Design of Cellular Layout

1. Design of Cellular Layout Operations Analysis and Improvement 2010 Fall Dr. Tai-Yue Wang Industrial and Information Management Department National Cheng Kung University

2. Presentation • The concept of manufacturing cells was presented as a specific case of product/process layout • The transformation requires a unique development and implementation methodology • The use of cells creates a unique set of production modules • Transforms the factory into a group of self-managed sub-factories or modules.

3. Presentation • This chapter presents some design and analysis tools focused on getting a company ready to progress to cellular manufacturing. • Including line balancing techniques.

4. Introduction • The first moveable assembly line was created by Ford to manufacture the Ford Model T. • A capstan and a thick cable to move the cars between assembly stations. • Before this development -> fixed position layout. • The required time to manufacture a car was reduced from 13 hours down to 6 hours. • Produced only identical black Ford Model T cars. • It was the color (Black) that dried the fastest. • Thereby allowing many Americans to afford a new car.

5. Introduction • Ford used many of the specialization ideas developed by Adam Smith • Nail manufacturing • Better having one worker manufacturing the head and another worker the tip. • Specialization and task decomposition lead to the birth of new professions. • Industrial engineers were devoted to component task design and to manufacturing scheduling.

6. Introduction • Alfred Aloan (General Motors) enhanced Ford’s work • Mix different models on the same assembly line. • During the 1960’s and 1970’s, Japan did not adopt the Ford, Aloan and Taylorist way of working • They expanded worker involvement to handle a variety of manufacturing processes.

7. Mass production • Some products are manufactured in very large batches to satisfy mass market demand. • Food products, which are manufactured in high volume, will not be addressed in this chapter. • Other products, such as toys, cars and electrical appliances will be studied in depth.

8. Mass production • Mass production has two basic characteristics. • Low product price as compared with the cost of the handmade products. • Product should be as standard as possible. • Reducing the number of different replacement parts. • Reduction in the number of spare parts. • Companies must maintain a spare parts accessibility for a certain amount of years.

9. Mass production • Simple product assemblies reduce process time. • The cost of each article decreases. • If the complexity of a product can be increased to accommodate additional replacement part applications then the part’s utility increases.

10. Mass production • Product-Quantity (P-Q) analysis tool helps to decide the optimal plant distribution for the different products manufactured. • Using Pareto Tool. • If products quantity is large and its variety is small, massive production fits perfectly • Focused assembly lines provide the most economic production alternative.

11. Flow or assembly lines • The recommended way to manufacture mass produced articles is by using flow or assembly lines. • Workstations dedicated to the progressive manufacture and assembly of parts. • Integrated using materials handling devices.

12. Flow or assembly lines • Creating flexible cells generally requires the duplication of machines. • Invest in several simple machines with low cost. • Instead of a versatile and more expensive machine.

13. Flow or assembly lines • Another advantage of assembly lines is that the work-in-process decreases. • Containers with waiting parts are placed in small queues. • This design eliminates excessive inventory required when using more versatile and expensive machines. • It is more critical to have higher machine utilization of more costly equipment. • It is more difficult to manage a greater variety of incoming material.

14. Flow or assembly lines • The most frequently used material handling device is the conveyor belt. • Size of the product -> Overhead conveyor systems. • Each product stops a predetermined amount of time in each station (synchronized flow). • Each product is removed at a workstation to be worked on (asynchronous flow).

15. M22 M21 M23 M24 M12 M11 M13 M14 Island layout • Advantages. • An effective layout when considering product movement. • Multifunctional workers. • One-piece-flow.

16. M22 M21 M23 M24 M12 M11 M13 M14 Island layout • Disadvantages. • Worker is specialized in an island or subject. • The islands are isolated with respect to each other. • Synchronization among them can be difficult.

17. Cell layout design justification • The manufacturing environment has changed significantly in the past several years. • Flow or assembly lines focus on high volume products

18. Cell layout design justification • The use of process focused systems or job shops is focused on very small volume products. • These systems maximize flexibility, but have other problems.

19. Cell layout design justification • Reasons to suggest the use of the manufacturing cells as opposed to process layout. • A high number of indirect workers is generally required in process layout. • A high level of work-in-process is also typical of process layout. • Product quality responsibilities are not as clear when using process layout.

20. Cell layout design justification • When cellular layout is properly employed… • Unnecessary product movement can be eliminated. • Product and manpower flexibility can be achieved. • A more economic production environment can be obtained. • The transition to cells is not easy.

21. Basic cells design nomenclature • Task-> a set of the necessary steps that the work gets decomposed into. • Is considered the smallest assignable unit to a cell. • Define the beginning and the end of the task in a precise way. • Methods and time methodologies facilitate the goal of optimizing a task. • Time study is also briefly presented in this chapter.

22. Basic cells design nomenclature • Workstations-> The logical organization of specific manufacturing or assembly equipment to perform task(s). • The number should be as small as possible. • Minimum workforce can be maintained along with a reduced work-in-process.

23. Basic cells design nomenclature • Takt-time and cycle time. • Takt is a German word for rhythm. • Is a critical term for manufacturing systems design. • Takt-time is the allowable time to produce one product at the rate a customer demands it. • This is NOT the same as cycle time.

24. Basic cells design nomenclature • Cycle time is the normal time to complete an operation on a product in each workstation. • Should be less than or equal to takt-time. • Cycle time is the sum of the task times that a product requires at each station. • During the cell design project, it is important that the takt-time and the cycle time be as close as possible.

