Abstract: A rotary milling cutter has a body with one or more insert seats on the body. A cutting insert is disposed in each seat and is locked in place by means of a pair of fasteners which are threaded into corresponding tapped holes in the cutter body adjacent the seat. Each insert contains a pair of spaced grooves on a surface of the insert adjacent the fastener. The axis of each fastener is disposed in the same plane as and at an acute angle to the axis of a corresponding groove. As each fastener is advanced into its threaded hole, a circular edge of the head of the fastener lodges in the corresponding groove exerting a locking force to lock the insert in place in its seat. The grooves have elliptically contoured cross sections conforming to the elliptical projections of the circular edges of the fastener heads when viewed axially of the grooves. There is also a counterbore at the near end of each threaded hole for receiving the head of the corresponding fastener.