Throwaway drill, insert of throwaway drill and cutting method using the same

A throwaway drill comprising:

• a throwaway drill holder having an inner edge insert pocket formed on the central axis side of a tip end of a drill main body and an outer edge insert pocket formed on the outer peripheral side of the tip end;
• and
• a first insert for the throwaway drill provided on the inner edge insert pocket in which at least part of the inner cutting edge projects from the tip end of the drill main body, the first insert for the throwaway drill comprising,
• a main body including a lower surface, an upper surface and a side surface, the insert having a supporting surface formed on the lower surface of the main body and a cutting edge formed at a crossed portion between the upper surface and the side surface of the main body,
• the cutting edge having an inner cutting edge and an outer cutting edge being continuous with each other and forming a corner portion of the upper surface,
• the inner cutting edge having a first inner cutting edge located on the corner portion side and formed linear, and a second inner cutting edge located oppositely to the corner portion with respect to the first inner cutting edge, the second inner cutting edge being inclined with respect to the first inner cutting edge and being closer to the supporting surface as being farther from the corner portion, and
• the outer cutting edge having a first outer cutting edge located on the corner portion side and being parallel to the first inner cutting edge;
• and
• a second insert for the throwaway drill provided on the outer edge insert pocket in which at least part of the outer cutting edge projects from the tip end of the drill main body, the second insert for the throwaway drill comprising,
• a main body including a lower surface, an upper surface and a side surface, the insert having a supporting surface formed on the lower surface of the main body and a cutting edge formed at a crossed portion between the upper surface and the side surface of the main body,
• the cutting edge having an inner cutting edge and an outer cutting edge being continuous with each other and forming a corner portion of the upper surface,
• the inner cutting edge having a first inner cutting edge located on the corner portion side and formed linear, and a second inner cutting edge located oppositely to the corner portion with respect to the first inner cutting edge, the second inner cutting edge being inclined with respect to the first inner cutting edge and being closer to the supporting surface as being farther from the corner portion, and
• the outer cutting edge having a first outer cutting edge located on the corner portion side and being parallel to the first inner cutting edge;
• wherein when viewed from the tip end side of the drill main body, the first inner cutting edge of the first insert and the first outer cutting edge of the second insert are arranged in parallel with each other.