80
CHAPTER 5
FEM SENSITIVITY ANALYSIS OF SHEAR FRICTION
Friction modeling in FEM metal cutting has been recognized as one of the most important and challenging tasks facing researchers engaged in modeling of machining operation as reported by ven Luttervelt et al (1998). Recently, Guoqin Shi et al (2002) employed the coulomb friction to study the effect of different frictional values and reported that the maximum temperature, shear angle and cutting force depend on frictional value. Therefore, a sensitivity analysis is performed to select the appropriate shear friction value for grooved tools in FE simulations.
5.1
SHEAR FRICTION VALUE
Yung-Chang Yen et al (2003 a) assumed shear friction value of 0.7 for positive rake angle tool and 0.3 for negative rake angle tool in FE simulations. Taylan Altan (2002) and Taylan Altan and Eugene Yen (2003) assumed 0.6 shear friction value in their FEM study. Tugrul Ozel (2003) and Eugene Y-C Yen et al (2003) assumed shear friction value 0.5 for FEM study. No one has suggested exact shear friction value for FE simulations. In this study, 0.4, 0.5 and 0.6 shear frictional values were selected to analyse the effect of friction values in terms of cutting force, thrust force, chip thickness and shear angle. The sensitivity analysis was performed for all the five tools at 0.16 mm/rev feed rate and cutting speed of 200 m/min.
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5.2
CUTTING FORCES
5.2.1
FE predicted cutting forces
The cutting forces versus the distance of tool travel for different tools of different shear friction values are shown in Figures 5.1(a-c) - 5.5 (a-c). At the beginning of the tool-workpiece engagement, the cutting forces (cutting force and thrust force) increases monotonically. Thereafter, small fluctuations in cutting force and thrust force were observed. This state is called steady state. The average values of the steady state cutting force and thrust forces are given in Table 5.1 and 5.2. The small fluctuations in the cutting forces are due to successive separation of the nodes in front of the tool tip. Every time, a node is split, the forces on the tool are suddenly reduced and then gradually increased until the next node is released. Most of the researchers, Lin and Lin (1992), Zhang and Bagchi (1994), Shih (1996), Oblikawa et al (1997), Movahnedy et al (2000), Liu and Guo (2000), Guoqin Shi et al (2002), Mamalis et al (2001) and Vahid Kalhori (2003) calculated the average values of the cutting forces in steady state in their simulations.
5.2.2
Comparison of experimental and FE predicted cutting force
Figure 5.6 shows the comparison of experimental and FE simulated cutting force for 0.4, 0.5 and 0.6 shear friction values and the values are given in Table 5.1. It can be noted from Figure 5.6 that cutting force increases as shear friction values increase (0.4-0.6) in all the tools. The deviation between experimental and FE simulated cutting force in 0.4 shear friction is 10-18%. In 0.4 shear friction value the FE simulated cutting force value is lower than the experimental cutting force in all the tools. FE simulated cutting force for Tool 1,
82
4 and 5 are good agreement with the experimental cutting force in 0.5 shear friction value and the deviation is approximately 5-7%. In case of Tool 2 and 3 the deviation between experimental and FE simulated cutting force for shear friction value 0.5 is marginally higher than the 0.6 shear friction value.
FE simulated cutting forces for Tool 1, 4 and 5 in the shear friction value 0.6 are higher than the shear friction value 0.5 cutting force. The deviation between experimental and FE simulated cutting force in 0.6 shear friction value is approximately 7-17%.
5.2.3
Comparison of experimental and FE predicted thrust force
Figure 5.7 shows the comparison of experimental and FE simulated thrust force for 0.4, 0.5 and 0.6 shear friction values and the values are given in Table 5.2. The FE simulated thrust force increases as shear friction value increases (0.4-0.6) in all the cutting tools (Figure 5.7). Higher deviation was observed in the 0.4 shear friction value for the experimental and FE simulated thrust force (approximately 36-40%). The deviation between experimental and FE simulated thrust force in the shear friction value 0.5 is 19-37%. The lower deviation is observed between the experimental and FE simulated thrust force in the 0.6 shear friction value (approximately 11-24%). In higher frictional values, FE simulated thrust forces are in good agreement with the experimental values. But the formation chip, cutting force, chip thickness and shear angle deviate drastically.
