In order to improve the durability of the cutting tool, feedrate adjusting machining at the corner is considered firstly. Because the commercial CNC (Computerized Numerical Control) system provides limited user settings, changing the feedrate is more suitable. The following measures can be adopted: (1) Change the feed rate at a point 6 mm before each corner, and then the commercial CNC system will complete the deceleration; after the tool passes through the corner, change the feed rate to the normal speed. (2) at the corner of B area, the feed-rate is slowed down to 1/3 of normal speed. At the corner of D area, the feed-rate is slowed down to 1/2 of normal speed. (3) Area C is a slot. Take 2 times for cutting and the axial depth is 2 mm each time. The feed-rate is slowed down to 1/2 of normal speed. Fig. 17b shows a single cavity that has been machined. The total machining time of a single cavity is 88 seconds. Upon checking the flank face of the cutting tool after machining the 18th cavity (Fig. 17c), the cutting lip has been found to be severely worn. Fig. 17d shows the milling force of one of the tool path loops. Compared to the original method (Fig. 16), the abruptly-changing load at the corner decreases, but the maximum load remains large. After the cutting tool exceeds the wear-out failure criterion (VB > 0.3 mm), 22 cavities can ultimately be machined. The number is higher when compared to the original method.

To obtain a better machining, trochoidal-aided machining could be considered. With the straight line area A as a reference and by controlling the average and maximum

engagement angles in trochoidal machining, the calculated results shown in Table 1 are obtained.

A machining method adopting perforative corner trochoid along the medial axis is shown in Fig. 18. The calculation of the parameters of the trochoid is shown in Table 1-

B. In the machining, the trochoid is first run in the middle of the cavity; then, the contour-parallel cutting path is run. Finally, the narrow slot in area C is processed. The total machining time of a single cavity is 113 seconds. The milling force during the process is shown in Fig. 18d. After the cutting tool exceeds the wear-out failure criterion (VB > 0.3 mm), 30 cavities can ultimately be machined. The number is much higher when compared to the original method.

In Fig. 19, a trochoidal path is added at the corner of each ring in the contour-parallel path. The calculation of the parameters of trochoid is shown in Tables 1-B and 1-D. The total machining time of a single cavity is 101 seconds. The milling force during the process is shown in Fig. 19d. The milling force during the process is effectively controlled. After the cutting tool exceeds the wear-out failure criterion (VB > 0.3 mm), 33 cavities can ultimately be machined.

The above four machining methods can be defined as M1, M2, M3 and M4, respectively. The number of cavities machined was 14, 22, 30 and 33, respectively. Obviously, by using the deceleration method (M2), tool life can be prolonged when compared with the original path method (M1), which, however, will be much longer by using methods M3 and M4. Considering that the change of loads at the corner is an important factor influencing tool life, a comparative analysis should be made for these methods. The maximum loads and average loads were observed during the machining

of Corner B when using each of these methods; Fig. 20a shows the results obtained. With M2, both the maximum and average loads decrease to a certain extent when compared to M1; however, both types of load still remain large. This indicates that because the cutting tool has a large amount of contact with the workpiece material in a unit of time, the loads on the tool are still heavy even if the tool has decelerated. However, with M3 and M4 where load optimization has been conducted, both the maximum and average load decrease significantly when compared to M1 and M2; therefore, there is less tool wear and fatigue and the tool life is extended greatly. Further compare M3 and M4; the result indicates that the tool wear of the method M3 is greater and the machining efficiency of the method M3 is much lower. The main reason for this is that the trochoidal track is longer in larger corner processing (the corners of D1 and D2 are close to 80 degrees). Therefore, by optimizing the trochoidal geometry and reducing the length of the trochoidal path, greater process efficiency can be obtained, and later research will be conducted in this aspect.