Machinability is a term indicating how the work material responds to the cutting process. In the most general case good machinability means that material is cut with good surface finish, long tool life, low force and power requirements, and low cost.
Machinability of different materials
Steels Leaded steels: lead acts as a solid lubricant in cutting to improve considerably machinability.
Resulphurized steels: sulphur forms inclusions that act as stress raisers in the chip formation zone thus increasing machinability.
Difficult-to-cut steels: a group of steels of low machinability, such as stainless steels, high manganese steels, precipitation-hardening steels.
Aluminum: easy-to-cut material except for some cast aluminum alloys with silicon content that may be abrasive.
Cast iron: gray cast iron is generally easy-to-cut material, but some modifications and alloys are abrasive or very hard and may cause various problems in cutting.
Cooper-based alloys: easy to machine metals. Bronzes are more difficult to machine than brass.
Selection of cutting conditions
For each machining operation, a proper set of cutting conditions must be selected during the process planning. Decision must be made about all three elements of cutting conditions,
Depth of cut
There are two types of machining operations:
Roughing operations: the primary objective of any roughing operation is to remove as much as possible material from the work piece for as short as possible machining time. In roughing operation, quality of machining is of a minor concern.
Finishing operations: the purpose of a finishing operation is to achieve the final shape, dimensional precision, and surface finish of the machined part. Here, the quality is of major importance. Selection of cutting conditions is made with respect to the type of machining operation. Cutting conditions should be decided in the order depth of cut - feed - cutting speed.