Machinability of popular automotive casting alloys is determined by (1) alloy chemistry, microstructure and properties, (2) the casting method employed, (3) various treatments that alter the microstructure, cleanliness or strength of the alloy, and (4) metallic and non-metallic impurities. Silicon and other hard phases act as abrasives in a relatively soft alloy matrix and tend to reduce cutting tool life. Copper and magnesium increase alloy hardness, improve machined surface finish and decrease the tendency of an alloy to build up on a cutting tool edge. Sand castings require more machining stock and have a coarser microstructure than either permanent mold castings or die castings and therefore are more costly to machine. Die castings are the least costly to machine. Treatments to refine primary silicon or modify the eutectic silicon morphology substantially improve tool life. Heat treatment, to increase hardness, reduces built-up-edge on the cutting tool and improves the surface finish of the machined part.