Overlapping twisted radial aerofoil blades or vanes (70) are formed on a compressor rotor or turbine disk or the like (68) by electro-discharge machining with a curved round section wire tool electrode (72) which is turned by a tool holder (28) as it is advanced on a helical path to form the inner surface of each blade or vane with the outer curve of the wire, or the outer surface of each blade or vane with the inner curve of the wire, the disk being indexed to a next rotary position after each blade surface is machined.