At the end of the experiment, the machined surface of the workpiece was observed by Keynes laser microscope (VK-X200K). In the process of observation, it is found that the cutting speed has little effect on the surface morphology, while the feed rate has a great influence on the surface morphology. The topography at each cutting speed (Fig. 1) when the feed rate is 0.05mm/tooth and the topography at each feed rate (Fig. 2) when the cutting speed is 800m/min are taken as examples. It can be seen from figures 1 and 2 that the texture of the machined surface is obviously directional, which coincides with the tool tip trajectory, and there are microcracks and pits formed by the spalling of graphite particles on the machined surface.

As can be seen from figure 1, the surface texture is obvious at low speed, and becomes lighter with the increase of cutting speed. When the cutting speed is 1600m/min, there is a smooth texture, and the change of micro-cracks and pits on the machined surface with the cutting speed is not obvious. As can be seen from figure 2, with the increase of feed, the surface pits increase obviously, and the surface pits are connected to each other, and the surface quality is poor. However, it is noted that with the increase of feed, the surface microcracks decrease. Combined with figure 1, it is inferred that the surface microcracks are caused by graphite particles, and the strength of graphite particles is small. In the process of processing, graphite particles are extruded, deformed or broken, resulting in cracks on the machined surface, and with the increase of feed rate, the extrusion of graphite particles is intensified, graphite particles are broken, and obvious surface pits are formed.

In a word, the change of the machined surface topography with the cutting speed is not obvious, but with the feed rate is obvious. It can be seen that the feed rate is the most important parameter affecting the surface topography.