Coolant-hole equipped drill

1. A coolant-hole equipped drill comprising:

a drill main body which is configured to rotate around an axis line during operation;

a cutting edge portion which has a tip flank formed at a tip side of the drill main body;

a plurality of chip discharging grooves, each of which is formed at an outer circumference of the cutting edge portion and provided with a front groove wall surface facing forward in the rotating direction of the drill main body;

a cutting edge which is formed at a ridge line portion where the front groove wall surface of the chip discharging groove and the tip flank of the cutting edge portion intersect with each other and in which an inner circumferential portion of the front groove wall surface is subjected to thinning;

a thinning blade formed by the thinning at an inner circumferential portion of the cutting edge;

a first thinning surface formed by the thinning at an inner circumferential portion of a tip side part of the front groove wall surface and formed forward of the thinning blade in the rotating direction;

a second thinning surface which is provided behind the tip flank in the rotating direction and inclined to the tip flank by the thinning so as to move backward to the base end of the drill main body, and which is formed forward of the first thinning surface in the rotating direction;

a coolant hole which is provided at the cutting edge portion so as to be opened at the tip flank and is constant in shape and dimension on a cross section orthogonal to the axis line over an entire length of the drill main body; and

a land portion which is provided between the chip discharging grooves adjacent to each other in the rotating direction, wherein

a first margin portion, a second margin portion and a third margin portion are formed on an outer-circumference wall surface of the land portion, so as to have intervals between each other in the rotating direction,

the first margin portion is provided behind the cutting edge in the rotating direction,

the second margin portion is provided behind the first margin portion in the rotating direction and intersects with the tip flank between two straight lines which pass through the axis line and circumscribe an opening portion of the coolant hole so as to hold the opening portion between two straight lines when viewed from the axis line of the drill main body, and

the third margin portion is provided behind the second margin portion in the rotating direction and at least a part backward in the rotating direction intersects with the tip flank, a tip of the third margin portion intersects with the second thinning surface, a part of the third margin portion forward in the drill rotating direction intersects with the second flank portion, and a part of the third margin portion backward in the drill rotating direction intersects with the second thinning surface, and

wherein the coolant hole includes:

a front hole wall surface which is positioned forward in the rotating direction of the drill main body;

a rear hole wall surface which is positioned backward in the rotating direction of the drill main body; and

an outer-circumference hole wall surface which is positioned on the outer circumferential side of the drill main body, and

wherein intervals between the front hole wall surface of the coolant hole and the rear hole wall surface thereof in the circumferential direction are gradually increased toward the outer circumferential side, a percentage for increasing the intervals between the front hole wall surface and the rear hole wall surface is gradually increased toward the outer circumferential side, and the front hole wall surface and the rear hole wall surface are formed in such a manner that both the cross sections are formed in a curved shape which is convex inside the coolant hole on the cross section orthogonal to the axis line, and

wherein parts at which the front hole wall surface, the rear hole wall surface and the outer-circumference hole wall surface intersect each other are smoothly connected by recessed curve portions.

2. The coolant-hole equipped drill according to claim 1 , wherein the third margin portion is greater in width in the rotating direction than the first margin portion and the second margin portion.

3. The coolant-hole equipped drill according to claim 1 , wherein the second margin portion is smaller in width in the rotating direction than the first margin portion and the third margin portion.

4. The coolant-hole equipped drill according to claim 1 , wherein the chip discharging groove has a rear groove wall surface which is positioned backward in the rotating direction and faces backward in the rotating direction, and

the coolant hole includes:

a front hole wall surface which is positioned forward in the rotating direction and constant in interval with the front groove wall surface of the chip discharging groove;

a rear hole wall surface which is positioned backward in the rotating direction and constant in interval with the rear groove wall surface of the chip discharging groove; and

an outer-circumference hole wall surface which is positioned on the outer circumferential side of the drill main body and constant in interval with a part of the outer-circumference wall surface of the land portion excluding the first, the second and the third margin portions.