Chapter 17. Synchronizing and Balancing Processes. Level the master production schedule. Production. Time

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1 Chapter 17 Synchronizing and Balancing Processes 1 Goal Achieve uniform flow Level the master production schedule Production Time Avg. Reduce the size of batches produced for final assembly Production A B C A B C A B C A B C Time Avg. 2

2 Synchronization Every upstream operation should produce at the same rate to satisfy demand That is, the rate of production and transfer should equal the rate of demand production rate = demand rate The final assembly production schedule is determined by the customer demand rate 3 Synchronized Cycle Times Set cycle times at every upstream operation according to the cycle time at final assembly. Do not produce at faster rates than the cycle time at final assembly. Why? The mixed model sequence must be frozen. Why? Cycle time is not a production rate. Why? 4

3 Synchronization Example 1 Available production time = 420 min/day Products Number W. Nissan Maxima GXE 58 X. Nissan Maxima SE 49 Y. Nissan Maxima SLE 30 Z. Nissan Maxima 20th Anniv. 14 All 150 Products Required CT (min) Required CT (sec) W. Nissan Maxima GXE X. Nissan Maxima SE Y. Nissan Maxima SLE Z. Nissan Maxima 20th Anniv All The mixed model sequence will be WXYWXWZYWXWXYWX. How will stations upstream have to perform? 5 Synchronization Example 2 Nissan runs for 420 minutes during each of two shifts. In this time, they produce the number of cars shown below. The number of parts are shown below each. The same tires and radios are used for all cars. However, the AC and doors have to be customized for each kind of car. The setup time for doors is 10 minutes and the setup time for AC is 30 minutes. Currently doors are made in batches of 25 units and AC units are built in batches of 25 units. Car Maxima (100) Altima (150) Sentra (300) Tires (5) AC Radio Doors (4) Tires (5) AC Radio Doors (4) Tires (5) Radio Doors (4) AC Radio Doors Tires 6

4 Synchronization Example 2 (cont.) What is the mixed model sequence for the cars? What is the overall cycle time, the cycle time for each kind of car and their components? 7 Synchronization Example 2 (cont.) Doors require.54 minute to complete and air conditioners require 1- _ minutes to complete. What is the batch size required in order to increase the cycle time? 8

5 Bottleneck Scheduling Assumption so far = Every operation has enough capacity to meet the production schedule If a station does not have enough capacity, it is the bottleneck If processes are long-running and stable, we may add capacity to the bottleneck However, most companies have product mixes and changing demand volume, therefore changing the bottleneck station. Bottleneck sets the pace Schedule the bottleneck to optimize capacity 9 Bottleneck Scheduling (cont.) Must increase capacity at the bottleneck Methods to increase capacity: Reduce setup time Increase the size of the process batch Methods to minimize lead time: Reduce the size of the transfer batch 10

6 Managing the Bottleneck Pace the process according to the capacity of the bottleneck Add a buffer upstream of the bottleneck to ensure that it always works when upstream operations are interrupted Do not release more jobs into the system than the bottleneck can handle (WIP will increase) Provide a signal from the bottleneck to release a job into the system when the bottleneck finishes a job 11 Drum-Buffer-Rope Line A Rope Bottleneck Operation Buffer Line B Rope Jobs are released into lines A and B upon release of order at the bottleneck Don t worry about operations downstream of the bottleneck because they work faster than the bottleneck Shipping schedule is based on the lead time through the process 12

7 Drum-Buffer-Rope (cont.) Goal: Increase the capacity at the bottleneck When the bottleneck is improved, another operation will become the bottleneck Where should the bottleneck be placed? When do we need to worry about the operations downstream from the bottleneck? 13 Pull from the Bottleneck Routing 1 Routing 2 Routing 3 Kanban cards are used to release material into the system when the bottleneck releases material Job orders are kept in the backlog at the bottleneck Kanban includes: Routing sequence Quantity a b c d e f p q g h i Backlog Job 1 Job 2. : Job j Similar to CONWIP except the bottleneck is connected to the start of the process 14

8 Pull from the Bottleneck (cont.) Release products based upon: b Time when the bottleneck will be ready for the job Time required to reach the bottleneck Buffer time (time before it is needed) at the bottleneck i L j W L + W b j = lead time for j to reach the bottleneck = buffer time = time when bottleneck is ready to process job i j 15 Scheduling from Bottleneck Example 1 Routing Lead Time Buffer Time Process Time at Routing up to Bottleneck (L) (W) Bottleneck (b) Job Routing L L+W b Assume that the first three orders have been prereleased. 16

9 Scheduling from Bottleneck Example 1 (cont.) Job Routing Release No Scheduling from Bottleneck Example 2 from page 13 Determine the schedule at the bottleneck with the given routings. Routing Lead Time Buffer Time Process Time at Routing up to Bottleneck (L) (W) Bottleneck (b) Assume that jobs 1 and 2 have been pre-released. Job Routing L L+W b

10 Scheduling from Bottleneck Example 2 from page 13 (cont.) 19 Balancing Definition: The procedure of adjusting the times at work centers to conform as much as possible to the required cycle time. Balanced process has equal processing times at each station. Balanced process is only appropriate in a paced line. It is inappropriate for job shops 20

11 Line Balancing Balance the line to achieve: Cycle time at each operation satisfies the required cycle time. Tasks assigned in the correct order. Efficient assignment of tasks. To balance the line: Develop a precedence diagram of tasks. Assign times for each task. From the beginning of the process, assign tasks to each station by using the largest task time first and adding on the next largest task time until the required cycle time is exceeded. 21 Line Balancing Example 1 7 hour workday = 420 min/day Operating time 420 min/day Required cycle time = = = 1.0 min/unit Demand 420 units/day Task Time (min) Total =

12 Line Balancing Example 1 (cont.) Station Time Available Tasks Eligible Assign Task Time for Task , ,3 none , none ,6 none , none Work Content Efficiency = (No. Workstations) (Required CT) = = = 0.76 or 76% Line Balancing Example 2 Task Time (min) Total = 4.5 Operating time = 480 min/day - 30 min/lunch - 2*(15 min/break) = 420 min/day Demand = 280 units/day What is the required cycle time?

