Insert type cutting instrument and insert, method for attaching insert to insert main body

An insert type cutting instrument, comprising:

• an insert formed as a flat polygonal plate, and which has an upper surface formed as a rake surface and a lower surface facing opposite the upper surface;
• an instrument main body allowed to rotate centering around an axis, and which has a recessed portion formed on an outer side surface of a head portion of the instrument main body;
• and
• an insert seat formed on a wall surface of the recessed portion facing in a normal rotation direction of the instrument main body, and to which the insert is attached so that the lower surface of the insert contacts the insert seat and the upper surface of the insert faces in the normal rotation direction of the instrument main body;
• wherein
• a first through-hole which goes through the upper surface of the insert and the lower surface, and a second through-hole which goes through the upper surface and the lower surface and which is located closer to a tip of the instrument main body than the first through-hole when the insert is mounted on the insert seat are formed in the insert,
• a first screw hole and a second screw hole located closer to the tip of the instrument main body than the first screw hole are formed in the insert seat of the instrument main body so that the first screw hole separates from the second screw hole in the axial direction,
• the insert is mounted on the insert seat so that the center of the first screw hole is located closer to the axis of the instrument main body than the center of the first through-hole and is located closer to a base end of the instrument main body than the center of the first through-hole, and the center of the second screw hole is located closer to the axis of the instrument main body than a center of the second through-hole and is located closer to the base end of the instrument main body than the center of the second through-hole, and
• a degree θ1 of a first inclination angle formed by the intersection of a straight line passing through the center of the first screw hole and the center of the first through-hole with the axis is equal to or less than a degree θ2 of a second inclination angle formed by the intersection of a straight line passing through the center of the second screw hole and the center of the second through-hole with the axis.