A Framework for a CAD-Integrated Tolerance Optimisation System

Abstract

Current assembly modelling and tolerance analysis functionality offered by most CAD vendors extends to the evaluation of a design in terms of its form and fit. There is no evidence of these tolerance optimisation tools offering the designer an 'in-context' perspective of process capability: an assessment of whether the required tolerances are achievable from the available manufacturing process. There is also a lack of provision for the direct use of experimental design data, within the CAD system environment. The aim of this research is to enable these prototype and production test results to be used in conjunction with a manufacturing capability model to establish the best compromise between product performance and process capability. This research program is to establish a framework with which process capability data can be captured, interpreted and subsequently used in a CAD system, making it readily available for the designer to use in the pursuit of a more robust product. The framework will be demonstrable in the context of vacuum pump design and manufacture in BOCE (BOC Edwards), the company hosting the research activity. The framework will have the potential to reduce the level of first time test failures that are attributed to products containing parts that are not made to specification. It will also eliminate the time and resource overhead incurred in redesigning parts, which cannot be manufactured due to the specification of unachievable tolerances. As a result, machining scrap will also be reduced. In addition, the designer mill be more confident that a 'robust' product has been produced, with reduced sensitivity to normal variations in the manufacturing process. Where a 'close' tolerance is absolutely vital, and the current manufacturing process is not capable, the framework will highlight this early in the design phase and allow the purchase of additional (and possibly long lead time) tooling or fixturing, thereby providing effective support to the new product introduction process.