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Part Three
Planning and control
Figure 15.7 The operation of the single-card kanban system of pull control
The arrival of the empty containers at stage A’s work centre is the signal for production to
take place at work centre A. The move kanban is taken from the holding box back to the
output stock point of stage A. This acts as authorization for the collection of a further full
container to be moved from the output stock of stage A through to the work centre at stage B.
Two closed loops effectively control the flow of materials between the stages. The move
kanban loop (illustrated by the thin arrows) keeps materials circulating between the stages,
and the container loop (illustrated by the thicker arrows) connects the work centres with the
stock point between them and circulates the containers, full from A to B and empty back
from B to A. This sequence of actions and the flow of kanbans may at first seem complicated.
However, in practice their use provides a straightforward and transparent method of calling
for material only when it is needed and limiting the inventory which accumulates between
stages. The number of kanbans put into the loops between the stages or between the stock
points and the work centres is equal to the number of containers in the system and therefore
the inventory which can accumulate. Taking a kanban out of the loop has the effect of
reducing the inventory.
Critical commentary
Just-in-time principles can be taken to an extreme. When just-in-time ideas first started
to have an impact on operations practice in the West, some authorities advocated the
reduction of between-process inventories to zero. While in the long term this provides the
ultimate in motivation for operations managers to ensure the efficiency and reliability of
each process stage, it does not admit the possibility of some processes always being
intrinsically less than totally reliable. An alternative view is to allow inventories (albeit small
ones) around process stages with higher than average uncertainty. This at least allows
some protection for the rest of the system. The same ideas apply to just-in-time delivery
between factories. The Toyota Motor Corp., often seen as the epitome of modern JIT, has
suffered from its low inter-plant inventory policies. Both the Kobe earthquake and fires
in supplier plants have caused production at Toyota’s main factories to close down for
several days because of a shortage of key parts. Even in the best-regulated networks,
one cannot always account for such events.