Abstract: A drill includes a substantially cylindrical bit body 10 having an axis O. The bit body 10 has a flange portion 13 which includes a converging portion 13a which reduces its diameter progressively towards the front end of the bit body and a flange 13b which is provided with a rearwardly facing annular surface 13c. The annular end surface is contactable with the front end surface of a drill holding portion of a machine too. Chip discharging flutes 14A,14B, each having an inner surface constituted by first and second inner surface portions 14a,14b and a third inner surface portion 14c arcuately recessed therefrom, are formed in the front end portion of the tip body 10. An inner peripheral cutting insert 16A and an outer peripheral cutting insert 16B are mounted on the ends of the chip-discharging flutes 14A,14B. The cross-sectional area S.sub.A of the first-mentioned chip-discharging flute 14A is determined to be greater than that S.sub.B of the second-mentioned chip-discharging flute 14B.

Abstract: An insert boring tool includes radially outward and inward cutting inserts mounted thereon. The radially outward and inward inserts are arranged such that end cutting edges of the respective inserts are intersected with each other at a point of intersection in loci of rotation of the respective inserts in a plane containing an axis of a tool body. The radially outward and inward inserts are positioned such that, assuming that a radial distance between the point of intersection and a radially outermost end of the end cutting edge of the radially outward insert is A, and a radial distance between the point of intersection and a radially innermost end of the end cutting edge of the radially inward insert is B, a ratio of A/B takes a value of from 2/8 to 45/55.

Abstract: A carbonizing furnace includes a heating chamber with a wall having channels therein, a carbon heater in the channels, a heat insulator composed of carbon fiber felt and a ceramic fiber felt which covers the heating chamber such that the carbon fiber felt is in contacting with the heating chamber and the ceramic fiber felt is separated by a space from the carbon fiber felt, and a housing enclosing the heating chamber and the heat insulator, and an inlet for filling the insulator with an inert gas. The carbonizing furnace can be run stably and continuously.