from all over the world. To survive this competition, companies are adapting different solutions in order to improve effectiveness, in their production processes. Cellular manufacturing (CM) has become a more popular way to organize manufacturing processes. However only cellular manufacturing is not enough for improving the effectiveness. First of all the manufacturing environment plays an important role in the performance of the manufacturing process itself. If the process is complex, requiring multiple cells, proper coordination of production between different cells has been recognized as essential. This research tries to find a solution to the problem of using the right production planning and control system for the right manufacturing system. Main objective of using the right production planning and control system is to manage the manufacturing system and produce more effectively. In order to search this solutions, firstly the fundamentals and goals of production planning and control systems MRP and JIT have been investigated. After that, by means of simulation, it has been tried to define right conditions for the manufacturing management systems. First of all, cellular manufacturing is required for effective manufecturing. However, CM not only improves the process flow by grouping machines into cells, but it also simplifies the manufacturing planning and control problem. This is so because the entire cell can be treated as a single scheduled unit. The internal cell scheduling can be handled largely by the cell crew. CM is made up of cells, each with dissimilar machines making a limited part or product family. The form of CM depends on internal and external flow patterns that vary from random to flow line and on whether machines are fully dedicated to assigned cell. When compared to a functional shop, the major operational advantages of CM are reduced lead times and reduced setup time. Reduced lead times are exhibited by the flow line configuration because of flow loading, small transfer lots, and no material handling. Reduced setup times are possible through setup families. Disadvantages of CM include vulnerability to peak loads at the single machines commonly used in a flow line. For functional job shops, "reshaping" the environment may offer more improvement potential than system choice. Indeed, CM is one example of that reshaping. Still, XIV once an environment is established (or anticipated), a decision must be made. How are part flows best coordinated? Selection of a Manufecturing Planning and Control (MPC) system usually defines the parts coordination method. Within JIT programs, Kanban has been used to control this flow. For American plants with CM, Material Requirements Planning (MRP) has been common. However, it is not clear under what conditions one of those systems would be better or even if MPC system choice makes any difference. That is in feet the one of the purpose of this thesis to explore what conditions, if any, favor one MPC system over another when used in complex flow CM environment. Major differences in the relative performance of the MRP, and Kanban systems depend on the manufecturing environment and the uniformity of the master production schedule. These differences are important in determining the appropriate application of MPC systems. Kanban is a part-usage rate-based system. A constant usage rate for each part is normally required by pull scheduling. Deviation from the planned usage rate or a part would be expected to reduce performance. One would expect that with a low part usage variation. Kanban system performance would be superior to MRP. Just in Time strategies are an example of providing constant production rates consistent with Kanban. JIT systems characterize manufacturing pull systems as they consider only firm order. JIT is not a technique like MRP, but a philosophy for manufacture considering production control, the manufecturing process and design, with the overriding desire to reduce manufecturing lead time. By substantially reducing batch sizes in a JIT environment, batches are processed more quickly, reducing queue times. The need for more machine setups, due to adoption of smaller batch sizes, is overcome by employing a flow type layout with machines, dedicated to specific tasks. Flexibility is obtained by the machines in the layout, having numerical control capability. This layout also helps to reduce the transport time, between operation. JIT, through design for manufacture, cell based manufacturing flow lines and flexibility within the manufacturing system enables a variety of products to be manufactured to customer demand in any batch size, but requires motivation and commitment from every employee in the company. The JIT system requires strong links to be developed with suppliers and due to short lead times any problems during production can have very serious repercussions. Major advantages of Kanban are : -Kanban systems provide visual information on the loading conditions of the work center. -Kanban systems imply decentralized control. The supervisor of each work center is responsible