This relates to a cutting insert (20) for use in a rotary cutting tool (1) having helically arranged teeth, comprising, a six sided block having two major parallel rhomboid-shaped plane surfaces (21, 22) constituting a seating face (22) and a clearance face (21) respectively, two opposed minor side surfaces constituting cutting faces (30) extending between the seating and clearance faces, each said cutting face forming an acute angle (A) with the clearance face and forming a cutting edge (32) along the intersection therewith, and two minor parallel plane (23, 24) end faces extending between the seating (22) and clearance faces (21) and the two cutting faces (30). Each cutting face (30) has a concave curvature centrally of its major dimension, and a convex curvature at each end of its major dimension to define a serpentine cutting face and cutting edge. When disposed along the helically arranged teeth of a rotary cutting tool, such as a milling cutter, the inserts form a cutting face, or a radial face, having a continuously variable positive radial rake angle, and a cutting edge of serpentine configuration which generally parallels the helical form of the teeth.