Machinability has different meanings for different situations. It is generally desired to produce a satisfactory part at the lowest possible cost.
The machinability characteristics of a material must then somehow relate to the minimum total cost required to produce a satisfactory part. Probably the most difficult system-machinability interaction involves the control and handling of the metal chips. For any type of automatic operation, the chips must be in the form of small, broken pieces rather than a long continuous chip. Chip breaking is generally achieved by causing the chip to curl tightly so that it interferes with either the tool of a portion of the work piece. The interference causes the chip to “uncurl” and fracture. The metallurgy of the casting can effect chip curl and is usually related with the strength and ductility of the metal. Traditional free machining additives reduce the ductility of the metal. This is not to ignore that chip curl can also be enhanced through tool geometry (chip-breakers), depth of cut (metal forming) and through reduced chip-tool friction (lubricants).
From the metals perspective, the higher the strength (hardness) and ductility (bendibility) the less machineable. It is not uncommon to here that a part is too hard when in fact it is really too ductile to give good machining characteristics. Cutting speed can also be related with the thermal conductivity of the cast part in conjunction with the cooling techniques and type of cutting tool.