17 Summary

The designer's job is to try to capture customer expectations and translate as many of these expectations as possible to the final product. The functional requirements of the design become detailed into dimensional tolerances or into attributes of the component or assembly. The ability of the manufacturing process, by which these products are made or assembled, to consistently provide dimensions within tolerance may be called its conformance to design. Understanding and controlling the variability associated with these design attributes then becomes a key element of developing a quality product.

Designers rarely fully understand the manufacturing systems that they are designing for, and subsequently they do not understand the variability associated with the design characteristics. Variability can have severe repercussions in terms of failure costs, appearing in production due to rework and scrap, and warranty costs (or worse!) when the product fails in service. There is need to try to anticipate the variability associated with the manufacturing processes used to produce the final product early in the design process. The designer needs to know, or else be able to predict the capability of the process and to ensure the necessary tolerance limits are sufficiently wide to avoid manufacturing defects. However, this has previously been difficult to achieve on concept design or where little detail exists.

Quality assurance systems demand not just tolerances, but process capable tolerances and characteristics that limit the potential failure costs incurred. The level of process capability is not necessarily mandated, but clearly the more severe the consequences of a defect, the lower the probability of occurrence of that defect should be. This 'risk' may be seen to bear a failure cost penalty and the expected cost must be limited. It should be clear that techniques such as SPC can only be applied to enhance the intrinsic capability of a process. Further, any inspection process to remove defective items can be only a compensation for a process with inadequate capability, or perhaps a design which has been specified with tolerances which are just too tight, or assemblies which are not quite practical.

From the above discussion, it follows that the quality and conformance to tolerance of the product characteristics should be 'designed in' and not left to the process engineer and quality engineer to increase to the required level. In order to do this, designers need to be aware of potential problems and shortfalls in the capability of their designs. They therefore need a technique which estimates process capability and quantifies design risks.

Reliability prediction techniques are a controversial but fundamental approach for designing for reliability. A key objective of these methods is to provide the designer with a deeper understanding of the critical design parameters and how they influence the adequacy of the design in its operating environment. These variables include dimensions, material properties and in-service loading. A key requirement is detailed knowledge about the distributions involved to enable plausible results to be produced in an analysis. It is largely the appropriateness and validity of this input information (and failure theory) that determines the degree of realism of the design process and the ability to accurately predict the behaviour and therefore the success of the design. The determination of an absolute value of reliability is impossible. In developing a product, a number of design schemes or alternatives should be generated to explore each for their ability to meet the target requirements. Evaluating and comparing designs and choosing the one with the greatest predicted reliability, or quality for that matter, will provide the most effective design solution.

The costs of quality experienced by manufacturing businesses are dominated by costs associated with failure, for example rework, scrap, design changes, warranty and product liability claims. These costs represent lost profit and potentially impact on the future opportunities of the business. Decisions made during the design stage of the product development process account for a large proportion of the problems that incur failure costs in production and service. It is possible to relate these failure costs back to the original design intent where variability, and the lack of understanding of variability, is a key failure costs driver. Current quality costing models are useful for identifying general trends in a long-term improvement programme, but are of limited use in the identification of the failure costs associated with actual design decisions. An important aspect of the discussion in the next section is to demonstrate a method whereby the estimation of failure costs at the design stage is useful in selecting the most cost effective, from a failure point of view, from a number of alternatives.

In the next chapter, we introduce the concepts of component manufacturing capability and the relationships between tolerance, variability and cost. The Component Manufacturing Variability Risks Analysis is then introduced, the first stage of the CA methodology, from which process capability estimates can be determined at the design stage. The development of the knowledge and indices used in the analysis are discussed within the concept of an 'ideal design'. The need for assembly variability determination and the inadequacy of the DFA techniques in this respect is argued, followed by an introduction to assembly sequence diagrams and their use in facilitating an assembly analysis. The Component Assembly Variability Risks Analysis is discussed next, which is the second stage of the CA methodology. Finally, a cost-quality model is presented, used to determine the failure costs associated with non-safety critical and safety critical applications in production and service through linkage with FMEA. Comprehensive guidance on the application of the technique is given and a number of case studies and example analyses are used to illustrate the benefits of the approach.

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