Drill head and drill

A drill comprising:

• a drill head;
• a holder;
• and
• a fixing member,
• wherein
• the drill head includes a first attachment surface attached to the holder, a head tip surface opposite to the first attachment surface in an axial direction along a central axis of the drill head, a head outer peripheral surface continuous with the first attachment surface and the head tip surface, and a shank shaft extending from the first attachment surface along the axial direction,
• the head outer peripheral surface is provided with a first flute and a second flute that extend helically about the central axis from the head tip surface to reach the first attachment surface,
• the first flute and the second flute are symmetric with respect to the central axis,
• when the first attachment surface is viewed along the central axis and in a direction from the first attachment surface toward the head tip surface, each of the first flute and the second flute has a first end and a second end on the first attachment surface, the first end and the second end facing each other with a space between them in a circumferential direction along a circumference centered on the central axis,
• the first attachment surface is provided with ridges that are radially asymmetrical with respect to the central axis, the ridges including a first ridge, a second ridge, and a third ridge that extend along a radial direction orthogonal to the axial direction,
• the first ridge is located between the first end of the first flute and the second end of the second flute in the circumferential direction,
• the second ridge and the third ridge are located between the second end of the first flute and the first end of the second flute in the circumferential direction,
• the second ridge is located closer to the second end of the first flute than the third ridge in the circumferential direction,
• an angle formed by a virtual straight line passing through a center of the first ridge in the radial direction and a virtual straight line passing through the central axis and the first end of the first flute is equal to an angle formed by the virtual straight line passing through the center of the first ridge in the radial direction and a virtual straight line passing through the central axis and the second end of the second flute,
• the shank shaft is provided with a notch,
• the holder has a second attachment surface in contact with the first attachment surface, and a holder outer peripheral surface continuous with the second attachment surface,
• the second attachment surface is provided with a first hole into which the shank shaft is inserted, and a first groove, a second groove, and a third groove that extend along the radial direction,
• the first groove, the second groove, and the third groove are fitted to the first ridge, the second ridge, and the third ridge, respectively,
• the holder outer peripheral surface is provided with a second hole continuous with the first hole, and
• fixing member is in contact with the notch by being inserted into the second hole.