The University of Alabama at Birmingham, in cooperation with the American Foundry Society, companies across North America, with support from the US Department of Energy, is conducting a project to develop an understanding of the factors that control the machinability of cast gray and ductile iron. Differences of as much as 500% have been found in machinability have been observed at the same strength. The most machinable irons were those with a high cell counts and few carbonitride inclusions. Additions of tin and copper can be added to both gray and ductile iron to stabilize the pearlite, but excessive additions (above those required to produce the desired pearlite content) degrade the machinability.