When the machine tool started to extract, a pinching effect occurred in the root section of the spiral flute. It was investigated and confirmed that the pinching started at the root section. Therefore, it had obvious meaning to choose the size of end mill. When the size of end mill is larger, the friction force with the workpiece increases, which imposes a greater load on the cutting edge, so wear damage of the cutting edge becomes serious. The cutting edge can bend and chip easily after the cutting of many times, and then crack occurred. For this reason, the concrete cutting process limit was the largest size of end mill. However, the root section of spiral flute was relatively large, so the increase of friction force between the root section of spiral flute and workpiece is relatively small. The root section of spiral flute had the main part of the friction force. It can be proved that wear damage of cutting edge was not serious and the length of crack was limited as shown in Figure 9. Therefore, a very small size of end mill can be used for concrete cutting.
Figure 4 to Figure 9(a) show the evolution of crack, Figure 9(b) to Figure 12 show the crack's shape and grow direction, and Figure 13 shows the complete fracture analysis. The evolution of crack, crack shape and its direction are mainly related to the roughness of the fracture surface and the contact surface. Figure 14 shows the engineering pictures of the fracture surface and its morphology. When the protrude tip of end mill strictly sticks with the notch, the main force acts on the notch to load on the steel ball. As shown in Figure 5, the steel balls were distributed evenly in the position a and b. It is found that the diameter of steel ball was 6mm. It is a sphere, so it has little elasticity. The protrude tip of end mill pressed it in the position c. There are several steel balls distributed in the position c. It is a hexagonal pyramid, so it can be found that it has obvious elasticity. The steel ball can locate in the position c for a while. The steel ball is pressed on the steel pad and then it will slide forward or backward in the notch. As shown in Figure 4, when the protrude tip of end mill indexes and rotates, the steel ball will move along the guide pin. d2c66b5586