Patent ID: 8690492

Claim:
A ball end mill comprising, a plurality of end cutting edges being formed on a tip section of an end mill body that is rotated around an axis, the plurality of end cutting edges being formed having, as its rotational trajectory around the axis, a hemisphere shape that is centered on the axis, the plurality of end cutting edges being formed in such a way as to extend from the inside of the point of the tip section at intervals in a circumferential direction to the outside of a rear end of the tip section; and a plurality of peripheral cutting edges that are continuous with outside ends of the end cutting edges and that extend to the rear end are formed at intervals in the circumferential direction on an outer circumferential portion of the end mill body; wherein a portion of the plurality of end cutting edges is formed into long end cutting edges that extend out from the vicinity of the axis at the tip section of the end mill body; an angle α is larger than an angle θ, the angle α on a cross-section perpendicular to the axis, being formed between a straight line that connects the axis to a peripheral cutting edge which is continuous with the long end cutting edge and a straight line that connects the axis to a peripheral cutting edge which is continuous with an end cutting edge which is positioned in forward of the long end cutting edge in the end mill rotation direction, and the angle θ, when viewed from the point in the axial direction, being formed between a tangent line at an inside end of the long end cutting edge and a tangent line at an inside end of the end cutting edge which is positioned in forward of the long end cutting edge in the end mill rotation direction, a helix angle relative to the axis of the long end cutting edges is larger than a helix angle relative to the axis of the end cutting edges that are positioned in forward of the long end cutting edges in the end mill rotation direction; and an angle, which is formed between the tangent line of the long end cutting edges and the tangent line of the end cutting edges, and the angle at positions on the long end cutting edges and the end cutting edges and at the same distance from the axis, becomes larger as these positions approach the rear end in the direction of the axis of the end mill body.