Abstract: In a radius end mill having a bottom edge formed on the end face thereof, a radius edge designed in a substantially quarter arc shape and formed at a corner portion thereof, and an outer peripheral edge formed spirally on the side surface thereof, the bottom edge and the radius edge being continuously connected to each other at a connecting point A while the radius edge is continuously connected to each other at a connecting point B, when a view taken along a plane that passes through the connecting points A and B and crosses a rake face of the radius edge is represented by an R cross-sectional view, the rake face of the radius edge is designed to have a convex curved line extending from the connecting point A to the connecting point B in the R cross-sectional view.

Abstract: A highly stiff end mill comprising a shank, a plurality of helix cutting edges on the periphery of the end mill, the end mill being characterized in that a radial rake angle of the helix cutting edges in a cross section perpendicular to the longitudinal axis of the end mill is set in the range of -29.degree. to 0.degree. and a core diameter of the end mill is set in the range of 70 to 90% of the outside diameter of the end mill.By selectively determining the negative rake angle in the range described above, this end mill is applicable to hard materials and has general-purpose properties. The improper factors of the end mill, such as increased cutting resistance, can be compensated for by setting the core diameter thereof to the range described above. This end mill is also structured to be able to cut materials with less hardness values as well as hard materials, thereby covering a wide range of applications.