The Decision Sciences Journal of Innovative Education

A Classroom Exercise

To Illustrate Lean Manufacturing Pull Concepts

This exercise is adapted from the Hewlett-Packard (1983) stockless production “traveling road show” training tape, and the Cellulose Aircraft, Inc. exercise in Heineke and Meile (1995), which provides the basis of paper airplane folding.  The HP tape is an excellent presentation of push and pull, however, the exercise in this paper engages the students in a hands-on production line that focuses specifically on the push vs. pull aspect of lean manufacturing.   

The goals of this exercise are to illustrate the significant reduction in WIP and throughput time using the pull system, to explain and illustrate the concepts of push, pull, kanban, bottleneck, cycle time, idle time, line balance, and to illustrate worker behavior in an operational setting. 

This exercise utilizes four students in an assembly line to build a paper airplane.  The work center tasks are assigned in such a way as to place the bottleneck at the third workstation.   The students first work in a standard push approach: each student works at their own comfortable pace as long as inventory is available, and they do not worry about inventory buildup.  The second run requires a pull of 3 units at a time, and the third run requires a pull of one unit at a time.  The reduction in inventory and throughput time is readily evident to all, providing a powerful learning experience. In addition, student discussion of a multitude of lean concepts and behavioral issues is easily elicited.

This exercise can be completed in about 50 minutes, making it feasible to do in a standard 1 hour class time block.  The basic multi-fold paper airplane using blank, standard 8 ½ x 11 inch paper is used.  The basic folds are easily explained, as shown in Figure 1. 

Material required:

·        100 sheets of white paper (probably more than you will actually use)

·        3 sheets of colored paper

·        Color marker

·        Forms for data collection:

¨      Task time sheets (12) – one for each work center for each of three runs – observer indicates times for 20 units.  See Form 1.

¨      Cycle time sheet (3) – one for each run of the game – observer indicates cycle time for 20 units that come off the end of the assembly line.  See Form 2.

¨      Throughput time sheet (1) – observer indicates the throughput time for the color sheet to move from raw material inventory to the end of the production line;  time once each run of the game.  See Form 3.

The four work centers are set in a line at tables in the classroom, with chairs for the workers to sit, as shown in Figure 2.   About 12 linear feet of table space is required to insure sufficient space for beginning raw material, WIP between each student, and finished goods inventory.  A stack of paper is placed before the first work center (raw material). 

Four students are asked to work on the production line.  The students are given the instruction sheet, Figure 1, and start a practice run to get any learning effects out of the way, insure correct folding, and to push some inventory into the system.  Since the objective in this exercise is the push and pull concepts, the quality is monitored by the instructor only to insure that they are not rushing improperly to try to balance the line.  During the practice, the instructor can correct any quality problems.  If the instructions are “work at a comfortable pace, do not speed, this is not a contest for maximum output, but quality will be monitored by the instructor,” students will work accordingly.

Seven students are asked to time the activities and collect WIP quantities.  Work centers one to four will have task times monitored using Form 1.  It is important to instruct students to time only the actual hands-on task time for the worker, since work center 4 will have idle time and will not be working all the time.  One student will collect the cycle time at the end of the line, the time between completion of successive units, using Form 2.  The student should be instructed to start timing when a plane is finished by work center 4, count the production of the next 20 planes, then stop timing when this last, 21^st, plane is finished.  Dividing by 20 will give the average cycle time.   The sixth student will note the throughput time of the color sheet using Form 3, discussed below.  The seventh student will count WIP on each run.

The remaining students should gather around and be instructed to keep an eye on the workers and the action, with knowledge that they are to provide insightful discussion at the conclusion. 

The first run is the push run:  let them make inventory as fast as they want and push to the next work center.  After a few minutes, as the inventory builds, have the observing student count the WIP without stopping the line.  We have found that writing the results in a table on the board will provide easy comparisons, as shown in Figure 4.

At an opportune time, insert the colored sheet onto the top of the raw material paper.  Make sure in the push run that this sheet is not conveniently placed on the top of subsequent piles of inventory, thus resulting in a relatively fast throughput.  Insist on a FIFO type of process between work centers;  the instructor can easily move the sheet to the bottom of a pile if the students try to use LIFO.  The student observer is instructed to track the throughput time:  the time from when the first worker takes the sheet from the raw material pile, to the time when the last worker completes the plane and places it in the finished goods inventory.  This throughput time should be written on the board as shown in Figure 4.  When the colored sheet is out of the line and the throughput time noted, the run can be stopped if at least 21 units have made their way through the end of the line and all the data collection has been completed. 

At the conclusion of each run, the observers are asked to find their average task times, cycle time, and throughput time.  These results will be added to the table on the white board as shown in Figure 4.  The tasks times should indicate that work center 3 is the bottleneck, and since the bottleneck is really the deciding factor in throughput, and thus the cycle time should be about the same as the bottleneck’s task time for each run. 

