Choosing Six Sigma or Cpk in Manufacturing Process
Although both six sigma and CPK are excellent measurement systems for quality improvement in design and manufacturing , a consensus has not been reached as to which system should be selected based on some of the issues discussed in this section. Currently, major industries and companies have either opted for one or the other , or for their own company brand of six sigma. In the latter case, a combination of rules from both systems is developed to clarity some of the issues, especially when dealing with internal manufacturing and the supply chain. This is important, since the requirement for six sigma or CPK levels are becoming part of the contractual agreements between companies and their supply chain, as well as performance measures for design and manufacturing centers in modern enterprises.
Some of the issues to be considered when a company plans to launch a quality program based on six sigma or Cpk approaches，and how they can converge, are:
• The classical definition of six sigma corresponds to the last line in Table 2.2. Six sigma is equivalent to Cp = 2 or Cpk = 1.5, while allowing a process average shift to the specification nominal of ±1.5 a. However, Cpk = 1.5 does not always equate only to six sigma. Many different conditions of specifications tolerance and process average shift can result in Cpk = 1.5, as shown in Table 2.2
• The implication of the six sigma average shift of 土 1.5 (T is that the production process variability will not improve beyond the ±1.5 a shift of the process average. This may be considered as a negative, since it does not encourage those in the supply chain to improve their process variability. By specifying a particular Cpk, a company can encourage its suppliers to minimize their variability, since it is apparent from Table 2.2 that the smaller the average sh^, the wider the specification tolerance can be.
• It is widely recognized that older manufacturing processes are more stable than newer processes, and shift. This has led to specifying a particular and then a different Cpk when the process matures, in 3 to 6 months after production start-up. In the auto industry, the starting Cpk is set at 1.67 and the mature CPK at 1.33. This was done to force the supply chain to pay attention to the process in the initial stage of production, a form of learning-curve-based improvements. This is sue of time improvements has long been recognized in the supply chain, with commonly used incentives for cost reduction base on time. The six sigma program maintains a constant ±1.5σallowable average shift, which is an easier goal to manage irrespective of time. It is the author’s opinion that it is better to manage quality with a single number and concept, as opposed to a time-dependant standard. In addition, the reduced life cycle of electronic products, and the emphasis on “doing it right the first time” should encourage the supply chain to set a goal for first production quality and then maintain it. This might prove less costly in the long run.
The choice of focusing on the process average shift correction to equal the specification nominal or reducing variability or both will be discussed in greater detail together with the quality loss function (QLF), discussed in Chapter 6.
Cpk and six sigma can have different interpretations when considering attribute processes. These are processes in production, where only the defect rates are determined and there are no applicable specification limits. Examples of attribute processes are assemblies such as printed circuit boards (PCBs) where rejects could be considered to be the result of implied specifications interacting with production variability of materials and processes. In these cases, the quality methodologies are centered around production defect rates and not specifications, thereby clouding the relationships and negotiations between design and manufacturing. Different levels of defect rates based on Cpk levels could be allowed for different processes, resulting in an overall product defect goal setting and test strategy based on these defects. Six sigma quality provides the power of the single 3.4 PPM defect rate as a target for all processes.
• A similar issue arises when using six sigma or Cpk for determining total system or product quality. This is the case when several six sigma designs and parts are assembled together into a system or product. Six sigma practitioners handle this issue by using the concept of rolled yield, that is, the total yield of the product based on the individual yields of the parts. Those using the Cpk terminology can continue to use Cpk throughout the product life cycle, assigning different Cpk targets as the product is going through the design and manufacturing phases.
