Page 640 - Design for Six Sigma a Roadmap for Product Development

Tolerance Design 593

Quality loss

A 0

A

T – 0 T – T T + T + 0
Figure 16.8 Quality loss function, customer tolerance, and producer tolerance.

where 0 is the customer tolerance limit and A 0 is the cost incurred to
the customer when the requirement level y is at the boundary of the
customer tolerance limit. Taguchi reasons that if the design can be
repaired or fixed before shipping to customers at a lower cost, say, A
and A A 0 , then the producer tolerance , which is the internal toler-
ance limit for shipping inspection, should be set at a narrower level
than the customer tolerance limits. Taguchi proposes that the specific
producer tolerance limit should be set according to the quality loss
function. Specifically, he thinks that at producer tolerance, the quality
loss should break even with the internal repair cost. Thus, when y
T or y T , by Eq. (16.29), the quality loss

A 0
L(y) 2
2
0
Then, setting the quality loss L(y) equal to the repair cost A, we obtain

A 0
A 2
2
0
Therefore, the producer tolerance limit can be computed by
0
A
(16.30)
0
0
A 0
A
A 0