Japanese approach: JIT and kanban

The Japanese approach, developed by Taiichi Ohno at Toyota, is the Toyota Production System. Its aim is to produce only what is needed, when it is needed, in the quantity needed (Just-in-Time, JIT), with the elimination of waste (muda).

Main elements

• Pull system: production starts only when the next process asks for parts, whereas MRP "pushes" parts according to a forecast schedule.
• Small lots and short set-up times (SMED).
• Levelled production (heijunka) and uniform loading.
• Elimination of seven wastes: overproduction, waiting, transport, over-processing, inventory, motion, defects.
• Total quality control and jidoka (stop on defect), multi-skilled workers, supplier partnership, U-shaped cells.

Kanban (card) system

Kanban means signboard or card. Two kinds are used:

• Production kanban: authorises the process to produce a container of parts.
• Withdrawal (move) kanban: authorises the next process to take a container from the preceding store.

 Process A ---> [Store A] ---> Process B
   ^   production    |  withdrawal    |
   |   kanban        |  kanban        |
   +---<-------------+-----<----------+

Working:

1. Process B uses a container of parts; its withdrawal kanban goes to the store of A and a full container is taken.
2. The production kanban that was on that container is freed and returned to Process A.
3. Process A produces one container to replace it.

The number of kanbans is $N = \dfrac{D\,(T_w + T_p)(1+\alpha)}{C}$ (D demand rate, $T_w$ waiting, $T_p$ processing time, $C$ container size, $\alpha$ safety factor). Fewer cards means less inventory.