EFFECT OF CUTTING FLUID AS MINIMUM QUANTITY …

EFFECT OF CUTTING FLUID AS

MINIMUM QUANTITY

LUBRICATION (MQL) ON

MACHINABILITY IN TURNING AISI

1060 STEELS USING COATED

CARBIDES.

Riton Kumer Das

DEPARTMENT OF MECHANICAL ENGINEERING

DHAKA UNIVERSITY OF ENGINEERING & TECHNOLOGY

GAZIPUR-1700

ii


MINIMUM QUANTITY




CARBIDES.

A Project

By

Riton Kumer Das

Department of Mechanical Engineering

Dhaka University of Engineering & Technology

Gazipur-1700

May 2015

iii


MINIMUM QUANTITY




CARBIDES.

A Project

By

Riton Kumer Das

Submitted to the Department of Mechanical Engineering, Dhaka University of

Engineering & Technology, Gazipur, in partial fulfillment of the requirements for the

degree of MASTER OF MECHANICAL ENGINEERING.


Dhaka University of Engineering & Technology

Gazipur-1700

May 2015

iv

The Project titled “Effect of Cutting Fluid as Minimum Quantity Lubrication

(MQL) on Machinability in Turning AISI 1060 Steels using Coated Carbides”,

submitted by Riton Kumer Das, Student No. 092304 (P) session 2009-2010, has been

accepted as satisfactory in partial fulfillment of the requirements for the degree of

Master of Mechanical Engineering on May 24, 2015.

BOARD OF EXAMINERS

1. Professor Dr. Md. Kamruzzaman Chairman and

Supervisor

Professor


and

Head, Department of Industrial & Production Engineering

DUET, Gazipur.

2. Professor Dr. Mohammad Asaduzzaman Chowdhury Member

(Ex-officio)

Head


DUET, Gazipur.

3. Professor Dr. Mohammed Alauddin Member


DUET, Gazipur.

4. Professor Dr. Mohammad Zoynal Abedin Member


DUET, Gazipur.

5. Professor Dr. Md. Nurul Absar Chowdhury Member

Department of Mechanical & Chemical Engineering

Islamic University of Technology (IUT)

Board Bazar, Gazipur.

(External)

v

Declaration

I do hereby declare that this work has been done by me and neither this project nor any

part of it has been submitted elsewhere for the award of any degree or diploma except

for publication.

Countersigned

Prof. Dr. Md. Kamruzzaman

Supervisor

&

Professor


and

Head

Department of Industrial & Production Engineering

DUET, Gazipur.

Riton Kumer Das

vi

This Project work is dedicated to

My beloved

Parents

vii

TABLE OF CONTENTS

Table of Contents vii

List of Figures ix

List of Tables xi

Notations xii

Acknowledgement xiii

Abstract xiv

CHAPTER 1 INTRODUCTION 1

1.1 Introduction 1

1.2 Literature Review 6

1.2.1 Dry Machining

6

1.2.2 Machining with conventional cutting fluids 7

1.2.3 Liquid Nitrogen Technology/Cryogenic Cooling

10

1.2.4 High Pressure Coolant

10

1.2.5 Minimum Quantity Lubrication

12

1.3 Summary of review 24

1.4 Scope of the Present Work 28

1.5 Objectives of the Present Work 29

CHAPTER 2 EXPERIMENTAL INVESTIGATIONS AND RESULTS 29

1.1 Introduction 31

2.2 Minimum Quantity Lubrication System 32

2.3 Experimental Procedure and conditions 33

2.4 Experimental Results 37

2.4.1 Cutting temperature

37

2.4.2 Machining Chips

39

2.4.3 Surface roughness

43

2.4.4 Dimensional Deviation

47

CHAPTER 3 DISCUSSION ON EXPERIMENTAL RESULTS 48

3.1 Cutting Temperature 48

viii

3.2 Machining Chips

51

3.3 Surface roughness

53

3.4 Dimensional Deviation

54

CHAPTER 4 CONCLUSIONS AND RECOMMENDATIONS 56

4.1 Conclusions 56

4.2 Recommendations 57

REFERENCES 59

ix

LIST OF FIGURES

Figure No. Description Page no.

Fig. 1.1 Classification of Near Dry machining 13

Fig. 1.2 Fishbone diagram showing cause and effect in MQL assisted

machining

14

Fig. 1.3 Distribution of Fabrication costs in engineering 26

Fig. 2.1 Photographic view of Mixing Chamber for MQL Supply 33

Fig. 2.2 Schematic view of the mixing chamber along with nozzle 33

Fig. 2.3 Photographic view of the MQL applicator 34

Fig. 2.4 Schematic view of the experimental set up 34

Fig. 2.5 Photographic view of surface roughness measuring technique 37

Fig. 2.6 Variation of average chip-tool interface temperature with different

cutting speed and feed under dry and wet condition during

machining AISI 1060 steel

38


speed and feed under different MQL condition during machining

AISI 1060 steel

38


speed under dry, wet, MQL with Vegetable oil and MQL with

Cutting Oil, VG 68 condition during machining AISI 1060 steel

38


speed under dry, wet, MQL with Vegetable oil and MQL with

Cutting Oil, VG 68 condition during machining AISI 1060 steel

39

Fig. 2.10 Variation of average chip thickness ratio (rc) with different cutting

speed and feed under Dry Condition during machining AISI 1060

steel

42


speed and feed under Wet Condition during machining AISI 1060

steel

42

x


speed and feed under MQL (Cutting Oil VG 68) condition during


43


speed and feed under MQL (Vegetable oil) condition during


43

Fig. 2.14 Variation of average surface roughness (Ra) with different cutting

speed and feed under Dry Condition during machining AISI 1060

steel

45


speed and feed under Wet Condition during machining AISI 1060

steel

45


speed and feed under MQL (Cutting Oil) condition during


46


speed and feed under MQL (Vegetable oil) Condition during


46

Fig. 2.18 Variation of dimensional deviation under Dry, Wet, MQL with

insoluble cutting oil and MQL with soluble cutting fluid condition

during machining AISI 1060 steel

47

xi

LIST OF TABLES

Table No. Description Page no.

Table 2.1 Experimental conditions

35

Table 2.2 Comparison of chip shape and color at different cutting speed and

feed under dry and different MQL conditions machining of AISI

1060 steel by SNMG coated carbide insert.

40

Table 3.1 Effectiveness of MQL with Vegetable oil 50

Table 3.2 Effectiveness of MQL with Cutting oil VG 68 51

xii

NOTATIONS

AISI American Iron and Steel Institute

BUE Built-up edge

CBN Cubic boron nitride

d Depth of cut

f Feed rate

HPC High Pressure Coolant

HSM High speed machining

MQL Minimum quantity lubrication

MRR Material removal rate

NIOSH National Institute of Occupational, Safety and Health

V Cutting Speed

xiii

ACKNOWLEDGEMENT

The author is deeply indebted to Prof. Dr. Md. Kamruzzaman, Professor of the

Department of Mechanical Engineering and Head, Department of Industrial &

Production Engineering, DUET for his guidance, help and encouragement throughout

the progress of the project work.

He also wants to thank especially to the Head of the Department of Mechanical

Engineering for the help rendered for allowing and providing Machine Shop facilities to

carry out the experiment whenever required. The help extended by the Director of

CRTS for providing research fund is highly acknowledged.

A special word of thanks is due to all the staff members of Machine Shop for their helps

in conducting the experimental work.

Finally, the author offer his sincere thanks to all those who either directly or indirectly

helped him in various ways to complete this project thesis.

May, 2015 Author

xiv

ABSTRACT

In metal cutting processes, high cutting velocity and feed generates large amount of heat

as well as high cutting temperature which increases tool wear, shortens the tool life and

deteriorates the job quality. To reduce this high cutting temperature the use of cutting

fluids is the most common strategy. The conventional cutting fluids are not that

effective in such high production machining particularly in continuous cutting of

materials like steels. Further, the conventional cutting fluids are not environmentally

friendly. The conventional cutting fluids which are used during machining changes the

performance of machining operations because of their lubrication, cooling and chip

flushing functions. The disposal of the cutting fluids often leads to local water pollution

and soil contamination. Recycling and reuse of conventional cutting fluids are also

problematic. In this decade, with increased environmental awareness, the researchers

are striving to develop environmentally friendly machining technology; one such

technology is to use minimum quantity lubrication with cutting oil. Minimum quantity

lubrication (MQL) or Minimum volume of oil (MVO) or Near dry machining presents

itself as a possible alternative for machining with respect to tool wear, heat dissertation

and machined surface quality. This research compares the mechanical performance of

dry cutting without coolant, soluble cutting fluid as conventional application, insoluble

cutting oil and vegetable oil both as MQL for the turning of AISI 1060 steel based on

experimental measurement of cutting temperature, Chips form (chip shape, chip color

and chip thickness ratio, rc), Surface finish (Ra) and Dimensional accuracy.

Compared to the dry and conventional turning, the use of cutting oil and vegetable oil as

MQL leads to reduce surface roughness, increase dimensional accuracy, and lower

cutting temperature, while also having favorable chip tool interaction.

Introduction

1.1 Introduction

Green manufacturing is one of the major research and development theme in

manufacturing sector in recent years due to the challenges raised by increased

environmental awareness, and strict protection laws and health regulations for

occupational safety. The main environmental and occupational health hazard problems in

metal cutting industry are related to the use and disposal of cutting fluids, which are used

to reduce force and power, increase tool life, improve surface finish and chip removal, and

reduce thermal distortion and subsurface damage [Devries, 1992]. Use of cutting fluid

provides numerous advantages in machining but suffers from serious drawbacks of

operator health hazard as well as environmental and economical problems. Improper

disposal of cutting fluids pollutes land, water, and air and thus disturbs the whole

environment [Jiang et al., 2008]. Contact of cutting fluid with skin and inhalation of its

vapor causes skin and respiratory problems due to presence of various additives such as

emulsifiers, biocides, rust inhibitors, stabilizers, etc. In addition, cutting fluid particles

remain suspended in the environment for a long period [Sutherland et al., 2000] and thus

affect other employees, which are not in direct contact with cutting fluids [Benett and

Benett, 1985]. National Institute of Occupational, Safety and Health (NIOSH), 1983

estimated that 1.2 million workers are potentially exposed to the hazardous/chronic

toxicology effects of metal working fluids. NIOSH recommended exposure limit of 0.5

mg/m3 for 10-hour time weighted average (TWA) for no requirement of respiratory

protection [Sutherland et al., 2000; Khan et al., 2009]. However, the oil mist levels on

shop floor are quite high; oil mist level of 1-56.5 mg/m3, 0.8-50 mg/m

3 and 20-90 mg/m3

was reported for flood type of cooling in automobile manufacture, manufacture of steel

products and automotive plants respectively [Benett and Benett, 1985]. Also, the use of

coolant fluid costs from 7 to 17% of the total manufacturing cost of work-piece [Weinert

et al., 2004] and requires additional time for work-piece/tool/machine cleaning [Dhar et

al., 2006a]. Moreover, a study conducted by Sutherland et al. [2000] reveals that 12-80

2

times more cutting fluid mist was generated with wet turning, than cast iron dust in dry

turning. Consequently, dry machining turned out to be the field of interest for many

researchers to deal with above-mentioned challenges. Dry machining was found successful

with some materials such as cast iron but less effectiveness was noticed when high

production rates and machining efficiency is required. Tools with high heat resistance and

wear resistance are required for dry machining. New tool coatings were found helpful to

some extent, as the machining cannot be still performed at the same rate as with cutting

fluids [Canter, 2009]. In addition, for continuous high speed machining of some materials

such as superalloys and titanium, cooling is necessary [Sreejith and Ngoi, 2000] to

improve tool life and surface finish. So, Near Dry Machining (NDM)/Minimum Quantity

Lubrication (MQL)/Micro-Liter Lubrication (μLL) emerged out as a plausible solution to

aforementioned problems as it reduces the drawbacks associated with flood cooling and

dry machining and even can perform better than flood cooling. The concept of MQL was

emerged nearly one and a half decade ago to meet out the strict environmental regulations,

issues related to operator health hazard and cost related to use of coolant. MQL has been

successfully applied in various machining operations such as turning [Dhar et al., 2006a],

drilling [Kelly and Cotterell, 2002], milling [Kishawy et al., 2005] and grinding

[Sadeghi et al., 2009]. Applications of MQL has resulted in better tool life, improved

surface finish, better chip forms and reduced cutting forces [Varadarajan et al., 2002;

Khan et al., 2009].

Machining is a material removal process that typically involves the cutting of metals using

different types of cutting tools in which a tool removes material from the surface of a less

resistant body through relative movement of the tool and application of force and is

particularly useful due to its high dimensional accuracy, flexibility of process, and cost-

effectiveness in producing limited quantities of parts. Due to removal of material in the

form of chips, new surfaces are cleaved from the workpiece accompanied by a large

consumption of mechanical energy which in turn transformed into heat, leading to

conditions of high pressure, high temperature and severe thermal conditions at the tool-

chip interface. The higher the tool temperature, the faster the flank wears. The use of

cutting fluids in machining processes either reduces the cutting zone temperature by its

cooling effect, or reduces of frictional heat generation due to its lubrication effect. The

3

third function that the cutting fluid serves is chip flushing or transportation of chips from

the cutting zone by the used / waste coolant stream.

