Lean Manufacturing: A Journey towards Customer Satisfaction
R. K. Nikkhil Rathnakar1, S. Karthikeyan2
2
B.E. Student, Dept. of Mechanical Engg., Thiagarajar College of Engineering,Madurai-15. Assistant Professor, Dept. of Mechanical Engg., Thiagarajar College of Engineering,Madurai-15. [email protected]
1
Abstract— Every industry tries to maximize its production and increase its profit. These two targets can be achieved by reducing waste (Lean Manufacturing) and improving management systems (Total Quality Management/ Total Productive Maintenance). Lean is a comprehensive set of techniques that, when applied, will allow more flexibility and more response by reducing the waste. ‘VMK Industries’ is a small production company in Madurai which manufactures output shaft on a contract basis. The production of the final product requires turning, key way milling and grinding. The raw material used is EN 9 steel. The industry has not been able to achieve the desired target. Lead time reduction has become a potential problem to be solved. After conducting brainstorming sessions, it is realized that key way milling involved high lead time. Fish Bone diagram has helped to find the root cause. Milling operation is the bottleneck. Moreover, turning, milling, grinding of targeted number of components are done separately. The three machines, namely CNC lathe, vertical machining centre and grinding centre are not arranged in a sequence. We have converted the process as a continuous one; turning of a component has been followed by milling which is in turn followed by grinding. A working holding fixture has been developed to hold the work piece so that two keyways are milled at a time. This has helped to reduce the lead time. The three machines are arranged in a sequential order to stream flow. Lead time has been reduced by 72%.The reduction in lead time has helped the company to meet the ever increasing customer demands. Keywords— Lean Manufacturing, Production Company, Lead time, Brainstorming, Fish Bone Diagram, Bottleneck, Customer Demand. I.INTRODUCTION Production industries are under great pressure to increase their profit by reducing the cost involved in value added and non–value added processes. Value added process needs to be optimized whereas non–value added process needs
to be eliminated. Quality and Lean have become a panacea to the production sector. Technology doesn’t make any difference among production industries. It is only the quality of management systems and adherence to lean principles that make an industry flourish. II. PROBLEM DEFINITION VMK Industries is a production company in Madurai which manufactures output shaft on contract basis. The raw material used is En 9 steel. The raw material requires three machining processes namely turning, milling and grinding to produce the end product. The process involved high lead time. Consequently, the company could not meet the customer demand. III. THE CONCEPT OF LEAN MANUFACTURING Lean is a comprehensive set of techniques, which when implemented, will help industries to reduce and eliminate the eight deadly wastes- Overproduction, Human Resources, Transportation, Inventory, Motion, Correction, Over-processing and Waiting.. Lean is called so because after implementing it, the process can run using less material, require less investment, and can also become more flexible and responsive to waste. [1] The main principle of lean manufacturing is to reduce waste, long lead time and promote flexibility of manufacturing systems. All Lean environments share certain critical characteristics. They are: Production is done to meet the customer demand. Material replenishment and work control are driven by signaling devices (for example, Kanbans) Synchronized production line Continuous improvement (Kaizen) procedures Trained professionals are employed to control the manufacturing process and they shift operations based on current demands. [2]
1
A. Strategies of Implementing Lean The overall approach to implement lean consists of four key strategies. They are: Synchronize supply to customer, externally: To synchronize externally is to supply the product to our customer at their needed demand rate, normalized to our production schedule. We want to supply all of the customer needs but we do not want to overproduce and create excess inventory. Tools like Takt Calculation allow this balance to be achieved. Synchronize production, internally: To synchronize production internally is to divide the necessary work in processing steps so that each processing step takes the same time. The ideal is that all processing steps perform at a cycle time equal to Takt. Basic Time Study and Balancing Graph are the tools helpful to achieve synchronization internally. [3] Create flow: The concept of flow is such that we do not want the production units to stop, except for value-added work. The flow concept has both overall measures and local measures. The local measure would be cycle time and the overall measure of flow is production lead time. In every case, if we can reduce cycle time or if we can reduce lead time, we will make process improvements. Establish Pull – Demand System: Pull systems have two characteristics. First, they have a fixed inventory; so the cycle stock, plus the buffer and safety stocks need to be determined. Second, they are activated when product is removed and this signals the upstream process to