a burr includes a spherical cutting element and a shield integrally placed behind the burr . the shield is circularly configured , has a rounded periphery and is provided with a smooth tooth - engaging surface . the shield thus insures that the depth of the penetration — into the tooth is precise , as predetermined , without inflicting deleterious harm to the tooth . the present invention further relates to a method , utilizing the shield , for precise tooth preparation of porcelain veneers and porcelain crowns .