The diffusion opening in the cooling hole of an airfoil is formed by an EDM process in which the outwardly flaring sidewalls of the opening, rather then having surfaces that are approximated to be smooth by having many small ribs formed therein, are formed with relatively few ribs with both longitudinally extending and radially extending surfaces that are substantially greater in dimension than those as normally formed. In this manner, the machining process is simplified and expedited, while at the same time, the cooling efficiency is increased.