【正文】 bar of aluminum exposed to the same set of test conditions would measure inches .In this case , the change in the aluminum bar length was nearly twice that of the steel bar .This is a clear indicition of the significant difference in thermal expansion coefficient between these materials . In terms of general machining practice ,those materilas with large thermal expansion coefficient will make holding close finish tolerance extremely difficult ,since a small rise in workpiece temperature will result in dimensinal change .The machining of these types of materials requires adequate coolant supplies for thermal and dimensional stability .I n addition,the use of positive cutting geometries on these materials will also reduce machining temperatures . Work Hardening :Many metals exhibit a physical characteristic which produces dramatic increase in hardness due to cold work .Cold work involves changing the shape of a metal object by bending ,shaping,rolling or forming .As the metal is shaped ,internal stresses develop which act to harden the part .The rate and magnitude of this internal hardening varies widely from one material to another .Heat also plays an important role in the work hardening of a material .When materials which exhibit work hardening tendencies are subjected to increased temperature ,it acts like a catalyst to produce higher hardness levels in the workpiece . The machining of workpiece materiaks with work hardening properties should be undertaken with a generous amount of coolant .In additon ,cutting speed should correlate specifically to the material machined and should not be recklessly altered to meet a production rate .The excess heat created by unusually high cutting speeds could be extremely by detrimental to the machning process by promoting work hardening of the workpiece .Low chip thicknesses should be avoided on these materials ,since this type of inefficient machining practice creates heat due to friction ,which produces the same type effect mentioned earlier .Positive low force cutting geometries at moderate speeds and feeds are normally very effetive on these materials . Machining The term ‘machinability’is a relative measure of how easily a material can be machined when pared to 160 Brinell AISI B1112 free machining low carbon steel .The 14 American Iron and Steel Institute (AISI) ran turning tests of this material at 180 surface feet and pared their results for B1112 against several other materials .If B1112 represents a 100% rating ,then materials with a rating less than this level would be decidedly more difficult to machine ,while those that exceed 100% would be easier to machine . The machinability rating of a metal takes the normal cutting speed ,surface finish and tool life attained into consideration to arrive at a final machinability rating .The fllowing chart shows a variety of materials and their specific machinability Rating: MaterialHardness Machinability Rating 6061T Aluminum 190% 7075T Alumimum 120% B1112 Steel 160BHN 100% 416 Stainless Steel 200BHN 90% 1120 Steel 160BHN 80% 1020 Steel 148BHN Machinability The factors affecting machinability have been explained 。 Some workpiece will not ‘ take a good finish’as well as others .The fundamental reason for surface roughness is the formation and sloughing off of parts of the builtup edge on the tool . Soft ,ductile materials tend to form a builtup edge rather easily .Stainless steeels ,gas turbine alloy ,and other metals with high strain hardening ability ,also tend to machine with builtup edges .Materials which machine with high shear zone angles tend to minimize buileup dege effects .These include the aluminum alloy ,cold worked steels ,freemachining steels ,brass , and titatnium allys .If surface finish alone is the chosen index of machinabiity ,these latter metals would rate higher than those in the first group . Chip Form :There have been machinability ratings based on the type of chip that is formed during the machining operation .The machinability might be judged by the case handling and disposing of chips .A material that produces long stringy chips would receive a low rating ,as would one which produces fine powdery chips .Materials which inherently from nicely broken chips ,a half or full turn of the normal chip helix ,would receive top rating . Chip handling and disposal can be quite expensive .Stringy chips are a menace to 16 the operator and to the finish on the freshly machined surface .However ,chip formation is a function of the machine variables as well as the workpiece material ,and the rationgs obtained by this method coould be changed by provision of a suitable chip breaker .