An apparatus for cutting a window in a casing pipe in a well, having only a single mill with full gage primary cutting inserts of a very hard, highly wear resistant, material. The primary inserts can be protected from damage by forming each blade on the mill with an outside edge profile having a shallower angle than the angle of the whipstock which is used to guide the mill through the casing, preventing the primary inserts from riding directly on the whipstock. The primary inserts on each blade can be further protected from damage by a row of secondary inserts of a relatively more durable material, arranged behind each row of primary inserts, or by a full diameter buffer section of mill body or blade.