Camera Ready Paper of -Ref. No AM-31

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Investigations on the Effects of Minimum Quantity Lubrication

on SurfaceRoughness and Cutting Force during Turning ofBearing Steel

Chakraborty, S.,1Behera, B.C2., Dinesh, S.,3Ghosh, S.,4

1Department of Mechanical Engineering, Indian Institute of Technology Bhubaneswar, [email protected],

2, 3, 4Department of Mechanical Engineering, Indian Institute of Technology Delhi, India2 [email protected], [email protected], [email protected],

Abstract: Environmental concerns call for the reduced use of cutting fluids in metal cuttingoperations. Minimum quantity lubrication (MQL) machining is one of the promising solutionsto the requirement for the decrease in cutting fluid consumption. This paper reports theresults of an experimental work performed to investigate the effect of an indigenouslydeveloped MQL set up for machining responses like cutting forces and surface finish of aturned hardened bearing steel. It was found that machining with the MQL set up resulted ingeneration of lesser cutting forces and better surface finish in comparison to dry and wetmachining.

Keywords : MQL machining; cutting force; surface finish

1 INTRODUCTION

Machining is the process of removing material inthe form of chips by means of a wedge shaped tool.

The need to manufacture high precision items and to

machine difficult-to-cut materials economically

leads to the development of improved machining

processes. The feasibility of dry cutting in the

manufacturing industries has received much

attention due to the high cost of cutting fluids,

estimated at 17% of the total manufacturing cost

which is about twice the tooling cost itself. Cuttingfluid waste needs to be treated prior to disposal and

furthermore, prolonged exposure to them is

hazardous to the machine operators due to risk of

skin cancer and breathing difficulties. Dry cutting isdesirable because not only it reduces manufacturing

costs, but also eliminates all the adverse effectsassociated with the usage of cutting fluids. In spite

of the noble idea to implement the process of dry

machining as mentioned above, the usage of cutting

fluids in machining several types of materials which

are difficult to machine offers several important

advantages, especially to increase productivity and

surface quality of the machined work piece and

hence cannot be totally eliminated as of now. This is

because cutting fluid allows cutting processes to be

carried out at higher speed, higher feed rate and

greater cutting depth due to increased lubricity andcooling at the chip-tool and chip-workpiece

interface. When used effectively, cutting fluids not

only improve surface finish and dimensional

accuracy, but also decrease the amount of powerconsumptions[1]. Furthermore, cutting fluid also

helps transport the excessive heat and chips

produced during the cutting processes away from the

cutting area, thus prolonging tool life [2]. Cuttingfluid related costs and health concerns associated

with exposure to cutting fluid mist and a growing

desire to achieve environmental sustainability in

manufacturing have caused industry and academia

to re-examine the role of these fluids and quantify

their benefits. In countries like USA stricter

legislations have been enforced to minimize the use

of cutting fluids during machining[3]. Hence some

noble approaches have been developed in thisdirection without compromising with the benefits

achieved from flood application of cutting fluids.

Proper selection of cutting fluids is a very important,

although complicated process as it includes various

aspects of machining conditions and parameters.

There has been a gradual shift from straight oils to

soluble oils and further to vegetable and synthetic

oils due to better cooling and lubricating propertiesand more importantly due to much safer handling

and disposal as they are eco-friendly [4]. Cutting

fluids also need to be managed meticulously after

use to reduce their health and environmental effect

and also to cut down disposal costs. Gradualreduction of cutting fluid usage by increasing the use

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of near-dry and dry machining is the most promising

step taken in this regard. The past few decades have

seen many efforts being undertaken to develop

advanced machining processes using less or no

cutting fluid.

Machines and tools designed for cutting fluidscannot be readily adapted for dry cutting[5]. New,

more powerful machines must be purchased, andspecial tooling is often needed to withstand the high

temperatures generated during dry cutting. The

quality of machined parts may be affected

significantly as the properties of the machined

surface are significantly altered by dry machining in

terms of its metallurgical properties and residualmachining stresses. High cutting forces and

temperatures in dry machining may cause the

distortion of parts during machining. Moreover,

parts are often rather hot after dry machining so their

handling, inspection gauging, etc., may present anumber of problems. Near-dry machining (NDM),also known as minimum quantity lubrication (MQL)

machining, is in the process of development to

provide at least partial solutions to the listed

problems with dry machining to wet machining. As

a result, tool life and finish of the machined surface

improved by 15 – 20%.

