【正文】 11 underlated to the current subject. By definitin,a material with high yield strengh (force required per unit of area to create permanent deformation) requires a high level of force to initiate chip formation in a machining implies that as a material’s strength yield increases,stronger insert shapes as well as less positive cutting geometries are necessary to bat the additional load encountered in the cutting hardness and yield strength increase simultaneously during heat ,materials with relatively high yield strengths will be more difficult to machine and will reduce toll life when pared to materials with more moderate strengths. Tensile strength:The tensile strength of a material increase along with yield strength as it is heat treated to greater hardness material condition is also establishde using a tensile strength (or ultimate strength) is defined as the maximum load that results during the tensile test,divided by the crosssectional area of the test ,tensile strength,like yield strength ,is expressed in value is referred to as a material condition rather than a property,since its level just like yield strength and hardness,can be altered by heat ,based on the material selected,distinct tensile and yield strength levels exist for each hrdness reading. Just as increased yield strength implied higher cutting forces during machining operations,the same could be said for increased tensile strength. Again,as the workpiece tensile strength is elevated,stronger cutting edge geometries are required for productive machining and acceptable tool life. Properties of Work Material Physical proterties will include those characteristics included in the individual material groups ,such as the modulus of elasticity ,thermal expansion and work hardening. Modulus of Elasticity :the modulus of elasticity can be determined during a tensile test in the same manner as the previously mentioned conditions .However ,unlike hardness ,yield or tensile strength ,the modulus of elasticity is a fixed material property and ,therefore,is unaffected by heat treatment .This particular property is an indicator of the rate at which a material will deflect when subjected to an external force .This property is stated in PSI and typical values are several million PSI for metals .A 2”x4”x8” 12 beam supported on either end ,with a 200 pound weight hanging in the middle ,will sag 17 times more than a beam of the same dimensions made out of steel and subjected to the same load .The difference is not because steel has a modulus of elasticity which is 17 times greater than wood . General manufacturing practice dictates that productive machining of a workpiece material with a relatively moderate modulus of elasticity normally requires positive or highly positive raked cutting geometries .Positive cutting geometries produce lower cutting enhanced on elastic material using these types of tools . Sharp positive cutting edges tend to bite and promote shearing of a material ,while blunt negative geometries have a tendency to create large cutting forces which impedd chip formation by severely pushing or deflecting the part as the tool enters the cut . Thermal Conductivity: Materials are frequently labeled as being either heat conductors or tend to transfer heat from a hot or cold object at a high rate ,while insulators impede the flow of heat .Thermal conductivity is a measure of how efficiently a material which has a relatively high thermal condicitivity would be considered a conductor ,while one with a relatively low level would be regarded as an insulstor . Metals which exhibit low thermal conductivitis will not dis sipate heat freely and therefore ,during the machining of these maerials ,the cutting tool and workpiece bee extremely hot . This excess heat accelerates wear at the cutting edge and reduces tool life .The proper application of sufficient amounts of coolant directly in the cutting zone (between the cutting edge and workpiece) is essential to improving tool life in metals with low thermal conductivities . Thermal Expansion :Many materials ,especially metals ,tend to increase in dimensional size their temperature rises .This physical property is referred to as thermal expansion .The rate at which metals expand varies , depending om the type or alloy of material under conderation .The rate at which metal expands can be determined using the material’s expansion coefficient ,the greater the value of this coefficient ,the more a material will expand when subjected to a temperature rise or contract when subjected to a temperaure reduction .For example ,a 100 inch bar of steel which encounters a 100 degree .Fahrenheit 13 rise in temperature would measure inches .A