Page 619 - Design for Six Sigma a Roadmap for Product Development

572 Chapter Sixteen

Customer Functional Design Manufacturing
Attributes Parameter Process
And Requirements Requirements Variables
And
Customer And And
Tolerances Tolerances Tolerances Tolerances

Figure 16.1 Tolerance development process.

Systematic application of DFSS principles and tools such as QFD
allows the identification of customer-sensitive requirements and the
development of target values for these characteristics to meet cus-
tomer expectations. It is vital that these characteristics be traced
down to lowest-level mappings, and that appropriate targets and
ranges be developed.
By definition, tolerance is the permissible deviation from a specified
value or a standard. Tolerances have different meanings at different
stages of the design process. Figure 16.1 illustrates the design toler-
ances at different design mappings (Chap. 8).
For a product or service, customers often have explicit or implicit
requirements and allowable requirement variation ranges, called
customer tolerance. In the next stage, customer requirements and tol-
erances will be mapped into design functional requirements and func-
tional tolerances. For a design to deliver its functional requirements to
satisfy functional tolerances, the design parameters must be set in cor-
rect nominal values and their variations must be within design para-
meter tolerances. For design development, the last stage is to develop
manufacturing process variable set points and tolerances.

Example 16.1: Power Supply Circuit The high-level functional require-
ment of a power supply circuit is to provide electrical power for small appli-
ances; output voltage is one customer requirement. Although few customers
will bother to measure the output voltage, the excessive deviation of output
voltage will affect the functional requirements of small appliances. The
larger the deviation, the more customers will notice the requirement degra-
dations of the appliances. Customer tolerance is usually defined as the tol-
erance limit such that 50 percent of customers will be unsatisfied. For
example, the nominal value of a power supply circuit could be 6 V, or T 6,
but if we assume that when the actual output voltage y is either 5.5 or
6.5 V, 50 percent of customers will be unsatisfied, then the customer tol-
erance will be 6 0.5 V. A power supply circuit consists of many compo-
nents, such as resistors, transistors, and capacitors. Setting the nominal