Abstract: In a known method of producing a quartz glass tube (22), a cylinder (21) of quartz glass which rotates about a rotational axis (27) is continuously fed to a heating zone (24), softened therein in portions starting from one end, and the softened portion (25) is drawn over a drill head (1) of a drill body arranged to be coaxial to the rotational axis and is thereby formed into the quartz glass tube (22). Starting therefrom, in order to indicate a method for forming a cylinder of quartz glass into a hollow cylinder of quartz glass having an inner bore that is as flawless as possible and straight and dimensionally stable, it is suggested according to the invention that a drill body should be used with a drill head (1) having a contact surface (2) of a convex curvature facing the cylinder (21), preferably a spherically shaped contact surface (2). The drill body according to the invention is characterized by a drill head (1) which has a contact surface (2) of a convex curvature (2).

Abstract: In a known method of producing a quartz glass tube (22), a cylinder (21) of quartz glass which rotates about a rotational axis (27) is continuously fed to a heating zone (24), softened therein in portions starting from one end, and the softened portion (25) is drawn over a drill head (1) of a drill body arranged to be coaxial to the rotational axis and is thereby formed into the quartz glass tube (22). Starting therefrom, in order to indicate a method for forming a cylinder of quartz glass into a hollow cylinder of quartz glass having an inner bore that is as flawless as possible and straight and dimensionally stable, it is suggested according to the invention that a drill body should be used with a drill head (1) having a contact surface (2) of a convex curvature facing the cylinder (21), preferably a spherically shaped contact surface (2). The drill body according to the invention is characterized by a drill head (1) which has a contact surface (2) of a convex curvature (2).

Abstract: A drill head carries a plurality of coaxially mounted drill crowns beginning with a first crown and followed by additional drill crowns which, upon rotation, describe concentric envelope circles of successively larger diameters. The first drill crown has cutting elements with a diamond coating with an average particle size of 200-1,000 .mu.m, and the additional drill crowns have cutting elements with a diamond coating with an average particle size of 200-300 .mu.m. The difference in diameter of succeeding drill crowns is successively small. Alignment of the drill crowns in a pilot hole is achieved by guide elements mounted behind the drill crowns and describing an inner envelope circle which is smaller than the outer envelope circle described by the largest drill crown, thereby defining a radial safety gap into which fluid is fed to form a cushion during rotation.