Drill with groove width variation along the drill and double margin with a thinning section at the tip

A drill bit comprising:

• a chip discharge groove in the shape of a spiral formed in an outer periphery of a tip section of a roughly cylindrical drill body that rotates around an axis, and
• a cutting edge formed on an intersecting ridgeline between a wall surface facing in a direction of drill rotation of the chip discharge groove and a tip flank of the drill body;
• wherein,
• the chip discharge groove includes a narrow width section, formed along the axis of the drill bit, in which a helix angle relative to the axis is constant, and groove width is constant,
• an expanding width section, formed in the chip discharge groove farther towards a rear of the drill bit than the narrow width section, in which groove width is gradually widened in the direction of drill rotation and towards the rear of the drill bit in the direction of drill rotation as it approaches the rear of the drill bit relative to a virtual groove in which the narrow width section is extended towards the rear end of the drill bit,
• wherein the expanding width section includes first and second expanding stages, and a non-zero angle at which the first expanding stage expands is different than a non-zero angle at which the second expanding stage expands.

A drill bit comprising:

• a chip discharge groove extending towards a rear of the drill bit, formed in an outer periphery of a drilling end section on a tip side of a drill body that rotates around an axis of the drill bit, a cutting edge formed on an intersecting ridgeline section between an inner wall surface that faces towards a direction of drill rotation of the chip discharge groove and a tip flank of the drilling end section, and
• a first margin section, which is adjacent to a rear side of the chip discharge groove in the direction of drill rotation, and a second margin section, which is adjacent to a front side of the chip discharge groove in the direction of drill rotation, the first and second margin sections being formed in a land section of the drilling end section;
• wherein,
• a thinning section formed on the tip side of an inner wall surface of the chip discharge groove that is continuous with an inner peripheral edge of the cutting edge and extends to the land section, and when the drilling end section is viewed from the tip side in an axial direction, an intersecting angle between a straight line X, which connects a point in the second margin section located the farthest towards the front in the direction of drill rotation and the axis of the drill bit, and a straight line Y, which connects an outer peripheral edge located in the land section at the intersecting ridgeline between the tip flank and thinning section and the axis, is set to be within the range of −5° to 10° when straight line X is taken to be positive when located farther towards the front in the direction of drill rotation than straight line Y.