produce—no signal, no production. Kanban and JIT (Just in Time) are the tools used to create pull system. IV. LEAD TIME REDUCTION In the 1960’s and 70’s, manufacturers competed on the basis of cost efficiency. In the 1980’s, quality was the rage and Zero Defects and Six Sigma came into vogue. Cost and quality are still crucial to world-class operations, but today, the focus is squarely on speed. Nearly all manufacturers today are under pressure from customers to cut lead times and rapid-response manufacturing pays big dividends. Customer lead time refers to the time span between customer ordering and customer receipt. Manufacturing lead time refers to the time span from material availability at the first processing operation to completion at the last operation. In many manufacturing plants, less than 10% of the total manufacturing lead time is spent actually manufacturing
the product. And less than 5% of total customer lead time is spent in the production process. The cumulative cycle times of the processes in the value stream are the theoretical limit to how much we can reduce lead times. Clearly, there is ample opportunity to reduce lead times in most organizations. Reducing lead times doesn't involve speeding up equipment to cut the cycle times or getting plant personnel to work faster. It involves rapid fulfillment of customer orders and the rapid transformation of raw materials into quality products in the shortest amount of time possible. [4] A. Methods of Lead Time Reduction The following guidelines will help reduction of lead times in an organization: Measure current lead times and set improvement targets: Lead times are not measured in most organizations. People don’t have a quantitative assessment of how long it takes for individual products to cross the value stream. [5] Things that are not measured cannot be improved. Change the organization from functional orientation to product orientation: If possible, all resources required to produce a product should be located close to each other. These product-focused groups are called work cells or cellular manufacturing. Cross-train plant personnel within cells in a number of operations for greater flexibility: Reducing the number of job classifications and maintaining multi-skilled teams on each shift is critical to rapid response manufacturing. Empower work cells and teams to take ownership for the entire value stream: Drive accountability for product cost, quality and delivery down to the lowest appropriate level. Continually reduce batch sizes between work centers: With operations in close proximity, transfer batches can be smaller and WIP inventories can be minimized. Institute local scheduling between work cells: Visual shop floor scheduling tools, like kanban systems, can be used to minimize WIP between cells and to eliminate queue time throughout the value stream. By employing these principles, many world-class manufacturers have shrunk lead times by 50-80%, gained market share, improved profitability and increased employee morale on the shop floor.
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V. AN OVERVIEW OF EXISTING PROCESS EN9 (unalloyed medium carbon steel) is the raw material used for production of output shaft. It is throughhardened before machining. The hardness is normally within the range of 180 to 230HB. A. Steps Involved in the Production of Output Shaft The stock is turned to different diameters along its length in a CNC lathe. The turned shaft is sent to the vertical machining centre where two key ways are milled on either ends of the shaft. After key way milling, the job is ground to produce the desired surface finish. The whole process of producing an output shaft takes 30 minutes to complete. They wanted to reduce the long lead time involved in the production of shaft.
Fig. 2 Existing Layout
VI. BRAINSTORMING AND ROOT CAUSE ANALYSIS In order to find out the root cause of the problem, a fish bone diagram is drawn. Brainstorming sessions are conducted to identify the causes of the problem. By and large, Man, Machine, Material and Method have been taken as the prime causes of the problem. The Fish Bone diagram shows four causes namely cycle time, machines, material and demand. The cycle time of turning, milling and grinding is compared. The machines are analysed for defects. Customer demand also could not be met. The machining operations have been performed separately. The raw material used is En 9 Steel. The hardness of the raw material is also found to be a cause for high lead time. The problem is further funneled by taking cycle time as the root cause of the problem. The cycle time of milling is high when compared to the other two processes and required to be debottlenecked. The fact that milling process is a bottleneck is well corroborated by the calculation of takt time.
Fig. 1 Existing Process Flow
B. Characteristics of Existing Process The targeted number of components was first turned, then milled and finally ground. The three machining centres were not arranged in a sequence. They were spread in different directions. Since the machining centres were far apart, flow was not streamlined. Of the three principal processes, Key way milling involves the longest lead time. Milling operation was the bottleneck. The existing process could not meet the customer demands. The whole process of producing an output shaft takes 30 minutes to complete.
