High speed milling or drilling tool of the type including a rotatable head which carries several circumferentially arranged cutting insert holders for reversible cutting inserts retained by dogs. Each insert has a pair of parallel top surfaces bounded by four orthogonal end surfaces, two of which are received against perpendicular sidewalls in an open pocket of the holder. Each top surface has at least one pair of elongate clamping indentations which are formed as surfaces of rotation and lie at a like angle .beta. to the adjacent end surface of the insert.