End mill and drilling method

An end mill for orbital drilling by which a workpiece is drilled by feeding a tool, in a tool axis direction offset to a central axis of a hole to be drilled, while rotating the tool around the tool axis direction and rotating the tool around the central axis of the hole,

• the end mill comprising:
• a shank;
• a first cutting edge formed in a peripheral portion of the shank and having a first end and a second end spaced apart in the tool axis direction;
• and
• a second cutting edge formed in a bottom portion of the shank,
• wherein at least a chamfered edge is formed on a first ridgeline between a first rake face and a first flank of the first cutting edge with the chamfered edge having a width W that is less than or equal to 150 μm,
• wherein the chamfered edge has an angle more than 0° and less than 30° to a cutting direction to the workpiece,
• wherein a first honed edge is formed in a first flank side of the first ridgeline, while the chamfered edge is formed in a first rake face side of the first ridgeline and borders a tip of the first cutting edge,
• wherein the first honed edge has a radius not less than 10 μm and not more than 30 μm,
• wherein a second honed edge is formed on a second ridgeline between a second rake face and a second flank of the second cutting edge, and
• wherein the second ridgeline has no chamfered edge, and
• wherein the first end of the first cutting edge originates at a location coinciding with a most proximal cutting edge region location in the shank, and extends continuously and without interruption on the periphery of the shank to a third honed edge extending between the second end of the first cutting edge and the second cutting edge, and
• wherein the first rake face defines a positive rake angle in the first cutting edge, and
• wherein the second rake face defines a zero or positive rake angle in the second cutting edge.