End mill and machining method

An end mill comprising:

• a shaft portion having a columnar shape and extending along a central axis;
• and
• a cutting portion positioned on a distal end side of the shaft portion,
• wherein eight peripheral cutting edges having a larger outer diameter than that of the shaft portion are provided in the cutting portion in a circumferential direction,
• each of the peripheral cutting edges is a twisted cutting edge extending helically around the central axis,
• in regard to one peripheral cutting edge of the eight peripheral cutting edges, when a cut length of the peripheral cutting edges along an axial direction is L, a twist angle of the twisted cutting edge is θ, and a circumferential distance at a lower end of the peripheral cutting edges between the one peripheral cutting edge and another peripheral cutting edge adjacent to a tool rotation direction rear side of the one peripheral cutting edge is a, n expressed by the following equation is from 0.9 to 1.1 for all of the eight peripheral cutting edges in at least a half region of the cut length from the lower end of the peripheral cutting edges,
• the twist angle is from 35° to 40°,
• n=(L×tan θ)/(2×a),
• a value obtained from L×tanθ corresponds to a circumferential width occupied by each of the peripheral cutting edges and has a unit of length,
• a value obtained from 2×a corresponds to a double length of a and has a unit of length, and
• n is a ratio of the circumferential width occupied by each of the cutting peripheral cutting edges and the double length of a and is a dimensionless quantity.