In the current study, different cutting tests were carried out to investigate and analyze the effect of the cutting conditions on the machining performance when using self-propelled rotary tools. The workpiece material was hardened steel AISI 4140 (46 ± 2 HRC). A tube shape workpiece was used to achieve homogeneous properties during the heat treatment process. The outer diameter of the workpiece was 100 mm, while the inner diameter was 50 mm, as shown in Fig. 3. Table 1 shows the chemical composition, thermal, and mechanical properties of AISI 4140 steel. The feed rate, cutting speed, and inclination angle were selected as design variables. The depth of cut was 0.2 mm, while the cutting length was 100 mm. A carbide round insert was used, and its outer diameter was 27mm. The rake angle was -5o, while the clearance angle was 5o. Fig. 4 shows the experimental setup of the current study.
Table 1. Chemical composite, thermal, and mechanical properties of AISI 4140
Chemical composition (wt.%): C: 0.38%-0.43%, SI: 0.15%-0.3%, Mn: 0.7%-1%,
Cr: 0.8%-1.1%, Mo: 0.15%-0.25%, Fe: 96.75%-97.84%
Density: 7850 Kg/m3
Young’s modulus (at 25 °C): 198 GPa
Poisson’s ratio (at 25 °C): 0.3
Tensile strength: 729.5 MPa
Yield strength: 379.2 MPa
Specific Heat (at 25 °C): 470 J/kg °C
Thermal Conductivity (at 25 °C): 42.7 W/m °C
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The average surface roughness was used to evaluate the machined surface quality. The average surface roughness was measured using Mitutoyo (SJ.201) portable surface roughness at a cut-off length of 2.5mm. The surface roughness was measured at three different locations, and the average value was calculated and used for the analysis. Mitutoya toolmaker’s microscope (TM-A505B) was used to measure the average flank wear of the insert after each run. The flank tool wear was measured at four different locations on the circular flank face of the insert, and the average value is obtained and used in the analysis. Fig. 5 shows a flow chart for the experimental procedures.
Taguchi’s approach was utilized in the current study to conduct a minimum number of experiments. Three design variables with four levels each were used in the present study. The selected design variables were inclination angle (i), feed rate (f), and cutting speed (V). The design variable and the assigned level for each variable are presented in Table 2. As there are three design variables with four levels each (i.e., 43), the full L64OA orthogonal array should be used. However, a fractional factorial, L16OA orthogonal array was employed to save cost and time [24]. Table 3 shows the 16 experiments of the current study.
Table 2. The levels assignment to the independent design variables
Design variables
|
Level 1
|
Level 2
|
Level 3
|
Level 4
|
A:i(°)
|
5
|
10
|
15
|
20
|
B:f (mm/rev) |
0.1 |
0.15 |
0.2 |
0.25 |
C:V (m/min) |
70 |
127 |
167 |
240 |
Table 3. The design of experiments for the machining runs
Test #
|
Inclination angle levels
|
Feed rate levels
|
Cutting speed levels
|
1
|
1
|
1
|
1
|
2
|
1
|
2
|
2
|
3
|
1
|
3
|
3
|
4
|
1
|
4
|
4
|
5
|
2
|
1
|
2
|
6
|
2
|
2
|
1
|
7
|
2
|
3
|
4
|
8
|
2
|
4
|
3
|
9
|
3
|
1
|
3
|
10
|
3
|
2
|
4
|
11
|
3
|
3
|
1
|
12
|
3
|
4
|
2
|
13
|
4
|
1
|
4
|
14
|
4
|
2
|
3
|
15
|
4
|
3
|
2
|
16
|
4
|
4
|
1
|
Table 4 shows the results of the average flank tool wear and average surface roughness during dry machining with self-propelled rotary tools. The minimum flank tool wear is observed at test 6, where the cutting speed was 70 m/min, the inclination angle was 10°, and the feed rate was 0.15 mm/rev. Besides, test 14 showed the highest flank tool wear, where the cutting speed was 167 m/min, the inclination angle was 20°, and the feed rate was 0.15 mm/rev. In general, the results showed that reducing the cutting speed leads to low flank wear, as expected. Unlike conventional cutting, increasing the feed rate leads to a decrease in the flank tool wear. That could be attributed to the fact that the cutting process becomes more stable at a high feed rate as a continuous chip was noticed.
