The quality tools in wide use today can easily be integrated within the six sigma definitions. The object of six sigma is to steadily increase the process capability index until it reaches the desired level: the specification limits of the design are equal to six sigma of the manufacturing variability.
Design engineers normally set the product specifications，whereas manufacturing engineers are responsible for production variability. The object of increasing the process capability to six sigma or Cp = 2 is twofold; increase' the product specifications, either by widening them or reducing the manufacturing variability. Either effort can have a positive perfect on reaching six sigma.
TIIP design specifications for any part or process are related to the top published product specifications. Ultimately, it is the customer that determines the relative importance of each specification and the desired level of performance. Good market research and project management for new products can determine the best level of specification. This level can be set to balance the wishes of the customer, tempered by what the competition is offering and considering inputs from design and manufacturing engineers as to the difficulty of meeting that specification level.
The quality of supplied parts and the efforts of the manufacturing engineers in production solely determine the denominator of the six sigma equation, or the manufacturing variability. Implementing the traditional quality tools of manufacturing, such as statistical quality control (SQC) and associated quality tools, can reduce the manufacturing variability. The tools of SQC and their relationship to process capability are discussed in Chapter 3.
The Cp formula can then be rewritten as