Drill for carbon-fiber composite material

A drill for drilling holes in a carbon fiber composite material, the drill comprising:

• two cutting edges formed symmetrically around a rotation axis of the drill, each of the two cutting edges having a respective main cutting edge formed from a drill forward end toward a side of a drill outer periphery and further having a respective thinned cutting edge formed at a side of the drill forward end of the corresponding main cutting edge,
• two relief surfaces formed symmetrically about the rotation axis of the drill, and
• two first margin portions and two second margin portions,
• wherein a respective back groove is formed from each of the two relief surfaces toward the drill outer periphery, and wherein each respective back groove is disposed in between one of two first margin portions and a corresponding one of the two second margin portions,
• wherein each of the two first margin portions has a respective first forward-end small-diameter portion with a diameter smaller than a diameter of the drill, and wherein each first forward-end small-diameter portion is formed from the side of the drill forward end toward a side of a drill rearward end,
• wherein each of the two second margin portions has a respective second forward-end small-diameter portion with a diameter smaller than the first forward-end small-diameter portion corresponding thereto, and wherein each second forward-end small-diameter portion is formed from the side of the drill forward end toward the side of the drill rearward end,
• wherein each first forward-end small-diameter portion and each second forward-end small-diameter portion has a respective inclination portion, the diameter of each inclination portion gradually increasing from the side of the drill forward end toward the side of the drill rearward end,
• wherein the inclination portion of each first forward-end small-diameter portion has a respective relief surface,
• wherein the inclination portion of each second forward-end small-diameter portion has a respective first inclination portion and a respective second inclination portion, each first inclination portion having a different inclination angle than the corresponding second inclination portion,
• wherein each first inclination portion is respectively formed from the side of the drill forward end in a direction toward the side of the drill rearward end and wherein each first inclination portion has an inclination angle greater than an inclination angle of the corresponding inclination portion of each first forward-end small-diameter portion,
• wherein each second inclination portion is respectively formed from the side of the drill rearward end of the corresponding first inclination portion in the direction toward the side of the drill rearward end and wherein each second inclination portion respectively has an inclination angle identical to the inclination angle of the corresponding inclination portion of each first forward-end small-diameter portion, and
• wherein, at a same location in a longitudinal direction of the drill, a diameter extending from a respective drill forward end of one second inclination portion of the drill to a respective drill forward end of another second inclination portion of the drill is smaller than a diameter extending from one of the two first margin portions of the drill to another of the two first margin portions of the drill.