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CHAPTER 4

APPLICATION OF SEMISOLID LUBRICANTS FOR IMPROVING RAKE

FACE LUBRICATION

4.1 INTRODUCTION

During minimal fluid application, since only a very small quantity of

cutting fluid is used for the dual purpose of cooling and lubrication, some

additional system of lubrication if available, will further improve the cutting

performance. It is reported that semi-solid lubricants can be effectively used during

metal cutting to achieve better cutting performance and it forms an alternative to

the conventional flood cooling techniques (Vamsi Krishna and Nageswara Rao,

2008). There is a great potential for enhancing cutting performance during minimal

fluid application with the aid of solid lubricants. Hence, it was decided to explore

whether the application of semisolid lubricant along with minimal fluid application

can improve cutting performance. An attempt was made to investigate the effect of

a semi solid lubricant such as grease in pure form as well as a mixture with 10%

graphite on the cutting performance during hard turning of AISI 4340 steel with

minimal fluid application and a comparison was made with wet and dry turning

under similar cutting conditions.

4.2 SILICON GREASE AS A SEMI SOLID LUBRICANT

Grease is a semi solid lubricant which is composed of calcium, sodium or

lithium soap base emulsified with mineral or vegetable oils. It is used in high

pressure applications and during metal cutting where liquid lubricants cannot be

retained. Greases are shear-thinning or pseudo-plastic fluids, which undergo

reduction in viscosity under shear. Greases are employed where heavy pressures

exist, where oil drip is undesirable, and/or where the motions of the contacting

surfaces are discontinuous so that it is difficult to maintain a separating lubricant

film in the contact zone. Grease-lubricated bearings have greater frictional

characteristics at the beginning of operation. Under shear, the viscosity drops to

give the effect of an oil-lubricated bearing of approximately the same viscosity as

the base oil used in the grease.

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In this research work, commercially available bearing grease having the

specification LGWA 2 (DIN 51825) was used as a semisolid lubricant in its pure

form and as a mixture with 10% graphite. It is a high load, wide temperature range

bearing grease and being recommended for a wide range of industrial and

automotive applications. Properties of LGWA 2 bearing grease are summarized

below.

Excellent lubrication at peak temperatures up to 220 °C for short

periods.

Effective lubrication in wet conditions

Good water and corrosion resistance

Excellent lubrication under high loads and low speeds

Graphite is widely used as a solid lubricant because of its low cost and

excellent lubricating action on account of its layered structure. Density of graphite

is 2.265 g/cm3 and its Mohs hardness ranges from 1.85 to 1.95. Figure 4.1 shows

the crystalline structures of graphite. The inter-planar spacing, i.e., the distance

between the adjacent interlayer for graphite is 3.35A˚. In graphite, the inter layer

bonding is very weak and one layer slides over the other under the application of

shear loads.

Figure 4.1 Crystalline structure of graphite

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4.3 DEVELOPMENT OF SEMI SOLID LUBRICANT APPLICATOR

A semi solid lubricant applicator was developed for applying silicon grease

at specific locations. Figure 4.2 shows a line sketch of the applicator. It consists of

a cylindrical container (C) with a piston (P) inside which can move forward

against the force of a stabilizing spring (S). When compressed air enters in to the

cylinder through the inlet (I), it forces a certain amount of semisolid lubricant

through the outlet (O) on the lid (L) of the semisolid container (C). The semisolid

lubricant coming out of the outlet (O) moves through the tube (T). A nozzle is

fixed at the free end of the tube which can deliver grease at specific contact zones.

The rate of delivery of the grease can be controlled by the control valve (V). A

relief valve is installed in the circuit to protect the system from accidental

overloads in the event of blocks in the nozzle. Fine adjustment of the rate of flow

of the semisolid lubricant can be achieved by adjusting the spring tension. This is

done by rotating the container lid in the proper direction. When the lid is rotated in

the clockwise direction, the spring gets compressed offering more resistance to the

motion of the piston (P) and thereby reducing the rate of flow of the semisolid

lubricant. Likewise rotation of the lid in the anti-clock wise direction increases the

rate of flow of the semisolid lubricant. Figure 4.3 presents a photograph of the

semisolid lubricant applicator. Fixtures were designed to locate the semisolid

lubricant applicator at three desired locations as shown in Figure 4.4.