Fig.3. Fish Bone Diagram
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VII. PROMOTING THE LEAN WAY In order to achieve lead time reduction and make the manufacturing strategies leaner, certain lean tools were used. Takt time calculation was done and balancing graphs were drawn. A. Takt Calculation Takt is the rate at which a product is produced. It is a true one piece flow, pull demand concept. In our case, Takt calculation helps to find out the time taken for production of a single component so as to meet the customer demand for a day. The company has two eight hour shifts per day. So the available work time is 16 hours. The customer demand is 116 components per day. The available work time in seconds = 57600 Takt = 57600 seconds / 116 components = 500 Seconds. B. Process Cycle time Estimation Turning = 360 seconds Milling = 720 seconds Grinding = 240 seconds Of the three processes, the process cycle time for milling is greater than takt time. The milling operation is the bottleneck.
VIII. ELIMINATION OF BOTTLENECK In order to eliminate bottleneck, few changes are done. The machining centres are arranged in a sequential order. This helped to streamline flow of workpiece. Process cycle time of Milling was the major problem because it exceeded the takt time. Milling process consists of cutting two keyways on either side of the shaft. The milling of each keyway took equal time of 360 seconds. Therefore the cycle time of milling was 720 seconds. A workholding fixture is developed to eliminate this hitch. With the new fixture, the two keyways are milled within 360 seconds. The process cycle time of milling is halved. An important modification in the existing process is that turning, milling and grinding of components are done consecutively rather separately. All process cycle times have been made lesser than Takt time.
Fig. 5 Modified Layout Fig. 4 Existing Balancing Graph
C. Estimation of Lead time – Phase I Lead time for any process = (Number of Components* Process cycle time) In the industry, the target number of component are turned, milled and ground independently. Turning = 116*360 = 41760 Seconds. Milling = 116*720 = 83520 Seconds. Grinding = 116*240 = 27840 Seconds. Product Lead time = 153120 Seconds.
Fig. 6 Redesigned Balancing Graph
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A. Estimation of Lead time – Phase II Among the processes, the highest cycle time is 360 seconds. Turning and milling takes 360 seconds respectively. Product Lead time = 116*360 = 41760 seconds. Reduction of Lead time in Percentage = (Existing Lead Time – Lead time after elimination of bottleneck) * 100 / (Existing Lead Time). Reduction of Lead time in Percentage = (153120 – 41760) * 100 / 153120 = 72.73 % After implementation of lean principles, Lead time reduction has been achieved. Now, the company has been able to meet the customer demand. IX. CONCLUSION The production of shaft involved three processes namely turning, milling and grinding. Of these processes, milling had the longest lead time. The introduction of new workholding fixture has helped to reduce the lead time involved in milling operation. The bottleneck has been eliminated. The flow of material has been synchronized. All process cycle times are below the takt time. Lead time has been reduced by 72 %. The reduction in lead time has helped the company to meet the ever increasing customer demands. The case study shows how a company can grow leaner and become more flexible by reduction of lead time. Similar case studies can be done to implement lean by reducing wastes. REFERENCES [1] Lonnie Wilson (1976), ‘How to Implement Lean Manufacturing’, McGraw-Hill, New York. [2] Bergman, L., Hermann, C., Stehr, J., & Sebastian T. (2007), ‘An Environmental Perspective on Lean Production’. The 41st CIRP Conference on manufacturing Systems, 2008. [3] Groover, M. (2007), ‘Work systems and the Methods, Measurement and Management of Work’, Pearson Prentice Hall. [4] Dennis, P. (2007), ‘Getting the Right Things Done’, The Lean Enterprise Institute, Brookline, MA. [5] Rother, M., & Shook J. (2003), ‘Learning to See: Value Stream Mapping to Create Value and Eliminate Muda.’ Brookline, MA. The Lean Enterprise Institute Brookline, MA.
AUTHORS BIOGRAPHY First Author, R.K.NIKKHIL RATHNAKAR R.K.Nikkhil Rathnakar is a third-year Mechanical Engineering student of Thiagarajar College of Engineering. His area of interest is Quality Engineering and Lean. He has presented three papers and a case study. He has great passion for B-Plan contests.
Second Author, S.KARTHIKEYAN S.Karthikeyan is an Assistant Professor of Mechanical Engineering, Thiagarajar College of Engineering. He has ten years of teaching experience. His area of interest is Quality Engineering. He has carried out an extensive research work on the above area and published many technical papers in national and international journals and conferences.
