To improve the chipping resistance of a sub cutting blade to chipping at the time of cutting machining.
This throw-away tip 50 is formed with a protruding part 80 on a third upper side sloped part 73 down-sloped from a third honing 63, whose ridge part intersecting the side surface thereof serves as a sub cutting edge 14, toward the central part of the throw-away tip 50. The protruding part 80 is formed by a protruding part upper side 81 of roughly triangular flat surface and a protruding part sloped side 82 intersecting the protruding part upper side 81 and the third upper side sloped part 73. A distance (h) from the base 81A of the protruding part upper surface 81 to the vertex 81B is set, relative to a distance H between the two sub cutting edges 14, 14 formed respectively at the opposing ridge parts of the throw-away tip 50, so as to be 0<h≤(1/2)H. The two basic angles α, β of the protruding part upper side 81 is set so as to be in the range of 0°<α<60°, 0°<β<60° respectively. The protruding part 80 is disposed in an area between the two bisectors T, T which bisect the angles formed by adjacent main cutting edge 13 and each of the sub cutting edges 14.
HASEGAWA TAKESHI
KITAGAWA YOSHIHIRO
SUZUKI YUICHI