The performance and service life of engineering component depends on their material,

dimensional or form accuracy and surface quality. The growing demand for higher

productivity, product quality and overall economy in manufacturing by machining and

grinding, particularly to meet the challenges of global cost competitiveness, insists high

material removal rate and high stability and long life of the cutting tools. But high

production machining with high cutting velocity, feed, and depth of cut is inherently

associated with generation of large amount of heat and high cutting temperature. Such high

cutting temperature not only reduces dimensional accuracy but also impairs the surface

integrity of the product by inducing tensile residual stresses and surface and subsurface

micro cracks in addition to rapid oxidation and corrosion. In high speed machining,

conventional cutting fluid application fails to penetrate into the chip-tool interface and thus

cannot remove heat effectively [Paul et al. 2000]. Addition of extreme pressure additives

in the cutting fluids does not ensure penetration of coolant at the chip-tool interface to

provide lubrication and cooling. Moreover, reaching to the boiling point conventional

coolant vaporizes and makes a non-conductive vapor barrier to enter the coolant

effectively into the interface. With the increase in cutting velocity, feed and depth of cut

this problem becomes acute and cooling process become ineffective gradually.

The challenge of modern machining industries is mainly focused on the achievement of

high quality, in terms of work part dimensional accuracy and surface finish, high

production rate and cost saving, with a reduced environmental impact. High temperature

generated during machining at high cutting speed and feed results in high tool wear,

reduced tool life, poor surface finish and dimensional accuracy and larger force is required

for machining. The mechanical energy consumed in the cutting area is converted into heat

and the main sources of heat are the shear zone, the interface between the tool and the chip

where the friction force generates heat and lower portion of the tool tip which rubs against

machined surface. The interaction of these heat sources combined with the geometry of the

cutting area, results in a complex temperature distribution. The temperature generated in

the shear plane is a function of the shear energy and the specific heat of the material.

4

Temperature distribution will be a function of the thermal conductivities of the workpiece

and the tool materials, the specific heat, cutting speed, depth of cut, the use of a cutting

fluid and cutting conditions. The temperature generation during machining is an important

factor since it affects the thermally activated mass transport phenomena in the cutting tool-

workpiece contact zone. While primarily dependent on the cutting speed and the

workpiece material properties, the cutting temperature is also affected by the cutting tool

properties. Almost all of the mechanical energy in metal cutting is transformed into heat.

The major portion of the produced heat is conducted into and removed with the chips from

the cutting region with nearly the entire remaining portion conducted into the workpiece

and cutting tool. At such elevated temperature the cutting tool if not enough hard may lose

their form stability quickly or wear out rapidly resulting in increased cutting force,

dimensional inaccuracy of the product and shorter tool life. Longer cut under high cutting

temperature causes thermal expansion and distortion of the job particularly if it is slender

and small in size, which leads to dimensional and form inaccuracy. On the other hand,

high cutting temperature accelerates the growth of tool wear and also enhances the chance

of premature failure of the tool by plastic deformation and thermal fracturing.

In general, the most important point in machining processes is the productivity, achieved

by cutting the highest amount of material in the shortest period of time using tools with the

longest lifetime. Combining all the parameters involved in the machining process to

maximize productivity is, nevertheless, a very complex task and becomes much more

difficult when working at high speed cutting in hardened steels. The temperature at the

cutting tool interface is one of the important factors influencing the machining process.

The primary function of cutting fluid is cooling and lubrication to avoid the high

temperature effect on a machined surface. A fluid’s cooling and lubrication properties are

critical in decreasing tool wear and tool wear and extending tool life. They have a strong

effect on the shearing mechanisms and, consequently, on the work part surface finish and

tool wear [Diniz et al., 2003]. A secondary function of cutting fluid is to flash away chips

and metal fines from the tool/work piece interface to prevent a finish surface from

becoming marred and also to reduce the occurrence of built-up-edge (BUE). The surface

quality of the machined parts also deteriorate with the increase in cutting temperature due

to built-up-edge formation, oxidation, rapid corrosion and induction of tensile residual

5

stress and surface micro cracks. Such problems become more acute and serious if the work

materials are very hard, strong and heat resistive and when the machined or ground part is

subjected to dynamic or shock loading during their functional operations. Therefore, it is

essential to reduce cutting temperature as far as possible.

In the recent years a lot has been done to avoid the cutting fluids from the production. Dry

cutting and semi-dry cutting such as minimum quantity lubrication (MQL) have been

favored by the industry. Dry and near-dry machining operations are the key technology of

environmentally friendly manufacturing process. The conventional flood supply system

demands more resources for operation, maintenance, and disposal, and results in higher

environmental and health problems. MQL machining has many advantages in this regard.

[Weinert et al., 2004]. By abandoning conventional cooling lubricants and using the

technologies of dry machining or minimum quantity lubrication (MQL), this cost can be

reduced significantly. However, it should also be noted that some of the benefits of cutting

fluids are not going to be available for dry machining and also dry machining will be

acceptable only whenever the part quality and machining times achieved in wet machining

are equaled or surpassed. In fact, the cooling lubricant performs several important

functions, which, in its absence, must be taken over by other components in the machining

process. For instance cooling lubricants reduce the friction, and thus the generation of heat,

and dissipate the generated heat. In addition, cooling lubricants are responsible for a

variety of secondary functions, like the transport of chips as well as the cleaning of tools,

work pieces and fixtures. In addition, cooling lubricants help to provide a uniform

temperature field inside the work piece and machine tool and help to meet specified

tolerances.

Most investigations of using MQL have been focused mainly on turning and drilling

operations. Most research has concluded positive effects to its lubrication ability. Thus,

this work is undertaken with the aim to evaluate the use of vegetable oil and cutting oil

through the application of MQL technique when turning medium carbon steel using solid

coated SNMG tool.

6

1.2 Literature Review

1.2.1 Dry Machining

Dry machining has been around for as long as traditional machining, but has seen a recent

surge in interest as more people are realizing the true cost of cutting fluid management.

Dry machining is ecologically desirable and it will be considered as a necessity for

manufacturing enterprises in the near future. Industries will be compelled to consider dry

machining to enforce environmental protection laws for occupational safety and health

regulations. The advantages of dry machining include: non-pollution of the atmosphere (or

water); no residue on the swarf which will be reflected in reduced disposal and cleaning

costs; no danger to health; and it is non-injurious to skin and is allergy free. Moreover, it

offers cost reduction in machining. Most industries apply cutting fluids/coolants when

their use is not necessary. By abandoning conventional cooling lubricants and using the

technologies of dry machining or minimum quantity lubrication (MQL), this cost can be

reduced significantly. However, it should also be noted that some of the benefits of cutting

fluids are not going to be available for dry machining and also dry machining will be

acceptable only whenever the part quality and machining times achieved in wet machining

are equaled or surpassed. In fact, the cooling lubricant performs several important

functions, which, in its absence, must be taken over by other components in the machining

process. For instance cooling lubricants reduce the friction, and thus the generation of heat,

and dissipate the generated heat. In addition, cooling lubricants are responsible for a

variety of secondary functions, like the transport of chips as well as the cleaning of tools,

work pieces and fixtures. In addition, cooling lubricants help to provide a uniform

temperature field inside the work piece and machine tool and help to meet specified

tolerances.

Many of the problems associated with dry machining occur because the metal reaches

higher temperatures than during machining with coolant lubricant. For example Klocke et

al. showed that the temperature of the tool can rise from 150 with flood coolant to nearly

400 when drilling AISI-1045. Without the lubrication more heat is generated during the

machining process and without the coolant effect, it cannot be as efficiently removed from

the interface of the tool and the work piece. The dimensional accuracy is often not as good

7

during dry machining because of the high temperature produced. Surface finish can also be

negatively affected. The increase in temperature increases the ductility of the metal,

changing the formation of chips [Klocke et al. 2006].

1.2.2 Machining with conventional cutting fluids

The primary function of cutting fluids is to reduce this cutting temperature and increase

tool life. In addition the cutting fluid has a practical function as a chip-handling medium.

Cutting fluids also help in machining of ductile materials by reducing or preventing

formation of a build-up-edge (BUE), which degrades the surface finish. Conventional

machining prevails [Ekinovic et al. 2005] plastically deformation in generating chips

whereas in high speed machining chip generation is followed by segmentation process.

Usually the high cutting temperature is controlled by profuse cooling. But such profuse

cooling with conventional cutting fluids is not able to solve these problems fully even

when employed in the form of jet or mist. With the advent of some modern machining

process and harder materials and for demand for precision machining, the control of

machining temperature by more effective and efficient has become extremely essential.

Generally, suitable cutting fluid is employed to reduce this problem through cooling and

lubrication at the cutting zone. But it has been experienced that lubrication is effective at

low speeds when it is accomplished by diffusion through the work piece and by forming

solid boundary layers from the extreme pressure additives, but at high speeds no sufficient

lubrication effect is evident. The ineffectiveness of lubrication of the cutting fluid at high

speed machining is attributed to the inability of the cutting fluid to reach the actual cutting

zone and particularly at the chip-tool interface due to bulk or plastic contact at high cutting

speed.

In metal cutting the material is subjected to extremely high strains in two principal regions:

the shear zone or primary deformation zone, and another zone, known as secondary

deformation zone. The majority of total deformation of work material during machining

takes place in the primary deformation zone. Most of the energy generated in the plastic

deformation converts into heat, which causes the temperature rise in the primary

deformation zone. However a small amount of heat transfer occurs between the work piece

8

and tools due to very short time of the deformation. Thus the temperature can be localized

in some areas of chips. This deformation and temperature localization will increase with

increasing cutting speed. Since vast amount of heat is generated during high speed

machining, low pressure conventional cutting fluid is vaporized due to high temperature

when it comes in contact with the work-tool-chip interface, makes a barrier (film).That’s

why no cutting fluid reach in the tool-chip interface or cutting zone [Ezugwu 2004]. The

film boiling temperature of conventional cutting fluid is about 3500

[Ezugwu and Bonney

2003]. The use of cutting fluids on machining operations has been questioned, due to

problems they may cause to the environment, due to damage to human health and also

more due to the severe laws regarding industrial waste that have been passed. Therefore,

industries are being forced to review the production processes aiming either, at elimination

or, when it is not possible, a sharp reduction in the use of these fluids. .Many articles

recommend the beneficial effects of cutting fluids. Despite the advantages, the detrimental

effect of cutting fluid associated with the working environment and waste disposal has

already been reported elsewhere. Operators’ health and ecology has become a major issue

for developing countries and stricter regulations and their enforcement are inhibiting the

use of flood coolant and subsequently increasing the cost of flood coolant use. It will be a

long time before cutting fluids can be considered totally armless and acceptable.

High production machining of metal inherently generates high cutting zone temperature

causes dimensional deviation and premature failure of cutting tools. The application of

cutting fluid during machining operation reduces cutting zone temperature to prevent

overheating and increases tool life acts as lubricant as well. It reduces cutting zone

temperature either by removing heats as coolant or by heat generation as lubricant. It is

believed that heat is carried away from the tool and the work by means of cutting fluid

which at the same time reduce the friction between the tool and the chip and work and also

facilitates the chip formation. The conventionally applied cutting fluid can do this job

properly at normal cutting conditions where the feed and depth of cut are very low. But the

main problem with conventional coolant is that it does not reach the real cutting area. In

case of ductile material and under high speed feed condition conventionally applied

coolant is completely ineffective to do so as the bulk and progressive contact between tool

face and the following chips cannot allow the coolant to enter into the interface where

9

maximum temperature attains. Moreover water soluble coolant is a major source of

environmental pollution, soil contamination and carrier of bacteria borne diseases like

lunch cancer, dermatitis and others. Cutting fluids are important causes of occupational

contact dermatitis which may involve either irritant or allergic mechanisms. Water mixed

fluids generally determine irritant contact dermatitis allergic contact dermatitis when they

are in touch with workers skin. Cutting fluids are widely used to reduce the cutting

temperature. But the major problems associated with the use of conventional methods and

types of cutting fluids, which are mostly oil based, are;

• Ineffectiveness in desired cooling and lubrications.

• Health hazards due to generation of obnoxious gases and bacterial growth.

• Inconvenience due to un-cleanliness of the working zone.

• Need of storage, additional floor space, pumping system, recycling and

disposal.

• Corrosion and contamination of the lubricating system of the machine tools.

• Environmental pollution and contamination of soil and water.

The cooling and lubricating effects by cutting fluid influence each other and diminish with

increase in cutting velocity. Since the cutting fluids do not enter the chip-tool interface

during high speed machining the cutting fluid action is limited to bulk heat removal only.

