A ceramic catalytic converter is made by first creating an electrode by cutting space apart holes through an electrode block, threading an electrode wire through each hole in the block and performing wire cutting electrical discharge machining to form a cutting surface with a plurality of cell walls converging at rounded corners. The electrode is used to cut an extrusion die, a feedhole is provided in the die (preferably connected to a corner of the cell), and ceramic material is extruded through the extrusion die to form an extruded substrate having a plurality of open cells defined by adjacent walls converging in a rounded corner to form the ceramic catalytic converter. The rounded corner cell walls allows a more uniform deposit of washcoat and precious metals and reduces the amount of precious metals necessary in the ceramic catalytic converter.