2 LITERATURE REVIEW

Filipovic and Stephenson[6] found similarity in

tool life in gun-drilling and cross-hole drilling of

crankshafts between wet machining and MQL.Using MQL and a diamond-coated tool in the

drilling of aluminium – silicon alloys, Braga et al. [7]

showed that the performance of the MQL process (in

terms of forces, tool wear and quality of machined

holes) was very similar to that obtained when usinga large amount of water-soluble oil, with both coated

and uncoated drills. Studying turning of brasses,

Davim et al.[8] concluded that, with proper selection

of the MQL system, results similar to flood lubricant

condition can be achieved. Although many research

papers have attempted to give explanation for the

distinctive results of MQL machining no direct

conclusive evidence are provided due to thecomplexity involved in the analysis. One of the most

feasible and better explanation is provided by

Astakhov in one of his work [9], which accredits the

reason to the embrittlement action of the cutting

fluid, which reduces the strain at facture of the work

material. This action is based on the rebinder effect,

directly concerned with the metal cutting process.

He suggests that atomized oil possesses greater

ability to enhance the embrittlement of the layer being removed and thus reduce the work of plastic

deformation during the chip production.

In MQL machining, very small quantity of cuttingfluid (CF) is supplied to the machining zone in very

small (atomized) droplet size with high velocity.

The amount of oil used is generally in the range of

10-100 ml/hr, which is about 1000 times smaller

than that used in conventional flood application of

oil. It was developed as an alternative to flood and

internal high pressure coolant supply to reduce the

CF consumption. The media is supplied as a mixtureof air and oil in the form of an aerosol (often referred

to as mist) with precise control over amount of oiland direction of spray to the cutting zone. The reason

for the shifting trend towards MQL machining is

that it is supposed to give the combined advantages

of dry and conventional flood machining in an eco-

friendly manner. The amount of oil used is so less

that parts engaged in metal removal are practicallydry, although the high velocity air jet carries the

small but sufficient amount of oil precisely to the

machining zone so as to provide the necessary

cooling and lubrication, besides removal of the chips

by the compressed air. Many research papers have been published which goes to show that MQLapplication involves much better machining

parameters than dry machining which are also on a

par or in some cases better than flood application of

oil. Ueda et al.[10] found that temperature reduction

in MQL turning is approximately 5%, while in MQL

end milling it is 10 – 15% and in MQL drilling it is

20 – 25% compared to the temperature in dry cutting.

Khan and Dhar [11] found that MQL with vegetable

oil reduced the cutting forces by about 5 – 15% from

that in dry cutting. The axial force decreased more

predominantly than the power force. They attributed

this reduction as well as the improved tool life andfinish of the machined surface to the reduction of the

cutting zone temperature. Similar results were

obtained during machining of 1040 steel [12]. Li and

Liang [13] found that the cutting forces during

machining of 1045 steel is lower under MQLmachining condition compared to the dry cutting.

They also attributed this reduction of forces to the

lowering of the cutting temperature which in turn

enables the tool to retain its sharpness over a longer

duration. Other groups of researches have compared

MQL with wet machining. Dhar et al. studied the

effect of MQL in tuning of 4340 steel using external

nozzle and aerosol supply to the tool. They foundthat the temperature at the tool – chip interface

reduced by 5 – 10% (depending upon the particular

combination of the cutting speed and feed) in MQL

compared

Herein bearing steel was chosen as the work

material because of its high hardness (above 58

HRC). It is difficult to machine because of its high

fatigue and bending strength [14]. For this reason, in present work machining operation was carried on

bearing steel material in MQL mode and the results

were compared with dry machining and wet

machining.


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3 EXPERIMENTAL SETUP

Block diagram for a Simple MQL setup designed

and fabricated indigenously is shown in figure 1. It

consists of two main systems - Fluid supply system

and the air supply system.

Fig 1. Block diagram of MQL setup

• Fluid supply system consists of a burette for oil

storage, a small pump for continuously supplying oil

from the burette and an intravenous (IV) set. Oil

level in the intravenous (IV) set, maintained at a

particular height throughout so as to overcome the back pressure from the oil inlet to the nozzle by

means of gravitational head. This ensures

continuous oil supply to the nozzle at the desired

small quantity which is also simultaneously

measured by a control valve.

