Closed-loop control of product properties in metal forming: A review and prospectus

Metal forming processes today operate with astounding productivity, repeatably creating precise parts in high volumes out of the stock sheet and bar products of the upstream metals industries. This achievement has come through decades of development of ever stiffer and more precise tooling used in fast-acting tightly controlled equipment, and yet in the wider context of manufacturing, metal forming processes seem to be less effective: tooling costs are high, and can only be justified by large batch production; the parts made by metal forming are usually not as required for assembly, and must be processed in further downstream machining operations; current processes do not respond well to process disturbances such as tool wear or unanticipated variation in material properties; twenty years of laboratory development of new flexible forming processes has led to little industrial take-up, due to a lack of precision. The missing ingredient in forming which gives rise to these problems is the absence of effective closed-loop control of product properties. The normal practice for blacksmiths and craft workers in former times – using their personal senses to adjust processing in response to evolving conditions – has been forgotten in the pursuit of process rigidity. This paper therefore aims to motivate a new wave of interest in applying closed-loop control of product properties to metal forming processes. A novel framework is developed to show metal forming processes at the heart of an outer control loop, and existing applications are reviewed. Surveys of sensors, actuators and modelling techniques reveal a rich seam of opportunities for new developments, and the paper concludes with some suggestions about near term opportunities for applying closed-loop control of properties to metal forming processes.