4.4 EXPERIMENTATION

Cutting experiments were carried out on a Kirloskar Turn master-35 lathe.

AISI 4340 steel with hardness of 45 HRC was used as work material. Multicoated

hard metal inserts with a specification of SNMG 120408 was used as cutting tool.

A specially formulated cutting fluid (Varadarajan et al., 2002b) was used as the

cutting fluid during minimal fluid application and was applied as a pulsing slug at

the tool work interface. The pressure at minimal fluid applicator was kept at 80 bar

and the frequency of pulsing was maintained as 300 pulses /min. Using the

pneumatic semisolid lubricant applicator, semi solid lubricant was applied at the

rate of 25 grams/min at the tool-chip interface, tool-work interface and at the top

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side of the chip as shown in Figures 4.4(a), 4.4(b) and 4.4 (c) respectively. A

photograph of the experimental set up is shown in Figure.4.5. An 18 run

experiment was designed to determine the effect of application of semi solid

lubricants on cutting performance. During the experiment, cutting speed, feed and

direction of application of semisolid lubricant were varied at three levels as in

Table 4.1. Parameters which were kept constant during the experiment are shown

in Table 4.2.

Figure 4.2 Line sketch of semi sold lubricant applicator

Figure 4.3 Photograph of semi sold lubricant applicator

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Figure 4.4 Application of semi solid lubricant at three different locations, (a) tool-

chip interface (D1), (b) tool-work interface (D2), (c) top side of the chip (D3)

Figure 4.5 Photograph of experimental set up for investigating the influence of

semisolid lubricant on cutting performance

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Table 4.1 Process variables and their values

Factor Level 1 Level 2 Level 3

Cutting velocity

(m/min) 70 80 90

Feed (mm/rev) 0.05 0.06 0.07

Mode of

lubrication

Minimal fluid

application

(L1)

Minimal Fluid

application with

semisolid

lubrication

(L2)

Minimal Fluid

application with

graphite

impregnated

semisolid

lubrication

(L3)

Direction of

semisolid

lubricant

Tool-chip

interface (D1)

(Figure 4.4(a))

Tool-work

interface (D2)

(Figure 4.4(b))

Back side of chip

(D2)

(Figure 4.4(c))

Table 4.2 List of parameters that were kept constant and their values

Parameters Values

Rate of fluid application 5 ml/min

Frequency of pulsing 300 pulses/min

Pressure at the fluid applicator 80 bar

Composition of cutting fluid 10% concentrate

Rate of semi solid lubricant application 25 grams/min

Depth of cut 0.5mm

The performance parameters such as surface roughness, main cutting force,

cutting temperature and the average flank wear were measured during each trial. A

stylus type perthometer was used for measuring surface roughness. The cutting

force was measured using a Kistler type lathe tool dynamometer. The cutting

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temperature was measured using an extrapolative prediction method (Varadarajan

et al., 2000) and the average flank wear was measured using a tool maker’s

microscope. In order to ensure the reliability of the results, all experiments were

repeated three times, and the average of these measurements was taken as the final

value. Observations during the experiment are summarized in Table 4.3. The

relative significance of the operating parameters was determined by response table

methodology using Qualitek-4 Software. ANOVA analysis was carried out to

assess the percentage influence of the individual parameters on cutting

performance.

4.5 RESULTS AND DISCUSSION

Figure 4.6 presents the relative significance of operating parameters on the

main cutting force. Figure 4.7 presents the relative significance of operating

parameters on cutting temperature. The relative significance of surface finish, tool

wear and tool-chip contact length are presented in Figures 4.8, 4.9 and 4.10

respectively. Table 4.4 presents the summary of the analysis carried out using

Qualitek-4. It presents a set of levels of operating parameters for achieving

minimum cutting force, minimum surface roughness, minimum cutting

temperature, minimum tool wear, minimum tool-chip contact length and maximum

surface finish.

