This simple game was developed by

James R. Holt, Ph.D., PE
Associate Professor Engineering Management
Washington State University

Feel free to use this game when you credit to the source.

Objective of Game

 The student will be able to understand how Drum-Buffer-Rope (DBR) scheduling applies to Job Shop environments.

•  
• Role Requirements:
   The Instructor should assign students to be the Scheduler, Work Center Operators, and Flow Control Monitor roles. The roles have the following requirements:

  Scheduler(one person) - This game requires the Scheduler to release the work according to the schedule. Each day the Scheduler writes the day the Job Shop Card was release in on the Release Day Line. The Scheduler will also have to keep track of how much work is int the system (be able to add up the buffer count ) and determine whether to release an order during the different versions of the Job Shop Game.

   Work Center Operator(four people) -There is one person at each Work Center (A, B, C or D). These Operators can perform one operation per day. If they have work, the Operator can write a number in one "Day Complete" box. This counts as the operation. In some scenarios of the game, the Operator will report certain information to the Scheduler. For example, in scenario 3, the operators are required to indicate the number of ‘B’ operations that have not been completed on the orders in their queue at the end of each day. This is easily done by indicated by either a closed fist for none, or the appropriate number of fingers.

  Flow Control Monitor(one person) - The Flow Control Monitor takes all completed products and calculates the Total Flow days (the date in the last Day Complete box minus the Release Day). The Flow Control Monitor records the Total Flow Days in two places. One is the Orders By Release Day Chart (where the Flow Time is plotted opposite the release day) and the other is the Flow Time Distribution Chart where a histogram is kept showing the number of Job that that were completed at a specific number of Flow Days.  Its best if the Flow Control Monitory can work and record as the days pass , but it is OK to accumulate work and record all the measurements at the end.

•  Scenarios Overview:
   The first two scenarios are run in a Traditional Job Shop scheduling manner releasing one new Job Order Card each day. Scenario #2 is a traditional job shop scheduling, but there is an attempt to 'optimize' work flow processes by trying to schedule which job comes when. Scenario #2 takes a lot of mental effort and may be skipped if time is short. Scenario #3 is a Drum-Buffer-Rope environment.

• Scenario #2 Instructions:

  The teacher makes 5 to 10 copies of the basic Job Order Sheets (Products 1-4 on one page) and cuts them into individual Job Order Cards.\

  The Scheduler, with the help of the other members of the team, tries to use intuition and thought about the different Job Order Routings and tries to sequence the Job Order Cards in a very thoughtful way in an effort to create a very predictable deliver time. The effort should try to help the bottleneck Work Center (‘B’) so it is not so plugged up with Inventory. The Scheduler will continue to release one Order card each day just as in Scenario #1.

  For each Shop Day, the Instructor will:

  1. Call out the Shop Day (ex. "Shop Day #7")
  2. Pause, then tell the students to write the number of the Shop Day (ex. "Write #7") on the next Order card in their queue waiting to be processed. Allow the students to determine the best way of handling the queue of cards they may receive. They can decide as a group or let individuals decide on their own.
  3. Pause, then tell the students to pass the Order card to the next Work Center on the Routing or to the Flow Control Monitor if the Order is finished (ex. "Pass the Order card")
  4. At completion, record the data. (See ‘Data Plotting Instructions’)
  Play continues even after Scheduler releases all the cards to the system. Continue until all the cards are complete.
  At the completion, discuss with the students:

  • Did you see any marked improvement?
  • Was your heavy duty thinking worth much?
  • Were you more predicable?
  • Was there any real improvement ? 
    
•  Scenario #3 Instructions:

  The teacher makes 5 to 10 copies of the basic Job Order Sheets (Products 1-4 on one page) and cuts them into individual Job Order Cards.

  An explanation of Drum-Buffer-Rope (DBR) is given before the third scenario is run. Depending on the depth of the knowledge the students have of DBR, they may be able to analyze the results of the previous exercises and make a determination of the correct buffer size. If not, the Instructor will have to give an explanation and guidance. The buffer size should be no less than 4 and no greater than 6. See the ‘Observations/Results’ for Scenario #3.

