Abstract: The rotary cutter tool assembly, including cutting insert, of the present invention is adapted to perform a finishing operation within a tube in such manner that tube wall thickness is ensured. To this end, the novel insert is provided with a guiding surface adjacent to the cutting edge of the insert, which guiding surface is displaced towards the central portion of the insert at a distance corresponding to the desired depth of cut. The insert is secured to the rotatable cutter head in such relation that said guiding surface precedes the cutting edge in the direction of rotation of the cutter head.