A rotor assembly for an axial magnetic bearing includes a shaft portion and a disc attached to the shaft portion. The disc includes a conical surface on a portion reaching radially from the aperture of the disc a distance away from the aperture. A fastening mechanism includes an aperture for the shaft portion so that the fastening mechanism includes a conical surface matching the conical surface of the disc. The fastening mechanism and the shaft portion are shaped to enable the fastening mechanism to be tightened axially against the conical surface of the disc so as to arrange the conical surface of the fastening mechanism to press the conical surface of the disc towards the center line of the shaft portion. Thus, the fastening mechanism acts against the centrifugal force and keeps the disc centric placed.