Page 410 - Design for Six Sigma for Service (Six SIGMA Operational Methods)

Design and Improvement of Service Processes—Process Management 369

C/T
In a flow shop, cycle time
is the time between two pieces
of finished product pieces,
or the production time between
two consecutive pieces.

Lead time

Lead time is the time it takes for one
piece of product to move all the way
from start to finish of the whole process

Value-added time
Value-added time is the time of those
work elements that actually transform
the product in the way that customers
are willing to pay for.

Figure 10.19 Some Important Process Metrics Used in Value Stream Mapping

10.5.3 Lean Operation Techniques

In many production systems, there are huge amounts of muda (the seven
wastes) in the process. From the examples given in Sec. 10.5.2, we can see that
out of the whole production lead time, the value-added time is usually only a
small fraction. The ratio of value-added time over production lead time can be
used as a measure of process efficiency. Specifically this can be stated as

Process efficiency = value-added time (10.1)
total leaad time

The major goal of lean operation is to increase process efficiency. A process
that has a high efficiency will have much less waste, a shorter lead time, and
lower cost. As a rule of thumb, a process is considered to be lean if the
process efficiency is more than 25 percent. Based on the research by
Michael George (2003), the typical process efficiency and world-class
efficiency for many types of processes are summarized in Table 10.2.

Clearly, the process efficiencies of typical processes are very low. A big pro-
portion of process lead time is not used to do value-added work, but to do
non-value-added work, that is, muda (the seven wastes). Lean operation
attempts to redesign the process flow and layout so that the portion of
process time spent doing non-value-added work is greatly reduced.