The effect of heat generated at the primary shear zone is less significant for its lesser

intensity and distance from the rake surface. But the heat generated at the chip-tool

interface is of much greater significance, particularly under high cutting speed conditions

where the heat source is a thin flow zone seized to the tool [Shaw M. C. 2005]. The

coolant can not act directly on this thin zone but only externally cools the chip, work piece

and the tool, which are accessible to the coolant. Removal of heat by conduction through

the chip and the work piece is likely to have relatively little effect on the temperature at the

chip-tool and work-tool interface. The disposal of used chemical coolants involves

incineration and partially contributes to global warming. Also use of flood coolants does

not inhibit the air boundary layer and a protocol was made for further investigation of

coolant flow mechanism [Ebbrell et al. 2000]. The presence of chemical substances like

sulfur, phosphorous, chlorine or any other extreme pressure additives in the coolant

introduces health hazard to the operator. It is also documented that 7-17% of machining

10

cost of a work-piece is due to coolant-lubricant deployment. Skin exposure is the dominant

route of exposure and it is believed that about 80 percent of all occupational diseases are

caused by skin contact fluids [Bennett et al. 1985].

1.2.3 Liquid Nitrogen Technology/Cryogenic Cooling

One solution to the problem of cutting fluid management currently under development is

the use of liquid nitrogen as a coolant and lubricant. The method currently under

development uses liquid nitrogen to perform the cooling and lubrication job of the cutting

fluid. Much of the part remains at ambient temperature while the flow of nitrogen is

carefully delivered to the point where it is needed. The small flow rate of liquid nitrogen

makes this technique a very attractive alternative. This technique can be used on

equipment that has been designed for use with cutting fluids. There is no cutting fluid to

dispose as the nitrogen evaporates harmlessly into air. If successful, this technique will

provide an alternative to business that want to eliminate the use of traditional cutting fluids

but cannot afford the capital expenditure required to purchase new dry-machining

equipment. Reportedly, tool life and finish quality are also improved by this technique due

to the low temperature at the tool/pat interface [Dhar et al. 2002]. Liquid nitrogen is an

expensive chemical that is environmentally inert. Nitrogen is the most abundant gas in

earth’s atmosphere and when liquid nitrogen warms, it simply mixes with and diffuses

harmlessly into the air. After applying this technique, the generated chips can easily be

recycled as there is no residual oil. Liquid nitrogen is hazardous to workers due to its

extremely low temperature. Exposure can result in mild to extreme frostbite. Nitrogen that

is stored in a sealed vessel will increase in pressure dramatically as it warms, potentially

resulting in a non-combustion explosion. Large spills can displace all of the oxygen in a

room in a short time. However, when proper equipment and handling technique are used

nitrogen is a very safe and environmental friendly alternative.

1.2.4 High Pressure Coolant

Coolants are used to reduce the amount of heat and friction at the point where a tool cut

into a metal work piece. This heat reduction allows the cutting tool to operate at higher

11

speeds and reduces tool wear. However, at the lower pressures typically used to deliver

cutting fluid, the coolant cannot effectively remove the majority of heat at the cutting point

because it does not reach the real cutting area. Instead the coolant washes over the tool

holder and work piece, cooling the surfaces somewhat, but not removing the intense heat

within the cutting area, itself. In fact most of this heat is conducted to the material around

the shear zone and to the tooling. Thus the temperature at the cutting point keeps higher

than desired. By directing the coolant stream more precisely and with the optimum

amount of pressure and flow rate, more heat can be removed dramatically. High pressure

assisted cooling is one of the preferred technologies, currently under exploitation

especially in the aerospace and power plant industries for machining exotic materials. The

creditability of high-pressure coolant assisted machining had been thoroughly investigated

over the years [Ezugwu et al. 2004 and Dhar et al. 2008]. This system not only provides

adequate cooling at the tool-workpiece interface but also provides an effective removal

(flushing) of chips from the cutting area. The coolant jet under such high-pressure is

capable of creating a hydraulic wedge between the tool and the workpiece, penetrating the

interface deeply with a speed exceeding that necessary even for very high speed

machining. The high-pressure coolant stream helps to break up chips and remove them

from the cutting zone area more effectively. The combination of reduced heat and more

efficient evacuation of chips prolong tool life and makes replacement more predictable

because the cutting tool wears out naturally, rather than failing prematurely because of

excessive heat or chip damage. Properly applied high-pressure coolant allows users to

achieve maximum performance. The coolant jet under such high-pressure is capable of

creating a hydraulic wedge between the tool and the work piece, penetrating the interface

deeply with a speed exceeding that necessary even for very high speed machining. This

phenomenon also changes the chip flow conditions. The penetration of high- energy jet at

the tool-chip interface reduces the temperature gradient and minimizes the seizure effect,

offering an adequate lubrication at the tool-chip interface with a significant reduction in

friction. It was found about the performance of high-pressure coolant jet on grind ability of

steel [Ueda et al. 2006 and Dhar et al. 2008] based on the experimental results that high-

pressure coolant jet reduces grinding zone temperature significantly due to effective

cooling and lubrication at the grinding zone area. HPC grinding yields to less significant

lamellar chips can be found due to change in feed rate.

12

1.2.5 Minimum Quantity Lubrication

In MQL process, oil is mixed with high-pressure air and the resulting aerosol is supplied

near to the cutting edge. This aerosol impinges at high speed on the cutting zone through

the nozzle. Air in the aerosol provides the cooling function and chip removal, whereas oil

provides lubrication and cooling by droplet evaporation. The flow of lubricant in MQL

process varies from 10 to 100 ml/h and air pressure varies from 4 to 6.5 Kgf/cm2 [Silva et

al., 2005]. Different ranges for flow rate were also reported in literature such as 50 to 500

ml/h [Dhar et al., 2006a] and 2 to 300 ml/h [Zhong et al., 2010]. However, in industrial

applications consumption of oil is approximately in the range of 10-100 ml/h [Kamata

and Obikawa, 2007]. When the flow rate of cutting fluid in MQL is less than or equal to 1

ml/h it is termed as Micro-Liter Lubrication (μLL) [Obikawa et al., 2008; Liu et al.

2011]. As the quantity of cutting fluid in MQL is very less (in ml/h instead of l/min) in

comparison to flood cooling, the process is also known as Near Dry Machining. If oil is

used as fluid medium in NDM, better lubrication is obtained with slight cooling effect

whereas, when emulsion, water or air (cold or liquid) were used, better cooling is achieved

with slight/no lubrication so, the processes were termed as Minimum Quantity Lubrication

and Minimum Quantity Cooling respectively [Weinert et al., 2004, Tawakoli et al. 2010].

NDM can be classified on the basis of method of aerosol spray and aerosol composition as

shown in Fig. 1.1. Detailed description is available with Astakhov, 2008.

In MQL, cooling occurs due to convective and evaporative mode of heat transfer and thus

is more effective than conventional wet cooling in which cooling occurs due to convective

heat transfer only. In addition, cutting fluid droplets by virtue of their high velocity

penetrates the blanket of vapor formed and provides more effective heat transfer than wet

cooling [Varadarajan et al., 2002]. However, according to Astakhov, [2008] aerosols do

not acts as lubricants or boundary lubricants as they do not have access to the tool-chip and

tool-work-piece interfaces due to too low penetration ability. In addition, the cooling

action due to droplet evaporation is also small due to very small flow rate of oil. MQL

action on forming chip is also negligibly small as compared to high pressure water soluble

metal working fluids due to low mass of aerosol. Astakhov, [2008] suggested that the

13

application of MQL enhances the Rebinder effect and thus reduces the work due to plastic

deformation.

Fig. 1.1 Classification of Near Dry machining [Astakhov, 2008; Weinert et al., 2004]

Possible parameters and machining conditions affecting the performance of MQL assisted

machining are illustrated in fishbone diagram as shown in Fig. 1.2. As little quantity of

cutting fluid was utilized in MQL process, the cutting fluid should possess significantly

higher lubrication qualities than mineral oil. Vegetable oil and synthetic ester oil are two

viable alternatives. Vegetable oils are nontoxic as they are based on extract from plants.

Molecules of these oils are long, heavy and dipolar in nature and provides greater capacity

to absorb pressure. Higher viscosity index provides stable lubrication in operating

temperature range and higher flash point provides opportunity to increase metal removal

rate due to reduced smoke formation and fire hazard [Krahenbuhl, 2002]. Wakabayashi et

al. [2006] introduced some synthetic esters, synthesized from a specific polyhydric

alcohol. These synthetic esters have high biodegradability, excellent oxidation stability,

good storage stability, and satisfactory cutting performance. Investigated synthetic esters

were suggested as satisfactory MQL cutting fluid on the basis of cutting performance and

14

optimal fluid for MQL machining on the basis of biodegradability, oxidation and storage

stability.

Fig. 1.2 Fishbone diagram showing cause and effect in MQL assisted machining

Some studies reported that application of MQL results in zero airborne mist levels as the

oil mist either vaporizes or clings to the work-piece or chips [Dasch and Kurgin, 2010].

However, Dasch and Kurgin [2010] found MQL mist level comparable to wet application

and proportional to the volume of oil entering the system. So, mass concentration and

particle size as well as composition and physical state of mist requires serious attention.

Advantages of MQL assisted machining are: fluid supplied to the cutting tool is consumed

at once so there is no need of fluid monitoring, maintenance or disposal [Dasch and

Kurgin, 2010]; reduction in solid waste by 60%, water use by 90%, and aquatic toxicity

by 80% due to delivery of lubricants in air instead of water [Clarens et al., 2008];

decreased coolant costs due to low consumption of cutting fluid; reduced toxicity and

hazardous effects as mostly vegetable oils are used which are nontoxic and biologically

inert [Khan et al., 2009]; reduced cleaning cost and time due to low residue of lubricant

on chip, tool and work-piece [Attansio et al., 2006]; better visibility of cutting operation

[Attansio et al., 2006].

Turning of AISI 52100 hardened steel was studied Diniz et al. [2003] using TiN coated

CBN inserts under dry cutting, wet cutting and minimum volume of oil (MVO). Mostly

15

similar values of flank wear and surface roughness were obtained with dry and MVO

cutting. Values of flank wear and surface roughness were always found better than wet

cutting. Based on study, dry cutting was concluded as the best technique for turning of this

material. The better performance of dry cutting was attributed to increased cutting zone

temperature that caused easier deformation and shearing of chip, reduced cutting forces

and vibration, and reduced tool wear. Attansio et al. [2006] studied tool wear in finish

turning of 100Cr6 normalized steel pieces under MQL and dry cutting conditions using

triple coated carbide tip (TiN outer layer, Al2O3 intermediate layer and TiCN inner layer).

MQL was applied on rake and flank face of the tool at constant cutting speed of 300 m/min

and depth of cut of 1 mm, and at feed rate of 0.2 and 0.26 mm/rev with cutting length of 50

mm and 200 mm. Equal or greater mean removed material was reported with flank MQL

as compared to dry and rake MQL. Tool life decreased with feed rate in all cutting

conditions however the tool life obtained in flank MQL was highest. Tool life increases in

flank MQL with increase in cutting length whereas it does not influence tool life in dry and

rake MQL. In rake MQL, lubricant was not able to reach the cutting area as no elements

indicating compounds from lubricant were seen on worn surface of tool tip. Tool wear and

surface roughness of AISI-4340 alloy was studied by Dhar et al., [2006b] with uncoated

carbide insert under MQL conditions. Principal flank wear and auxiliary flank wear were

selected to study the tool wear as former affects the cutting force, and latter affects the

surface finish and dimensional deviation. Reduced tool wear and improved surface finish

was achieved with MQL as compared to dry and wet machining mainly due to effective

reduction in cutting temperature.

In another study on same alloy, chip tool interface temperature and dimensional deviation

were also monitored along with surface roughness and tool wear. At low cutting speeds the

chip makes partially elastic contact with the tool but with increase in cutting speed chip

makes fully plastic or bulk contact with the rake face of the tool. So, at low cutting speeds

more effective cooling was observed as MQL was dragged due to capillary effect in the

elastic contact zone. While, at high cutting speed less reduction in cutting temperature was

observed due to reduction in time to remove accumulated heat and due to fully plastic or

bulk contact preventing the MQL to reach the hot chip-tool interface. Decrease in feed

improves the cooling effect to some extent particularly at low cutting speed possibly due to

16

slight lifting of the thinner chip. About 5 to 10% decrease in average cutting temperature

was recorded depending upon the level of cutting speed and feed rate. Reduced

dimensional deviation with machining time was observed with MQL as compared to that

in dry and wet turning [Dhar et al., 2007]. Rahman et al. [2009] reported about 5 to 10%

reduction in average cutting temperature in MQL turning of AISI 9310 alloy depending

upon the levels of cutting speed and feed.

In MQL turning of AISI-1040 with uncoated carbide, MQL jet was targeted on the rake

and flank face of the auxiliary cutting edge to achieve better dimensional accuracy. With

MQL application the cutting temperature is effectively reduced and blue colored spiral

shaped chips produced under dry and wet conditions became metallic colored and half

turn. Also the back surface of chip under MQL is much brighter and smoother indicating

the favorable chip tool interaction and elimination of built–up edge formation. Reduced

value of chip compression ratio and improved dimensional accuracy was also achieved

with MQL [Dhar et al., 2006a]. Similar improved results were also reported in MQL

turning of AISI-9310 and AISI-1060 alloy by using vegetable oil-based cutting fluid

[Khan and Dhar., 2006; Khan et al., 2009]. Physics based models for MQL was

developed by Li and Liang [2007] to predict the cutting temperature, cutting force, tool

wear and aerosol generation rate.

