Characterization of smearing patterns in ball nose end milling process

Very shallow depth of cut are used in ball end milling finishing operation in order to minimize the chip thickness, therefore reducing the surface roughness. When the chip dimension reaches a certain thickness threshold (the minimum uncut chip thickness), the cutting dynamic switches from effective material removal to ploughing and smearing. Smearing effects can significantly enhance the surface roughness. Another important factor is the choice of the appropriate tool path, which strongly affect the tool engagement condition and surface roughness. In this work, smearing pattern produced on mould steel by cBN ball end mill, are characterized by means of SEM analysis. The results show that the location of the smearing pattern is strictly connected with the relative direction of cutting speed and feed and step over direction. Furthermore, a comparison with the theoretical chip thickness distribution confirms that the smearing of the material occurs in the area in which the chip thickness approaches to zero.