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Six Sigma and Lean Fundamentals 43

Raw Product
material

Figure 2.16 One-piece flow.

layouts since they reduce travel distance, required floor space, and
total throughput times. A single-piece flow means that the work is
done one piece at a time, not one batch at a time. This will eliminate
the work-in-process inventory completely.
On the other hand, in a single-piece line flow process, any error or
defect in any process step will cause the whole line to stop. In tradi-
tional Western operation management, the work-in-process inventory
or buffer inventory is used to temporarily “feed” the downstream
process steps so the line will not stop. However, the Toyota production
system believes that the buffer inventory has more disadvantages
than benefits; buffer inventory ties up money and hides problems. In
the Toyota production system, zero buffer inventory is used to expose
all the hidden problems in the production process; it forces you to
debug all hidden problems so eventually you will have a zero defect
production process.

2.5.3 Work cell (cellular manufacturing)
In the Toyota production system, the ideal production process set
up for lean operation is a one-piece flow work cell, as illustrated by
Fig. 2.17.
A work cell is a U-shaped layout of several different kinds of
machines that form a one-piece flow line. The U shape is used here
because it saves floor space and shortens travel distance for operators.
Cell

Raw
materials

Products

Figure 2.17 One-piece flow work cell.