From Figures 4.6 to 4.10, it is observed that the direction of application of

semi solid lubricant forms the most significant parameter influencing the cutting

performance in terms of main cutting force, cutting temperature, surface finish,

tool wear and tool chip contact length. Among the type of semisolid lubricants, it

is seen that the application of semisolid lubricant impregnated with graphite in

accompaniment with minimal fluid application brought forth the least cutting force

when compared to conventional minimal fluid application. It was also observed

that application of semisolid lubricant at the tool chip interface in accompaniment

with minimal fluid application corresponding to D1 brought forth lower cutting

force when compared to the other two directions.

85

Table 4.3 Observations during 18 run experiment

Trial

No.

Cutting

velocity

(m/min)

Feed

(mm/rev)

Mode of

Lubrication

Direction

of

semisolid

lubricant

Cutting

Force

(N)

Cutting

Temp (ºC)

Surface

Finish

(µm)

Tool-chip

contact

length

(mm)

Tool wear

(mm)

1 70 0.05 L1 D1 136 251.85 1.13 0.24 0.09

2 70 0.06 L2 D2 119 205.10 1.04 0.28 0.08

3 70 0.07 L3 D3 103 165.40 0.92 0.22 0.05

4 80 0.05 L1 D2 161 273.90 1.08 0.24 0.08

5 80 0.06 L2 D3 112 206.86 0.92 0.23 0.07

6 80 0.07 L3 D1 63 167.20 0.78 0.18 0.06

7 90 0.05 L2 D1 114 220.97 0.93 0.24 0.08

8 90 0.06 L3 D2 100 155.70 0.88 0.23 0.07

9 90 0.07 L1 D3 142 245.67 1.24 0.24 0.09

10 70 0.05 L3 D3 106 198.04 0.88 0.21 0.08

11 70 0.06 L1 D1 165 197.16 1.25 0.27 0.08

12 70 0.07 L2 D2 146 232.44 1.04 0.24 0.07

13 80 0.05 L2 D3 113 175.11 0.94 0.23 0.09

14 80 0.06 L3 D1 93 192.22 0.74 0.21 0.05

15 80 0.07 L1 D2 137 217.44 0.90 0.25 0.07

16 90 0.05 L3 D2 112 152.25 0.95 0.25 0.06

17 90 0.06 L1 D3 188 296.11 1.09 0.27 0.08

18 90 0.07 L2 D1 124 190.06 0.92 0.22 0.07

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Figure 4.6 Relative significance of operating parameters on Cutting force

Fig. 4.7 Relative significance of operating parameters on cutting temperature

87

Figure 4.8 Relative significance of operating parameters on surface finish

Figure 4.9 Relative significance of operating parameters on tool wear

88

Figure 4.10 Relative significance of operating parameters on tool -chip contact

length

Table 4.4 Levels of operating parameters for optimum performance

Desired Outcome

Cutting

velocity

(m/min)

Feed

(mm/rev)

Mode of

lubrication

Direction of

application

of semisolid

lubricant

Low Cutting Force 80 0.07 L3 D1

Low Cutting Temperature 80 0.07 L3 D1

Better Surface Finish 80 0.07 L3 D1

Minimum Tool Wear 80 0.07 L3 D1

Min.Tool Chip Contact Length 80 0.07 L3 D1

The cutting fluid particles entering at the tool-work interface can reach the

tool-chip interface through the micro cracks that exist on the work near the tool tip

(as explained earlier). But extreme thermal conditions that prevail at the tool-chip

interface can adversely affect the lubricating ability of the cutting fluid. But when

89

the solid lubricant was applied at the tool-chip interface, it takes the latent heat of

fusion from the tool-chip interface .This reduces the severity of the thermal

conditions that prevail at the tool chip interface and prevents the complete

degradation of the lubricating properties of the cutting fluid particles present at the

tool-chip interface. The graphite particles present in the semisolid lubricant further

reduces friction at the tool-chip interface. Moreover a mixture comprising of

cutting fluid particles, molten semi solid lubricant and the graphite particles act as

a dielectric that prevents intermolecular and inter atomic interaction between the

chip and the tool surfaces. This prevents adhesion of the chip to the tool surface

and changes the conditions prevailing at the tool-chip interface from sticking to

one of sliding leading to drastic reduction in cutting force and reduces tool-chip

contact length which further reduces the cutting force.