for lead time control and inventory control. This is, in many way, the basic driving force behind the inherent improvement mechanism which is realized by a process of gradual inventory reduction and quality improvement. XV -Kanban/pull scheduling is recognized as being simpler than detailed computer based scheduling. Because of the relationship between inventory level, lead times and production, kanban systems can be self-regulating: there may be little need for rescheduling although loading is still necessary. We can summarize limitations on the pull approach: -It has rather limited applicability at present since the basic mechanism relies on conditions of repetitive manufacturing and a reliable production table demand environment. -Information flow is tied to material As a result valuable information (e.g. on the demand trend) is not sent to all stages of production as soon as it is available. Pull systems can therefore be characterized by large information lead times, especially where there are large material flow lead times. The original MRP was and is material requirement planning, a calculation technique for planning purchase orders and manufacturing orders according to what is needed to complete a high level master production schedule(MPS). By linking other business management applications to MRP, MRP II creates a complete closed-loop management facility mat touches all of the major functional areas of the business and coordinates all activities toward the single goal of producing the right product at the right time. MRP II typically includes planning applications, customer service applications (order entry, finished goods inventory, forecasting, sale analysis), execution systems (production control, purchasing, inventory, product data management), and financial functions (cost accounting, general ledger, payables, receivables, payroll). MRP systems characterize manufacturing push systems as they consider both firm and forecast future sales, held in the MPS. MRP is essentially a mathematical process, which produces the net requirements to complete future sales, at the correct time phased intervals for individual components. Closed loop MRP systems were developed to furnish feedback MRP through the inclusion of capacity requirements planning (CRP), enabling the feasibility of the MPS to be assessed by providing a detailed capacity profile needed as a simulation toll, MRP II systems were devised to include a further feedback loop incorporating the capacity module, rough cut capacity planning. The technique was also extended to include financial features, thereby offering a consolidated approach to the management of all manufacturing resources. We can summarize the advantages of this approach: -All relevant information (including material requirements, work in process levels, machine status and inventory levels) is stored in a central computer, implying a centralized control and a coordination among the work centers. -Implementation has stimulated the development and use of well organized information systems. XVI Major criticisms of the MRP approach include its long planning horizon associated with the MPS, its reliance on fixed lead times, its treatment of batch sizes and its approach of scheduling to infinite capacity. MRP systems are costly and time consuming to install. The creation of databases and tailoring of software is such that negligible benefits occur during the first two years of development. The main disadvantages of this approach are as follows: -The inability to maintain data information at a level of high reliability. -A lack of an inherent improvement mechanism. -A lack of real-time coordination among the consecutive stages means that frequent rescheduling is necessary to keep the total system under control -Approximations in the approach can cause excess safety stocks. MRP can accommodate any part usage variation in any time period. It makes no assumption regarding either part or group variation in production rates. Consequently the application of MRP is quite general While independently timed orders and the resulting load variation would be expected to detract from MRP performance, and Kanban ratebased characteristic would likely cause them to fere even worse. The apparent failure of MRP II implementations derives from an inappropriate measure of success and failure. While it is true that the majority of implementations do not result in complete, controlled closed loop systems that are an integral part of everyday operations an all major areas of the company, it is also true that the vast majority of MRP II efforts result in a commendable return-on-investment. Furthermore, it is unfair to blame the software for lack of results. There are many examples of the same types of the companies installing the same package where some succeed and others fail Industry consultants, who get to observe and participate in multiple implementations, will quickly tell, mat it is the people issues, and how they are managed, that are the primary determinants of success. Included in this list are: -Top management commitment -Management