A drill includes a cutting edge having a ridgeline. The ridgeline has a part of an ellipse and a straight line. It is assumed that the coordinates of a separation point of an ellipse portion from a straight line portion are (x 0 , y 0 ). The straight line is a tangent to the ellipse at the separation point. It is assumed that a 1 is an area from a tip end of the drill to x 0 , a 2 is an area from x 0 to the origin of the ellipse, and a 3 is a negative area, along the x-axis. The ridgeline of the cutting edge is formed straight along the tangent in the area a 1 , and is formed along the ellipse in the area a 2 . A margin is formed in the area a 3 continuously from a relief face of the cutting edge.
1. A drill comprising:
a cutter edge section comprising a cutting edge having a ridgeline, a maximum diameter position, and a predetermined point located closer to a tip end of the drill than the maximum diameter position,
wherein at least a part of the ridgeline is formed along a part of a reference ellipse, with a portion of the ridgeline continuously extending from the predetermined point toward the tip end of the drill formed straight along a tangent to the reference ellipse at the predetermined point,
wherein the cutting edge has a relief angle δ 1 at a position closest to the tip end of the drill and a relief angle δ 2 at the maximum diameter position, the relief angles δ 1 and δ 2 having a relationship δ 1 >δ 2 ,
wherein, in at least a part of an area from the position closest to the tip end of the drill to the maximum diameter position, the relief angle gradually decreases from the relief angle δ 1 to the relief angle δ 2 ,
wherein the cutter edge section comprises a length L from the maximum diameter position to the tip end of the drill and a maximum diameter ΦD at the maximum diameter position, with the length L and the maximum diameter ΦD satisfying a ratio L/ΦD from 1.4 to 1.6, and
wherein a point angle of the drill is in a range from 100° to 140°.
2. The drill according to claim 1 , wherein the part of the ridgeline continuously extends to the maximum diameter position from the predetermined point located closer to the tip end of the drill than the maximum diameter position.
3. The drill according to claim 2 , wherein the reference ellipse is an ellipse having a minor axis located at the maximum diameter position.
4. The drill according to claim 2 , wherein the cutting edge has a constant relief angle in a predetermined area extending from a position closest to the tip end of the drill.
5. The drill according to claim 2 , wherein the cutting edge has a constant relief angle in an area closer to the tip end of the drill than the predetermined point.
6. The drill according to claim 1 , wherein, in an area from the predetermined point to the maximum diameter position, the relief angle gradually decreases from the relief angle δ 1 to the relief angle δ 2 .
7. The drill according to claim 1 , wherein the drill has a margin continuously extending from a relief face of the cutting edge toward a rear end of the drill, wherein the margin has the relief angle δ 2 .
8. The drill according to claim 1 , wherein the cutter edge section comprises a diameter ΦA at the predetermined point, and the maximum diameter ΦD and the diameter ΦA satisfy a ratio ΦA/ΦD from 0.333 to 0.089.
9. A drill comprising:
a cutter edge section comprising a cutting edge having a ridgeline, a maximum diameter position, and a predetermined point located closer to a tip end of the drill than the maximum diameter position,
wherein at least a part of the ridgeline is formed along a reference curve that is defined by connecting end points of a plurality of arcs, each of the arcs having end points on a reference ellipse, with a portion of the ridgeline continuously extending from the predetermined point toward the tip end of the drill formed straight along a tangent to the reference curve at the predetermined point,
wherein the cutting edge has a relief angle δ 1 at a position closest to the tip end of the drill and a relief angle δ 2 at the maximum diameter position, the relief angles δ 1 and δ 2 having a relationship δ 1 >δ 2 ,
wherein, in at least a part of an area from the position closest to the tip end of the drill to the maximum diameter position, the relief angle gradually decreases from the relief angle δ 1 to the relief angle δ 2 ,
wherein the cutter edge section comprises a length L from the maximum diameter position to the tip end of the drill and a maximum diameter ΦD at the maximum diameter position, with the length L and the maximum diameter ΦD satisfying a ratio L/ΦD from 1.4 to 1.6, and
wherein a point angle of the drill is in a range from 100° to 140°.
10. The drill according to claim 9 , wherein a tangent to at least one arc in the plurality of arcs is common to a tangent to the reference ellipse.
11. The drill according to claim 9 , wherein at least one arc in the plurality of arcs has a common point with the reference ellipse at a position other than the end points.
12. The drill according to claim 9 , wherein the part of the ridgeline continuously extends to the maximum diameter position from the predetermined point located closer to the tip end of the drill than the maximum diameter position.
13. The drill according to claim 12 , wherein the reference ellipse is an ellipse having a minor axis located at the maximum diameter position.
14. The drill according to claim 12 , wherein the cutting edge has a constant relief angle in a predetermined area extending from a position closest to the tip end of the drill.
15. The drill according to claim 12 , wherein the cutting edge has a constant relief angle in an area closer to the tip end of the drill than the predetermined point.
16. The drill according to claim 9 , wherein, in an area from the predetermined point to the maximum diameter position, the relief angle gradually decreases from the relief angle δ 1 to the relief angle δ 2 .
17. The drill according to claim 9 , wherein the drill has a margin continuously extending from a relief face of the cutting edge toward a rear end of the drill,
wherein the margin has the relief angle δ 2 .