13 Line Balancing Example 2 (cont.) What tasks should be assigned to each station and how many stations will we have? What is the efficiency of the process? 25 Balancing for Mixed Model Production Problem with the line balancing problem is that it is for one product In mixed model production, the task time may be different for each product Use the weighted average time rule Determine the percent of products that are comprised of product j Use the percentages to develop a weighted task time for task i Add the task times until they exceed the required cycle time 26

14 Weighted Average Rule Example 1 Product j Task i A B C Work Content Product D (daily) Required CT (min) A B C 60 7 All products What are the weighted average task times? 27 Weighted Average Rule Example 1 (cont.) Answer: Weighted Product Average A B C All products Task i Weighted Average

15 Weighted Average Rule Example 2 Continuing from the example on page 24, determine the task times utilizing the weighted average rule. Product j Task i A B C D Work Content Product D (daily) Required CT (min) A B 70 4 C 35 8 D 35 8 All products Weighted Average Rule Example 2 Will the stations still meet the required cycle time of 1.4 minutes/unit? 30

16 Why does mixed model production work? Products with more time-consuming and less timeconsuming tasks are paired together. One task time may be greater than the required cycle time if and only if another task time lowers the weighted average below the required cycle time. Proof: The required cycle time is 2 minutes/unit. Products A, B, and C have task times of 2.5, 1.6, and 1.75 minutes. Product Task Times Demand Percentage Weighted Average A B C Total The average task time is 1.95 minutes which is below the required cycle time of 2 minutes/unit. 31 More Ways to Achieve Balance Dynamic Balance Move tasks between stations for different products to achieve balance. Causes a loss of flow. Parallel Line Double capacity by adding a parallel station for the task that exceeds the required cycle time Must be sure to maintain the mixed model sequence The task time is not divided in half, instead, two products are machined at the same time Efficiency will probably be poor 1 2a 2b 3 32

17 Balancing for Synchronous Flow Fabrication and final assembly should all be balanced. Balance the entire process by: Adding or subtracting tasks at operations (level loading) Moving workers between tasks and operations (flexing) Adjusting the time to perform work tasks (standard work) Goal: Match the required cycle time to minimize waste. 33 Balancing Through Worker Reassignment Since every task time is below the required cycle time, every worker will be idled sometimes so the cell will not overproduce The preferred option is to give most workers tasks that equal the required cycle time and then to try to reduce the task times to move a worker out of the cell Use worker E to refill kanbans from WIP stores until the worker is removed from the cell Worker Task Times Possible Worker Task Times Preferred Worker Task Times Time A B C D E Workers Time A B C D E *Note: The required cycle time = 1 min/unit Workers Time A B C D E Workers 34

18 Balancing Through Worker Reassignment (cont.) Balancing operations is a continuous process To maintain synchronous production, utilize: Visual signals to inform workers (andon lights and status boards) Multi-skilled workers Flexible operations Reduction in setup times Scheduling below capacity for delays 35 Adapting to Schedule Changes As the schedule changes, do not adjust worker and machine assignment, but lengthen the production day or shorten the takt time instead Adjust the mixed model production sequence according to the percentage of demand of each product May adjust the production rate with the required cycle time May increase the production rate and the length of the day Let management take care of the long-term capacity planning issues and let the shop floor deal with the daily issues 36

19 END 37 Answers for Synchronized Cycle Time page 7 Sequence = S A S M S A S M S A S time/shift = 420 min # of shifts = 3 shifts total time = 1260 min # of changeovers in mixed model set = 10 Setup time for doors = 10 min Batch size for doors = 25 # of setups for doors = 80 total time for doors = 460 min Setup time for AC = 30 min Batch size for AC = 25 # of setups for AC = 20 total time for AC = 660 min Tires Number Cycle Time Maxima Altima Sentra All Cars Number Cycle Time Maxima Altima Sentra All Doors Number Cycle Time Maxima Altima Sentra All AC Number Cycle Time Maxima Altima Sentra All Radio Number Cycle Time Maxima Altima Sentra All

20 Answers for Synchronized Cycle Time page 8 Trick question! Doors cannot be improved unless we change the mixed model sequence to SSAASSMMSSA which only has 6 setups. Setup time for doors = 10 min Batch size for doors = 200 # of setups for doors = 6 total time for doors = 1200 min Doors Number Cycle Time Maxima Altima Sentra All Air conditioner may use the original sequence with 10 setups but increase the batch size from 25 to 50. # of changeovers in mixed model set = 10 Setup time for AC = 30 min Batch size for AC = 50 # of setups for AC = 10 total time for AC = 960 min AC Number Cycle Time Maxima Altima Sentra All Scheduling from Bottleneck Example 2 on page Routing Lead Time Buffer Time Process Time at Routing up to Bottleneck (L) (W) Bottleneck (b) Job Routing L L+W b Assume that jobs 1 and 2 have been released. Job 3 has L+W>2+3, so release job 4 instead. Job 1 is done at hour 2 and job 3 has L+W=3+3, so release job 3. 40

21 Scheduling from Bottleneck Example on page (cont.) Release No Routing Lead Time up to Bottleneck (L) Waiting or Buffer Time (W) Process Time at Bottleneck (b) Note: Schedules tend to become more inaccurate with time and hence less dependable. 41