Abstract: A cutting tool has a cutting head and a shank. The cutting head has a body, a head threaded portion, a first helical head surface and a second helical head surface. The shank has a cylindrical portion, a shank threaded portion, a first helical contact surface and a second helical contact surface. The head threaded portion screw-engages the shank threaded portion. The first helical head surface is parallel to the thread of the head threaded portion and the first helical contact surface is parallel to the thread of the shank threaded portion. The second helical head surface and the second helical contact surface are inclined reversely to the first helical head surface and the first helical contact surface, respectively. The first helical head surface comes into surface contact with the first helical contact surface. The second helical head surface comes into surface contact with the second helical contact surface.

Abstract: The threaded engagement device includes a female thread portion and a male thread portion, which are engaged to each other through a plurality of threads formed along the axis direction. Each thread of the female thread portion and the male thread portion of the threaded engagement device includes a crest, a root, a stab flank and a load flank. The root of the female thread portion has a straight portion extending along the axis direction and a concave rounded portion. The root of the male thread portion has a concave rounded portion, wherein the radius of curvature of the rounded portion formed in the root of the male thread portion is larger than the radius of curvature of the rounded portion formed in the root of the female thread portion.