83
LO
oo o
Oo Lf)
oo ro
co
o in
o o o
Cutting forces (N)
Cutting force......... Thrust force
to
Tool displacement (mm)
oi
o
'-v j
iq
cb
in
CM
a>
co
CD
c\i
o o
CO
cn
Cutting forces (N)
Cutting force......... Thrust force
Tool displacement (mm)
Cutting forces (N)
Cutting force......... Thrust force
Tool displacement (mm)
Figure 5.1 (a-c)
FE simulated cutting forces versus tool displacement for Tool 1 a) m = 0.4 b) m = 0.5 c) m = 0.6 shear friction values
84
Cutting forces (N)
Cutting force............ Thrust force
(a)
Cutting forces (N)
Cutting force.......... Thrust force
Cutting forces (N)
Cutting force............ Thrust force
Figure 5.2 (a-c)
(c) FE simulated cutting forces versus tool displacement for Tool 2 a) m = 0.4 b) m = 0.5 c) m = 0.6 shear friction values
85
-Cutting force Cutting forces (N)
• Thrust force
03
ro
o o
o
bo
Cutting forces (N)
Cutting force.......... Thrust force
Tool displacement (mm)
(b)
Cutting forces (N)
Cutting force...........Thrust force
(c)
Figure 5.3 (a-c)
FE simulated cutting forces versus tool displacement for Tool 3 a) m = 0.4 b) m = 0.5 c) m = 0.6 shear friction values
86
Cutting forces (N)
Cutting force......... Thrust force
(a)
CM
Cutting forces (N)
Cutting force......... Thrust force
(b)_________________
CM
Cutting forces (N)
Cutting force......... Thrust force
Tool displacement (mm) (C)
Figure 5.4 (a-c)
FE simulated cutting forces versus tool displacement for Tool 4 a) m = 0.4 b) m = 0.5 c) m = 0.6 shear friction
87
values
ooin oo
cn
o oo
Cutting forces (N)
-Cutting force......... Thrust force
o o
1.6
2.9
Tool displacement (mm)
■ Cutting force
■ Thrust force
0.9
2.8
O O
cn
Cutting forces (N)
(a)
ooo ooto o o
1.9
3.5
Tool displacement (mm)
(b)
o
o
CV J
CD
GO
CO
7*4
Cutting forces (N)
Cutting force......... Thrust force
Tool displacement (mm)
(c) Figure 5.5 (a-c) FE simulated cutting forces versus tool displacement for Tool 5 a) m = 0.4 b) m = 0.5 c) m = 0.6 shear friction values
88
■ Experiment im0.4im0.5am 0.6 g- 1500
|
.p
1000
D>
c
500
E
o Tool 1
Tool2
Tool3
Tool4
ToolS
Cutting tools
Figure 5.6
Comparison of experimental and FE simulated cutting force for 0.4, 0.5 and 0.6 shear friction values
Table 5.1
Experimental and FE simulated cutting force for 0.4, 0.5 and 0.6 shear friction values
Experimental Cutting cutting force tools (N)
FE simulated cutting force(N) Shear frictional Values m 0.4 m 0.5 m 0.6
% of deviation Shear frictional values m 0.4
m 0.5
m 0.6
Tool 1
1140
1026
1198
1215
10.0
5.1
6.6
Tool 2
1280
1051
mi
1167
17.9
13.3
8.8
Tool 3
1200
1002
1078
1115
16.5
10.2
7.1
Tool 4
1180
897
1138
1201
16.9
5.4
11.2
Tool 5
1070
1056
1154
1251
1.3
7.9
16.9
12.5
8.4
10.1
Mean
89
i Experiment ■ m 0.4 ■ m 0.5 I m 0.6 z O O o *— CO
800 -I 600 400
□ 200 0 -1
k. .C i-
Tool 1
Tool2
Tool3
Tool4
Tool5
Cutting tools
Figure 5.7
Comparison of experimental and FE simulated thrust force for 0.4,0.5 and 0.6 shear friction values
Table 5.2 Experimental and FE simulated thrust force for 0.4, 0.5 and 0.6 shear friction values
Experimental Cutting thrust force tools (N)
FE simulated thrust force(N) Shear frictional Values m0.4 m 0.5 m 0.6
% of deviation Shear frictional values m 0.4
m 0.5
ni 0.6
Tool 1
610
388
496
480
36.4
18.8
21.4
Tool 2
410
256
290
339
37.6
29.4
17.4
Tool 3
580
374
420
442
35.5
27.7
23.7
Tool 4
520
124
327
414
76.1
37.2
20.4
ToolS
520
314
421
462
39.5
19.1
11.2
Mean
45.0
26.4
18.8
90
5.3
CHIP THICKNESS
Deformed chip thickness value is very important in metal cutting to calculate the shear angle. Figure 5.8 shows the comparison of experimental and FE simulated chip thickness for 0.4, 0.5 and 0.6 shear friction values and the corresponding chip thickness values are given in Table 5.3. It can be noted from Figure 5.8 that the chip thickness value increases as shear frictional values increase in all the tools. FE predicted chip thickness values are lower then the experimental thickness values in the shear friction value of 0.4. The higher deviation value of 10-22% observed between experimental and FE simulated chip thickness in the shear friction value of 0.4. in Figure 5.8. It can be observed that, FE simulated chip thickness values are in good agreement with the experimental chip thickness values in the shear friction value of 0.5 and deviation is approximately 3% (except Tool 2). This deviation is lower than the 0.4 and 0.6 shear friction deviation values. The deviation between experimental and FE simulated chip thickness values in the shear friction value 0.6 is approximately 3-6%.