A discussion at this point should result in the identification of the bottleneck, the piles of inventory, and observation that work center 4 is idle some of the time while the other work centers worked continuously.  This is a good time to reinforce the concepts of bottleneck, cycle time, throughput time, push, line balance and worker behavior.

The second run is a pull 3 run.  Designate a space between each work center as an imaginary kanban box where inventory will be held as shown in Figure 3.  Place 3 units of WIP in each of the 3 kanban areas.  Indicate that one worker’s in box is the previous worker’s out box.  A worker can start work at their work center only when 1) there are 3 units in the incoming space, the kanban box, between their work center and the previous work center (except for work center 1, who has a stack of raw material paper), and 2) when there is no inventory in the box out of their work center.   When these conditions are met, the empty out box is considered a kanban signal that more production is needed.  The worker can then pull the 3 units out of the in box (only when there are 3 units in the in box) and do the work.  When they are done they place the 3 units in the out box and wait until the box is empty again.  Work center 4 can pull the 3 out of the in kanban box only when he or she has completed the last batch of 3.  This is the time to discuss the kanban as a visual signal that is pulling the production of planes.

Since this is very different from the push system, students may need some observation and help to make sure they are following the pull discipline.  After a few trials to make sure the pull is working, start the run.  Repeat the insertion of the color sheet, and the collection of the WIP quantity, throughput time and cycle time.  During the pull run, an instructor can have some fun asking the bottleneck worker what the problem is since this is holding up the output, while also asking the idle workers why they are sitting around doing little work. 

The pull system requires 3 of the 4 work centers to be idle part of each cycle, a process that is counter to our usual thinking.  It is actually good, in this case, to be idle!  The alternative is a build-up of inventory.   If you use Goldratt’s The Goal, this is a perfect time to discuss similar key concepts in that book.

After this pull 3 run and debriefing, repeat with a kanban box holding one unit, showing even more performance improvement.  Discussion will focus on the dramatic reduction in the performance measures of inventory and throughput time. 

In addition to the discussion of the key learning objectives noted earlier, students should be able to discuss the following.  Discussion of many other topics will follow.

·        Students are often astounded to discover that as the inventory is reduced, the throughput time is also reduced.  In addition, three workers are idle.  How can this be?

·        Work center 3 has the longest task time and is the bottleneck.  The bottleneck worker can be asked how he or she felt as the only one working all the time while the other three workers had idle time.

·        If this is a work team, how will they go about resolving the obvious imbalance in work loads?

·        In the pull system, inventory build-up hid the fact that the first two work centers were out of balance with the bottleneck. 

·        The line can be balanced in a variety of ways.  Use the task times to determine how to combine tasks to balance the line.

·        Work centers 1 and 2 were often idle in the pull runs, but not the push system.

·        Workers often do not communicate with one another. Worker communication must be enabled if this team is to improve the production process.

·        The concept of pull is readily apparent.

Other classroom material can be used effectively in conjunction with this exercise.  Goldratt’s The Goal is natural for bottlenecks and throughput, and the issues of people (or machines) sitting idle because they are driven by the bottleneck speed.   Why do we keep those people (or machines) busy when they really should be working at the pace of the bottleneck? 

At the conclusion of a recent semester, the author surveyed students on the most important learning aspects of the course.  The airplane exercise was rated one of the most important learning activities in the class.  Students noted that the basic lessons learned were remembered through to the conclusion of the course.  The most remembered concept was that as the inventory was reduced so was the throughput time.

References

Heineke, Janelle. N, and Larry C. Meile, eds., Games and Exercises for Operations Management, Prentice Hall,  Englewood Cliffs, New Jersey, 1995.

Hewlett-Packard Corporation, Stockless Production: Push out the Old, Pull in the New, video tape, 1983.

 Figure 1

Paper Airplane Work Center Assignments

Work Center 1

Fold up in half long way.

                                                                        Long fold

Work Center 2

Fold first corner on each side.               Fold down on each side.

Work Center 3                                                                       Long fold

Second fold of wing on each side.                Fold down on each side.

                                                                 Draw a star with a colored marker on one side

Work Center 4

Third fold of wing on each side.  Place in finished goods inventory.

                                                              Fold down on each side.

Figure 2

Push Production Line Setup

Table length ~12 feet

Material Flow

Figure 3

Pull Production Line Setup

Table length ~12 feet

Material Flow

Figure 4

White Board Data Collection

Form 1 (abridged)

Paper Airplane Manufacturing

Task Time

Work Center ___________

Form 2

Paper Airplane Manufacturing

Cycle Time

Run _________

Form 3

Paper Airplane Manufacturing

Color Sheet Throughput Time