A Kanban system is a means to achieve Just-in-time (JIT) production. It works on the basis that each process on a production line pulls just the number and type of components the process requires, at just the right time. The mechanism used is a Kanban card. This is usually a physical card but other devices can be used. Two kinds of Kanban cards are mainly used:

Using Kanbans on a production line

Each process (area, cell) on the production line has two Kanban `post-boxes', one for withdrawal and one for production-ordering Kanbans. At regular intervals a worker takes withdrawal Kanbans that have accumulated in his process post-box, and any empty pallets, to the location where finished parts (components, assemblies) from the preceding process are stored. Each full pallet has attached to it one or more production-ordering Kanbans which he removes and puts in the appropriate post-box belonging to the process that produced the parts. The worker now attaches a withdrawal Kanban to the pallet and takes it back to his own process area. When this new pallet begins to be used, its withdrawal Kanban is put back into the withdrawal post-box. At each process on the line, production-ordering Kanbans are periodically removed from their post-box and used to define what parts and quantities to produce next.

When and How Kanbans are effective

Kanbans help simplify planning and to fine-tune production to meet changing customer demand of up to + or - 10%. The system requires planned monthly and weekly production schedules. Kanbans simplify day to day flexibility, and changes to the production schedule need only to be given to the final assembly process and will then automatically work their way back up the line. Kanban systems can be tightened by removing cards or by reducing the number of parts on a pallet. The effect will be to speed the flow through the process and hence reduce lead times. However it also makes the system more vulnerable to breakdowns and other causes of dislocation. By identifying the areas within the line that are causing disruption, efforts can be made to improve them. Thus the overall efficiency of the line is raised by tackling the key points.

Other types of Kanban also used are Supplier Kanbans - to withdraw goods from external suppliers, and two types of Signal Kanban, which are inserted near the bottom of a stack of items. These automatically initiate production of batch produced items when the stock reaches a pre-set re-order level.

Single card Kanban systems

In a single-card Kanban system, parts are produced and bought according to a daily schedule, and deliveries to the user are controlled by a `conveyancing' (withdrawal) Kanban. In effect, the single-card system is a push system for production coupled with a pull system for delivery to the point of use. Single-card Kanban controls deliveries very tightly, so that the using work centre never has more than a container or two of parts and the stock point serving the work centre is eliminated. Single-card systems work well in companies in which it is relatively easy to associate the required quantity and timing of component parts with the schedule of end products. These are usually companies with a relatively small range of end products, or products which are not subject to rapid, unexpected changes in demand levels.

References

• Yasuhiro Monden, 1993, Toyota Production System: an integrated approach to Just-In Time. Second edition, Industrial Engineering and Management Press, Institute of Industrial Engineers, Norcross, Georgia.
• Schonberger. R.J., 1982, Japanese Manufacturing Techniques: nine hidden lessons in simplicity, Free Press, New York, London.

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