The models were validated with the experimental results obtained in turning of AISI 1045

material. MQL was supplied on the flank face of the tool by a 0.762 mm diameter opening

in the tool holder. Cutting forces for MQL are found smaller than dry cutting but higher

than wet cutting. At lower cutting speeds lubrication was effective but at high cutting

speed (228.75 m/min) ineffective lubrication was observed. MQL was most effective in

reducing the tangential cutting force among the cutting force components. MQL also

reduced the cutting temperature for the entire range of speed and provided a lower wear

rate in comparison to dry cutting. However, it was expected that MQL will generate more

cutting fluid aerosol than flood cooling due to splash mechanism. Tasdelen et al. [2008]

investigated the affect of different cooling techniques such as MQL, compressed air and

emulsion on tool chip contact length in turning of 100Cr6 steel with different engagement

times of inserts. Lower contact lengths were observed with MQL and compressed air as

17

compared to dry cutting. However, emulsion assisted cutting provided the shortest contact

length. For long engagement times, MQL and compressed air have same contacts lengths,

as the cooling effect was mainly from air constituent in aerosol. For short engagement

times, lubrication effect of oil drops decrease the friction in the sliding region and

overcomes the cooling effect resulting in shorter contact lengths than compressed air. Also

at short engagement times, increase in quantity of oil decreases the contact length. More

up curled chips were obtained with emulsion than MQL and air assisted cutting. Chips

obtained from MQL and compressed air have almost same radius of curvature. Whereas,

chips obtained from dry cutting have largest radius of curvature. Chips obtained in dry

cutting were wider than the chips obtained with other methods due to side flow in the shear

plane and have side curl due to difference in speed at outer and inner diameter of work-

piece. Shorter contact lengths were observed with TiN coated tool due to different friction

and temperature distribution in the cutting zone. Effect of oil drops were found even at

reduced engagement time for TiN coating than uncoated carbides. On the basis of study it

was concluded that for short engagement time machining MQL is very suitable.

A study was conducted by Hwang and Lee [2010] to predict the cutting force and surface

roughness and to determine the optimal combination of cutting parameters in turning of

AISI 1045. To determine the significant parameters among supplied air pressure, nozzle

diameter, cutting speed, feed rate and depth of cut a two level fractional factorial design is

employed. It was reported that except supplied air pressure all the parameters significantly

affected the surface roughness. Models are then developed for prediction of cutting speed

and surface roughness in MQL and wet turning using Central Composite Design. From the

validation experiment cutting force equations are found valid whereas surface roughness

equations were not appropriate for accurate prediction. The mismatch in experimental and

predicted values was attributed to uncontrolled parameters such as work material defect,

lathe vibration and measuring errors. Nozzle diameter of 6 mm, cutting speed of 361

m/min, feed rate of 0.01 mm/rev and depth of cut of 0.1 mm were found optimal for MQL

turning whereas nozzle diameter of 6 mm, cutting velocity of 394 m/min, feed rate of 0.02

mm/rev and depth of cut of 0.1 mm were found optimal for wet turning considering

surface roughness and cutting forces simultaneously. MQL turning was found to be more

18

advantageous than wet turning when only surface roughness and cutting forces were

considered.

The cutting fluids are used in machining processes to improve he characteristics of

tribological processes, which are always present on the contact surfaces between tool and

work pieces. Due to several negative effects, a lot has been done in the recent past to

minimize or even completely avoid the use of cutting fluids [Sokovic and Mijanovic,

2001]. A number of attempts were made in the past to improve cooling/lubrication in high

speed machining and in the case of machining of difficult -to-machine materials by the use

of a minimum quantity lubricant jet to overcome these problems. The results achieved by

these investigators were very encouraging [Suda et al., 2004, McCabe, 2001]. Cutting

forces were reduced, chip shape, surface quality and tool life improved, thereby increasing

the metal removal rate and improving the overall performance of the machining operation.

In particular, minimal quantity lubrication (MQL) machining has already played a

significant role as successful near-dry machining in a number of practical applications

[Sutherland et al. 2000, Rahman et al., 2002]. Minimum quantity of lubrication (MQL)

in machining is an alternative to completely dry or flood lubricating system, which has

been considered as one of the solutions for reducing the amount of lubricant to address the

environmental, economical and mechanical process performance concerns [Heinemann et

al., 2006]. The concept of MQL sometimes referred to as near dry machining that is based

on the principle of total use, without residues, applying lubricant between 10 to 100 ml/h at

a pressure from 4 to 6.5 Kgf/cm2 [Silva et al., 2005]. The minimum quantity lubricants

have the advantages of advanced thermal stability and lubricating capability over

conventional cutting fluids. In this technology, the lubricating function is ensured by the

oil and the cooling function is provided mainly by the compressed air. From the

viewpoints of environment, safety, health, performance and cost, the vegetable based oils

are considered as alternatives to petroleum-based metalworking cutting fluids because of

its high biodegradability, high viscosity indexes and good thermal stability [Gawrilow,

2003]. The vegetable-based oils could produce better results than the mineral reference oil

in view of increased machining performance as well as renewable sources [Belluco and

Chiffre, 2004]. Many researchers [Belluco and Chiffre, 2004; Rahman et al., 2002;

Itogawa et al., 2006; Khan and Dhar, 2006; Heinemann et al., 2006] have conducted

19

various machining investigations using MQL with both synthetic oils and vegetable oils.

Many researchers have suggested the MQL techniques in machining processes [Rahman

et al., 2001; Davim et al., 2006]. Lugscheider et al. [1997] applied this technique in

reaming process of grey cast iron and aluminium alloy with coated carbide tools. The

significant reduction in tool wear and improvement in surface quality of the holes have

been observed using MQL technique when compared to dry cutting. Dhar et al. [2006]

employed MQL technique in turning of AISI 1040 steel and the results clearly indicated

that a mixture of air and soluble oil machining is better than conventional flood coolant

system. Braga et al. [2002] reported that the holes obtained during drilling of aluminium–

silicon alloys with uncoated and diamond coated K10 carbide tools using MQL technique

presented either similar or better quality than those obtained with flood lubricant system.

The investigations carried out by Kelly and Cotterell [2002] on aluminium alloy revealed

that the MQL technique is preferable for higher cutting speeds and feed rates. Davim et al.

[2007] carried out experimental investigations on machining of brass under different

conditions of lubricant environments. The primary function of cutting fluid is cooling and

lubrication. The cooling and lubricating properties of cutting fluids are critical in

decreasing tool wear and extending tool life. Cooling and lubrication are also important in

achieving the desired size, finish and shape of the workpiece. In respect of profit, safety

and convenience a number of alternatives to traditional machining are currently under

development. Minimum quantity lubrication (MQL) is an obvious and very intricate

balance between dry machining and traditional methods. MQL (Minimum Quantity

Lubricant) technique, a very small quantity of oil is applied to the cutting point and it

improves the cutting performance as compared to dry cutting. Some investigations have

been performed to examine the effect of MQL technique on the reduction of tool wear and

the improvement of surface roughness of work materials, but there are few studies which

have investigated the influence of MQL technique on the tool temperature [Byrne et al.

2003].

The reduced utilization of cooling lubricants, in order to improve environmental

protection, safety of machining processes and to decrease time and costs related to the

number of machining operations, can be pursued performing machining processes with

the MQL (minimum quantity of lubricant) technique or without any cutting fluid (dry

20

cutting) [Diniz et al. 2003]. Such approaches can allow the obtaining of the product

specifications, in terms of surface roughness and dimensional accuracy, by the shortening

conventional process cycles (i.e. avoiding grinding). The effect of the lubrication-cooling

condition on the surface quality of the machined part strongly depends on the type of

machining operation to be performed (e.g. turning, milling, etc.), as well as on the process

parameters to be used. In particular, in face milling operations cutting takes place with

high frequency tooth impacts, depending on the cutting speed, and discontinuously due to

the presence of several teeth; for such reasons dry and MQL face milling can be performed

over a wide field of workpiece materials [ Weinert et al. 2004, Vieira et al. 2001]. In line

with growing environmental concerns involved in the use of cutting fluids in machining

processes, as reported by several researchers and manufacturers of machine tools, strong

emphasis is being placed on the development of environmentally friendly technology, i.e.,

on environmental preservation and the search for conformity with the ISO 14000 standard.

On the other hand, despite persistent attempts to completely eliminate cutting fluids, in

many cases cooling is still essential to the economically feasible service life of tools and

the required surface qualities. This is particularly true when tight tolerances and high

dimensional and shape exactness are required, or when the machining of critical, difficult

to cut materials is involved. This makes the minimum quantity of lubrication an interesting

alternative, because it combines the functionality of cooling with an extremely low

consumption of fluids (usually < 80ml/h). These minimal quantities of oil suffice, in many

cases, to reduce the tool’s friction and to prevent the adherence of materials. The

minimization of cutting fluids has gained increasing relevance in the past decade [Dörr &

Sahm, 2000]. This small amount of fluid suffices to reduce friction in cutting, diminishing

the tendency of adhesion in materials with such characteristics. A comparison with

conventional cooling revealed numerous advantages [Dörr and sahm 2000 and Diniz and

Micarony 2002]. However, compared with the conventional technique, MQL involves

additional costs to pressurize the air and technological supports needed in the process in

order to overcome the technological restrictions of the MQL technique. For instance,

special techniques for transporting chips may be necessary, and productivity may decrease

due to the thermal impact on the machined components. Oil vapor, mist and smoke

generated during the use of MQL in machining can be considered undesirable byproducts,

since they contribute to increase the index of airborne pollutants. This has become a factor

21

for concern, requiring a adequate exhaustion system in the machine tool. A compressed air

line that works intermittently during the process is used for atomization. These compressed

air lines generate noise levels that usually exceed the legally established limits [Machado

& Diniz, 2001]. Based on the known costs of wet machining and MQL machining, a

comparison was made of the costs of investments and annual fixed and proportional costs

at the BMW company. The comparison of the total investment costs in the transfer line,

results of the process still merit further in-depth studies. Minimum quantity lubrication

systems employ mainly cutting fluids that are no soluble in water, especially mineral oils.

Due to the very small amounts of cutting fluids used, one must consider that the costs

should not prevent the use of high technology compositions in the field of basic and

additive oils. Vegetal-based materials are being increasingly used. These oils, inhaled in

the form of aerosol, reduce the health hazard factor [Novaski & Dörr, 1999].

Kamata and Obikawa [2007] investigated finish turning of Inconel 718 under MQL with

three types of coated carbide tool (TiCN/Al2O3/TiN (CVD), TiN/AlN superlattice (PVD)

and TiAlN (PVD)). Biodegradable synthetic ester was supplied with compressed air on

both the rake and flank face of the tool. On the basis of tool life and surface finish,

TiCN/Al2O3/TiN (CVD) and TiN/AlN superlattice (PVD) were found suitable for finish

turning of Inconel 718 with MQL. They reported that optimization of air pressure is

required for appropriate application of MQL in finish turning of Inconel 718. They also

reported that carrier gas plays a vital role in cooling of cutting point as short tool life were

obtained with argon gas as compared to air. Increase in cutting speed from 1m/s to 1.5 m/s

resulted in drastic decrease of tool life and worse surface finish for both the coatings under

MQL condition. Also, increase in lubricant quantity, increased the tool life and surface

roughness for TiCN/Al2O3/TiN coating, whereas it decreased the tool life and surface

roughness slightly for TiN/AlN coated tool.

Su et al. [2007] used cooled air (at a temperature of -200C) with MQL at a pressure of 6

bar in finish turning of Inconel 718 alloy. Application of cooled air and cooled air with

MQL resulted in 78% and 124% improvement respectively in tool life over dry cutting.

Improvement in tool life was attributed to reduction in cutting temperature resulting in

reduced abrasion, adhesion and diffusion wear. Surface roughness was also reduced

22

drastically in both conditions due to reduction in tool wear. Significant improvement in

chip shape was also reported as short continuous tubular chips were obtained under both

conditions.

Obikawa et al., [2008] observed that control of oil mist flow and decrease in distance

between nozzle and tool tip enhances the cutting performance of MQL particularly in

Micro-liter lubrication range (oil consumption less than 1 ml/h). Finish turning of Inconel

718 was investigated in micro-liter lubrication (μLL) range with biodegradable ester using

three different types of nozzles: ordinary type, cover type for normal spraying and cover

type for oblique spraying.

Effectiveness of MQL (eMQL) was computed by the relation (1)

eMQL=

TMQL- Tdry

Twet- Tdry

.........................................................(1)

where TMQL, Tdry and Twet are tool life for MQL, dry and wet turning.

Ordinary nozzle and cover type nozzle for normal spraying are not found suitable for μLL.

