Page 277 - Lean six sigma demystified
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Chapter 8 Su S taining i mpr ovement 255
Results chart
15.0
UCL 14.49
14.0
13.0
CL 12.03 Results
UCL
12.0 +2 Sigma
Results Unstable conditions +1 Sigma
Average
11.0 –1 Sigma
–2 Sigma
LCL
10.0
LCL 9.57
9.0
8.0
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Batch
Figure 8-4 • Unstable XmR chart of sample variation.
The Bad News
Because the control chart of the data (Fig. 8-4) shows that the process is unstable
(shown in red in the QI Macros), you may not be able to meet your customers’
requirements consistently and predictably.
In other words, you just got lucky. The process may not deliver on the custom-
ers’ requirement next time. The question is Does it matter? Taguchi wondered
about this as well and did some research.
The Taguchi Loss Function
Taguchi suggests that every process have a target value and that as the product
moves away from target value, there’s a loss incurred by society. This loss may
involve delay, waste, scrap, or rework. Look at the histogram in Fig. 8-3. Sure,
the product fits within the specification limits, but as you can see, the customer
might have to reset their production machines several times to accommodate
the changes in specifications. Loss!
