Kanban is often heard in discussions when talking about Japanese JIT philosophy. Most common translation for kanban is card. In Toyota’s factory cards were big part of managing the material flow.
Push or pull production?
Before going further to Toyota’s kanban system we should separate push and pull productions. Push production, like MRP, is based on pre-scheduled production. Pull production is based on permission to start production from the next workstation. Biggest difference is that push production is pre-scheduled and pull production better follows changes in demand.
In push production we can easily tell when products are ready but changes are difficult because whole production has to be scheduled again. Pull production is always based on need and conforms better for changes.
In kanban system production is always made for need. When part is removed from stock, which can be WIP or end products stock, previous workstation feeding the stock is given authorization to manufacture more parts. This workstation sends another production order to previous station etc. In kanban worker needs both production order from following workstation and parts from the previous station.
Two card system
Toyota used two card system in order to manage both work and movement of parts. Idea is that when workstation is free for next duty worker takes production card. Card works as an production order and tells what next workstation needs. Worker checks WIP stock if there is parts to make that order. If there is parts worker takes their movement card and puts it in the other box. If there is not parts worker takes another production card that matches with the parts he has. Anytime worker finds production card and parts that belongs to that he does his job and puts finished part into the sending area.
Warehouse worker regularly checks movement cards and replaces production cards with movement cards. Then he moves parts from sending area to next receiving area. Removed production cards are returned to the workstations which made the orders.
Toyota’s two card system is based on the idea that workstations are scattered and it’s not possible to have needed parts immediately. That’s why every station has its own receiving and sending area. Movement cards are signal to the warehouse that parts need to be moved from sending area into the next receiving area.
One card system
If workstations are close together parts can be handed from workstation to another and WIP stocks are not needed. Then kanban works with only one card. Basic idea is the same, worker needs both parts and production card in order to do his job. But in one card system worker sends production card into the previous station, not movement card.
Handling the work in process is the only difference between one and two card systems. When WIP handling is fast and smooth it is not necessary to control the amount of WIP (one card system, workstations close each other). If WIP handling is slow and irregular it might be necessary to control the amount and movement of the WIP.
In some applications card are not used at all. WIP can be regulated by limited amount of baskets or pallets which are used for parts storage and moving. In some places WIP parts are stored in small areas that are painted on the floor and can’t store more parts. Electronical tracking is one way to to follow parts and their coming and leaving can be logged at workstations for instance by barcode or RFID tag.
Kanban is based on limiting the work in process. Take a closer look and you’ll discover that this is familiar with the reorder point method. When inventory goes below the signal level production or movement card is sent to the previous stations and we will receive new parts.