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doc_916617356.pdf
R. K. Nikkhil Rathnakar1, S. Karthikeyan2
2
B.E. Student, Dept. of Mechanical Engg., Thiagarajar College of Engineering,Madurai-15. Assistant Professor, Dept. of Mechanical Engg., Thiagarajar College of Engineering,Madurai-15. [email protected]
1
Abstract— Every industry tries to maximize its production and increase its profit. These two targets can be achieved by reducing waste (Lean Manufacturing) and improving management systems (Total Quality Management/ Total Productive Maintenance). Lean is a comprehensive set of techniques that, when applied, will allow more flexibility and more response by reducing the waste. ‘VMK Industries’ is a small production company in Madurai which manufactures output shaft on a contract basis. The production of the final product requires turning, key way milling and grinding. The raw material used is EN 9 steel. The industry has not been able to achieve the desired target. Lead time reduction has become a potential problem to be solved. After conducting brainstorming sessions, it is realized that key way milling involved high lead time. Fish Bone diagram has helped to find the root cause. Milling operation is the bottleneck. Moreover, turning, milling, grinding of targeted number of components are done separately. The three machines, namely CNC lathe, vertical machining centre and grinding centre are not arranged in a sequence. We have converted the process as a continuous one; turning of a component has been followed by milling which is in turn followed by grinding. A working holding fixture has been developed to hold the work piece so that two keyways are milled at a time. This has helped to reduce the lead time. The three machines are arranged in a sequential order to stream flow. Lead time has been reduced by 72%.The reduction in lead time has helped the company to meet the ever increasing customer demands. Keywords— Lean Manufacturing, Production Company, Lead time, Brainstorming, Fish Bone Diagram, Bottleneck, Customer Demand. I.INTRODUCTION Production industries are under great pressure to increase their profit by reducing the cost involved in value added and non–value added processes. Value added process needs to be optimized whereas non–value added process needs
to be eliminated. Quality and Lean have become a panacea to the production sector. Technology doesn’t make any difference among production industries. It is only the quality of management systems and adherence to lean principles that make an industry flourish. II. PROBLEM DEFINITION VMK Industries is a production company in Madurai which manufactures output shaft on contract basis. The raw material used is En 9 steel. The raw material requires three machining processes namely turning, milling and grinding to produce the end product. The process involved high lead time. Consequently, the company could not meet the customer demand. III. THE CONCEPT OF LEAN MANUFACTURING Lean is a comprehensive set of techniques, which when implemented, will help industries to reduce and eliminate the eight deadly wastes- Overproduction, Human Resources, Transportation, Inventory, Motion, Correction, Over-processing and Waiting.. Lean is called so because after implementing it, the process can run using less material, require less investment, and can also become more flexible and responsive to waste. [1] The main principle of lean manufacturing is to reduce waste, long lead time and promote flexibility of manufacturing systems. All Lean environments share certain critical characteristics. They are: Production is done to meet the customer demand. Material replenishment and work control are driven by signaling devices (for example, Kanbans) Synchronized production line Continuous improvement (Kaizen) procedures Trained professionals are employed to control the manufacturing process and they shift operations based on current demands. [2]
1
A. Strategies of Implementing Lean The overall approach to implement lean consists of four key strategies. They are: Synchronize supply to customer, externally: To synchronize externally is to supply the product to our customer at their needed demand rate, normalized to our production schedule. We want to supply all of the customer needs but we do not want to overproduce and create excess inventory. Tools like Takt Calculation allow this balance to be achieved. Synchronize production, internally: To synchronize production internally is to divide the necessary work in processing steps so that each processing step takes the same time. The ideal is that all processing steps perform at a cycle time equal to Takt. Basic Time Study and Balancing Graph are the tools helpful to achieve synchronization internally. [3] Create flow: The concept of flow is such that we do not want the production units to stop, except for value-added work. The flow concept has both overall measures and local measures. The local measure would be cycle time and the overall measure of flow is production lead time. In every case, if we can reduce cycle time or if we can reduce lead time, we will make process improvements. Establish Pull – Demand System: Pull systems have two characteristics. First, they have a fixed inventory; so the cycle stock, plus the buffer and safety stocks need to be determined. Second, they are activated when product is removed and this