The mechanism is intended for increasing the twist of radial blades (46) which are integrally forged with the perimeter rim of a rotor disc. Two guide assemblies are arranged at axially opposite sides of the blades. Each comprise circumferentially spaced channels (72,76) for slidable blade forming tools (80,82). One tool (80) of each pair has its end (84) contoured to engage the suction side of a blade, whilst the other tool end contour (90) engages its pressure side as the two tools are moved together by a press assembly. The rotor is located between the assemblies by a clamp mechanism. The rotor and twist mechanism are pre-heated.