Curvature control in radial-axial ring rolling

Radial-axial ring rolling (RARR) is an industrial forging process that produces seamless metal rings with uniform cross-section using one radial and one axial rolling stage. Conventionally, the ring products are circular and the process is tightly constrained using guide rolls for stability, and to ensure the circularity and uniformity of the ring. Recent work has shown that when guide rolls are omitted, stability can be maintained using differential speed control of the roll pairs. However, achieving uniform curvature in this unconstrained configuration was not always possible when the controller only centred the ring within the rolling mill. In addition to the regulation of constant curvature in circular rings, differential speed control in unconstrained rolling offers an opportunity to bend the ring about the mandrel to create shapes with nonuniform curvature, for example: squares, hexagons, rings with flat sections, etc. We describe a control technique for creating non-circular rings using the rolling hardware of a conventional RARR mill, machine-vision sensing and differential speed control of the rolling stages. The technique has been validated for an industrial material in numerical simulations using the finite element method and also demonstrated on a desktop-scale RARR mill using modelling clay to simulate metal at elevated process temperatures.