The invention discloses an improved tool and inserts for concurrently milling multiple splines on a round (rod) material. Replaceable inserts having teeth formed to have their cutting edges shaped in the arc of a circle are disclosed. The inserts each include at least two sets of rough milling teeth and a fine set of milling teeth. The tool holder shank includes recesses which provide a common or universal unit for receiving various sized groupings, each comprising an insert, a wedge for securing the insert, and a supporting member for the insert.