Jaws are essential elements to clamp a piece. Soft jaws usually are used in last operations because they guarantee a higher clamping precision than the hardness jaw. Soft jaws are machined to the diameter at the same diameter as the working piece.
Normally used for first operations, they do not guarantee a high precision clamping. Heat treated and with a special teeth on clamping areas, to make sure a safe clamping. When centering precision required, the hard jaws must be special, with the clamping areas grinded at the same diameter as the working piece.