Abstract: The knife assembly consists essentially of a relatively light two-edged cutting insert and a heavier holder. The insert is symmetrical about its longitudinal axis. In cross-section the two cutting edges of the insert represent the acute corners of a parallelogram, the obtuse corners of which are occupied by two longitudinal grooves that are hook-shaped in cross section. The holder includes a base plate having a projection complementary to either of said grooves. A clamping plate secures the insert on the base plate and binds said projection and said hook-like groove together. In the assembled knife the cutting edge of the insert is bounded by two plane surfaces one of which is flush with a plane surface on the holder and the other of which is flush with a plane surface on the clamping plate.