BORING TOOL

PURPOSE:To improve the dischargeability of a core by forming at least a part of the section, which is extended along the rearward in the rotational direction from the contact of an inscribed circle of the fellow wall faces, in inclination toward peripheral side to the tangent of this inscribed circle, among the wall faces which face each other with an axis of the opening between. CONSTITUTION:When cut processing is started, the cutting is realized by an inside edge 13a and a cut blade 13 on the peripheral side. On the other hand, the core grows up in the central part of the material to be cut and when it grows up to some degree, it will be twisted off by the cut resistance and discharged from an opening 14. Then the space between the fellow wall faces 16 of the opening 14 is gradually widened according to the tangent 1 direction from the contact P deg. of this inscribed circle. As a result, the free degree of the core becomes large and the wall face 16 becomes hard to be abrased by the core, whereby the core can be discharged favorably. Accordingly, there never occurs such a case that the wall face 16 should be abnormally abrased or chipped.