System modeling to support variation reduction in a sheet stretch forming manufacturing system

Quality is a major issue in the production of sheet stretch-formed aircraft skins. A typical stretch-forming system is composed of a series of operations, including heat treatment, stretch forming, chemical milling, and trimming. Each of these operations is subject to significant input and process variation, leading to variation on the work-in-process. This variation then results in fit-up problems at the assembly stage and high scrap or rework cost. In this paper we use a mathematical model of a stretch-forming manufacturing system to aid in reducing end-of-line variation. For the paper we reduce the system to two key operations: heat treatment and stretch forming. Physics-based mathematical models of each are used to derive a variational model of the system, which maps output quality characteristic variations to input variations. This variational model is used to identify the major sources of variation in the system: aging time and forming force. To reduce the effects of these variables, a feed-forward control strategy is proposed, in which aging time is measured after heat treatment and used to determine force during stretch forming. Feedback control is not effective. Analytical expressions are developed to predict the effects of the system control strategy on end-of-line variation. Finally, the results of a shop floor implementation of this strategy are presented, showing a 30% reduction in strain variation on production parts.