Determining Short- and Long-Term Process Capability
An important part of new product development is the development of process capabilities and specifications for new parts and products. De* sign engineers work with the general specification of products that are set by marketing or the customer, but these specifications do not necessarily flow down to all of the parts and to all of their attributes. It is necessary for design engineers to always question the relevance of each part specification, and whether it is too tight for its proper use in the customers hands. It is always desirable to use tools such as quality function deployment or QFD? discussed in Chapter 1, to attempt to relate each specification for every part to the customer^ wishes.
For six sigma designs of new products, process capability should be determined in the prototype stage of parts manufacturing. Some large consumer and mass product companies normally plan for large prototype runs to fully simulate the variability of the production process. This may not be feasible for many industries, due to the cost of parts or the volume of expected sales, so that process capability has to be derived from low volumes, using the techniques discussed in this chapter.
Process capability for new products can follow one of the following three scenarios:
1. The product represents an evolutionary increase in technology, and engineers build the prototypes with tight control, in special prototype shops. In this case, the process capability of the prototypes might actually be of higher quality that the early production runs.
2. For state of the art products, the part specifications are set aggres^ sively, with the implication that the early production runs will have a poor yield. The parts in this case will attain the desired level of quality through rigorous testing against specifications. Eventually, their process capability will improve over time, thus achieving the specified first-time yield sometime after product release.
3. Using six sigma procedures for process capability implies that every purchased or manufactured part or assembly meets the six sigma requirements. Process capabilities might not be available for many of the new purchased parts and may have to be calculated from prototype purchases. For major companies, this issue is less of a problem, as they can specify the process capability or six sigma directly in the purchasing contracts for parts.