Reduction in frictional forces brought about by better rake face lubrication

can bring forth reduction in cutting temperature, reduction in tool wear and

improvement in surface finish. When the cutting fluid was applied at the tool work

interface, some quantity of the cutting falls on the uncut work surface which forms

the top side of the chip during the next rotation (Figure 4.11). The top side of the

chip is characterized by myriads of micro cracks with nascent crack tips. In normal

case, the micro cracks can coalesce due to intense surface interaction.

When they coalesce, the chip becomes stronger and shows a tendency to

bend towards the rake surface which leads to increased tool-chip contact length

and associated increase in the main cutting force, tool wear, and surface roughness.

But when tiny droplets of cutting fluid get adsorbed on the top side of the chip

owing to their small size and high velocity, they dope the nascent surfaces

generated and prevent the coalesce of crack tips. This leads to the weakening of

the top side of the chip and the chip tends to bend away from the tool rake face

resulting in reduction of tool-chip contact length and associated benefits such as

lower cutting force, lower tool wear, and lower cutting temperature.

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Figure 4.11 Presence of fluid particles on the uncut surface forms the top side of

the chip

When semisolid lubricant was applied at the tool-chip interface it takes the

latent heat of fusion from the tool chip interface and reduces the severity of the

thermal conditions that prevail there and prevents the complete degradation of the

lubricating properties of the cutting fluid particles presents at the tool chip

interface. The effectiveness of heat transfer on the tool rake face depends on the

duration for which the agency that removes heat remains in contact with the

surface. More the time of contact, more will be the effectiveness of heat transfer.

Since the semisolid lubricant can stick on the contact surface it remains there for a

longer duration than is possible for a droplet of cutting fluid and extracts more heat

from the rake face and preserve the lubricating capabilities of the fluid particles

that reach the tool-chip interface via the capillaries on the work surface near the

tool tip. This enhanced lubricity on the rake face reduces cutting force, cutting

temperature and hence tool wear. Moreover, the mixture consisting of fluid

particles and traces of molten semisolid lubricant can act as a dielectric preventing

surface interaction as explained in the previous section. Reduction of surface

interaction between the surfaces of the tool and the back side of the chip can

further reduce the tool-chip contact length and improve the cutting performance

(Figure 4.12 (a) and (b)).

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(a)

(b)

Figure 4.12 Tool chip contact length (a) in the absence of semisolid lubricant, (b)

in the presence of semi solid lubricant, L2<L1

Tool-chip contact

length

Tool-chip contact

length

92

Table 4.5 Comparison of performance during dry turning, wet turning and turning

with minimal fluid application

Desired Outcome Dry

Turning

Wet

Turning

Turning

with

Minimum

Fluid

Application

(MFA)

MFA

with silicon

grease

impregnated

with 10%

graphite

Cutting Force (N)

162 146 117 109

Cutting Temperature(ºC)

317 283 247 236

Surface Finish(µm)

1.42 1.27 1.21 0.93

Tool Wear(mm)

0.084 0.079 0.073 0.0616

Tool Chip Contact Length

(mm) 0.293 0.272 0.245 0.236

(V=80 m/min, F=0.07 mm/rev, DOC=0.5 mm, Pressure of pulsing=80 bar,

Frequency of pulsing=300 pulses/min, Rate of application of silicon grease

impregnated with 10% graphite =25 grams/min)

Hence, the application of semisolid lubricant at the tool-work interface does

not bring forth improvement in cutting performance. Similarly when the semisolid

lubricant was applied at the back side of the chip, it facilitates cooling at the back

side of the chip and promotes the chip curl leading to reduction in tool-chip contact

length. But the presence of semisolid lubricant on the top side of the chip does not

contribute much to the reduction of friction at the tool-chip interface. But when the

semi solid lubricant application was done at the tool-chip interface and the

minimal fluid application at the tool-chip interface (Figure 4.4(a)), the mechanism

responsible for reduction of friction at the tool–chip interface and the mechanism

that is responsible for bending of the chip away from the tool rake face become

active simultaneously as explained earlier. The cumulative impact of the two

mechanisms can bring forth improvement in cutting performance.

