A SINIFI MALZEMELERİN TALEP MODELLEMESİ VE STOK KONTROLÜ: BİR YÖNTEM VE UYGULAMA ÇALIŞMASI

Abstract

Nowadays, companies need to keep a wide range of spare parts for both production and service and/or maintenance repair purposes in order to meet customer requests and stay where they want in the market share. It is of great importance to keep spare parts in stock for the service or integrated logistics support (ILS), after-sales services within the company. ILS inventory policy should minimize the cost within the terms of the contract concluded with the customer while meeting the customer's needs on time. When realizing the purpose of cost minimization, attention should be paid to the cost of out-of-stock or the maximum acceptable level of out-of-stock. Therefore, it is necessary to provide a certain level of reliability depending on the material being considered. Some of the materials that are kept in stock are produced by the company, while others are supplied from external manufacturers. For both cases, there are many restrictions accompanying the cost minimization goal in determining the best inventory amounts that the company should keep for these materials. Within the framework of the problem under consideration, all purchases within the ILS are evaluated within the scope of procurement and are divided into two groups, internal and external procurement. Internal supply is the demand for the company's own manufactured materials, and external supply is the supply for the purchased materials. The materials in both groups vary according to the costs and demands. In project-based companies, there may be differences in the quantities to be stored as the restrictions and parameters they use in inventory calculation and control change. These influences indicate the sensitivity of the material and its flexibility to situations. They are class A materials that make up a very large part of inventory systems in terms of cost/return and can explain them.

Within the scope of this thesis, it is aimed to make repairs with a customer warehouse for a class A spare part selected within the scope of defense industry integrated logistics support unit activities; to determine the distribution of incoming requests, changing lead time distribution changes for material supply, being affected by the change of set-up, holding and backorder costs, and its sensitivity to all these restrictions. For this purpose, an inventory model was created and assumptions were determined. Mathematical models suitable for unit and compound Poisson were created in the Matlab program with the equations created in the light of the assumptions, and by calculating the optimal parameter values for the determined parameter scenarios, they were run in the simulation models coded in the Matlab program for the repair situation and demand distributions whose sensitivity was desired to be analyzed. Performance measures were compared and their sensitivity was examined by establishing hypothesis tests. While the sensitivity levels and differences were examined according to the results, the difference with the current policy was also evaluated. As a result of all evaluations and sensitivity reviews, an application study and comparison was made. In this application study, the numerical effect of using a stock policy appropriate to the process and product is also demonstrated through mathematical and simulation models.