We have considered one of their factors to find out the mechanism of generating surface roughness, that is, replica on the finished surface by roughness of the cutting edge. In order to estimate the degree of replica we use multiple correlation coefficient, r
i·18 and r
i·12…i-1 ; in Fig. 4 R
1 is the roughess of the edge before cutting, R
8 is that of after cutting and R
2, ……, R
7 are those of the surfaces at six sorts of cutting depth in axial direction. In the results the deeper the surface is cut, the rougher its profile becomes and the degree standardized by roughness of edge is in proportion to their Brinell hardness. While replica is better in oil cutting than in dry and the order for these materials is the folloming ; -brass, copper, mild steel, hard steel, and cast iron. Also cutting depth at maximum value of r
i·12…i-1 is most suitable one for the material because r
i·12…i-1 means stability of generating roughness.
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