Values of eMQL decreased to 0.50, 0.47 and 0.36 as oil consumption (Q) decreased to 1.1 ,

0.5 and 0.2 ml/h for ordinary nozzle whereas with cover type nozzle for normal spraying

increase in eMQL is 0.22 and 0.18 for Q =0.50 and 0.20 ml/h over ordinary nozzle. The

cover type nozzle for oblique spraying provided significant improvement as value of eMQL

was 0.80,0.94 and 0.97 for Q =0.2, 0.5 and 1.1 ml/h respectively. Good surface finish and

tool life of 47 min was obtained at an oil consumption of 0.5 ml/h and cutting speed of 1.3

m/s.

A study on effects and mechanisms in MQL intermittent turning of Aluminum alloy

(AlSi5) was conducted by Itoigawa et al. [2006]. MQL was studied with oil and oil film on

water droplet using rapeseed oil and synthetic ester as lubricant. MQL with rapeseed oil

showed only a small lubrication effect in light loaded machining conditions. MQL with

synthetic ester shows a lubrication effect but there was significant tool damage and

aluminum pick-up on the tool surface. MQL with water droplets using synthetic ester

23

provided good lubrication. They reported that influence of water for good frictional

performance depends not on the film chemi-sorption process but on water’s chilling effect

to sustain boundary film strength. In MQL machining of 6061 aluminum alloy the quantity

of adhered material to the tool was more as compared to flooded coolant and less as

compared to dry cutting. No considerable reduction in material adhesion and flank wear

was observed by increasing the lubricant quantity to two times. Significant increase in

flank wear was reported with increase in cutting speed. Cutting forces were found highest

under dry cutting and lowest under flooded condition. The variation of cutting forces with

different machining strategies is attributed to the amount of adhesion. Surface roughness

obtained by MQL is found to lie between dry cutting and flooded condition [Sreejith,

2008].

Davim et al. [2007] conducted a study on turning of brass with MQL to study the effect of

the quantity of cutting fluid. They compared the cutting power, specific cutting force,

surface roughness and chip form with MQL at Q =50, 100, 200 ml/h and with flood

cooling at Q =2000 ml/h. Cutting parameters in the experimental test are cutting speed (v)

=100, 200 and 400 m/min, feed rate (s) =0.05, 0.10, 0.15 and 0.2 mm/rev, depth of cut (t)

=2 mm. Slightly higher cutting power was observed with MQL lubrication at 50 ml/h and

flood lubrication at 2000 ml/h whereas almost same power is noticed with MQL at flow

rate of 100 and 200 ml/h. This suggests that similar/better cutting conditions can be

achieved with MQL as compared to flood lubrication. The specific cutting force is found

lower at a cutting velocity of 200 m/min except for fluid lubrication and reported it to be a

critical speed for brass machining. At Q =200 ml/h specific cutting force is found to be

lowest. Surface roughness decreased with increase in flow rate. Similar surface roughness

is observed with MQL at Q =200 ml/h and flood lubrication. Also for all the machining

conditions the relation between Rt and Ra was found maintained. Similar chip forms were

observed MQL and flood lubrication. In further work by Gaitonde et al., [2008], quantity

of lubricant, cutting speed and feed rate were determined for simultaneously minimizing

surface roughness and specific cutting force by using Taguchi method and utility concept.

They reported that Q =200 ml/h, v =200 m/min and s =0.05 mm/rev are optimal process

parameters. Feed rate is found to be most significant factor followed by quantity of MQL

lubricant and cutting speed in optimizing the machinability characteristics.

24

Effect of dry cutting, flood coolant, and minimum quantity lubrication were studied in

continuous and interrupted turning of Ti-6Al-4V alloy with uncoated carbide inserts. It

was reported that in continuous cutting, MQL seems to be more effective than flood

cooling at high cutting speed and feed rate due to its better lubrication ability. In

interrupted cutting MQL was also found more effective than dry and flood coolant

particularly in two slots cutting [Wang et al., 2009]. The main problem with machining of

titanium alloys is related to high heat generation at tool-chip interface due to which

machining of these alloys is recommended only with copious amount of cutting fluid. As

the main concern in titanium alloy machining is to remove the heat generated during the

process, Minimum quantity cooling (MQC) seems to be more appropriate than MQL. A

sequential procedure for determining operating parameters in MQC assisted turning of Ti-

6Al-4V alloy is presented in following section.

1.3 Summary of review

During machining temperature is the apprehensive element and it is essential to control

this cutting temperature for good surface finish. Cutting fluid reduces cutting temperature

and also provides lubrication effect between the tool and work interface. There are

different types of lubrication as well as cooling system available to minimize the cutting

temperature. Many researchers also made their investigations using these different types of

cooling and lubrications systems like flood cooling, near dry cooling, or micro lubrication,

high pressure jet cooling, cryogenic cooling and MQL cooling. Flood cooling reduces

temperature to some extent by bulk cooling but is not very much effective because it cools

only the top surface of the job and the due to its overhead application. It has some bad

effects too, when cooling fluid comes in contact with the human body it creates skin

irritation, lung cancer etc. Near dry cooling is based on air coolant, a little amount of

temperature is reduced. Cryogenic cooling effectively reduces temperature from the

cutting zone but it is very costly and in nitrogen rich atmosphere notch wear of the tool

takes place. High pressure coolant has been reported to provide some reduction in cutting

temperature [Aronson 2004]. It reduces temperature very quickly due to high pressure jet

coolant reaches very easily in the chip-tool interface. It has been experienced that

25

lubrication is effective at low speeds when it is accomplished by diffusion through the

work piece and by forming solid boundary layers from the extreme pressure additives, but

at high speeds no significant lubrication effect in evident. The ineffectiveness of

lubrication of the cutting fluid at high speed machining is attributed to the inability of the

cutting fluid to reach the actual cutting zone and particularly at the chip-tool interface due

to bulk or plastic contact at high cutting speed. On the other hand the cooling and

lubricating affects of cutting fluid influence each other and diminish with increase in

cutting velocity. The machining temperature could be reduced to some extent by

improving the machinability characteristics of the work material metallurgical, optimizing

the tool geometry and by proper selection of process parameters. Some recent techniques

have enabled partial control of machining temperature by using heat resistance tools like

coated carbides, CBN etc. the thermal deterioration of the cutting tools can be reduced by

using CBN tools. But CBN tools are very expensive. Although the modified inserts offer

reduced cutting force, their beneficial effect on surface finish is marginal. It was reported

that that coolant injection offers better cutting performance in terms of surface finish, tool

force and tool wear when compared to flood cooling.

Coolants are frequently used in machining processes. They reduce tool wear, dissipate heat

from the workpiece and machine, assist in the removal of swarf and release cutting

residues attached to the workpiece, tool and equipment. The increased use of coolants in

recent years has caused a considerable increase in costs for procurement, maintenance and

disposal. The compatibility of coolants with the environment and the potential health risks

to the machine operator generated by the latter have increasingly become the subject of

criticism. The disposal of used coolants is not entirely sound ecologically and is also

causing rising costs. These costs are often underestimated because they are mainly

included in general overheads. Thus in fabrication operations with centralized systems,

they reach a level of 7 to17 % of the total fabrication costs. In comparison with the latter,

the tool costs in general amount to just 2 to 4 % of overall costs (Figure 1). Minimizing

lubricant consumption therefore has to be the ecological and economic objective of a

fabrication operation which looks towards the future. Against the background of modern

cutting materials and advanced coatings, the question arises as to whether machining

without lubricant i.e. “dry” is possible on an industrial scale.

26

Fig. 1.3 Distribution of Fabrication costs in engineering

The influence of cutting speed and feed rate on machinability aspects were studied and

concluded that flood lubrication can be successfully replaced by MQL type of lubrication.

However, in manufacturing machining industries, the temperature and its detrimental

effects are generally reduced by:

• Proper selection of process parameters and geometry of the cutting tools.

• Proper selection and application of cutting fluid.

• Using heat and wear resistant cutting tool materials like carbides, coated carbides

and high performance ceramics (CBN and diamond are extremely heat and wear

resistive but those are too expensive and justified for very special work materials

and requirements where other tools are not effective.

Coolant plays a significant role in improving lubrication as well as minimizing

temperature at the tool-chip and tool-workpiece interfaces, consequently seizure during

machining. Flood cooling is not effective in terms lowering cutting temperature when

machining exotic materials. The coolant does not readily access the tool-workpiece and

tool-chip interfaces that are under seizure condition as it is vaporized by the high cutting

temperature generated close to the tool edge. Machining of medium carbon based alloys at

high-speed conditions can therefore be achieved by combination of the appropriate tool

material, machining technique and the choice of a suitable cooling technology. Minimum

quantity lubrication is one of the preferred technologies currently under exploitation.

27

In general, when machining steel with coated carbide tools different tool wear mechanisms

occur, such as: abrasion, adhesion, oxidation and even some diffusion, which act

simultaneously and in proportions depending mainly on the temperature. The task of

defining which of those mechanisms is the predominant one has become a very complex

task. However, some researches relating wear mechanisms to the cutting speed have been

made and some important results have been published. For example, the raise in

temperature at the cutting zone occurs basically due to the cutting speed increase. The

abrasion phenomenon occurs predominantly at low cutting speeds, adhesion at medium

ones, and oxidation/diffusion at high ones. The limit of growing for cutting speed depends

on several other factors, such as tool-workpiece combination, contact time between them

and the presence of cutting fluids. However, those findings are only indications and may

not offer more than recommendations for practical applications. By superimposing wear

mechanisms and their relations with cutting speed, it is possible to explain most of the tool

wear observed in practice; although in some cases the causes may depend on some other

factors occurring at the cutting zone and tool/workpiece contact area, [Arsecularatne et

al., 2006]. In machining of steels the use of proper coating structure can contribute to

substantial reduction of the friction action between the rake and chip and result in a

decrease in heat generation and lower the tool-chip interface temperature. The selection of

work piece material with low thermal conductivity and low heat capacity an a coating

material with low thermal conductivity leads to a reduction in the contact length, resulting

in the effect of a thermal barrier. As a consequence, heat is concentrated within the thin top

layer of the coating to protect the tool against diffusion.

The application of MQL or dry cutting techniques requires that the functions of the cooling

lubricant are carried out by the other components of the machining system. To this

purpose, the cutting material and tool coating play a key role. In the last decades, research

has led to the development of cutting materials with improved performances, such as ultra-

fine grain cemented carbides, cermets, ceramics, cubic boron nitrides and diamond, in

order to withstand the higher temperatures occurring under MQL and dry machining

conditions, and provide a longer tool life [Weinert et al. 2004; Poulachon et al. 2001;

Noordin et al. 2001]. A further contribution has been given by the development of tool

coatings that can compensate for the lack of the cooling lubricant; in particular, they can

28

improve the tool wear behavior, reduce the thermal load of the cutting tool by acting as

thermal barrier and improve the sliding behavior on the flank and rake faces by acting as a

solid lubricant. Improvements in coating technology have led to the development of

multilayer coatings, nanolayer coatings, supernitrides, self-lubricating coatings, CBN

coatings and diamond coatings [Weinert et al. 2004, Grzesik et al. 2006; Zhang et al.

2007; Arndt 2003; Belmonte 2003].

At present, industry and researchers are looking for ways to reduce the use of lubricants

because of ecological and economical reasons. Therefore, metal cutting is to move toward

dry cutting or semi-dry cutting. This project presents an investigation into MQL

(Minimum Quantity Lubrication) machining with the objective of deriving the optimum

cutting conditions for the turning process of 42CrMo4. To reach these goals several finish

turning experiments were carried out, varying cutting speed, feed rate and Cutting fluid

with MQL. The surface roughness and tool wear results of tests were measured and the

effects of cutting conditions were analyzed by the graphical presentation. From the

experimental results, it is found that a better surface roughness can be obtained by

decreasing oil quantity and feed rate.

1.4 Scope of the Present Work

There are a lot of scope and necessary to carry out intensive research and development

work for more effective and efficient machining of such increasingly used steels. Such

research and development work through understanding of mechanism and mechanics of

machining of this critical steel will essentially enable enhanced productivity, product

quality, tool life and overall economy in machining though optimum selection of process

parameter, tool material and geometry and environment.

Chapter 1 comprises with Introduction and Literature Review of the previous

works. the objective of the present work is included at the end of the chapter.

Chapter 2 comprises with Experimental set-ups, Experimental procedures, data

collected and Investigations on experiment.

Chapter 3 comprises with the detail Discussion on Experimental Results. The

experimental results are plotted or tabulated where necessary and presented in this chapter.

29

In Chapter 4 pin pointed conclusions are drawn from the research outcomes.

Finally a list of references are provided in separate Chapter.

1.5 Objectives of the present work

It is exposed from the aforementioned literature review that minimum quantity lubrication

(MQL) assisted machining is starting to be established as a method for substantial increase

of removal rate and productivity in the metal cutting industry. The economy of machining

steel is strongly connected to effective chip control, for higher utilization of machines and

temperature reduction in the tool, for raising the rates of metal removal. The growing

demands for high MRR, mainly the high cutting temperature restrains precision and

effective machining of exotic materials. Thorough investigation is essential to explore the

potential benefits of minimum quantity lubrication (MQL) in such cases. But enough work

has not been done systematically yet in this direction.