93

Figure 4.13 (a) Variation of cutting force with cutting velocity

Figure 4.13 (b) Variation of cutting temperature with cutting velocity

0

50

100

150

200

250

80 90 100 110 120

Cu

ttin

g Fo

rce

(N

)

Cutting Velocity (m/min)

Feed=0.1mm/rev, DOC=1.25mm

Cutting Velocity = Variable

Dry turning

Wet turning

Conventional minimal fluid application

Semis solid lubricant application

0

100

200

300

400

500

600

80 90 100 110 120

Cu

ttin

g Te

mp

era

ture

(oC

)

Cutting Veocity(m/min)

Feed rate=0.1mm/rev DOC = 1.25mm

Cutting Velocity =Variable

Dry turning

Wet turning

Conventional minimal fluid application

Semis solid lubricant application

94

Figure 4.14 (a) Variation of cutting force with feed rate

Figure 4.14 (b) Variation of cutting temperature with feed rate

Comparison of the cutting performance during dry turning, conventional

wet turning and hard turning with minimal fluid application in the presence of

semisolid lubricant impregnated with graphite is available in Table 4.5. Cutting

performance during hard turning with minimal fluid application in the presence of

silicon grease impregnated with graphite was compared with dry, wet and

0

50

100

150

200

250

0.04 0.05 0.06 0.07 0.08

Cu

ttin

g Fo

rce

(N

)

Feed rate(mm/rev)

Cutting Velocity =80m/min DOC = 1.25mm

Feed rate = Variable

Dry turning

Wet turning

Conventional minimal fluid application

Semis solid lubricant application

0

50

100

150

200

250

300

350

400

0.04 0.05 0.06 0.07 0.08

Cu

ttin

g Te

mp

era

ture

(oC

)

Feed rate (mm/rev)

Cutting Velocity = 80m/min DOC = 1.25mm

Feed rate = Variable

Dry turning

Wet turning

Conventional minimal fluid application

Semis solid lubricant application

95

conventional minimal fluid application by conducting variable speed and variable

feed tests at the optimal cutting condition and the results are presented in Figures

4.13 (a), 4.13 (b), 4.14 (a) and 4.14 (b). Further improvement in cutting

performance was noticed (in Figure 4. 13 (a) to (d)) when the minimal fluid

application was carried out along with application of silicon grease impregnated

with graphite. The improvement in cutting performance is attributed to the

enhanced lubricity at the tool chip interface offered by the graphite on account of

its structure as described in section 4.2.

(a) Dry Turning (b) Turning with MFA

(c) MFA with Grease (d) MFA with Grease mixed with

10% Graphite

Figure 4.15 SEM photograph of worn out inserts during (a) dry turning, (b)

conventional turning with minimal fluid application, (c) turning with minimal fluid

application in the presence of silicon grease applied at the tool-chip interface and

turning with minimal fluid application with silicon grease impregnated with 10%

graphite applied at the tool-chip interface under identical cutting conditions (V=80

m/min, f=0.07 mm/rev and DOC=0.5 mm)

96

Figures 4.15 (a), 4.15 (b), 4.15 (c) and 4.15 (d) present the SEM photograph

of worn inserts during pure dry turning, conventional minimal fluid application

and minimal fluid application in the presence of silicon grease and silicon grease

impregnated with graphite. It was observed that damage on the tool was minimum

during turning with minimal fluid application in the presence of silicon grease

impregnated with 10% graphite applied at the tool-chip interface.

4.6 SUMMARY

1. It was observed that the introduction of silicon grease at the rate of 25

grams/min at the tool-chip interface improved cutting performance during hard

turning with minimal fluid application. There was 14 % reduction in cutting force,

14% reduction in surface roughness and 3 % decrease in cutting temperature when

compared to conventional minimal fluid application.

2. When silicon grease was impregnated with 10% graphite, there was

further improvement in cutting performance. There was 20% reduction in cutting

force, 49% reduction in tool wear, 23% reduction in surface roughness and 4%

decrease in cutting temperature when compared to conventional minimal fluid

application.

3. The present study illustrates the technique of application of semi solid

lubricants in accompaniment with minimal fluid application as a potential

performance enhancer for hard turning with minimal fluid application.