A set of 20 steel mandrels for forming jump rings. Each mandrel is 4" long overall and will form a coil up to 2.5" long. Mandrels range in diameter from 2.5 mm to 12 mm in increments of 1⁄2 mm. All but the smallest have holes and are marked with their diameters in millimeters. The mounting end of each of the larger mandrels is stepped down to 9.5 mm (3/8”).