Page 63 - Design for Six Sigma a Roadmap for Product Development

40 Chapter Two

2.5.1 Waste elimination in process
In observing the mass production, Tachii Ohno (Ohno 1990, Liker
2004), an engineering genius of Toyota, who is the pioneer of the
Toyota production system, identified the following “seven wastes” in
production systems:
1. Overproduction: producing too much, too early
2. Waiting: workers waiting for machines or parts
3. Unnecessary transport: unnecessary transporting of moving parts
4. Overprocessing: unnecessary processing steps
5. Excessive inventory: semifinished parts between operations and
excessive inventory of finished products
6. Unnecessary movement: unnecessary worker movements
7. Defects: parts that need rework or are scrap
These seven wastes are called muda, which is a Japanese term for
missed opportunities or slack. These items are considered waste
because in the eyes of customers, these activities do not add value to
the products that they wanted.
In lean operation principles, the seven wastes can be identified by the
value stream mapping method. The waste caused by overproduction
can be reduced or eliminated by a pull-based production system. The
waste caused by excessive inventory, waiting, unnecessary transport,
and unnecessary movement can be greatly reduced by one-piece flow
and work cell (cellular manufacturing). A setup time reduction tech-
nique is often necessary to make one-piece flow possible. Use of one-
piece flow and work cell also makes defect detection easier. Besides lean
operation principles, other techniques, such as ergonomics, poke yoke
(foolproof), and statistical process control, should be applied to reduce
the waste caused by defects and unnecessary movements.
Figure 2.13 is a simplified value stream map for a production process.
Clearly, based on the definition of seven wastes, the staging, transporta-
tion, setup, and inspection are nonvalue-added steps; casting, machining,
and assembly are value-added steps. In Fig. 2.13, the horizontal length of
each step is proportional to the time required to do the step. The total
time duration from the beginning of the process to the end of the process
is often called the process lead time. Clearly, in the above example, the
value-added time is a small portion of the total lead time. In many pro-
duction systems, the ratio of value-added time to production lead time
can be used as a measure of process efficiency, specifically:

P ocess efficiency = value-added time (2.2)
r
total leaad time