Page 618 - Design for Six Sigma a Roadmap for Product Development

Chapter
16

Tolerance Design

16.1 Introduction
In the DFSS algorithm, the objective of tolerance design is to gain further
optimization beyond what has been gained via parameter optimization
and transfer function detailing to achieve a Six Sigma–capable design.
In this step, the DFSS team determines the allowable deviations in
design parameters and process variables, tightening tolerances and
upgrading only where necessary to meet the functional requirements.
Where possible, tolerances may also be loosened. This is the objective
of step 11 of the DFSS algorithm in the optimization phase (O) of the
ICOV process (Chap. 5). In tolerance design, the purpose is to assign
tolerances to the part, assembly, or process, identified in the physical
and process structures, based on overall tolerable variation in the func-
tional requirements, the relative influence of different sources of vari-
ation on the whole, and the cost-benefit trade-offs.
Tolerance design can be conducted analytically on the basis of the
validated transfer function obtained empirically or via testing. In
either case, the inputs of this step are twofold: the DFSS team should
have a good understanding of (1) the product and process require-
ments and (2) their translation into product and process specifications
using the QFD.
The going-in position in the DFSS algorithm is to initially use toler-
ances that are as wide as possible for cost considerations, and then to
optimize the function of the design and process through a combination
of suitable design parameters (DPs). Following this, it is necessary to
identify those customer-related functional requirements (FRs) that are
not met through parameter design optimization methods. Tightening
tolerances and upgrading materials and other parameters will usually
be required to meet Six Sigma FR targets.

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