The microhardness of the machined surface was measured by MH- 6 microhardness tester. During the test, the pressure load was 100g, and then the pressure was kept for 10s. The microhardness is calculated according to the grip = 0.1891XF/D2, where f (n) is the load and d (mm) is the average length of the two diagonals of the quadrilateral indentation. Under each set of parameters, 10 points are selected for measurement, and one maximum value and one minimum value are removed from the measurement results, and then the average value is taken as the final result.
The picture shows the surface microhardness of PCBN tool when milling vermicular graphite cast iron. As can be seen from the figure, the surface microhardness increases with the increase of feed rate, and the microhardness increases only slightly when the feed rate increases from f=0.05mm/tooth to f=0.1mm/tooth and from f=0.15mm/tooth to f=0.2mm/tooth. Although the microhardness increases greatly from f=0.1mm/tooth to f=0.15mm/tooth, this phenomenon is consistent with the analysis results of cutting force. In the case of small feed (f=0.05mm/tooth and f=0.1mm/tooth), the microhardness increases obviously with the increase of cutting speed, this is because the thermal softening effect is not obvious under the condition of small feed, and the cutting force increases with the increase of cutting speed, which aggravates the plastic deformation of the machined surface and leads to the increase of microhardness. Under the condition of large feed (f=0.15mm/tooth and f=0.2mm/tooth), the microhardness changes with the cutting speed, showing a trend of first increasing, then decreasing, and then increasing. The reason for the first increase is that the cutting force increases, which aggravates the deformation of the machined surface, and then decreases because the cutting temperature increases, and the thermal softening effect resists the strain rate strengthening effect obviously. The surface microhardness decreases and increases again because the tool wear increases and the cutting force increases, which leads to the increase of microhardness.
The higher the microhardness value, the better the wear resistance of the machined surface, but the impact resistance decreases, which is easy to produce cracks and fatigue damage. In a word, in order to obtain small microhardness and high cutting efficiency, the appropriate cutting parameters are v=1200m/min, f=0.15mm/tooth or f=0.2mm/tooth.