Idexable drill insert

1. A quadrangular indexable drill insert comprising:

a substantially flat top surface and bottom surface, a side surface adjoining the top and bottom surfaces from four sides of the drill insert, the bottom surface being smaller than the top surface to provide positive cutting geometry;

four identical cutting edges of the side surface intersecting the top surface, each cutting edge comprising:

a first part edge;

a second part edge extending directly from the first part edge, the second part edge being disposed tangential to an imaginary inscribed circle disposed about a central axis of the insert;

a third part edge extending away from the second part edge, the second and the third part edges being joined together by a transition part edge having a concave curved portion adjacent the second part edge and a convex curved portion adjacent the third part edge; and

a curved nose portion of predetermined radius disposed at a corner of the insert adjoining the first part edge of one cutting edge with the third part edge of a succeeding cutting edge of the four identical cutting edges;

a through hole disposed at the center of the drill insert extending between the top surface and the bottom surface of the drill insert; and

a chamfer provided at the bottom edge of the drill insert,

wherein each of the first part edge, the second part edge and the third part edge are linear,

wherein the first part edge is disposed at an angle in the range of from about 0.5° to about 5° with respect to a horizontal axis of the insert such that the first part edge extends inwardly and is closest to the horizontal axis about the second part edge,

wherein the second part edge is disposed at an angle in the range of from about 5° to about 15° with respect to the horizontal axis such that the second part edge generally extends toward the horizontal axis when moving away from the first part edge; and

wherein the third part edge is disposed generally parallel with respect to the horizontal axis.

2. The drill insert of claim 1 wherein each curved nose portion has a radius in the range of from about 0.4 mm to about 1.2 mm.

3. The drill insert of claim 1 wherein the through hole comprises a mouth portion and a stepped portion structured to accommodate an insert screw.

4. The drill insert of claim 1 wherein the chamfer is disposed at an angle of about 30° with respect to a vertical axis of the drill insert.

5. The drill insert of claim 1 wherein the top surface is provided with a protruded portion about the mouth portion.

6. The drill insert of claim 1 wherein corners of the edge surface of the insert form an angle in the range of from about 0° to about 11° with respect to a vertical axis of the drill insert.

7. A drill comprising:

a drill body having a peripheral pocket; and

a quadrangular indexable drill insert disposed in the peripheral pocket, the quadrangular indexable drill insert comprising:

a substantially flat top surface and bottom surface, a side surface adjoining the top and bottom surfaces from four sides of the drill insert, the bottom surface being smaller than the top surface to provide positive cutting geometry;

four identical cutting edges of the side surface intersecting the top surface, each cutting edge comprising:

a first part edge;

a second part edge extending from the first part edge, the second part edge being disposed tangential to an imaginary inscribed circle disposed about a central axis of the insert;

a third part edge extending away from the second part edge, the second and third part edges being joined together by a transition part edge having a concave curved portion adjacent the second part edge and a convex curved portion adjacent the third part edge; and

a curved nose portion of predetermined radius disposed at a corner of the insert adjoining the first part edge of one cutting edge with the third part edge of a succeeding cutting edge of the four identical cutting edges;

a through hole disposed at the center of the drill insert extending between the top surface and the bottom surface of the drill insert; and

a chamfer provided at the bottom edge of the drill insert,

wherein each of the first part edge, the second part edge and the third part edge are linear,

wherein the first part edge is disposed at an angle in the range of from about 0.5° to about 5° with respect to a horizontal axis of the insert such that the first part edge extends inwardly and is closest to the horizontal axis about the second part edge;

wherein the second part edge is disposed at an angle in the range of from about 5° to about 15° with respect to the horizontal axis such that the second part edge generally extends toward the horizontal axis when moving away from the first part edge; and

wherein the third part edge is disposed generally parallel with respect to the horizontal axis.

8. The drill of claim 7 wherein the drill body further comprises a central pocket having an insert different from the quadrangular indexable drill insert disposed therein.

9. The drill of claim 7 wherein each curved nose portion has radius in the range of from about 0.4 mm to about 1.2 mm.

10. The drill of claim 7 wherein the through hole comprises a mouth portion and a stepped portion structured to accommodate an insert screw.

11. The drill of claim 7 wherein the chamfer is disposed at an angle of about 30° with respect to a vertical axis of the drill insert.

12. The drill of claim 7 wherein the top surface is provided with a protruded portion about the mouth portion.

13. The drill of claim 7 wherein corners of the edge surface of the insert form an angle in the range of from about 0° to about 11° with respect to a vertical axis of the drill insert.