signals the upstream process to produce—no signal, no production. Kanban and JIT (Just in Time) are the tools used to create pull system. IV. LEAD TIME REDUCTION In the 1960’s and 70’s, manufacturers competed on the basis of cost efficiency. In the 1980’s, quality was the rage and Zero Defects and Six Sigma came into vogue. Cost and quality are still crucial to world-class operations, but today, the focus is squarely on speed. Nearly all manufacturers today are under pressure from customers to cut lead times and rapid-response manufacturing pays big dividends. Customer lead time refers to the time span between customer ordering and customer receipt. Manufacturing lead time refers to the time span from material availability at the first processing operation to completion at the last operation. In many manufacturing plants, less than 10% of the total manufacturing lead time is spent actually manufacturing
the product. And less than 5% of total customer lead time is spent in the production process. The cumulative cycle times of the processes in the value stream are the theoretical limit to how much we can reduce lead times. Clearly, there is ample opportunity to reduce lead times in most organizations. Reducing lead times doesn't involve speeding up equipment to cut the cycle times or getting plant personnel to work faster. It involves rapid fulfillment of customer orders and the rapid transformation of raw materials into quality products in the shortest amount of time possible. [4] A. Methods of Lead Time Reduction The following guidelines will help reduction of lead times in an organization: Measure current lead times and set improvement targets: Lead times are not measured in most organizations. People don’t have a quantitative assessment of how long it takes for individual products to cross the value stream. [5] Things that are not measured cannot be improved. Change the organization from functional orientation to product orientation: If possible, all resources required to produce a product should be located close to each other. These product-focused groups are called work cells or cellular manufacturing. Cross-train plant personnel within cells in a number of operations for greater flexibility: Reducing the number of job classifications and maintaining multi-skilled teams on each shift is critical to rapid response manufacturing. Empower work cells and teams to take ownership for the entire value stream: Drive accountability for product cost, quality and delivery down to the lowest appropriate level. Continually reduce batch sizes between work centers: With operations in close proximity, transfer batches can be smaller and WIP inventories can be minimized. Institute local scheduling between work cells: Visual shop floor scheduling tools, like kanban systems, can be used to minimize WIP between cells and to eliminate queue time throughout the value stream. By employing these principles, many world-class manufacturers have shrunk lead times by 50-80%, gained market share, improved profitability and increased employee morale on the shop floor.
2
V. AN OVERVIEW OF EXISTING PROCESS EN9 (unalloyed medium carbon steel) is the raw material used for production of output shaft. It is throughhardened before machining. The hardness is normally within the range of 180 to 230HB. A. Steps Involved in the Production of Output Shaft The stock is turned to different diameters along its length in a CNC lathe. The turned shaft is sent to the vertical machining centre where two key ways are milled on either ends of the shaft. After key way milling, the job is ground to produce the desired surface finish. The whole process of producing an output shaft takes 30 minutes to complete. They wanted to reduce the long lead time involved in the production of shaft.
Fig. 2 Existing Layout
VI. BRAINSTORMING AND ROOT CAUSE ANALYSIS In order to find out the root cause of the problem, a fish bone diagram is drawn. Brainstorming sessions are conducted to identify the causes of the problem. By and large, Man, Machine, Material and Method have been taken as the prime causes of the problem. The Fish Bone diagram shows four causes namely cycle time, machines, material and demand. The cycle time of turning, milling and grinding is compared. The machines are analysed for defects. Customer demand also could not be met. The machining operations have been performed separately. The raw material used is En 9 Steel. The hardness of the raw material is also found to be a cause for high lead time. The problem is further funneled by taking cycle time as the root cause of the problem. The cycle time of milling is high when compared to the other two processes and required to be debottlenecked. The fact that milling process is a bottleneck is well corroborated by the calculation of takt time.
Fig. 1 Existing Process Flow
B. Characteristics of Existing Process The targeted number of components was first turned, then milled and finally ground. The three machining centres were not arranged in a sequence. They were spread in different directions. Since the machining centres were far apart, flow was not streamlined. Of the three principal processes, Key way milling involves the longest lead time. Milling operation was the bottleneck. The existing process could not meet the customer demands. The whole process of producing an output shaft takes 30 minutes to complete.
