Drill

A drill comprising:

• a bar-shaped tool main body to be rotated around a shaft center;
• a discharge flute provided in a helical shape in an outer peripheral surface of the tool main body from a leading end portion toward a rear end portion thereof;
• a cutting edge formed at a ridge section between an inner face, of the discharge flute, oriented toward the front in a rotation direction of the tool main body and a flank at the leading end portion;
• an oil hole provided at the flank, the oil hole supplying a cutting fluid to the side of the cutting edge;
• and
• a gash portion, a ridge line between the gash portion and the flank extending from an inner end of the cutting edge toward an outer side in a radial direction while curving in a circular arc shape toward the front in the rotation direction, and connected to the discharge flute, wherein
• the oil hole is provided with a fan-shaped cross-section as a result of being surrounded by
• a front-side inner wall surface positioned to a front side in the rotation direction of the tool main body,
• a rear-side inner wall surface positioned to a rear side in the rotation direction of the tool main body, and facing the front-side inner wall surface in a circumferential direction,
• an outer peripheral-side inner wall surface formed by a partial cylindrical surface centered on a center line of the tool main body and having a radius of curvature corresponding to a radius of a circumscribed circle of the oil hole, and
• an inner peripheral-side inner wall surface formed by a partial cylindrical surface centered on the center line of the tool main body, having a radius of curvature corresponding to a radius of an inscribed circle of the oil hole and being smaller than the radius of curvature of the outer peripheral-side inner wall surface, and facing the outer peripheral-side inner wall surface in the radial direction,
• in the flank, a reference line extending from the shaft center of the tool main body toward the outer side in the radial direction is defined as a first reference line wherein an R3 corner formed between the inner peripheral-side inner wall surface and a rear-side inner wall surface is located on an intersection point between the first reference line and the inscribed circle, a reference line rotated by a first angle to the front in the rotation direction around the shaft center with respect to the first reference line is defined as a second reference line wherein the front-side inner wall surface is located on the second reference line and between the inscribed circle and the circumscribed circle, a reference line rotated by the first angle to the rear in the rotation direction around the shaft center with respect to the first reference line is defined as a third reference line wherein an R4 corner formed between the outer peripheral-side inner wall surface and the rear-side inner wall surface is located on the third reference line, and the outer peripheral-side inner wall surface is located between the second reference line and the third reference line, and a reference line rotated to the rear in the rotation direction around an intersection point between the inscribed circle and the first reference line, by a second angle greater than the first angle with respect to the first reference line and facing the gash portion is defined as a fourth reference line wherein the rear-side inner wall surface is located between the inscribed circle and the circumscribed circle, and both a rearmost end and a foremost end of the rear-side inner wall surface are located on the fourth reference line,
• the oil hole is provided between the second reference line and the third reference line,
• the front-side inner wall surface is disposed along the second reference line, and
• the rear-side inner wall surface is disposed on the fourth reference line, and is curved in a circular arc shape to the same direction side as the gash portion toward the front in the rotational direction, at a position separated from the gash portion.