Craftsmanship and automation in flexible metal spinning

Abstract

Industry is responsible for over a third of global carbon emissions, and the production of steel and aluminium components accounts for over a fifth of all these emissions. If we wish to mitigate global heating, emissions from these sectors must be reduced. A promising strategy is to cut process waste by developing flexible and efficient manufacturing techniques. Flexible metal spinning is an incremental forming technique which makes axisymmetric sheet metal components without part-specific tooling and with little material waste. The process has the potential to challenge more established techniques such as stamping and deep drawing in the prototyping, initial development and small-volume production of steel and aluminium parts. However, despite advances in computer power, flexible spinning has not been automated yet. The design of toolpaths by trial and error results in waste of time and material; the complexity introduced by the additional rollers has not been addressed properly yet; and only axisymmetric parts have been made so far, so the range of application is limited. In this thesis, three studies are performed to expand the capabilities of spinning and address the issue of automatic toolpath design. In the first study, an accurate and flexible toolpath generation algorithm is developed to spin multiple axisymmetric and asymmetric components without dedicated tooling. Using this new algorithm, elliptical and square parts are spun for the first time on a mandrel-free spinning machine. The influence of part asymmetry on the achievable forming height is found to be minimal in the range investigated, which implies that spinning can successfully produce many part geometries flexibly. In the second study, the first complete rulebook for toolpath design is written by capturing the skills and wisdom of experienced spinning craftsmen using a haptic interface. The validity of the rules is confirmed by a statistical analysis of the results of over 70 experimental trials with craftsmen. Control of both the roller force and the workpiece shape are found to be fundamental for the craftsmen to spin a part without failure. In the third and final study, the mechanics of the traditional craft of hand raising are replicated using a flexible spinning system, and a new configuration called ‘raising by spinning’ is tested. Force control is implemented to apply a pure couple to the workpiece, and an upper bound yield-line model is developed and verified to design force-paths that are stable against wrinkling. The results show that raising by spinning is more stable than conventional spinning and that wrinkling can successfully be avoided in the first stages of the spinning process. A conical part is successfully spun in this way using a virtual mandrel. Inspired by craftsmen and their manual wisdom, this thesis makes a significant contribution in the direction of toolpath design automation in spinning. The combined results of the three studies provide the fundamental features of a toolpath design algorithm that could be applied successfully to a variety of axisymmetric and asymmetric shapes, thicknesses and materials. Such an algorithm would employ force-controlled toolpaths that account for the asymmetry of the target part, and it would implement the results of the upper bound model and the craftsmen’s spinning rules to avoid both wrinkling and tearing. In this way, spinning could begin to outperform competing forming techniques without losing the soul of its craftsmanship history