Fig.3. Fish Bone Diagram
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VII. PROMOTING THE LEAN WAY In order to achieve lead time reduction and make the manufacturing strategies leaner, certain lean tools were used. Takt time calculation was done and balancing graphs were drawn. A. Takt Calculation Takt is the rate at which a product is produced. It is a true one piece flow, pull demand concept. In our case, Takt calculation helps to find out the time taken for production of a single component so as to meet the customer demand for a day. The company has two eight hour shifts per day. So the available work time is 16 hours. The customer demand is 116 components per day. The available work time in seconds = 57600 Takt = 57600 seconds / 116 components = 500 Seconds. B. Process Cycle time Estimation Turning = 360 seconds Milling = 720 seconds Grinding = 240 seconds Of the three processes, the process cycle time for milling is greater than takt time. The milling operation is the bottleneck.
VIII. ELIMINATION OF BOTTLENECK In order to eliminate bottleneck, few changes are done. The machining centres are arranged in a sequential order. This helped to streamline flow of workpiece. Process cycle time of Milling was the major problem because it exceeded the takt time. Milling process consists of cutting two keyways on either side of the shaft. The milling of each keyway took equal time of 360 seconds. Therefore the cycle time of milling was 720 seconds. A workholding fixture is developed to eliminate this hitch. With the new fixture, the two keyways are milled within 360 seconds. The process cycle time of milling is halved. An important modification in the existing process is that turning, milling and grinding of components are done consecutively rather separately. All process cycle times have been made lesser than Takt time.
Fig. 5 Modified Layout Fig. 4 Existing Balancing Graph
C. Estimation of Lead time – Phase I Lead time for any process = (Number of Components* Process cycle time) In the industry, the target number of component are turned, milled and ground independently. Turning = 116*360 = 41760 Seconds. Milling = 116*720 = 83520 Seconds. Grinding = 116*240 = 27840 Seconds. Product Lead time = 153120 Seconds.
Fig. 6 Redesigned Balancing Graph
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A. Estimation of Lead time – Phase II Among the processes, the highest cycle time is 360 seconds. Turning and milling takes 360 seconds respectively. Product Lead time = 116*360 = 41760 seconds. Reduction of Lead time in Percentage = (Existing Lead Time – Lead time after elimination of bottleneck) * 100 / (Existing Lead Time). Reduction of Lead time in Percentage = (153120 – 41760) * 100 / 153120 = 72.73 % After implementation of lean principles, Lead time reduction has been achieved. Now, the company has been able to meet the customer demand. IX. CONCLUSION The production of shaft involved three processes namely turning, milling and grinding. Of these processes, milling had the longest lead time. The introduction of new workholding fixture has helped to reduce the lead time involved in milling operation. The bottleneck has been eliminated. The flow of material has been synchronized. All process cycle times are below the takt time. Lead time has been reduced by 72 %. The reduction in lead time has helped the company to meet the ever increasing customer demands. The case study shows how a company can grow leaner and become more flexible by reduction of lead time. Similar case studies can be done to implement lean by reducing wastes. REFERENCES [1] Lonnie Wilson (1976), ‘How to Implement Lean Manufacturing’, McGraw-Hill, New York. [2] Bergman, L., Hermann, C., Stehr, J., & Sebastian T. (2007), ‘An Environmental Perspective on Lean Production’. The 41st CIRP Conference on manufacturing Systems, 2008. [3] Groover, M. (2007), ‘Work systems and the Methods, Measurement and Management of Work’, Pearson Prentice Hall. [4] Dennis, P. (2007), ‘Getting the Right Things Done’, The Lean Enterprise Institute, Brookline, MA. [5] Rother, M., & Shook J. (2003), ‘Learning to See: Value Stream Mapping to Create Value and Eliminate Muda.’ Brookline, MA. The Lean Enterprise Institute Brookline, MA.
AUTHORS BIOGRAPHY First Author, R.K.NIKKHIL RATHNAKAR R.K.Nikkhil Rathnakar is a third-year Mechanical Engineering student of Thiagarajar College of Engineering. His area of interest is Quality Engineering and Lean. He has presented three papers and a case study. He has great passion for B-Plan contests.
Second Author, S.KARTHIKEYAN S.Karthikeyan is an Assistant Professor of Mechanical Engineering, Thiagarajar College of Engineering. He has ten years of teaching experience. His area of interest is Quality Engineering. He has carried out an extensive research work on the above area and published many technical papers in national and international journals and conferences.
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doc_916617356.pdf