【正文】 thin,hard,sheets. B For medium hardness low and medium carbon steels in the annealed condition. C For material ＞ than Rockwell‘ B’100 In terms of general machining pratice,low material hardness enhances productivity,since cutting speed is offten selected based on material hardness(the lower the hardness, the higher the speed).Tool life is adversely affected by an increase in workpiece hardness,since the ctting loads and temperatures rise for a specific cutting speed with part hardness, thereby reducing with toll life .In drilling and turning ,the added cutting temperature is detrimental to tool life ,since it produces excess heat causing accelerated 10 dege milling,increased material hardness produces higher impact loads as inserts enter the cut,which often leads to a premature breakdown of the cutting edge. Yield Strengh:Tensile test work is used as a means of parison of metal material tests can establish the yield strengh ,tensile strengh and many other conditions of a material based on its heat addition,these tests are used to pare different workpiece tensile test involves taking a cylindrical rod or shaft,and pulling it from opposite ends with a progressively larger force in a hydraulic machine .Prior to the start of the test ,two marks either two or eight inches apart are made on the rod or the rod is systematically subjected to increased loads,the marks begin to move farther material is in the socalled ‘elastic zone’when the load can be removed form the rod and the marks return to their initial distance apart of either two or eight the test is allowed to proresss,a point is reached where ,whenthe load is removed the marks will not return to their initial distance this point,permanent set or deformation of the test specimen has taken 2 shows how yield strength is measured. Figure 2 Yield strength is measured by pulling a test specimen as shown. Yield strengh is measured just prior to the point before permanent deformation takes strengh is stated in pounds per square inch(PSI) and is deternined by dividing the load just prior to permanent deformation by the cross sectional area of the test material property has been referred to as a condition ,since it can be altered during heat part hardness produces an increase in yield strengh and therefore,as a part bees harder,it takes a larger force to produce permanent deformation of the should not be confused with fracture strengh,cracking or the actual breaking of the material into pieces,since these properties are quite different and 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 wh