CHAPTER 5 FEM SENSITIVITY ANALYSIS OF SHEAR FRICTION

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...
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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,

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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.

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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

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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

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-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

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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

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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

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■ 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

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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%.

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■ 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

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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

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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.