Machinability can sometimes be difficult to predict and be impacted by different characteristics of the castings. Inoculation, a key step in foundry process, is well-known to be a powerful tool to adjust the characteristics of castings in both grey and ductile irons. Some metallurgical cases were investigated to understand different machinability behaviors in correlation to their metallographic characteristics. Thus, two examples taken from grey iron processes are analyzed to illustrate how inoculation is required to adjust characteristics of iron castings, especially the modification of graphite characteristics and the prevention of micro-shrinkage, both parameters are well-known to have an impact on iron machinability. The study also illustrates the importance of the ratio Mn/S to guarantee the presence of manganese sulfides in the matrix, beneficial for iron machinability.