The objective of the present work is to make an experimental investigation on the role of

minimum quantity lubrication (MQL) by different types of cutting fluids (soluble cutting

fluid, insoluble cutting oil and vegetable oil) on the cutting performance of AISI 1060 as

compared to completely dry cutting in respect of

i. Average chip-tool and work-tool temperature

ii. Chip morphology (Chip shape, color, Chip thickness ratio)

iii. Surface finish (Ra) and

iv. Dimensional accuracy

In machining steel by the industry used coated carbide tool (SNMG 120408) at different

speeds and feeds combination. In this study, the minimum quantity lubrication (MQL) was

provided with a spray of air and cutting fluids at a pressure 8 bars and coolant flow rate of

100 ml/hr. The results indicated that the use of minimum quantity lubrication (MQL) by

cutting oil (VG-68) leads to reduced surface roughness, delayed tool wear and lowered

cutting temperature significantly in compare to other environments.

These results are being expected to improve machinability due to reduction in cutting zone

temperature enabling favorable chip formation and chip-tool interaction. It will also

30

provide reduction in tool wear which will enhance the tool life, dimensional accuracy and

surface finish.

Experimental Investigations

2.1 Introduction

High speed machining are those technologies, which are recently being increasingly

applied in the production industries especially in mould producing. These machining

process are characterized by their productivity, good surface finish quality and higher

dimensional tolerances. The economy of machining steel is strongly connected to effective

chip control, for higher utilization of machines and temperature reduction in the tool, for

raising the rates of metal removal. Cutting with the excess amount of cutting fluids is still

very common in conventional machining to control high cutting temperature which

adversely affects, directly and indirectly, chip formation, cutting forces, tool life and

dimensional accuracy and surface integrity of the products.

The effectiveness, efficiency and overall economy of machining any work material by

given tool depend largely on the machinability characteristics of the tool-work material

under the recommended conditions. The conditions are

• Cutting temperature; which affects product quality and cutting tool performance

• Chip formation pattern

• Magnitude of cutting forces; which affects power requirements, vibration and

dimensional accuracy

• Surface finish

• Tool wear and tool life

For achieving substantial technological and economical benefits in addition to

environmental friendliness, Minimum quantity lubrication system needs to be properly

designed considering the following important factors:

• Effecting cooling by enabling cutting liquid jet reach as close as to the actual hot

zones as possible.

32

• Avoidance of bulk cooling of the tool and the job, which may cause unfavorable

metallurgical changes.

• Minimum consumption of cutting fluid by pin-pointed impingement and only

during chip formation.

The machining responses have been studied and evaluated for assessing the machinability

characteristics of the steel specimen under both dry and minimum quantity lubrication

(MQL) conditions.

2.2 Minimum Quantity Lubrication System

The beneficial role of minimum quantity lubrication system on environmental friendliness

has already been established. The aim of the present work is primarily to explore and

evaluate the role of such cooling on machinability characteristics of some commonly used

tool-work combinations mainly in terms of cutting temperature, surface finish and chip-

forms, which governs productivity, product quality and overall economy.

A MQL supply system typically consists of a compressor, container for cutting fluid, fluid

supply pump, mixing chamber, nozzle, separate pipes for supply of cooling lubricant and

air for their independent adjustment. However, the components of the MQL system may

vary depending upon the type of fluid delivery system and atomization. The schematic

diagram of the experimental set-up fabricated at Machine Tool Laboratory, Dhaka

university of Engineering & Technology, Gazipur is presented in Fig. 2.1. The schematic

of mixing chamber is also shown in Fig. 2.2 An air compressor was used to increase

pressure of air. In order to deliver metered supply of air at desired pressure, pressure

regulator and flow meter are fitted in air supply line. The pressurized air from nozzle can

be directed on rake face depending upon experimental requirement. A fluid chamber fitted

with a pump was used to store and deliver the coolant to the nozzles. A commercially

available flexible tube and float type flow controller was used to control the supply of

cutting fluid to the nozzle. Commercially available gas welding nozzles were used to

impinge aerosol at high velocity in the cutting zone. The inlet side of nozzles was modified

by drilling for air and coolant.

33

For MQL supply it is very important to design MQL mixing system. First of all it is

necessary to mix air and lubricant to obtain the mixture to be spread on the cutting surface.

Two different mixing methods can be used: mixing inside nozzle and mixing outside

nozzle. Using the mixing inside nozzle equipment, pressurized air and lubricant are mixed

into the nozzle by a mixing device. The Fig. 2.1 and Fig. 2.2 show the photographic and

schematic views of mixing chamber for MQL supply. The lubrication is obtained by the

lubricant, while a minimal cooling action is achieved by the pressurized air that reaches the

cutting surface. Several advantages derive applying this method. Mist and dangerous

vapors are reduced and the mixture setting is very easy to control.

Fig. 2.1 Photographic view of Mixing Chamber for MQL Supply

Fig. 2.2 Schematic view of the mixing chamber along with nozzle

2.3 Experimental Procedure and conditions

Machining medium carbon steel by coated carbides is a major activity in the

manufacturing industries. Machining of steel involves more heat generation for their

ductility and production of continuous chips having more intimate and wide chip-tool

contact. In this experiment MQL conditions are used during machining to compare the

results with the same by dry condition. For MQL supply the positioning of nozzle tip is

Mixing chamber

Compressed air

Cutting fluid

Nozzle

MQL

34

very important and that has been settled after a number of trials. During machining the

MQL jet is directed along the main cutting edge to reach at the principal flank and partially

under the flowing chips to the cutting edges.

The photographic view and schematic view of the experimental setup are shown in Fig. 2.3

and Fig. 2.4 respectively. In the figure it is shown that a minimum quantity lubrication jet

was injected through the tool rake face, consists of a compressor, MQL applicator, centre

lathe machine, tool-wear thermocouple and experimental steel. In this study, the minimum

quantity lubrication (MQL) was provided with a spray of air and cutting fluids at a

pressure 8 bars and coolant flow rate of 100 ml/hr.

Fig. 2.3 Photographic view of the MQL applicator

Fig. 2.4 Schematic view of the experimental set up

35

Effectiveness of cooling and the related benefits depend on how closely the minimum

cutting fluid can reach the chip-tool and work-tool interfaces where apart from the primary

shear zone, heat is generated. The tool geometry is reasonably expected to play significant

role on such cooling effectiveness. Considering the view of tool configuration (WIDIA)

namely SNMG 120408 has been undertaken for the present investigation. The inserts were

clamped in a PSBNR-2525 M12 (WIDIA) type tool holder.

The experiment was carried out on lathe, which has a 7.5 kW spindle and maximum

spindle speed of 1600 rpm. The work material was medium carbon steel having external

diameter of 105 mm, and length of 500 mm. The tool holder provided negative 6° side and

back rake angles and 6° side cutting-edge and end cutting-edge angles. The ranges of the

cutting speed (V), feed (f) and depth of cut (d) were selected based on the tool

manufacturer’s recommendation and industrial practices. The following cutting conditions

were employed in this investigation:

Table 2.1 Experimental conditions

• Machine tool Lathe, 7.5 kW

• Work material AISI 1060 steel

Chemical Composition by weight percent

(Fe ꞊98.67, C ꞊ 0.58, Si ꞊ 0.07, Mn ꞊ 0.6, P ꞊ 0.03 and S

꞊ 0.05)

Physical Properties : Density 7.85 g/cm3, Melting point 1510ºC.

Mechanical Properties : Ultimate tensile strength 620 MPa, Yield strength 485 MPa.

Hardness, Brinell: 183

• Cutting insert Coated carbide, SNMG 120408, WIDIA

Process parameters

• Cutting speed, V 67, 84, 105, 134, 167 and 211 m/min

• Feed, f : 0.10, 0.125, 0.15 and 0.175 mm/rev

• Depth of cut, d : 1mm

• Working tool geometry -6°,-6°,6°,5°,15°,75°,0.8 (mm)

• Coolant type : soluble fluid, Cutting oil (VG 68) and Vegetable oil

• Coolant delivery

methods

: Minimum quantity lubrication with a spray of air and cutting

fluids at a pressure of 8 bars and flow rate of 100 ml/hr

36

A number of Cutting velocity V and feed rate f have been taken over relatively wider

ranges keeping in view the industrial recommendations for the tool-work materials

undertaken and evaluation of role of variation in V and f on the effectiveness of MQL.

The average cutting temperature was measured by a noncontact type laser operated spot

infrared thermometer. An infrared thermometer is a thermometer which infers temperature

from a portion of the thermal radiation sometimes called blackbody radiation emitted by

the object being measured. They are sometimes called laser thermometers if a laser is used

to help aim the thermometer, or non-contact thermometers or temperature guns, to describe

the device's ability to measure temperature from a distance. By knowing the amount of

infrared energy emitted by the object and its emissivity, the object's temperature can often

be determined. Infrared thermometers are a subset of devices known as "thermal radiation

thermometers".

The most basic design consists of a lens to focus the infrared thermal radiation on to a

detector, which converts the radiant power to an electrical signal that can be displayed in

units of temperature after being compensated for ambient temperature. This configuration

facilitates temperature measurement from a distance without contact with the object to be

measured. As such, the infrared thermometer is useful for measuring temperature under

circumstances where thermocouples or other probe type sensors cannot be used or do not

produce accurate data for a variety of reasons.

The thickness of the chips directly and indirectly indicates the nature of chip-tool

interaction influenced by the machining environment. The chip samples were collected

during short run and long run machining for the cutting speed-feed combinations under dry

and MQL conditions. The thickness of the chips was repeatedly measured by a slide

caliper to determine the value of chip thickness ratio. The roughness of the machined

surface after each cut was measured by a Talysurf (Sutronic 25 , Tylor Rank Hobson

limited, UK) which is shown in Fig. 2.5. the results are documented and plotted at various

MQL environment having different combinations velocities and feed.

37

Fig. 2.5 Photographic view of surface roughness measuring technique

2.4 Experimental Results

2.4.1 Cutting temperature

Heat is generated at the (i) primary deformation zone due to shear and plastic deformation

(ii) chip-tool interface due to secondary deformation and sliding and (iii) work-tool

interface due to rubbing during machining of any ductile materials,. All such heat sources

produce maximum temperature at the chip-tool interface. The chip formation mode,

cutting forces and tool life are substantially influenced by this temperature. Therefore,

many attempts are made to reduce this detrimental cutting temperature. Conventional

cutting fluid application, may or to some extent cool the tool and job in bulk but can not

cool and lubricate expectedly at the chip-tool interface where the temperature is maximum.

This may be happened due to the bulk contact of flowing chips with the tool rake surface

or due to elastic contact with the tool before leaving the chips. Elastic contact allows slight

penetration of the cutting fluid only over a small region by capillary action. The cutting

fluid action becomes more and more ineffective at the interface with the increase in cutting

speed when the chip-tool contact becomes almost fully plastic or bulk.

38

60 80 100 120 140 160 180 200 220

360

400

440

480

520

560

600

640

680

720

760

800

Work material : AISI 1060 Steel

Cutting tool : SNMG 120408

Depth of cut : 1.0 mm

Envoronment

Dry

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Ave

rag

e C

hip

-to

ol in

terf

ace

te

mp

era

ture

, 0

C

Cutting Velocity, VC m/min

Dry

60 80 100 120 140 160 180 200 220

360

400

440

480

520

560

600

640

680

720

760

800




Envoronment

Wet

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Ave

rag

e C

hip

-to

ol in

terf

ace tem

pe

ratu

re, 0C


Wet

Fig. 2.6 Variation of average chip-tool interface temperature with different cutting speed

and feed under dry and wet condition during machining AISI 1060 steel

60 80 100 120 140 160 180 200 220

360

400

440

480

520

560

600

640

680

720

760

800




Envoronment

Cutting oil VG 68

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Avera

ge C

hip

-tool in

terf

ace te

mpera

ture

, 0

C


60 80 100 120 140 160 180 200 220

360

400

440

480

520

560

600

640

680

720

760

800




Envoronment

Vegetable oil

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Avera

ge C

hip

-tool in

terf

ace tem

pera

ture

, 0

C


MQL with Vegetable oil MQL with Cutting oil VG 68

Fig 2.7 Variation of average chip-tool interface temperature with different speed and

feed under different MQL condition during machining AISI 1060 steel

60 80 100 120 140 160 180 200 220

360

400

440

480

520

560

600

640

680

720

760

800

Aver

age

chip

-to

ol

Inte

rfac

e T

emper

ature

, 0

C

Average Cutting Speed, m/min

Dry

Wet

MQL Veg oil

MQL Cut. Oil

60 80 100 120 140 160 180 200 220

360

400

440

480

520

560

600

640

680

720

760

800

Aver

age

chip

-tool

Inte

rface

Tem

per

ature

, 0

C


Dry

Wet

MQL Veg oil

MQL Cut. Oil

Feed 0.10 mm/rev Feed 0.125 mm/rev

Fig 2.8 Variation of average chip-tool interface temperature with different speed under

dry, wet, MQL with Vegetable oil and MQL with Cutting Oil, VG 68 condition


39

60 80 100 120 140 160 180 200 220

360

400

440

480

520

560

600

640

680

720

760

800

Avera

ge

chip

-tool

Inte

rface

Tem

per

ature

, 0

C


Dry

Wet

MQL Veg oil

MQL Cut. Oil

60 80 100 120 140 160 180 200 220

360

400

440

480

520

560

600

640

680

720

760

800

Av

erag

e ch

ip-t

oo

l In

terf

ace

Tem

per

atu

re, 0

C


Dry

Wet

MQL Veg oil

MQL Cut. Oil

Feed 0.15 mm/rev Feed 0.175 mm/rev

Fig. 2.9 Variation of average chip-tool interface temperature with different speed under

dry, wet, MQL with Vegetable oil and MQL with Cutting Oil, VG 68 condition


2.4.2 Machining Chips

Machining is a process of shaping by the removal of material which results in chips and

the geometrical and metallurgical characteristics of these chips are very representative of

the performance of the process because the form (Shape, color) and thickness of the chips

directly and indirectly indicate the nature of chip-tool interaction influenced by the

machining environment. In general chip formation in machining can be categorized as

forming continuous, discontinuous or serrated chips.

