Six sigma, like many new trends or initiatives, is not without its critics and detractors. The author has run into several issues brought up by engineers and managers struggling with six sigma concepts, and has attempted to address these concerns by writing this book. Some of the most frequent critiques of six sigma, and the authors approach to addressing these problems are listed below.
1. The goal of six sigma defects，at 3.4 PPM, and some of its principles, such as the ±1.5 sigma shift of the average manufactured part from specification nominal, sound arbitrary. In addition, there is no solid evidence as to why these numbers have been chosen.
These are reasonable assumptions that were made to implement six sigma. There are other comparable systems, such as Cpk targets used in the auto industry, that could substitute for some of these assumptions. Discussions of these concepts are in Chapters 2 and 3.
2. The cost of achieving six sigma might result in a negative return on investment. Conventional wisdom once held that higher quality costs more, or that there is an optimum point at which cost and quality balance each other, and any further investment in quality will result in negative returns (see the discussion of the quality loss function in Chapter 6).
These beliefs are based on the misconceptions that more tests and inspections are needed in the factory prior to delivery to the customer, in order to deliver higher quality. Six sigma advocates the identification of these costs daring the design stage, prior to the manufacturing release of the product, so that these costs are well understood. In addition, it has been demonstrated in six sigma programs that the cost of changing the product in the design stage to achieve higher quality, whether through design changes, different specifications, better manufacturing methods, or alternate suppliers, are much lower than sub- sequent testing and inspection in manufacturing. These issues are discussed in the chapters on product testing (Chapter 4) and cost (Chapter 6).
3.Many companies fee] that the six sigma programs only work well for large-volume, well-established, and consumer-oriented companies such as Motorola and GE, but do not work for other industries such as aerospace, defense, or medical, since their volumes are small or they are more focused on maximizing the performance of products or reducing the time of development projects.
There are many statistical methods that can be used to supplant the sampling and analysis required for six sigma, allowing smaller companies the full benefits of six sigma in product design and manufacturing. Six sigma methods can be used successfully to introduce new low-volume products as well as quantifying marginal designs. These methods will be discussed in the chapters on high and low volume (Chapter 5) and six sigma current and new products (Chapter 8),
4.Many engineers feel that six sigma is for manufacturing only, not for product design, and that it is very difficult to accomplish and cannot be achieved in a timely manner.
In this book, there will be many examples of using six sigma and its associated tools, such as design of experiments (DoE), in product design, These methods can help in realizing the six sigma goals and targets in a timely and organized manner in design and manufacturing.
In addition, there are many examples where design engineers were
surprised to find out that they are already achieving six sigma in current designs. Six sigma can also be used to flush out “gold plated” designs: designs that are overly robust, beyond the six sigma limits, and therefore costing more than required. These issues are discussed in Chapter 7 on DoE and Chapter 8 on designing current and new products.