Abstract: In the fabricating of a mandrel bar, a plurality of used mandrel bar segments are provided, each of which has a central bore comprising a smooth surface portion and a threaded portion. Each segment has a circumferential recess at an end thereof and a bevelled surface adjoining the recess and extending inwardly of the segment at an angle to the vertical. A coupling has a smooth central boss section and threaded ends. A connection is made by threadedly connecting the ends of the coupling with a pair of opposed mandrel segments, with the smooth boss of the coupling fitting inside the smooth portion of the bore of each segment to effect, with the threaded joint, a straight, tight joint. A weld groove of gull-shaped cross-section is provided between the opposed ends of a pair of mandrel segments, having a body portion formed by a pair of opposed bevelled surfaces and wings formed by a pair of opposed recesses.