2 - Designing capable components and assemblies

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This chapter discusses the decisions that are made during the design stage of the product development process, accounting for a large proportion of the problems that incur failure costs in production and service. One of the basic expectations of the customer is conformance to specification—that is, the customer expects output characteristics to be on target with minimum variation. The designer must aim to achieve the standards demanded by the specification, but at the same time not exceed the capabilities of the production department. A good set of design rules can indicate process capable tolerances for a design. The designer needs to understand when required tolerances are pushing the process to the limit and to specify where capability should be measured and validated. Designers have always had to deal with the fact that parts cannot be made perfectly; or if they could, they would not remain perfect for long during use. Therefore, defining the “ideal” component is only one aspect of the designer's job. In analyzing a design at the concept development or early detailing stage, it is necessary to focus on the main variability, associated with manufacturing processes. Common-cause or inherent variability is due to the set of factors that are inherent in a machine/process by virtue of its design, construction, and the nature of its operation, for example, positional repeatability and machine rigidity, which cannot be removed without undue expense and/or process redesign. Assignable-cause or special-cause variability is due to identifiable sources, which can be systematically identified and eliminated.