A method and apparatus for forming a closely spaced radial array of overlapping twisted airfoil blades or vanes. The apparatus comprises an electro-discharge machine that articulates an electrode (150), electrode (150) being in the form of an elongate and substantially rigid (non-wire) curved element which is turned by a tool holder (130) as it is advanced on a helical path. As the electrode (150) is advanced toward the edge surface of the blank it forms the inner surface of each airfoil blade with its outer curve, and forms the outer surface of each airfoil blade with its inner curve. As the electrode (150) is advanced through the blank it is twisted to give the first groove a helical form, thereby forming a twisted airfoil blade.