25. Basic cells design nomenclature • Takt-time and cycle time. • Takt-time can never be smaller than the largest workstation cycle time. • A rhythm becomes faster than the production system is capable of handling in the upper diagram.

26. Basic cells design nomenclature • Takt-time and cycle time. • Cycle time is a measure of how much time it takes for a particular operation.

27. Basic cells design nomenclature • Total workstation cycle time (pi). • The sum of the process times for each one of the tasks assigned to a workstation.

28. Basic cells design nomenclature • Idle time (hi) • Difference between the takt-time and the cycle time or total work at each workstation. • It can never be less than zero. (That would imply that a station has assigned more work than the takt-time.) • Reality for many assembly companies can be quite different than theory. (Cell flexibility allows a company to rebalance tasks where the maximum cycle time exceeds the required takt-time.)

29. Basic cells design nomenclature • Precedence diagram. • Precedence restrictions that exist when assembling the product.

30. Basic cells design nomenclature • Precedence diagram. • Characteristics. • There are only left to right arrows. • There are no precedence relationships between tasks in the same level. • In the determination of a level, all the tasks without precedence among those that have not be placed yet are placed in that level. • The figure also include the task duration.

31. Cell design methodology • The methodology to develop production cells is very straightforward. • Form product families. • Change the machines location. • Calculate the output production rate, the task assignment in each workstation and the necessary number of workers in the cell. • Planning and controlling the cell.

32. Cell design methodology The difficulty lies in the high number of prerequisites needed to be able to carry out this layout transformation.

33. Prerequisites for cell designing • Multifunctional workers. • Determine the required space for the cells (specially U-shaped cells). • Invest in new machinery. • Improve the set-up time of dies.

34. Prerequisites for cell designing • Look for simple methods for production automation. • Choose new production planning and control systems so that future planning or control of the line is not required.

35. Line Balancing • Main goal of line balancing techniques. • To assign tasks to workstations so that the minimum number of workstations can be achieved. • Each task needed to produce a part are assigned to only one workstation. • In each workstation the assigned work (pi) is less than the takt-time. • The idle time is minimal. • This assignment should not violate any of the precedence relationships

36. Line Balancing • Line balancing for some companies may require specific algorithms. • The workers should be assigned to a fixed number of workstations. • The next methodology does not apply.

37. General steps in the line balancing • STEP 1. Define the tasks and their times (ti). • STEP 2. Specify the precedence relationships • Building the precedence diagram. • STEP 3. Determine the takt-time (Tc).

38. General steps in the line balancing • STEP 4. Calculate the minimum number of workstations (Mmin). • Supposes the lower limit of the number of workstations that can be created.

39. General steps in the line balancing • STEP 5. Choose a task assignment rule. • This is explained later on. • STEP 6. Assign tasks. • Until the assigned time is equal to the takt-time • Until it is no longer possible to assign a task due to the restrictions of time or sequence. • In this case it will be necessary to create a new workstation and continue with the assignment.

40. General steps in the line balancing • STEP 7. Determine the total idle time and the line efficiency. • STEP 8. If the obtained solution is not considered acceptable -> choose another assignment rule.

41. Line balancing--Tasks assignment rules • A task is eligible if it has not yet been assigned and all those that precede the task (preceding tasks) have been. • It will be necessary to decide among several eligible tasks. • There are different heuristic methods.

42. Line balancing--Tasks assignment rules • Heuristic methods are simple rules that propose two selection criteria. • The second criteria will only be used if there are several tasks that coincide in the first criteria. • The obtained solutions can be compared by analyzing the idle time share among the workstations.

43. Line balancing--Heuristics • Total number of following tasks heuristic. • Among the eligible tasks -> choose the task that has the largest total number of following tasks. • If two or more tasks coincide -> select the task with the longest time ti.

44. Line balancing--Heuristics • Individual durations heuristic. • Among the eligible tasks -> choose the task with the longest time ti. • If two or more tasks coincide -> select the task that has the largest total number of following tasks.

45. Line balancing--Heuristics • Largest positional weight heuristic. • Among the eligible tasks -> choose the task that has the largest positional weight. • Positional weight is the sum of the task time and the time of all its following tasks • If two or more tasks coincide -> select the task with the longest time ti.

46. Line balancing--Special cases • Task time larger than the takt-time. • Accept that the task dictates the takt-time. • Supposes the loss of possible product sales or to increase the number of pending orders.

47. Line balancing--Special cases • Solve the problem. • Divide the task into two tasks. • Analyze and define the task again. • Improve the task or the product. • In case of an assembly task incorporate an assistant. • Place two workstations in parallel. • It increases the work-in-process. • This solution will be adopted if and only if the other choices are not possible.

48. tc=5 S2’ 0 P2’=10 p1=5 S1 0 S3 0 p3=5 S2’’ 0 p2’’=10 0 5 10 15 20 25 30 35 S3 S2’’ S2’ S1

49. S1 S2 S3 S4 S5 S1 S2 S3 S6 S5 S4 Line balancing in U-shaped cells • The line balancing method, sometimes, causes an unequal time assignment. • The U-shaped layout, with shared tasks, helps to solve these unequal times assignment situations.

50. S1 S2 S3 S6 S5 S4 Line balancing in U-shaped cells • U-shaped cells avoid constant displacements to the start of the line and solves many of the island distribution problems.