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William A. Levinson, P.E.  Principal
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The problem with "push" production control

Two-card kanban

One-card kanban

Kanban without cards

Production control at the Watertown Arsenal

Pull production control in a blacksmith shop

Visual controls

Kanban Systems and Visual Controls

The Problem with Push Production Control Systems
Production control and pig-swallowing
"OK, you've swallowed the pig; now what do  you do with it?" This is a common problem in push production systems.
Constrictor snakes such as the one shown at the right are designed to process meals in the indicated manner, and they often sleep for several months while they digest their meals. This is not how we want a factory to operate; pig-swallowing should be left to systems that are designed to handle it.
Standard, Charles, and Davis, Dale. 1999. Running Today's Factory: A Proven Strategy for Lean Manufacturing. Cincinnati, OH, Hanser Gardner Publications (1999, 111-112) uses the phrase "pig in a python" to describe large inventory bubbles that move through a factory. "If smaller orders are released more often, the factory resources are loaded much more easily. …This is analogous to the python swallowing dozens of little piglets instead of one large pig. …Surprisingly, many factories prefer to 'stretch the python' so it can swallow an even larger hog!" The reference mentions an explorer who saw an anaconda swallow an adult hog in a Peruvian rain forest. (Dale Davis studied anthropology and specialized in Maya culture.) However, Levinson (editor), Leading the Way to Competitive Excellence (1998, ASQ Quality Press) coined the phrase "pig-swallowing," and even included an actual photo of a recently-fed python from The Serpent's Den in Milford, PA.

Two-Card Kanban

Reference: CPIM Participant Guide, "Execution and Control of Operations," Version 1.1, January 2001, from APICS
  • Uses a move card and a production card.
    • Move card: requests movement of a specific number of units from an outbound stockpoint to an inbound stockpoint.
      • Attached to a standard container of parts when the container is moved.
    • Production card: indicates that items should be made for use or to replace pipeline stock.
      • Tells what to produce
      • Used only at the work center and its outbound stockpoint.
  • Suitable when workstations are not close together.
One card (move or production) stands for one standard container of parts. The production card remains at its workstation. When a container of parts is taken (pulled downstream), the production card authorizes production of another container. When the requesting (pulling) workstation empties a container of parts, return the move card to the supplying operation and attach it to the next container. The following figures are from a PowerPoint presentation for a 1-day lean manufacturing workshop.

One-Card Kanban
  • This system uses only a move card. It is appropriate when the workstations are close together.
  • An empty space in the outbound stockpoint becomes the production order for another container.
  • When the downstream (pulling) workstation takes a container, the attached kanban is sent back to the upstream workstation to authorize production of a replacement.
Kanban Without Cards
Kanban-style pull production control can be achieved without using cards. The setup is very similar to a two-bin replenishment system.
Production Control at the Watertown Arsenal (1908-1915)
The Watertown (Massachusetts) Arsenal operated under scientific management as originated by Frederick Winslow Taylor during the early twentieth century. The Watertown Arsenal did not use a direct pull system. The job-shop nature of the factory, in which each job might follow a different route, probably precluded a direct pull system anyway. In a work cell or assembly line, each station has one upstream supplier and one downstream customer. In a job shop, a drill press (for example) might have several different upstream suppliers and downstream customers. It cannot therefore send a kanban to one upstream operation. Some form of central production control is probably necessary and today it would be achieved with a computerized information system. As a government supplier, Watertown produced to order, not to market forecasts.
Production control took place in a planning room that received information from the factory and sent instructions to it. The system used the following forms:
  1.  A job card for each operation. The job card included the name and suborder number of the part, the number of the machine on which it was to be run, and the machine's location. The operator was to mark the time required to perform the operation. The card served two purposes:
    • Its issuance to the shop told the operator which operations were to be performed on the material or part.
    • When the operation was complete, its return to the planning room provided information for calculation of labor and overhead costs. Remember, however, that labor is essentially a sunk cost unless the factory is paying overtime, and that overhead is a sunk cost.
  2. A move card for each part. The move card (apparently a routing or lot ticket) listed the entire sequence of operations for the job. When the planning room wanted to begin work on a part it sent the move card to the storehouse with instructions to deliver the raw materials to the first workstation in the sequence. The storehouse then returned the move card to the planning room as notification that the work was now at the first operation. The workstation could work on the job but could not send it anywhere until the planning room returned the move card
    • The move card went to and from the planning room instead of staying with the job. This let the planning room know exactly where any job was at any given time. This is an important characteristic of modern computerized production control systems.
    • The move card also assured that jobs received the required inspections because the inspector had to initial the card and return it to the planning room. Only when the planning room returned the card to the inspector could the job move to the next operation. This covered a requirement of ISO 9000 (which did not, of course, exist then) that the quality system assure that all required inspections are performed.
    • Although the Watertown system was not a classical pull system, it did not allow work to be pushed blindly either. The card that authorized production came not from a downstream operation but from the planning room. This may have given the planning room some control over inventory buildup in the factory; it probably didn't start jobs until capacity was available for them.
  3. A move tag told the unskilled laborers who transferred work in the factory where to take the job next.
  4. Instruction cards gave operators detailed instructions on how to perform job assignments.
    • The concept is similar to that of modern work instructions or operating instructions, and instruction cards were usually a standard feature of the Taylor system.
    • The reference says that Watertown did not use many instruction cards because of the availability of skilled labor. Watertown's job cards and suborders contained general instructions, which might be supplemented by verbal direction from shop foremen or gang bosses. Their roles might have been similar to those of manufacturing technicians and other shop floor technical support personnel.

