Page 622 - Design for Six Sigma a Roadmap for Product Development

Tolerance Design 575

Low-level requirements and tolerances
T ± Δ
i x i

Y= f(x , x , …, x ) T ± Δ 0
y
1
n
2
High-level requirement and tolerances
Figure 16.2 A typical stage of tolerance design.
transfer function relationship between higher- and lower-level parame-
ters. In a typical stage of tolerance design, the main task is, given the tar-
get requirement of y and its tolerance (i.e., T y 0 ), how to assign the
tolerances for x i values. There are three major issues in tolerance design:

1. Manage variability.
2. Achieve functional requirements satisfactorily.
3. Keep life-cycle cost of design at low level.

From an economical point of view, functional requirements should be
satisfied with minimum variation. From the earlier discussion of cus-
tomer tolerances, customer tolerance is defined as “the tolerance limits
for which 50 percent of customers will be unhappy if they [the limits]
are exceeded.” For each individual customer, tolerance varies from per-
son to person; a very selective customer will not tolerate any deviation
of requirement from its ideal state. If a design requirement is at the
boundary of customer tolerance, there are already 50 percent of cus-
tomers unhappy with the design. Minimizing functional variations
will maximize customer satisfaction and will also certainly reduce
rework, warranty cost, and aftersale service cost. On the other hand,
it is also highly desirable that design parameter tolerances and
process variable tolerances be set at wider intervals. Obviously, loose
tolerances of design parameters and process variables will make man-
ufacturing easier and cheaper. Taguchi’s parameter design is trying to
minimize requirement variation with the presence of noise factors; the
noise factors also include piece-to-piece (piecewise) variation. Therefore,
a very successful parameter design could “loosen up” some tolerances.
However, if a parameter design is insufficient to limit the FR varia-
tion, tolerance design is very essential.
In tolerance design, cost is an important factor. If a design parame-
ter or a process variable is relatively easy and cheap to control, a
tighter tolerance is desirable; otherwise, a looser tolerance is desir-
able. Therefore, for each stage of tolerance design, the objective is to
effectively ensure low functional variation by economically setting
appropriate tolerances on design parameters and process variables.