  The Instructor will ensure that the Scheduler arranges the Order cards in the same release sequence as Scenario #1. (That is, the Instructor gets a clean set of Job Order Cards and puts them in the same order as the random set from Scenario #1. This is done by examining the previous release dates. If you wish, you can reuse the same cards as used in Scenario #1 and have each person write with a different color pen.). Explain to the students that the reason you are using the exact same sequence is to provide a direct comparison of results between the two scheduling methods. The Scheduler will release orders into the shop based on how much work is in the buffer. The amount of work in the buffer is defined as the number of Work Center ‘B’ operations which have not been completed for all Order cards on the shop floor. At the beginning of each Shop Day, when the Instructor says "Buffer Count", the Scheduler will count all  the number of ‘B’ operations which have not yet been completed no matter where they are located (include uncompleted B work on cards at the other Work Centers also). If the number of ‘B’ operations yet to be completed is less than the buffer size, the Scheduler will plan to release one Order card. If the number of ‘B’ operations is equal to or greater than the buffer size, the buffer is considered full and no Order card is released

  For each Shop Day, the Instructor will:

  1. Call out "Buffer Count" and the students will raise one finger for each ‘B’ operation which has not been completed on the Orders in their queue - use a closed fist if they have none. The Scheduler will count the fingers and make a determination of whether or not to release an order for the day.
  2. Pause
  3. Call out the Shop Day (ex. "Shop Day #7")
  4. Pause, then tell the students to write the number of the Shop Day (ex. "Write #7") on the next Order card in their queue waiting to be processed. Note: Now is a good time to explain what Priority Means in DBR. Tell the students the Priority is determined by the market. And, that means the earliest started job has the highest priority. So should they have a queue of jobs, they should stack them with the EARLIEST RELEASE DATE on the top. This way, the oldest job in the system is moved most quickly through the system.
  5. Pause, then tell the students to pass the Order card to the next Work Center on the Routing or to the Flow Control Monitor if the Order is finished (ex. "Pass the Order card")
  6. At completion, record the data. (See ‘Data Plotting Instructions’)

   At the completion of the third scenario, compare the results to the first two scenarios in the same terms used in Scenario #1.

  • Did the last job complete Scenario #3 about the same day as in Scenario #1 and #2?
  • What is the slope of the Trend Line for Flow time versus Release Day?
  • What is the difference in the shape of the Histograms?
  • Which is more predictable? Why?
  • Was any work lost because of tying the Rope?
  • What is the level of Chaos in the DBR solutions?
  • What was the level of Work-In-Prograess.
    

•  Data Plotting Instructions: There are two charts for plotting Order card data (see examples below).
  1. ‘Number of Flow Days vs. Order Release Days (in Release Day sequence)’ (top chart). This chart represents the number of Total Flow Days for each Job Order Card shown in sequence of the day released.

   A. Arrange the Order cards in sequence by Release Day. Each order will have its own column.

  B. Plot the ‘Total Flow Days’ by putting an ‘X’ in the appropriate column at the ‘Total Flow Days’ height.

  C. ex. the ninth Order released had Total Flow Days = 15.
  2. ‘No. Of Orders vs. Flow Days (Distribution)’ (bottom chart). This chart represents the number of Order cards that have the same number of Total Flow Days.

  A. For each Order card, put an ‘X’ in the column (starting at the bottom of the chart and working up 1, 2 , 3 and so on) representing the ‘Total Flow Days’ for each order.

  B. ex. Six orders had Total Flow Days = 7

Summary

• Observations / Results:

(Results documented by Chuck Gauthier, Alpha West, Portland, OR, Summer 1998)

Conclusion:

 DBR is an appropriate technique for scheduling a complex manufacturing environment.
  Advanced Simulations 

• Luck of the Draw
   Set up the Cards as for Scenario #1 (shuffling the cards) and play again only using DBR like for Scenario #3. Clever students will try to spend a lot of time arranging the cards into a nice sequence. They may think that the performance of the system is because of their cleverness.  Of course, cleverness is not always possible in real life.  By shuffling the cards in LUCK OF THE DRAW, you see that it is the DBR controls that give the predictable outcome not cleverness (although, cleverness on first release and last to release can give a slightly tighter flow day distribution when used with DBR).
   
• Change Over
   Inserting DBR on the fly: Often students ask the question, "How can I implement DBR in our Job Shop?"  A fun simulation is to run Scenario #1 for 20 days which builds up a healthy Queue in the system and THEN immediately implement DBR from Scenario #3 with the queue already there.  You have to hold back releasing work until the selected buffer size is reached.   After that, things are smooth as silk., chaos diminishes, stress is reduced and you are very predicatable.  Continue playing until about 40 cards are released. The visuals show very nicely how the Flow days improve.
   
• Variability
   Using Dice: After you gain proficiency in using the Job Shop Game, you may want to add more variability.  The basic game is deterministic.   That is, the only variability is the shuffle of the cards before the start of the game.  This is not realistic.  To simulate variability, you can give each Work Center a single die.  Each day the Work Center rolls the die.  If the roll is 2,3,4, or 5, the Work Center has productivity that day (acts normally) and can write one number (if they have work to do).   If the roll is a 1, the Work Center has a bad day and is not productive at all (cannot do any work at all - cannot write a number-any work in progress at that work center just sits there).  On the other hand, if the roll is a 6, the Work Center has a really good day with double production capability and can write TWO numbers (if they have that much work to do). On the average, each Work Center produces the same as with the deterministic game. But the variability is enough to upset work flow a bit. You may want to play this game yourself before working with the students.   You will find that using dice both Scenario #1 and Scenario #3 perform almost the same as before only with a bit more variability in the outcomes.  The parallelism is surprising. Running a good Drum-Buffer-Rope solution will give the same results for the variable model as the deterministic model.
  

