TABLE OF CONTENTS It can be seen from this chapter that the power of the process capability index is the cooperative joining of responsibility for quality improvements between manufacturing and design engineers. Design engineers are responsible for setting the specification limits for new products as broad as possible and still permit the proper functioning of the product. Manufacturing engineers have to narrow the manufacturing process distribution, as measured by the standard deviation of the product characteristics. This can be achieved by more frequent maintenance schedules, improving incoming inspection methods, working with suppliers, increased operator training, and performing design of experiments (DoE) to reduce the variability of the process.
The formal definitions of six sigma and other quality measuring systems such as Cp and Cpk were introduced. In addition, their relationship to determining the defect rate and examples of calculations were also shown, from both variable and attribute manufacturing processes. An important part of these quality systems is the understanding of the assumptions underlying each system. The choice of the proper system should be compatible with the type of business the enterprise is engaged in and its competition.
The assumption that all manufacturing and supply data are normally distributed was examined, and methods to prove normality were shown. In the case of nonnormality, alternate methods for transforming data to normal distribution, performing six sigma calculations, and then converting the data back to the original distribution were also shown.
