Tolcerance margins as constraining factors of changes in complex products
Engineering changes to a product are important for it to reach and maintain market competitiveness. The nature of today’s market has seen the development time for many products reduced considerably. Companies strive to accelerate their development process while trying to maintain a high quality. Performing engineering changes is not a simple task, as small changes can cause severe disruptions to manufacturing and scheduling processes. Another potential major impact of engineering changes, perhaps more common in complex systems, is the effect it may have within the product itself. Changes to one component could necessitate further changes to other components of the design [CLA-01]. Change propagation in design causes forms of disruptive effects such as high amounts of rework and delayed schedules. Engineers need support with evaluation of the effects of proposed changes. This paper looks at the different mechanism through which change can propagate within a system with the aim of applying this understanding to developing a method for evaluating the effects of proposed changes to a design. The method is based on the understanding that the components in a product are built to within certain tolerances and that change will only propagate if these tolerances are violated. These change margins are not the same as manufacturing margins. The rest of this paper begins with a background discussion on existing research into factors responsible for change propagation in products and the techniques adopted to reduce its effects (section 2). The definition for “tolerance margins” and explanations of the relationships between these margins and change propagation are highlighted in section 3. A tolerance-based method for evaluating changes is discussed in section 4. The evaluation was carried out using a technique developed at the Cambridge University Engineering Design Centre known as the Change Prediction Method (CPM). This paper concludes with a brief summary of the usefulness of the method and highlights areas for further development.