Lecture 23 Machinability

Definition by ‘Woldman and Gibbons’: “The most machinable metal is one which will permit the fastest removal of the largest amount of metal per tool grind (without re-sharpening of the cutting tool) with satisfactory surface finish”

It is the material property that indicates how machinable a material is an attempt to express this property quantitatively by a single parameter has eluded the tool engineers for a year. It is considered that machinability is a property mainly concerned with the work material but is also affected by the cutting speed, dimension of cut, tool material, nature of tool work engagement, tool geometry, coolant, the rigidity of the machine tool and any particular criterion to evaluate machinability. The work material variable that influence machinability are its chemical composition, material property (such as yield strength, hardness, work hardening characteristics, toughness etc.), metallurgical characteristics (microstructure, grain size, heat treatment, inclusions etc.), shape and dimensions of the workpiece and its condition (cast and forged).

 

Evolution of Machinability

No direct method of determining the ease and difficulty of removing metal and therefore use of indirect methods have been made to evaluate machinability of metal. Some of the methods suggested: -

        i.            Tool life / Tool wear or the cutting speed for a given two life.

       ii.            Specific power consumption

     iii.            The magnitude of the cutting force generates during machining 

    iv.            Quality of the machined surface

       v.          Dimensional stability of the finished work

     vi.            Shear angle or chip thickness ratio

   vii.            Ease of chip disposal

   viii.            Chip hardness

     ix.            Surface finish, etc.