• Kanban
   Just In Time: Doing Kanban flow in a flow line is easy to watch.  But, doing Kanban in a Job Shop is hard and takes a lot of careful control.  If you are brave, you can try to do Kanban with the Job Shop Game as a comparison with DBR.  I suggest using a Kanban size of two cards (one in process and one waiting).  This way, the maximum theoretical work in process would be 8 cards.  There are a lot of different possible outcomes depending upon your control of the system.  In the Job Shop, cards often flow from two Work Centers to one Work Center.  This leads to a decision of who gets to move there and who doesn't if the Kanban for the receiving Work Center is full.  With students, this often becomes 'whoever is fastest'.  As such, you can get a nice distribution with a few way out outliers.  The key to running Kanban is the release of work.  The scheduler calls out the day but only writes the release date if it is possible to release work on that day.  Since this is similar to the rope in DBR, you will get a tight distribution of flow times.   However, rather than having a safe feeling buffer, the work in process queue at Work Center B can vary drastically and often approach zero.  If you play Kanban with dice, you will surely starve Work Center B once or twice in 36 cards.
  
• Balance Line
   Lean Production: Some students will argue that whole problem is the B Operation. There is unbalanced Capacity. What if the work released to the Job Shop had equal work load for each work station. That is, instead of releasing six days of work for Work Center B every four days, what if in four days you released four days of work for each Work Center? Wouldn't that smooth everything out? This is the type of problem that happens in a large job shop environment where it is clear every work center has capacity to meet the market demand, and yet they still cannot seem to get things delivered on time. It turns out you can not run a Job Shop with a balanced line without tying the rope. The Job Shop just won't work. To illustrate this impact of Lean on a Job Shop, play the Job Shop Game using Job Orders Cards named Balanced #1, Balanced #2, Balanced #3 and Balanced #4. These four Job Order cards contain 4 operations for each of A, B, C and D. Copy 5-10 copies of the Order Card Sheet with Balanced #1-4 Job Orders. Cut the sheet into individual Job Order Cards making sure to have the same number of each Job Order product (1-4). Shuffle the cards. Release the work as in Scenario #1. You will be surprised at the time it takes to complete all the jobs and the huge variability in when the jobs are completed. While the Flow Times do not continually increase as they did in the earlier Scenario #1, there will me quite a few significant outliers. And, you will not be able to accurately predict when a Job Order will be delivered or be an outlier. Consequently, predictability is lost. No matter what you do, the Lean Production line will take longer to produce the same number of Jobs per day ( on the average). To remedy this, tie the rope to control the buffer. But in this case, count the number of Jobs (number of cards) released to the system as the buffer. You will get remarkable predictability. Experiment with different buffer sizes.
   
• Dynamic Buffering
   Advance Buffer Management:  In general, buffer sizes are fixed.  But, there are some cases where it is justified to change the buffer size (adjust up and down).   One such incident is when the work load on the system shifts and causes a non-constraint to become temporarily more heavily loaded than the chosen constraint for a time.  When this happens, you have a choice: 1) to change the constraint of the company (not the recommended option since it changes the whole measurement system and subordination process) or 2) to increase the buffer size to accommodate the temporary change in product mix.  This is a hard thing to teach and difficult for students to understand, however, with the Job Shop Game, it is easy to demonstrate.  Here is how.  Create another Product #5 that flows to C->A->C->D.  Take the basic 36 cards and shuffle them to play Scenario #3 as usual.  The scheduler will insert the Product #5 card for the Product #2 card as follows:  After the tenth day (and until the 25th day), every time a Product #2 card comes up, exchange it for the new Product #5.  This will shift the system constraint to Work Center C temporally.  If you have too small a buffer for the real constraint (Work Center B), the buffer will not be enough to assure continued production on the constraint.  Allow the students to determine how to increase the buffer size during the temporary shift in product mix such that Work Center B does not run out of work.  Encourage them to return the buffer size to its original size after the day 25. 

I hope you enjoy playing the Job Shop Game as much as I do.  It is a robust exercise that really opens the eyes to the necessity of the rope in DBR. The cleaver instructor can use the Job Shop game (with variability) as an effective introduction to problems with Multi Tasking in Projects and the Critical Chain Project Management techniques.

Of course, I'm always looking for improvements. Please contribute your suggestions.

Dr. Holt

Support Materials:

Blank Charts

Basic Jobs (1-4)

Advanced Jobs (5-8)

Balanced Line Cards (B1-B4)

Ó Copyright Washington State University 2005