Designing and implementation of lean assembly systems

Nowadays, many companies investigate increasing productivity in assembly line to increase machine usage rate and reduce buffer stock in processes thanks to demand fluctuations and competition. These researches which aimed increase productivity, remark assembly line balancing problems. Goal of this master thesis is redesign assembly lines via lean philosophy and lean approach. Fundamental thinking of lean approach is "doing more with less". It means optimize current system outputs by decreasing wastes. According to Fujio Cho; everything excluding required machine, materials, parts, facility area and work force which provide creating value, represents waste. In lean approach, waste means non – value added everything and customers refuse paying additional cost for them. General overview based on product family have high importance in terms of total efficiency. Improvement for parts cannot be done without assess total system. Total efficiency includes that; • Reduction cell and process cycle times to equal takt time, • Reduction of lead times. Non – value added operations should be eliminated to provide total efficiency. In addition, customer expectations which are changed fast, can be met with less cost. Value stream mapping is an important tool in order to show non – value added operations and wastes. Value stream mapping are created by following all operations in manufacturing system for a determined product. Value stream mapping helps to visualize material and information flow during all operations. Map focus whole picture in manufacturing facility. In this master thesis, office seats manufacturing facility was examined. Value stream map is created after completing some statements such as following process, making time studies, tracking buffer stocks in manufacturing facility. Cycle time is determined until one product finished via time studies and lead time is calculated via Little method. According to Little method (L= λW); current stock amount is divided with daily demand and lead time is obtained by calculating the stock amount as a day between two processes. Value stream mapping provides that; • Visualization of all manufacturing processes, • Visualization of not only wastes but also source causing wastes, • Creation of a common language in manufacturing facility, • Determining where lean manufacturing approaches use. Value stream map which belongs to current situation, describes bottlenecks and process failure rates clearly in manufacturing facility. After that, improvement projects and action plan are created for future. Office seat demands for ten months are examined and required office seats amount in a day to produce are calculated for each seat type and model. Then, takt time is obtained. Improvement and kaizen plans are prepared in order to equal cycle time and takt time for future situation. Three alternative cell structure are designed. Cell production is not only an organizational approach, but also a change culturally. That means creating a work environment which has production ability and capacity for whole of the unit are shipped directly internal and external customers. A cell should be designed with many qualifications such as; • Product flexibility, • Operator flexibility, • Manufacturing speed flexibility, • External routing flexibility, • Internal routing flexibility, • Process flexibility. When creating value stream maps for future situations, different lean manufacturing tools are used in addition to cell production. This master thesis aims reducing set up time in laser cutting process via SMED (Single Minute Exchange of Die), TPM (Total Productive Maintenance), decreasing cycle times via standardization process, Poka – Yoke, providing more efficient and suitable working environment via ergonomics. Moreover, eliminating buffer stock is provided FIFO which means "First In First Out". Value stream maps which belong to future situation is prepared with three alternative cell structures and their system results are compared. Alternative cell structure 2 presents the most optimal solution in terms of lead times when compared the other alternatives. Cell structure 2 has the highest improvement rate on the manufacturing system. In cell structure 2, lead time is reduced from 3,31 days to 1 day 0,55 minutes. That means approximately 70% improvement. Value stream map which belongs to current situation, includes reliability and failure rates. "Adding product components" process has the highest failure rate with 1% in current. One office seat in hundred seats are sent with incorrect or missing components for customers. 1% has an extremely important effect for a company which produces 62 office seats in a day in order to meet customer demands. This master thesis studies to describe main reason of high failure rate in adding product components process via root – cause analysis. Main reason is determined as operation complexity. There are too many model and type components and each of them located on a different container. An operator should find suitable container and choose the required and right component in takt time. So, that causes operation complexity. Kitting method is suggested as a reactive precaution. Kitting has many benefits like that; Saving space from manufacturing area and reduction of buffer stocks by storing components and subassemblies at a central storage area. Providing better system control and higher flexibility by routing only the kit containers for assembly system. Making easier material delivery to workstations by eliminating the need to supply individual component containers. Providing better visibility and control for high cost or violable components and assembly parts. Make easier product changeover and supporting small batch size operations with a large product variety. Approaches of some companies for kitting; Time and effort are required to prepare kits as a non – value added process. (waste) Kits are prepared before assembly, so storage space requirement is increased. When different kits contain common components, an assignment of available components to kits needs to be done. When shortages of components, all efficiency performance of kitting is affected negatively. Spare components should be needed at the assembly line for probability of wrong or defected kit. In the contrary case, the production will be disrupted. If a kit has shortage of components, the other components which belong to same kit, can get cannibalized. But, it may causes complexity in components accountability and tracking. Operators should be trained about kitting to prepare kit with right components. Stable kits are use in the master thesis. 25 square meter area is gained by implementation kitting method in future layout plan of office seats manufacturing company. After review of operator task distribution, work force for 2 operators is saved. Although these saving, takt time is not exceeded. So, all processes can be continued without troubles. These 2 operator will be used for different areas to support company development.