The theoretical findings suggest that a reduction in the surface roughness spectrum at the ball end mill facilitate a reduction in the compressive stress on the tool by 44%, 75% and 94% when the surface roughness reduces from 0.8 to 0.6 µm, 0.8 to 0.4 µm and 0.8 to 0.2 µm respectively. The actual magnitude of compressive stress may not be accurate as thermal effects are likely to predominate at the tool-work interface. However, a reduction trend of tool-work interface stress is well expected.
The preliminary AFM polishing results indicate that the surface finish was improved from Ra =0.
215 ?m, Rt =1.29 ?m to Ra =0.157 ?m, Rt =1.16 ?m respectively between 0 to 60 minutes of polishing time. Throughout the process, other polishing conditions such as: Media pressure, media concentration, Media: SiC powder + elastomer were maintained same. However, the material removal mechanism is more towards series of micro-indentations and micro-fracture. It is critical to meet the ductile regime conditions to improve the surface finish which can be achieved by applying controlled force on the WC ball nose end mill.
The media should be focussed to a small critical depth which can be calculated using the following equation 3 for establishing the brittle-ductile transitions. [3]
(3) where, Kc =Fracture toughness, 9.8 MPa.(m)1/2; E = Young’s modulus, 600 GPa; H = Hardness, 17.45 GPa for WC. It should be noted that K10 grade WC was assumed for the above properties of the WC ball nose end mill. By using above values the critical depth was calculated to be 1.62 ?m. Critical depth of cut can be indirectly controlled by varying the different process parameters in AFM process.
Counter check on transition from brittle to ductile can be made sure by comparing the applied and required force. In general, required shear stress to move a dislocation was calculated by using Peierls stress equation 4. is given below [3]
(4) Where, G = Elastic shear modulus (N/m2) 250 GPa; ? = poisson ratio 0.2 and for brittle materials such as WC.
By substituting the above material properties, ?c was calculated to be 23.72 MPa, for WC and it was compared with the AFM pressure which is 7.5 MPa. Therefore, the AFM pressure was found to be below the expected values for the WC ball nose end mill polishing. .
In this experimentation, Abrasive flow finishing machine (AFFM -150 D) as shown in the figure 4, is used for polishing the ball nose end mills. The process involves extruding an Abrasive filled semisolid media through a intricate parts of workpiece. The clamping of the work holding fixture was done by applying hydraulic pressure between two vertically opposed media cylinder and these cylinders move the media to and fro vertically through the workpiece(s). Two cylinder strokes i.e, the lower cylinder and the upper cylinder, forms one process cycle. The concentration of media and abrasive are in the weight ratio of 1:1.
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