In order to obtain the specific wear laws of the end cutting edge, in this section the end-milling experiments of CFRP are conducted. By micro-observing the end cutting edge each time after it cutting for a certain distance, the wear extent of each wear form is quantitative characterized and summarized, so that the wear laws and mechanisms can be obtained.
3.1 Experimental setup for end-milling
In the study, T800 multi-directional CFRP workpieces with the stacking-sequence of (-45/0/45/90)s are used. The K44 cement carbide tool is used whose specific structural parameters are listed in Table 1.
Parameter
|
Value
|
Table 1
Structural parameters of tool
Diameter (mm)
|
10
|
Minor cutting edge angle (°)
|
2
|
Helix angle (°)
|
25
|
Rake angle of end cutting edges (°)
|
25
|
Clearance angle of end cutting edges (°)
|
7
|
Second clearance angle of end cutting edges (°)
|
14
|
The experiments are conducted on the Mikron HSM500 machining center, as shown in Fig. 7, and the machining parameters are set as listed in Table 2. Each time after the tool cutting for a certain distance, the end cutting edge is micro-observed in order to measure the wear extent, and at the same time the machined surface quality is measured for analyzing the influence of the tool wear which will be discussed in Section 4. The certain distance is defined as the cutting interval length in this study. In order to accurately acquire the initial wear laws of the tool’s corner, the first nine cutting interval distance is set as 25mm, while the one after that is set as 50mm.
Parameter
|
Value
|
Table 2
Machining parameters for end milling experiments
Cutting speed Vc (m/min)
|
251
|
Feed per tooth fr (mm/r)
|
0.06
|
Axial depth of cut ac (mm)
|
1.2
|
Radial depth of cut ae (mm)
|
5
|
3.2 Quantitative characterization and law analysis on end cutting edge wear
According to the analysis in the Section 2, the minor cutting edge angle leads to the rapid wear of the tool’s corner, as the micro-observing image shown in Fig. 8. Since the corner wear directly leads to the increase on the length of the end cutting edge involved in cutting process, which facilitates the wear on the cutting edge and the flank and thereby influences the machining quality of the bottom surface of the blind slots, the effects of the corner wear can be considered as an indirect-effect on the surface quality [14]. Hence, in this study the wear laws of the cutting edge and the flank are emphatically analyzed.
3.2.1 Wear laws of cutting edge
It can be found from Fig. 10 that the changing trend of the cutting edge radius is not monotone, but presents the changing laws that keeping stable at first (cutting distance from 0 to 50mm) and then fluctuating (cutting distance further than 50mm). This changing trend relates to the three wear forms of the end cutting edge analyzed in Section 2. Namely, at the early stage of tool wear, the main wear form is the corner wear. Since the length of the end cutting edge involved in cutting process is relatively small, the worn part of the cutting edge has not reached the position marked in Fig. 9(a), so the change of the cutting edge radius is not obvious [15, 16]. On the other hand, the small contact length of the cutting edge on the unmachined surface cause the very rapid wear of the part of edge involved in cutting, which turns the tool’s corner into the arc shape, as Fig. 9(b) shows.
With the wear of the tool’s corner, the length of the end cutting edge involved in cutting process is increasing. At this time, the part of end cutting edge involved in cutting will suffer the wear both on the cutting edge and the flank. First, the cutting edge radius will increase rapidly due to the severe friction of the strong abrasive carbon fibers on the cutting edge, causing the blunting of the cutting edge and thereby decline the cutting effect on the fibers. These ineffective-cut fibers will spring back after the tool moving away, which will rub the flank of the end cutting edge and cause the flank wear. But since the flank is wear, the flank angle increases, which in turn decreases the cutting edge radius. Taken together, in the end-milling process of the CFRP, the cutting edge of the end cutting edge will be under two opposite effects. One is the abrasive effects derived from the fibers, the other one is the sharpening effects derived from the flank wear. Hence, when the cutting edge wears to some extent, namely after the cutting distance reaching 50mm as shown in Fig. 10, the cutting edge radius will fluctuate with the increase of cutting distance.
In addition, according to the analysis in Section 2, except for the rub from the spring back of the ineffective-cut fibers, the chip which is brought into the narrow space between the flank and the machined surface will also lead to the flank wear. Since the chip is continuously formed in the end-milling process of CFRP, the flank wear induced by the rub of chip is on going, so the cutting edge is sharpening all the time during the end milling process [15, 17]. Therefore, the cutting edge radius is decreasing overall, as shown in Fig. 10.
On the other hand, with the decrease of the cutting edge radius and the continuous wear on the flank, the stiffness of the cutting edge will decline, which leads to the tipping of cutting edge, as shown in Fig. 11.
3.2.2 Wear laws of flank
In addition to the measurement of the wear extent of the cutting edge, since the initial flanks gully-like striations after the action of the grinding wheel, the worn flank does not have a gully-like streak and shows a bright band. The wear of the flank is also quantitative characterized through the micro-observation. As Fig. 12 shows, the flank wear width can be calculated through Eq. (1), just
$$VB=(20 \times L)/0.0322$$
1
In which, 20 is the real length of the ruler, while 0.0322 is the measuring length of the ruler in the figures. L is the measuring length of the flank wear width. The units of these values in Eq. (1) are all micrometer. By this way, the changing law of the flank wear width with the increase of the cutting length is summarized in Fig. 13.
According to the results in Fig. 13, it is obvious that the overall changing trend of the VB is increasing, which is consistent with the results in Part 3.2.1 and the analysis in Section 2. In addition, it can also be found that with the same increasement of the cutting distance, the increasement of the VB varies. Namely, at the early stage of the flank wear, the wear is very rapid, as segment AB shown in Fig. 13. But when the cutting distance exceeds 75mm, the wear rate of the flank appears to be fast and slow alternant, as segment BC. At the late stage of the wear, the VB nearly keep constant.
The changing laws of VB above can be explained through combining the wear laws of the cutting edge and the two origins of the flank wear. At the early stage of the wear, due to the small contact length of the cutting edge on the unmachined surface, the wear is rapid. With the process of cutting, as the tool’s corner becomes blunt, the length of the cutting edge involved in cutting increases, which facilitates the cutting edge wear and the flank wear, as analyzed above. At this time, the cutting edge radius fluctuates significantly which in turn influences the wear rate of the flank. That is, when the cutting edge radius is relatively small, the tool is sharp and the most of the fibers will be cut effectively. In this situation, the flank wear mainly derives from the rub of the chip, so the wear rate is small. However, with the cutting edge radius increasing due to the severe rub from the fibers, the cutting effects on the fibers are declining and thereby the origins of the flank wear are not only the abrasion of the chip, but also the rub of the spring-back ineffective-cut fibers. In this situation, the wear rate of the flank is relatively high, leading to the sharpening trend of the cutting edge. Hence, the wear rate of the flank appears to be fast and slow alternant.
At the late stage of the tool wear, since the cutting edge radius is rather small according to Fig. 10, the main source of the flank wear is the rub of chip. Because the length of the cutting edge involved in cutting is very large, the contact area between the flank and the chip is large enough that the flank is rubbed evenly and mildly. But on the other hand, the large contact area with the chip facilitates the adhesion of the chip on the flank under the adhesive effect of the softening resin [7, 16, 18], as shown in Fig. 14.