Abstract: A substantially square-shaped exchangeable milling insert for mounting in a cylindrical milling cutter tool for millint 90.degree. recesses in a workpiece and having four indexable cutting edges from respective side edges of said base and intersecting said rake surface, each cutting edge comprising first, second and third successive angularly disposed portions being such that during cutting, the first and second portions of a cutting edge cut an upright surface of a substantially 90.degree. recess in a workpiece with a third portion of an adjacent cutting edge serving as a wiper for a base surface of the recess, at least the first and second portions of each cutting edge being so located that all points thereon lie on a cylindrical envelope generated by a line parallel to a rotary axis of the cutting tool and rotating with respect to the rotary axis at a radius equal to the cutting radius of the cutting tool.