Drill and drilling method for workpiece

1. A drill comprising:

a cutting edge;

lands formed between helical flutes; and

margins at edges of the lands along the helical flutes,

wherein the cutting edge has a point angle which is continuously decreased from a center-position point angle A at a center position to a maximum-diameter-position point angle of 0° at a maximum diameter position,

wherein the cutting edge has a radial relief angle which is continuously decreased from the center position to the maximum diameter position,

wherein the cutting edge has a rake angle of 0°,

wherein a radial relief angle ε at the maximum diameter position satisfies a condition ε<15°,

wherein the point angle A at the center position satisfies a condition 90°<A <150°,

wherein a length of the cutting edge along an axis of the drill is at least 1.6 times a maximum diameter of the drill, and

wherein a width of each of the margins is from 0.1 to 1.5 mm.

2. The drill according to claim 1 , wherein a ridgeline of the cutting edge is a parabolic ridgeline formed of a part of a parabola.

3. The drill according to claim 1 , wherein a ridgeline of the cutting edge is an arcuate ridgeline formed of a single arc.

4. The drill according to claim 1 , wherein an axial relief angle at the center position is δ, and the center-position point angle A and the axial relief angle δ at the center-position satisfy a relationship of δ>(180°-A)/2.

5. The drill according to claim 1 , wherein, at a position between the center position and the maximum diameter position, the radial relief angle is at least 20°.

6. The drill according to claim 5 , wherein, at a position between the center position and the maximum diameter position, the radial relief angle is at least 30°.

7. A drilling method for a workpiece, comprising:

a first step of drilling a hole in the workpiece by using a drill including a cutting edge, lands formed between helical flutes, and margins at edges of the lands along the helical flutes, and by cutting the workpiece by the cutting edge at the center-position; and

a second step of reaming by cutting the drilled hole formed in the first step, by the cutting edge at the maximum-diameter-position while gradually shifting from the first step,

wherein the cutting edge has a point angle which is continuously deceased from a center-position point angle A at a center position to a maximum-diameter-position point angle of 0° at a maximum diameter position,

wherein the cutting edge has a radial relief angle which is continuously decreased from the center position to the maximum diameter position,

wherein the cutting edge has a rake angle of 0°,

wherein a radial relief angle ε at the maximum diameter position satisfies a condition of ε<15°,

wherein the point angle A at the center position satisfies a condition 90°<A<150°,

wherein a length of the cutting edge along an axis of the drill is at least 1.6 times a maximum diameter of the drill, and

wherein a width of each of the margins is from 0.1 to 1.5 mm.

8. The drilling method of claim 7 , wherein

the drilled hole is formed in a multilayer component member comprising a reinforced composite material layer.

9. The drilling method of claim 8 , wherein the step of reaming includes removing delaminated portions of the multilayer component member.

10. The drilling method of claim 9 , wherein

the drilled hole is formed in the multilayer component member such that the hole is substantially free of burrs and substantially free of delamination between layers of the multilayer component member.