• Air supply system consists of a compressor of

maximum output air pressure capacity of 8 bars and

a pressure gauge fitted close to the air inlet to the

nozzle to measure the delivery pressure to the nozzle

connected by hose pipes.

4 EXPERIMENTAL CONDITIONS

Experiments were conducted under dry, wet and

MQL environment on a CNC lathe machine (Lea

dwell-Fanuc Series Oi Mate-TD).MQL nozzle

arrangement is shown in figure 2.

Fig.2 MQL nozzle arrangement

Bearing steel was chosen as the workpiece

material due to its high hardness. Nominal

composition of the bearing steel used is given in

Table 1. TiAlN coated carbide cutting tool

(CNMG12408MS) have been used for this

experiment and the process parameters are given in

Table 2.Each experiment (dry, wet and MQL) has

been conducted under the same cutting condition.

The cutting forces have been recorded using the

Kistler make dynamometer and an associatedsoftware by the name DynoWare.

Table 1 Nominal Composition of the Bearing Steel

Table 2 Components and parameters of machining

Component/

parameter

Description

Cutting Fluid Water Soluble Oil (1: 80 of

oil by volume)

Nozzle Internally mixed micro-nozzle with twin fluid

atomization

MQL flow rate 250 ml/hr

Air pressure 3 bar

Cutting velocity 60, 80, 100 m/min

Feed 0.14, 0.18, 0.22 mm/rev

Depth of cut 0.5 mm

Dynamometer KISTLER DynoWare (model

no.-9129AA).

5 RESULTS AND DISCUSSION

The cutting forces were measured and recorded in

all three Cartesian coordinate directions for each

combination of cutting velocity and feed with a

constant depth of cut under dry, wet and MQL mode

of cooling. A total of 27 experiments have been

performed (9 each in dry, wet and MQL).The

variation of cutting force (FC ) with change of feed(f)and cutting velocity (Vc) are shown in Figure 3 &

Figure 4respectively.

Fig.3 Variation of cutting force with feed, at

cutting speed 60m/min and depth of cut 0.5mm

C Mn Si P and S Cr

0.55-

1.1

0.1-1.15 0.15-2.0


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Fig.4 Variation of cutting force with cutting speed,at feed rate 0.22mm/rev and depth of cut 0.5mm.

Fig.5 Variation of Ra with cutting velocity, at feed

rate 0.22mm/rev and depth of cut=0.5mm

From the above figures, it is observed that thecutting force maintains an increasing trend with an

increasing feed and a decreasing trend with an

increase in cutting velocity under all types of

machining conducted, but the lowest values have

been obtained under the MQL mode because the

lubricity effect of atomized coolant significantlyreduces the coefficient of friction at the tool-chip

interface which has resulted in the reduction in the

forces. The atomized fluid also maintained the

sharpness of the cutting edge by transferring heat

from the cutting zone. An average reduction of about

13% from dry machining and about 6% from wet

machining has been achieved in the cutting force

when MQL machining has been performed. The

surface roughness of the workpiece has also been

examined for all the machined surfaces under a

Talysurf surface profilometer. The variation in

average surface roughness parameter (R a) with

change of feed is shown in Figure 5. Figure 5 revealsthat the surface finish shows a deteriorating trend

with an increase in feed. However the best results

have been achieved under both dry and MQL

machining, because under wet condition the chip

flow direction got deviated towards the workpiece

surface resulting in the poor surface finish. This

verifies that the MQL setup has been successful in

providing a reasonably good surface finish (almost

comparable to dry machining) of the workpiece

CONCLUSION

Application of MQL during turning of bearing

steel has been experimentally studied and it has been

observed that a substantial decrease in cutting force

could be achieved with the indigenously developed

MQL setup compared to dry and wet machining.Also the surface finish under MQL machining was

quite close to that obtained under dry machining andit has been much better than the finish obtained

under wet machining. Hence from the results

obtained it may be concluded that the utility of

Minimum Quantity Lubrication during machining

of the bearing steel has been established and much

better results can be achieved if a study is done tooptimize the process parameters such as cutting

velocity, feed, oil composition and its consumption

rate, inlet pressure of compressed air .

REFERENCES

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[9] V. P. Astakhov, "Ecological Machining: Near-

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Camera Ready Paper of -Ref. No AM-31