Provided is a step-shaped drill bit including at least two stepped cutting edges that are axially provided. A cutting edge of a first step from a tip is formed so as to satisfy at least one of the following conditions. That is, a number of blades is smaller, a point angle of the cutting edge is larger, or a clearance angle of the cutting edge is larger than that of cutting edges of a second and subsequent step(s). As a result, a thrust resistance during a perforation can be reduced compared with a case in which the first step cutting edge is formed such that the number of blades, the point angle of the cutting edge and the clearance angle of the cutting edge are the same as those of the second and subsequent step(s).
1. A step-shaped drill bit comprising at least two stepped sections that are axially provided, wherein
a cutting edge of a first stepped section from a tip has a point angle that is larger than a point angle of a cutting edge of a second stepped section, and
a cutting edge of the first stepped section has a circumferential clearance angle that is larger than a circumferential clearance angle of a cutting edge of the second stepped section.
2. The drill bit as claimed in claim 1 comprising at least three stepped sections that are axially provided, wherein
each of the second and subsequent stepped sections satisfies at least one condition selected from the group consisting of:
having the same or larger number of blades than a neighboring stepped section on the tip side,
comprising a cutting edge having the same or smaller point angle than a cutting edge of a neighboring stepped section on the tip side, and
comprising a cutting edge having the same or smaller circumferential clearance angle than a cutting edge of a neighboring stepped section on the tip side.
3. The drill bit as claimed in claim 1 comprising at least three stepped sections that are axially provided, wherein
each of the second and subsequent stepped sections satisfies at least one condition selected from a group consisting of:
having a larger number of blades than a neighboring stepped section on the tip side,
comprising a cutting edge having a smaller point angle than a cutting edge of a neighboring stepped section on the tip side, and
comprising a cutting edge having a smaller circumferential clearance angle than a cutting edge of a neighboring stepped section on the tip side.
4. The drill bit as claimed in claim 1 , wherein an axial distance between the tip of the drill bit and the second stepped section is the same as or larger than a diameter of the first stepped section.
5. The drill bit as claimed in claim 1 , wherein the first stepped section is two-bladed.
6. The drill bit as claimed in claim 1 , wherein the second stepped section is four-bladed.
7. The drill bit as claimed in claim 1 , comprising a third stepped section having four blades.
8. The drill bit as claimed in claim 1 , wherein
a last stepped section comprises a cutting edge for finally enlarging a perforated hole has a point angle, at a front end thereof, that is smaller than a point angle of a cutting edge of a stepped section closer to the tip of the drill bit, and
the point angle of the cutting edge of the last stepped section decreases gradually along a front-rear direction such that the cutting edge of the last stepped section has a smoothly curved profile.
9. The drill bit as claimed in claim 8 , wherein a point angle at a rear end of the cutting edge of the last stepped section is zero.
10. The drill bit as claimed in claim 9 , wherein a diameter of the drill at the position where the point angle is zero is a maximum diameter of the drill bit.
11. The drill bit as claimed in claim 8 , wherein the last stepped section is a third stepped section of the drill bit.
12. The drill bit as claimed in claim 1 , wherein the first stepped section comprises a number of blades and a number of flutes arranged between adjacent blades.
13. The drill bit as claimed in claim 1 , wherein the cutting edge of the second stepped section comprises a ridgeline having a minimum diameter at a front end and a maximum diameter at a rear end.
14. The drill bit as claimed in claim 1 , further comprising a third stepped section, wherein the second stepped section comprises a number of blades, the third stepped section comprises a number of blades, and the number of blades of the third stepped section is the same as the number of blades of the second stepped section.
15. The drill bit as claimed in claim 1 , wherein
the second stepped section comprises a first set of blades and a second set of blades, the second set of blades being oriented at a 90° orientation from the first set of blades, and
in an axial cross-section, the profile of the first set of blades differs from the profile of the second set of blades.
16. The drill bit as claimed in claim 1 , further comprising a cutting edge of a third stepped section, wherein a length from the tip of the drill bit to a front end point of the cutting edge of the second stepped section is greater than a length from the front end point of the cutting edge of the second stepped section to a front end point of the cutting edge of the third stepped section.
17. A method for perforating a fiber reinforced resin composite material comprising
drilling a work piece including a fiber reinforced resin composite material with the drill bit as claimed in claim 1 .
18. A step-shaped drill bit comprising at least two stepped sections that are axially provided, wherein
a cutting edge of a first stepped section from a tip has a point angle that is larger than a point angle of a cutting edge of a second stepped section, and
a cutting edge of the first stepped section has a circumferential clearance angle that is larger than a circumferential clearance angle of a cutting edge of the second stepped section, wherein
a cutting edge of a last stepped section for finally enlarging a perforated hole has a point angle, at a front end thereof, that is smaller than a point angle of a cutting edge of a stepped section closer to the tip of the drill bit,
the point angle of the cutting edge of the last stepped section decreases gradually along a front-rear direction such that the cutting edge of the last stepped section has a smoothly curved profile,
the point angle at a rear end of the cutting edge of the last stepped section is zero, and
a difference of a minimum diameter at a front end of the cutting edge of the last stepped section and a maximum diameter at the rear end of the cutting edge of the last stepped section is not less than 1 mm.
19. A step-shaped drill bit comprising at least two stepped sections that are axially provided, wherein
a cutting edge of a first stepped section from a tip has a point angle that is larger than a point angle of a cutting edge of a second stepped section,
a cutting edge of the first stepped section from the tip has a circumferential clearance angle that is larger than a circumferential clearance angle of a cutting edge of the second stepped section, and
the first stepped section has a fewer number of blades than the number of blades at the second stepped section.
20. A step-shaped drill bit comprising at least two stepped sections that are axially provided, wherein
a cutting edge of a first stepped section from a tip has a circumferential clearance angle that is larger than a circumferential clearance angle of a cutting edge of a second stepped section, and
the first stepped section has a fewer number of blades than the number of blades at the second stepped section.