a vibration - resistant , thread - forming screw . the screw has an elongated shank made of a solid , one - piece body and provided with an interior core portion which , in any plane normal to the shank axis , has a circular configuration . integral with and projecting radially from this core portion are a plurality of lobes uniformly distributed circumferentially about the shank axis and extending along spirals which progress angularly about the shank axis in a direction opposite to the thread . each lobe has , in the direction in which the screw is turned into a body , an elongated leading leg which progresses gradually away from the shank axis in the latter direction of turning and a relatively short trailing leg which returns abruptly toward the shank axis before joining the leading leg of the next lobe . a specially shaped die for forming the screw .