Investigations on hole-flanging by paddle forming and a comparison with single point incremental forming

Abstract

The use of dedicated dies to carry out hole-flanging operations is costly for small batch production. As a result, hole-flanging by single point incremental forming (SPIF) was put forward because it offers advantages such as low equipment cost, high formability, and flexibility compared to conventional hole-flanging. However, drawbacks of hole-flanging by SPIF such as poor geometrical accuracy and long process times are obstacles to its industrial take-up. This paper presents a novel incremental hole-flanging method which uses paddle-shaped tools rotating at high speeds while being fed axially into the sheets to form flanges, with the aim of preserving the advantages of incremental forming while reducing the drawbacks related to process time. The results of the experiments show higher formability for the paddle forming process compared to hole-flanging by multi-stage SPIF and conventional hole-flanging. The high formability in paddle forming is attributed to the presence of shear which delays fracture. Furthermore, heat generated due to friction between the paddle and metal blanks leads to a rise in the blank temperature which enhances formability. Optical strain measurements of the formed flanges reveal that the paddle speed and feed have major influences on the forming mechanism of the process and the process borders. A maximum hole expansion ratio (HER) of 3.3 was obtained from the experiments in a process time of ∼2 s, showing that the process combines high formability and low process times.