In the present work, the chip samples are collected while turning the AISI 1060 steel by

SNMG insert at different speed-feed combinations under dry, conventional coolant and

MQL conditions by the cutting oil (HC straight run, VG 68) and vegetable oil have been

visually examined and categorized with respect to their shape and color. The form and

color of all these chips were noted down based on ISO 3658-1977 (E) standard chip forms.

The results of such categorization of the chips produced at different environments have

been shown in Table 2.2.

40

Table

2.2

Comparison of chip shape and color at different speed and feed under dry and

different MQL conditions machining of AISI 1060 steel by SNMG coated carbide

insert.

Feed

f

Cutting

Velocit

y

V

Dry Wet MQL(Vegetable

Oil)

MQL(Cutting Oil)

Chip

Shape

Color Chip

Shape

Color Chip

Shape

Color Chip

Shape

Color

0.10

67 Long

Tubular

Metallic Loose

Arc

Metallic Loose

arc

Metallic Loose

arc

Metallic

84 Long

Tubular

Metallic Loose

Arc

Metallic Loose

arc

Metallic Loose

arc

Metallic

105 Long

Spiral

Metallic Loose

Arc

Metallic Loose

arc

Metallic Loose

arc

Metallic

134 Ribon Metallic Tubular Golden Loose

arc

Metallic Loose

arc

Metallic

167 Ribbon Golden Tubular Golden Loose

arc

Bluish Loose

arc

Metallic

211 Snarled

Ribon

Golden Tubular Bluish Loose

arc

Blue Loose

arc

Bluish

0.125

67 Loose

Arc

Metallic Loose

Arc

Metallic Loose

arc

Metallic Loose

arc

Metallic

84 Long

Helical

Metallic Loose

Arc

Metallic Loose

arc

Metallic Loose

arc

Metallic

105 Long

Helical

Golden Loose

Arc

Metallic Loose

arc

Metallic Loose

arc

Metallic

134 Long

Tubular

Golden Long

Tubular

Golden Loose

arc

Metallic Loose

arc

Metallic

167 Long

Spiral

Golden Long

Tubular

Golden Loose

arc

Golden Loose

arc

Golden

211 Ribon Burn

Blue

Long

Spiral

Bluish Loose

arc

Bluish Loose

arc

Bluish

0.15

67 Loose

Arc

Burn

Blue

Loose

Arc

Metallic Loose

arc

Golden Loose

arc

Metallic

84 Long

Tubular

Metallic Loose

Arc

Metallic Loose

arc

Metallic Loose

arc

Metallic

105 Long

Spiral

Burn

Blue

Loose

Arc

Metallic Loose

arc

Metallic Loose

arc

Metallic

134 Long

Spiral

Blue Long

Spiral

Golden Loose

arc

Golden Loose

arc

Golden

167 Long

Spiral

Burn

Blue

Long

Spiral

Golden Loose

arc

Blue Loose

arc

Bluish

211 Loose

Arc

Golden Long

Spiral

Blue Loose

arc

Blue Loose

arc

Bluish

0.175

67 Long

Tubular

Metallic Loose

Arc

Metallic Loose

arc

Golden Loose

arc

Metallic

84 Long

Tubular

Golden Loose

Arc

Metallic Loose

arc

Golden Loose

arc

Metallic

41

105 Long

Tubular

Golden Long

Helical

Golden Loose

arc

Golden Loose

arc

Golden

134 Long

Helical

Burn

Blue

Long

Helical

Golden Loose

arc

Bluish Loose

arc

Golden

167 Long

Helical

Burn

Blue

Long

Helical

Bluish Loose

arc

Blue Loose

arc

Bluish

211 Long

Tubular

Burn

Blue

Long

Helical

Blue Loose

arc

Blue Loose

arc

Bluish

Chip Shape

Group Loose Arc Chips Long Tubular Snarled Tubular

Chips

Snarled Ribbon

In machining another important machinability index is chip reduction coefficient, rc (ratio

of chip thickness before and after cut). For a given tool geometry and cutting conditions,

the value of rc depends upon the nature of chip-tool interaction, chip contact length and

chip form all of which are expected to be influenced by MQL conditions to the level of

speed and feed. The thickness of the chips was repeatedly measured by a digital slide

caliper to determine the value of chip thickness ratio, rc (ratio of chip thickness before and

after cut). The variation in value of rc with speed and feed at various MQL conditions has

been plotted which are shown from Fig. 2.9, Fig. 2.10, Fig. 2.11, Fig. 2.12 respectively.

42

60 80 100 120 140 160 180 200 220

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70




Envoronment

Dry

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Ave

rage

Ch

ip T

hic

kne

ss R

atio

, r c


Fig. 2.10 Variation of average chip thickness ratio (rc) with different speed and feed

under Dry Condition during machining AISI 1060 steel

60 80 100 120 140 160 180 200 220

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70




Envoronment

Wet

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Ave

rage

Ch

ip T

hic

kne

ss R

atio

, r c


Fig. 2.11 Variation of average chip thickness ratio (rc) with different speed and feed

under Wet Condition during machining AISI 1060 steel

43

60 80 100 120 140 160 180 200 220

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70




Envoronment

Cutting oil VG 68

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Ave

rage

Ch

ip T

hic

kne

ss R

atio

, r c


Fig 2.12 Variation of average chip thickness ratio (rc) with different speed and feed

under MQL (Cutting Oil VG 68) condition during machining AISI 1060 steel

60 80 100 120 140 160 180 200 220

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70




Envoronment

Vegetable oil

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Ave

rage

Ch

ip T

hic

kne

ss R

atio

, r c


Fig 2.13 Variation of average chip thickness ratio (rc) with different speed and feed

under MQL (Vegetable oil) condition during machining AISI 1060 steel

2.4.3 Surface roughness

The performance and service life of any machined part are governed largely by the product

quality of that product, which for a given material is generally assessed by dimensional

and form accuracy and surface integrity of the product in respect of surface roughness,

44

oxidation, corrosion, residual stresses and surface and subsurface micro cracks. Surface

roughness is predominantly considered as the most important feature of practical

engineering surfaces due to its crucial influence on the mechanical and physical properties

of machined parts. So, characterization of surface topography is essential in applications

involving friction, lubrication and wear, contact resistance etc. Surface finish is also

important index of machinability or grindabiity because performance and service life of

the machined/ground component are often affected by its surface finish, nature and extent

of residual stresses and presence of surface or subsurface micro cracks particularly when

the component is to be used under dynamic loading or in conjunction with some other

mating parts.

Although roughness is usually undesirable it is difficult and expensive to control in

manufacturing. To decrease roughness it is necessary to increase manufacturing cost. This

often results in a trade-off between the manufacturing cost of a component and its

performance in application. Several factors will influence the final surface roughness in a

machining operation. The final surface roughness might be considered as the sum of two

independent effects: (1) the ideal surface roughness is a result of the geometry of the tool

and feed rate and (2) the nature of surface roughness is a result of the irregularities in the

cutting operation. Factors such as spindle speed, feed rate and depth of cut that control the

cutting operation can be setup in advance and factors such as tool geometry, tool wear,

chip loads and chip formations, or the material properties of both tool and work piece are

uncontrolled. Even in the occurrence of the chatter or vibrations of the machine tool,

defects in the structure of the work material, wear of tool, or irregularities of chip

formation contribute to the surface damage in practice during machining. Surface

roughness has been measured at two stages- (i) after a few seconds of machining with the

sharp tool while recording the cutting forces and (ii) with the progress of machining while

monitoring growth of tool wear with machining time.

The variation in surface roughness observed with advancement of machining of steel by

the coated carbide SNMG insert at a particular set of cutting speed (V), feed rate (f), and

depth of cut (t), under dry, wet and different MQL conditions which have been shown in

Fig. 2.13.

45

60 80 100 120 140 160 180 200 220

2.0

2.5

3.0

3.5

4.0

4.5

5.0Work material : AISI 1060 Steel



Envoronment

Dry

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Avera

ge S

urf

ace

Ro

ug

hn

ess, R

a (µm

)


Fig. 2.14 Variation of average surface roughness (Ra) with different speed and feed

under Dry Condition during machining AISI 1060 steel

60 80 100 120 140 160 180 200 220

2.0

2.5

3.0

3.5

4.0

4.5

5.0




Envoronment

Wet

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Avera

ge S

urf

ace

Ro

ug

hn

ess, R

a (µm

)



under Wet Condition during machining AISI 1060 steel

46

60 80 100 120 140 160 180 200 220

2.0

2.5

3.0

3.5

4.0

4.5

5.0




Envoronment

Cutting oil VG 68

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Avera

ge S

urf

ace

Ro

ug

hn

ess, R

a (µm

)



under MQL (Cutting Oil) condition during machining AISI 1060 steel

60 80 100 120 140 160 180 200 220

2.0

2.5

3.0

3.5

4.0

4.5

5.0




Envoronment

Vegetable oil

Feed rate

0.10 mm/rev

0.125 mm/rev

0.15 mm/rev

0.175 mm/rev

Avera

ge S

urf

ace

Ro

ug

hn

ess, R

a (µm

)



under MQL (Vegetable oil) Condition during machining AISI 1060 steel

47

2.4.4 Dimensional Deviation

0 50 100 150 200 250 300 350 400 450 500

0

50

100

150

200

250

300

350

400

450

500



Cutting velocity : 105 m/min

Feed : 0.125 mm/rev


Dim

en

sio

nal D

evia

tio

n, (µ

m)

Length of Cut, mm

Dry

Wet

MQL (Vegitable oil)

MQL (Cutting oil, VG 68)

Fig. 2.18 Variation of dimensional deviation under Dry, Wet, MQL with insoluble

cutting oil and MQL with soluble cutting fluid condition during machining

AISI 1060 steel

Discussion on Experimental Results

3.1 Cutting Temperature

In machining material removal rate (MRR) increases with increase in velocities and feed

rates. Cutting temperature also increases with the increase in MRR and specific energy

consumptions. Such high cutting temperature adversely affects the chip formation, cutting

forces, tool life, dimensional accuracy and surface finish of the products. Therefore,

application of minimum quantity lubrication (MQL) is expected to improve upon the

aforesaid machinability characteristics which play an imperative role on productivity,

product quality and overall economy. It also provides an environmental friendliness

system in machining particularly when the cutting temperature is very high.

The average chip-tool interface temperature is usually measured using reliable tool-work

thermocouple technique. But in this project work the average cutting temperature is

measured by using a non-contact type laser operated infrared thermometer which infers

temperature from a portion of the thermal radiation and plotted against cutting velocity for

different work-tool combinations, feeds and environments undertaken.

Infrared thermometers can be used to serve a wide variety of temperature monitoring

functions. A few examples provided include:

• Monitoring materials in process of heating and cooling, for research and

development or manufacturing quality control situations

• Checking mechanical equipment or electrical circuit breaker boxes or outlets for

hot spots

• Checking heater or oven temperature, for calibration and control purposes

• Detecting hot spots / performing diagnostics in electrical circuit board

manufacturing

• Checking for hot spots in fire fighting situations

• Measuring temperature of active and/or dormant volcanos

• Detecting clouds for remote telescope operation

49

Sometimes, especially near ambient temperatures, false readings will be obtained

indicating incorrect temperature. This is most often due to other thermal radiation reflected

from the object being measured, but having its source elsewhere, like a hotter wall or other

object nearby - even the person holding the thermometer can be an error source in some

cases. It can also be due to an incorrect emissivity on the emissivity control or a

combination of the two possibilities.

Fig. 2.6, Fig. 2.7, Fig. 2.8 and Fig. 2.9 are showing how and to what extent temperature

has decreased due to application of cutting fluids in minimum quantity lubrication (MQL)

system under the different experimental conditions by coated SNMG insert. With the

increase in cutting speed and feed, average chip-tool interface temperature increased as

usual, even under minimum quantity lubrication (MQL) system, due to increase in energy

input.