of change, including helping people to accept change -Adapting the organization and its processes to exploit the system's capabilities -Providing sufficient user training and education Manufacturers can use the advantages of both JIT and MRP systems by integrating both systems. We can call such systems as hybrid systems. Main idea of integrating push and pull systems is to find a point, which you can see as a border for push and pull systems. Up to that point you can push or pull the manufacturing system, and again from that point on the inverse. An integration of push and pull principles would imply that some part of the material flow is pushed and some other part is pulled. XVll -Kanban/pull scheduling is recognized as being simpler than detailed computer based scheduling. Because of the relationship between inventory level, lead times and production, kanban systems can be self-regulating: there may be little need for rescheduling although loading is still necessary. We can summarize limitations on the pull approach: -It has rather limited applicability at present since the basic mechanism relies on conditions of repetitive manufacturing and a reliable production table demand environment. -Information flow is tied to material As a result valuable information (e.g. on the demand trend) is not sent to all stages of production as soon as it is available. Pull systems can therefore be characterized by large information lead times, especially where there are large material flow lead times. The original MRP was and is material requirement planning, a calculation technique for planning purchase orders and manufacturing orders according to what is needed to complete a high level master production schedule(MPS). By linking other business management applications to MRP, MRP II creates a complete closed-loop management facility mat touches all of the major functional areas of the business and coordinates all activities toward the single goal of producing the right product at the right time. MRP II typically includes planning applications, customer service applications (order entry, finished goods inventory, forecasting, sale analysis), execution systems (production control, purchasing, inventory, product data management), and financial functions (cost accounting, general ledger, payables, receivables, payroll). MRP systems characterize manufacturing push systems as they consider both firm and forecast future sales, held in the MPS. MRP is essentially a mathematical process, which produces the net requirements to complete future sales, at the correct time phased intervals for individual components. Closed loop MRP systems were developed to furnish feedback MRP through the inclusion of capacity requirements planning (CRP), enabling the feasibility of the MPS to be assessed by providing a detailed capacity profile needed as a simulation toll, MRP II systems were devised to include a further feedback loop incorporating the capacity module, rough cut capacity planning. The technique was also extended to include financial features, thereby offering a consolidated approach to the management of all manufacturing resources. We can summarize the advantages of this approach: -All relevant information (including material requirements, work in process levels, machine status and inventory levels) is stored in a central computer, implying a centralized control and a coordination among the work centers. -Implementation has stimulated the development and use of well organized information systems. XVI BOLUM 1 GÎRÎŞ Ürünler ve firmalar arası rekabetin geçmiş ile kıyaslanamayacak kadar fazlalaştığı günümüz dünyasında, firmalar ayakta kalabilmek için değişik stratejiler uygulamak zorunda kalıyorlar. Bu stratejilerin pazarlama, satış, tanıtım, üretim gibi farklı boyutları vardır. Pazarlama, satış gibi dışa dönük konularda şirketler agresif tutumlar izlerken; üretim konusunda maliyetlerin düşürülmesi, kalitenin arttırılması gibi hususlarda azami dikkat sarf ediyorlar. Mamulün aranılır olmasında satış ve pazarlama gibi dalların etkisi tartışılmaz şekilde var olsa da, kalite ve müşteri talebinin tam olarak zamanında, istenen şekilde karşılanması da mamulün aranılırlığına mutlak ki çok büyük katkı sağlıyor. Maliyetlerin düşürülmesi unsuru da şirketler için tartışılmaz öneme haizdir. Üretim konusu ele alındığında; üretime yönelik taleplerin karşılanabilmesi için imalatın isteklere karşılık verebilecek imkanlara sahip olması, akışların düzgün olması ve düşük ayar zamanlan gereklidir, ama yeterli değildir. Bugün Üretim Yönetimi Sistemleri ile üretimin performansım arttırdığımızı biliyoruz. Üretim Yönetimi Sistemleri ile malzemenin imalat süresine - teslim süresine -, malzeme imal edilirken onu oluşturan alt parçaların stok miktarlarına, üretim emirlerine ve imalat miktarlarına müdahale etmek mümkündür. Bu şekilde müşteri talepleri istenilen zamanda, istenilen miktarda karşılanmaya çalışılırken; imalat hücreleri arasında koordinasyon sağlanmış olur. Stok seviyeleri düşük tutularak, maliyetlere olumlu katkı sağlanabilir ve imalatta hücreler arası bilgi alışverişi sağlanabilir.