Plunge milling insert

An on-edge indexable cutting insert for a milling cutter, comprising:

• a block of hard cutting material of generally parallelepiped form having two parallel major plane surfaces serving respectively and interchangeably as a major seating face and a clearance face of the insert;
• a pair of opposed plane parallel minor edge surfaces perpendicular to said major surfaces and serving respectively and interchangeably as a cutting face and an edge seating surface of the insert;
• and
• a hole passing through the insert between said major surfaces and countersunk from each to receive a countersink headed retainer to secure the insert to the cutter;
• said minor edge surfaces each having therein a pair of grooves each extending the full width of said minor edge surfaces parallel to said major surfaces adjacent thereto with a plane mid-portion of said edge surface between said pair of grooves;
• said major plane surfaces having thereon adjacent to each of said minor edge surfaces a narrow land making an acute exterior angle with said major plane surface and intersecting the adjacent groove of the adjacent minor edge surface at an acute angle to form therewith straight and parallel cutting edges along at least two opposite sides of each of said major plane surfaces.