The cutting temperature generally increases with the increase in cutting speed and feed,

though in different degree, due to increased energy input and it could be expected that MQL

would be more effective at higher values of cutting speed and feed. The average chip-tool

interface temperature have been determined by using non-contact type infrared

thermometer and plotted against cutting speed for different work-tool combinations, feed

rates and environments undertaken. The figures from Fig. 2.6 to Fig. 2.9 are showing how

and to what extent cutting temperature has decreased due to application of MQL

application under the different experimental conditions. With the increase in cutting speed

and feed, cutting temperature increased as usual, even under MQL conditions, due to

increase in energy input. The difference in cutting temperature noted for the different

work-tool combinations under dry machining and same cutting speed-feed conditions has

been mainly due to difference in specific energy requirement.

Apparently more drastic reductions in cutting temperature are expected by employing

MQL. But practically it has not been so because the MQL has been employed in the form

of thin jet along the cutting edge and towards only the chip-tool interface instead of bulk

cooling. Also the jet, like any cutting fluid, could not reach deeply in the chip-tool

50

interface for plastic or bulk contact, particularly when cutting speed and feed are large. The

MQl is more effective using cutting oil VG 68.

Table 3.1: Effectiveness of MQL with Vegetable oil

Feed Speed TMQL TDry TMQL-

T

TWet TWet-

T

Effectiveness

0.10

67 450 570 -120 480 -90 1.33

84 500 650 -150 540 -110 1.36

105 540 670 -130 580 -90 1.44

134 580 710 -130 620 -90 1.44

167 590 720 -130 630 -90 1.44

211 600 730 -130 640 -90 1.44

0.125

67 490 615 -125 535 -80 1.56

84 570 690 -120 610 -80 1.50

105 580 690 -110 620 -70 1.57

134 600 725 -125 645 -80 1.56

167 610 735 -125 655 -80 1.56

211 620 740 -120 660 -80 1.50

0.15

67 540 650 -110 580 -70 1.57

84 605 710 -105 640 -70 1.50

105 600 715 -115 645 -70 1.64

134 610 735 -125 650 -85 1.47

167 640 755 -115 685 -70 1.64

211 650 755 -105 690 -65 1.62

0.175

67 590 695 -105 635 -60 1.75

84 640 730 -90 670 -60 1.50

105 645 735 -90 675 -60 1.50

134 650 751 -101 690 -61 1.66

167 660 765 -105 705 -60 1.75

211 670 765 -95 715 -50 1.90

In comparison between Table 3.1 and table 3.2, it is evident that the effectiveness of

cutting oil VG 68 is more effective under all the condition and tool-work condition

undertaken during this project work. under low pressure situation vegetable oil itself

creates an obstacle in its flow. Regarding the reduction in cutting temperature all the

speed-feed condition undertaken cutting oil VG 68 performs better basicallt due to its

higher cooling and lubricating capability than that of vegetable oil.

51

Table 3.2: Effectiveness of MQL with Cutting oil VG 68

Feed Speed TMQL TDry TMQL-

T

TWet TWet-

T

Effectiveness

0.10

67 390 570 -180 480 -90 2.00

84 440 650 -210 540 -110 1.91

105 480 670 -190 580 -90 2.11

134 520 710 -190 620 -90 2.11

167 530 720 -190 630 -90 2.11

211 540 730 -190 640 -90 2.11

0.125

67 430 615 -185 535 -80 2.31

84 510 690 -180 610 -80 2.25

105 520 690 -170 620 -70 2.43

134 540 725 -185 645 -80 2.31

167 570 735 -165 655 -80 2.06

211 560 740 -180 660 -80 2.25

0.15

67 490 650 -160 580 -70 2.29

84 555 710 -155 640 -70 2.21

105 550 715 -165 645 -70 2.36

134 560 735 -175 650 -85 2.06

167 590 755 -165 685 -70 2.36

211 595 755 -160 690 -65 2.46

0.175

67 540 695 -155 635 -60 2.58

84 600 730 -130 670 -60 2.17

105 605 735 -130 675 -60 2.17

134 610 751 -141 690 -61 2.31

167 620 765 -145 705 -60 2.42

211 620 765 -145 715 -50 2.90

3.2 Machining Chips

The pattern of chips in machining ductile metals are found depending upon the mechanical

properties of the work material, tool geometry particularly rake angle, levels of cutting

speed and feed and cutting environment.

Chip thickness ratio, rc (ratio of chip thickness before and after cut) is an important

machinability index of chip formation and specific energy consumption for a given tool-

work combination. For given cutting conditions, the value of chip thickness ratio depends

upon the nature of chip-tool interaction, chip contact length and chip form all of which are

expected to be influenced by MQL in addition to the levels of cutting speed and feed rate.

52

In machining conventional ductile metals and alloys producing continuous chips, the value

of rc is generally less than 1.0 because chip thickness after cut becomes greater than chip

thickness before cut due to almost all sided compression and friction at the chip-tool

interface. Smaller value of rc means larger cutting forces and friction and is hence

undesirable.

The effect of increase in cutting speed and feed and the change in environment on the value

of chip thickness ratio, rc obtained during turning AISI 1060 steel are shown in Fig. 2.10

which depict some significant facts;

i. values of rc has all along been less than 1.0

ii. reduction of cutting zone temperature by the application of MQL increased

the value of rc further

iii. the value of rc increased with increase in cutting speed and feed

In machining any conventional ductile metals like steels also rc increases with increases in

cutting speed due to plasticization and shrinkage of shear zone. The figures from Fig. 2.10

to Fig. 2.13 clearly show that throughout the present experimental domain the value of rc

gradually increased with the increase in cutting speed though in different degree for the

different tool-work combinations, under both dry and MQL conditions. The value of rc

usually increases with the increase in cutting speed particularly at its lower range due to

plasticization and shrinkage of the shear zone for reduction in friction and built-up edge

formation at the chip-tool interface due to increase in temperature and sliding velocity.

Fig. 2.10 to Fig. 2.13 show that application of MQL has increased the value of rc

particularly at lower values of cutting speed and feed when the AISI 1060 steel rod was

machined by SNMG insert. By MQL applications, rc is reasonably expected to increase for

reduction in friction at the chip-tool interface and reduction in deterioration of effective

rake angle by built-up edge formation and wear at the cutting edges mainly due to

reduction in cutting temperature.

Table 2.2 shows that AISI 1060 steel, whose strength and hardness are much less

compared to that of the other steels, when machined by the pattern type SNMG insert

53

under dry condition produced loose arc chips. The geometry of the SNMG insert is such

that the chips of this softer steel (AISI 1060 steel) first came out continuously, got curled

along normal plane and then hitting at the principal flank of this insert broke into pieces

with regular size and shape. When machined under MQL conditions the form of these

ductile chips did not change but their back surface appeared much brighter and smoother.

This indicates that the amount of reduction of temperature due to application of MQL

enabled favourable chip-tool interaction and elimination of even trace of built-up edge

formation. The colour of the chips have also become much lighter i.e. metallic from blue

due to reduction in cutting temperature due to cooled MQL jet especially when employed

MQL with Cutting oil VG 68.

3.3 Surface roughness

Surface finish is also an important index of machinability because performance and service

life of the machined/ground component are often affected by its surface finish, nature and

extent of residual stresses and presence of surface or subsurface microcracks, if any,

particularly when that component is to be used under dynamic loading or in conjugation

with some other mating part(s). Generally, good surface finish, if essential, is achieved by

finishing processes like grinding but sometimes it is left to machining. Even if it is to be

finally finished by grinding, machining prior to that needs to be done with surface

roughness as low as possible to facilitate and economize the grinding operation and reduce

initial surface defects as far as possible. The major causes behind development of surface

roughness in continuous machining processes like turning, particularly of ductile metals

are:

i. feed marks or scallop marks inherently left by the tool tip

ii. built-up edge formation, if any

iii. vibration in the tool work system

iv. irregular deformation of the tool nose due to chipping, wear and fracturing

The level of feed rate directly and almost proportionately governs the surface roughness in

machining by single point cutting tools but the value of cutting speed also affects the

pattern and extent of surface finish, though indirectly through deformation of the tool nose

profile, BUE formation and vibration.

54

Fig. 2.14 to Fig. 2.17 show the variation of Ra values of surface roughness after 45

seconds of turning the AISI 1060 steel rod at different cutting speed-feed combinations

under dry, wet and MQL conditions by coated SNMG insert.

Fig. 2.14 to Fig. 2.17 clearly show that surface roughness increased with the increase in

feed rate and decreased with the increase in cutting speed. Reduction in Ra with the

increase in V may be attributed to smoother chip-tool interface with lesser chance of built-

up edge formation in addition to possible truncation of the feed marks and slight flattening

of the tool-tip. The improvement in surface finish by MQL might be due to reduction in

break-in wear and also possibly reduction or prevention of built-up edge formation

depending upon the work material and cutting condition.

However, it is evident that MQL jet with cutting oil VG 68 substantially improves surface

finish depending upon the work-tool materials and mainly through controlling the

deterioration of the auxiliary cutting edge by abrasive, chipping and built-up edge

formation.

3.4 Dimensional Deviation

MQL provided remarkable benefit in respect of controlling the increase in diameter of the

finished job with machining time as can be seen in Fig. 2.18. In plain turning the finished

job diameter generally deviates from its desired value with the progress of machining i.e.

along the job-length mainly for change in the effective depth of cut due to several reasons

which include wear of the tool nose, over all compliance of the Machine-Fixture-Tool-

Work (M-F-T-W) system and thermal expansion of the job during machining followed by

cooling. Therefore, if the M-F-T-W system is rigid, variation in diameter would be

governed mainly by the heat and cutting temperature. With the increase in temperature the

rate of growth of auxiliary flank wear and thermal expansion of the job will increase.

High-pressure coolant takes away the major portion of heat and reduces the temperature

resulting decrease in dimensional deviation desirably.

The machining tests conducted for studying the role of MQL on dimensional inaccuracy

were separately carried by fresh inserts. Dimensional deviation, for each tool-work-

55

environment combination, was measured along the job axis after one complete pass. Fig.

2.18 shows how dimensional deviation decreased spectacularly in case of machining AISI

1060 steel by coated SNMG inserts at 105 m/min under to MQL. Under such conditions

MQL with cutting oil was found to reduce dimensional deviation quite substantially,

particularly in case of SNMG inserts, as can be seen in Fig. 2.18.

56

Conclusion and Recommendation

4.1 Conclusions

This study led to the evaluation of the effectiveness of the MQL system. The results

clearly indicate the advantages of using MQL over dry and flood cooling while

machining the material. Based on the observations made and the experimental results

obtained, the following conclusions are made:

i. The present MQL system enabled reduction in average chip-tool interface

temperature and even such apparently small reduction, unlike common belief,

enabled significant improvement in some machinability indices.

ii. Due to MQL jet, the form and colour of the steel chips became favourable for

more effective cooling and improvement in nature of interaction at the chip-tool

interface. In the case of application of MQL jet through an external nozzle,

friction is reduced at the chip-tool interface due to the fragmentation of the chip

by the impinging jet which prevents intimate contact at the chip-tool interface,

consequently leading to bending and self breakage of chips.

iii. The surface finish obtained is much better than that obtained in the case of dry

machining. The welding of hot chip to the cutting edge which is a common

problem while machining ductile steel is completely eliminated with the

application of MQL jet, leading to an improvement in surface quality.

iv. Dimensional accuracy also substantially improved mainly due to significant

reduction in cutting temperature due to application of MQL jet.

57

v. Further, the enhanced effectiveness of the coolant/lubrication by applying the

cutting oil at high velocity in the form of a narrow jet, leads to a reduction in the

quantity of the cutting oil being used, reducing the amount of disposal, which is a

primary concern of environmental protection authorities.

4.2 Recommendations

i. In this research work only one MQL jet is applied along the rake surface. Other

application methods, for example, along the main cutting edge and flank surface,

can be further investigated in the future. The best solution of application methods

to control cutting temperature, forces, tool wear and product quality can be offered

through studying those configurations.

ii. All testing presented in this work used coated carbide insert with SNMG tool

geometry, although it is not expected that this geometry is optimal for any or all

cases. Previous work has shown that tool types and geometry affects nearly

everything about the process: chip formation mode, cutting temperature, cutting

forces, tool wear and failure, surface finish, residual stresses, and white layer

generation. The types and geometry of the cutting tool should also be considered

in dry and MQL cutting because the type and geometry of the cutting tool is an

important factor for machinability of steel.

iii. In this work, the pattern of flow is not considered. So for future investigations the

pattern of flow of jet can be measured, i.e., whether it is laminar or turbulent.

Though turbulent flow is able to transport more heat in comparison to laminar jet,

but for more thinning of jet lamina flow jet is preferable.

iv. This research work only focused on the effect of MQL jet on machinability of

steels like chip formation mode, cutting temperature and product quality. To

achieve a better understanding of the machining process planning with

environmental concerns as a factor of consideration, the cutting fluid atomization

58

behavior in MQL turning process in order to estimate the resulting air quality can

be further investigated in the future.

59

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