(1) Station A finishes, sends the job to B, and sends the move card to the planning room.
(2) B can work on the job but cannot send it to C until the planning room returns the move card.
(3) B finishes, sends the job to C, and sends the move card to the planning room. Work was not started until the purchasing division reported that all necessary materials were available. When this was done, the planning room circulated the job cards and move tickets to the shops that were to perform the work. The planning room also used a stores-issue ticket to direct the storeroom to send the material to the first machine. When it arrived, the shop posted the first job card. This could well have been a form of visual control or visible management because it showed what each machine was doing.

Aitken, Hugh G. J. 1960. Scientific Management in Action: Taylorism at Watertown Arsenal, 1908-1915. Princeton: Princeton University Press

Modern two-card kanban systems work as follows (CPIM Participant Guide, Execution and Control of Operations, from APICS)

  • The move card requests movement of a specific number of units from an outbound stockpont to an inbound stockpoint. The move card travels with the parts.
  • The production card indicates that items should be made for use or to replace pipeline stock. Production cards do not travel with the parts or container; they remain at the work center and its outbound stockpoint.
Pull Production Control in a Blacksmith Shop (1911 or earlier)
A large blacksmith shop has cleverly solved the problem of keeping the workmen supplied with jobs without loss of time to the pieceworker.
Near each shear or cut-off in the shop is a push button connected with an indicator in the department office. A few minutes before the shear man is through with a job, he pushes a button, which indicates to the storekeeper that shear No. 12, for example, will soon be ready for stock for another job.
…The storekeeper then pushes a second button, which calls the man who has charge of the iron house. To him is given the original memo order, and he delivers the material to shear No. 12, in ample time to keep the workman busy, and prevent any loss of time on the shear" (The System Company. 1911. How to Get More Out of Your Factory. London: A. W. Shaw Company, Ltd.).
Visual Factory Controls

The metal pockets (empty or occupied) serve as visual controls and forms of kanban squares. The system apparently limits the queued work at each machine.

Source: The System Company. 1911. How Scientific Management is Applied. London: A. W. Shaw Company, Ltd.

"Individual instruction cards are assembled on hooks allotted to machines to which jobs are assigned. Jobs in process and ahead are given different rows. Out in the shops may be a board like that in lower cut where the gang boss can keep work tickets."

This is for a job shop, not a production line, so downstream operations do not call directly for work from upstream ones. The instruction cards could easily have had the effect, however, of preventing the accumulation of work-in-process inventory.
An empty hook, like an empty kanban square, meant the machine could accept more work. An occupied hook meant the machine was busy. This also constitutes visual control because the presence or absence of a ticket on a machine's hook indicates the machine's status.

Source: The System Company. 1911. How Scientific Management is Applied. London: A. W. Shaw Company, Ltd.

There actually doesn't seem to be much difference between the picture on the top left and Photo 10-6, "Kanban Board for Peebles 'Small Parts' Production Area" in Jeffrey Liker's Becoming Lean (1998, Productivity Press), except for the fact that the latter image is not yet in the public domain.

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