Regarding the average surface roughness, the results revealed that the variation of the cutting conditions has a corresponding effect on the surface roughness. The optimal surface roughness was obtained at test 13, where the cutting speed was 240 m/min, the inclination angle was 20°, and the feed rate was 0.1 mm/rev. In general, the surface roughness values were relatively low compared to the conventional cutting processes. That could be attributed to the large radius of the round insert compared to the nose radius of traditional tools. Increasing the cutting velocity lead to a reduction in the surface roughness value, as expected. Besides, increasing the feed rate leaded to deterioration of the machined surface due to the increase of the chip load.
Table 4. Average surface roughness and tool wear results
Test
#
|
i(°) |
f (mm/rev) |
V (m/min) |
VB (μm) |
Ra (μm) |
1
|
5
|
0.1
|
70
|
16
|
0.83
|
2
|
5
|
0.15
|
127
|
38
|
1.08
|
3
|
5
|
0.2
|
167
|
20
|
0.78
|
4
|
5
|
0.25
|
240
|
22
|
0.95
|
5
|
10
|
0.1
|
127
|
61
|
1.00
|
6
|
10
|
0.15
|
70
|
3
|
1.13
|
7
|
10
|
0.2
|
240
|
59
|
0.84
|
8
|
10
|
0.25
|
167
|
14
|
0.90
|
9
|
15
|
0.1
|
167
|
51
|
1.18
|
10
|
15
|
0.15
|
240
|
25
|
0.93
|
11
|
15
|
0.2
|
70
|
5
|
1.17
|
12
|
15
|
0.25
|
127
|
40
|
1.48
|
13
|
20
|
0.1
|
240
|
12
|
0.56
|
14
|
20
|
0.15
|
167
|
71
|
0.94
|
15
|
20
|
0.2
|
127
|
51
|
1.11
|
16
|
20
|
0.25
|
70
|
4
|
1.83
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A comparison between the fixed and rotary tools was performed to study the effect of the tool motion on the studied machining responses. Fig. 6 shows the tool wear results for fixed and rotary tools at the best and worst conditions (i.e., test 6 and test 14). The wear of the rotary tool was reduced by 37% at test 14 (where the maximum tool wear occurred) compared to the fixed tool. Besides, at test 6, the tool wear of the rotary tool was reduced by 22% compared to the fixed tool. That could be attributed to the benefits of the tool rotational motion. Fig. 7 shows a schematic of machining using a self-propelled rotary tool. Part of the generated heat at the secondary shear zone (Qsz) goes to the cutting tool (Qtool). A portion of this heat is dissipated by conduction (Qcond), and the large portion is carried away from the cutting zone by the tool motion (Qmotion).Therefore, as a result of the tool motion, every portion of the cutting edge will have a chance to cool down during the disengagement period before engaging again with the workpiece. That will prevent the tool from any expected thermal damages by maintaining the tool temperature within acceptable limits even under dry machining as a result of the self-cooling feature of the rotary tools. In addition, the tool motion allows the tool wear to be distributed over the whole round edge instead of being concentrated at a single point, as occurs in the conventional tools, which dramatically increases the tool life.
Fig. 8 shows surface roughness results for fixed and rotary tools at the best and worst conditions (i.e., test 13 and test 16). Better surface roughness was provided by the fixed tool, especially at the worst condition (i.e., test 16), where the surface roughness of the rotary insert achieved 1.83 µm. That can be due to different possible factors include machining stability, which is significantly affected by the dynamic nature of the rotary tool. Besides, the surface roughness is also affected by the generated marks in the direction of the relative cutting velocity as a result of the tool motion, as discussed in previous work [5].
To study the effect of the inclination angle on the tool wear and the surface quality, a comparison was performed between two cutting tests under certain conditions of cutting speed and feed rate (i.e., v=167 m/min and f=0.15 mm/rev) with two different inclination angles of 5° and 20°, as shown in Fig. 9. It was observed that when using a low inclination angle (i.e., 5°), the chips were collided and pushed into the workpiece surface, as shown in Fig. 9(b). Afterward, the cutting edge crushes the adhered chips, which increases the tool wear, as can be seen in Fig. 9(d). On the other hand, no chips adhesion was observed in the machined surface at 20° inclination angle (see Fig. 9(a)), and accordingly, lower tool wear was obtained compared to the case of 5o inclination angle (see Fig. 9(c)). That can be attributed to the increase in the chip flow angle based on the oblique cutting principles, as confirmed by Yamamoto et al. [25].