91
■ Experimental 0 m 0,4 lm 0.5 Hm 0.6
Tool 1
Tool2
Tool3
Tool4
Tool5
Cutting tools
Figure 5.8
Comparison of experimental and FE predicted chip thickness for 0.4, 0.5 and 0.6 shear friction values
Table 5.3
Experimental and FE predicted chip thickness for 0.4, 0.5 and 0.6 shear friction values
Experimental
FE predicted chip
Cutting
chip
thickness (mm)
tools
thickness
Shear frictional values
% of deviation Shear frictional values
(mm)
m 0.4
m 0.5
m 0.6
Tool 1
0.33
0.28
0.34
0.35
15.2
3.0
6.1
Tool 2
0.28
0.31
0.32
0.36
10.7
14.3
28.6
Tool 3
0.31
0.26
0.3
0.32
16.1
3.2
3.2
Tool 4
0.31
0.24
0.32
0.32
22.6
3.2
3.2
Tool 5
0.31
0.28
0.32
0.33
9.7
3.2
6.5
14.9
5.4
9.51
Mean
m 0.4
m 0.5
m 0.6
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5.4
SHEAR ANGLE
The angle of inclination of the shear plane to the direction of cutting is termed as the shear angle. In metal cutting, the shear angle is considered as a fundamental parameter that defines the plastic deformation and the geometry of the process as reported by Valery R. Marinov (2001 a and b). Shear angle is calculated for the measured chip thickness from experimental and FE simulated by using equation (5.1).
1 -rc cosy
(5.1)
where #
= Shear angle
rc
= Chip thickness ratio
Y
= Tool rake angle
Figure 5.9 shows the comparison of experimental and FE simulated shear angle for 0.4, 0.5 and 0.6 shear friction values and the corresponding shear angle values are given in Table 5.4. Kluft et al (1979) reported that the shear angle is increased by the decrease of the coefficient of friction between the tool rake face and chip. A similar result is observed in Figure 5.9, the highest shear angle values are observed in the 0.4 shear friction value and lower shear angle values are observed in the shear friction value of 0.6. Guoqin Shi et al (2002) pointed out that the shear angle strongly depends on the coefficient of friction. Higher deviation is observed between experimental and FE
93
simulated shear angle values in the shear friction value of 0.4 (approximately 8-22%). In the shear friction value of 0.5 the deviation between experimental and FE predicted shear angle values are 2-4%. This deviation values are lower than the 0.4 and 0.6 shear friction shear angle deviation values. In the shear friction value of 0.6 the deviation between experimental and FE predicted shear angle values are approximately 2-6 % (except Tool 2). Based on the shear angle sensitivity results, the shear friction value 0.5 provides the good agreement between the experiment and FE predicted of shear angle values for all the tools.
ro
O O
—I
K)
Shear angle (deg)
CO
o o io tn o o io u i
04
B1 Experimental Elm 0.4 Elm 0.5 Elm 0.6
Tool2
Tool3
Tool4
Tool5
Cutting tools
Figure 5.9
Comparison of experimental and FE predicted shear angle for 0.4, 0.5 and 0.6 shear friction values
94
Table 5.4
Experimental and FE predicted shear angle for 0.4, 0.5 and 0.6 shear friction values
Experimental Cutting shear angle tools (deg)
FE predicted shear angle (deg)
% of deviation
Shear frictional values
Shear frictional values
m 0.4
m 0.5
m 0.6
m 0.4
m 0.5
m 0.6
Tool 1
0.33
24.9
28.5
24.2
14.5
2.5
4.9
Tool 2
0.28
28.4
26.1
25.5
8.1
10.2
16.2
Tool 3
0.31
26.2
30.2
26.9
15.2
2.7
2.7
Tool 4
0.31
26.2
32.0
25.1
22.1
4.1
2.7
Tool 5
0.31
26.2
28.4
25.5
8.4
2.7
5.1
Mean
13.7
4.4
6.3
5.5
SELECTION OF SHEAR FRICTION FOR FEM STUDY In metal cutting, the FE output results mainly depend on chip-tool
interface frictional conditions. The chip-tool interface friction value affects the FE simulation results drastically. A smaller change in friction value leads more variation in the cutting forces, chip thickness and shear angle. Therefore, the accuracy of the FE simulation results is based on the chip-tool interface friction value. A sensitivity analysis is performed to select the appropriate shear friction value for grooved tools in FE simulations. The 0.4, 0.5 and 0.6 shear frictional values are selected in this study. The shear friction value 0.5 provides the good agreement between experimental and FE simulation of all the tools. Hence, the shear friction value 0.5 was employed in all the FE simulations.