Drill

1. A drill comprising:

a tip cutting edge provided at a tip end; and

a main cutting edge connected to a rearward end portion of the tip cutting edge,

wherein:

a point angle of the tip cutting edge is a predetermined acute angle,

a point angle of the main cutting edge decreases from a front end toward a rear end of the main cutting edge,

a point angle at the front end of the main cutting edge is an acute angle greater than the point angle of the tip cutting edge,

the main cutting edge includes a first part, a second part, and a third part in this order from a tip end side,

the first part has a constant first point angle,

the second part has a constant second point angle smaller than the first part, and

the third part has a third point angle, decreasing continuously toward the rear end thereof, from an angle equal to or smaller than the second point angle.

2. The drill according to claim 1 , wherein the point angle of the tip cutting edge is in a range of 30° to 50°, and the point angle at the front end of the main cutting edge is in a range of 60° to 90°.

3. A drill comprising:

a tip cutting edge provided at a tip end; and

a main cutting edge connected to a rearward end portion of the tip cutting edge,

wherein:

a point angle of the tip cutting edge is a predetermined acute angle in a range of 30° to 50°,

a point angle of the main cutting edge, in relation to the center axis of the drill, decreases from a front end toward a rear end of the main cutting edge, and

a point angle at the front end of the main cutting edge is an acute angle greater than the point angle of the tip cutting edge and is in a range of 60° to 90°.

4. The drill according to claim 3 , wherein the point angle of the main cutting edge decreases continuously from the tip end toward the rear end of the main cutting edge.