6

CHAPTER 2

LITERATURE SURVEY

2.1 INTRODUCTION

This chapter presents a detailed review on the literatures related to this

research work. Such a review of literature is essential to understand the

fundamental concepts of hard turning process and to acquire knowledge on the

latest development in the related areas. It helps to understand the basic

mechanisms of the process and to analyse the results obtained. Basic concepts of

hard machining and the details of cutting tools used are also discussed. The review

also presents information on various methods to reduce cutting fluid during

machining and details on design of experiments based on Taguchi techniques

which is extensively used in this research work.

2.2 HARD TURNING

Hard turning is a process of turning of hardened steel with hardness above

45 HRC. Hardened steels are widely used in automobile, tool and die industries.

Traditionally, hardened materials are machined using a process cycle consisting of

turning in the soft state, heat treating to the desired hardness and subsequently

finish grinding to the final dimension. Hard turning eliminates some steps involved

in the conventional process cycle and the components are machined to their final

dimension in the hardened state. Hard turning has gained popularity in machining

industries as an alternative to grinding process as it has several advantages over

grinding process. The various advantages of hard tuning over grinding are higher

productivity (Huddle, 2001), reduced set up times, surface finish closer to grinding

and ability to machine complex parts. A qualitative comparison between hard

turning and grinding (Klocke et al., 2005) is shown in Fig 2.1.

During hard turning, high hardness of work pieces results in large cutting

forces and high temperatures at the cutting zone. Hence, successful hard turning

requires ultra hard cutting tools and machine tools of high rigidity. Because of

these requirements hard turning cannot be easily adaopted on the shop floor

without major modification on the existing setup.

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Figure 2.1 Comparison between hard turning and grinding (Klocke et al., 2005)

2.2.1 Mechanism of chip formation in hard turning

Proper understanding of the mechanism of chip formation during hard

cutting is essential for process optimization and evaluation. The formation of saw-

tooth chips is one of the primary characteristics in the machining of hardened

steels with geometrically defined cutting tools (Tonshoff et al., 2000; Guoa et al.,

2004). Saw-tooth chip formation has been explained in literatures using two

mechanisms namely adiabatic shearing (Mabrouki et al., 2006; Hou et al., 1997)

and surface crack propagation (Vyas et al., 1999; Matsuo et al., 1991).

In adiabatic shearing, the root cause of saw-tooth chip formation is a

catastrophic thermoplastic instability. The decrease in flow stress is caused by

thermal softening which increases the strain. Increase in strain by thermal

softening is more pronounced than that associated with strain hardening. There are

two stages involved in this process. The first stage involves plastic instability and

strain localization in a narrow band in the primary shear zone. The second stage

involves gradual buildup of the segments with negligible deformation by the

upsetting of the wedge-shaped work material ahead of the advancing tool. The

8

gradual bulging of the chip segment slowly pushes the previously formed chip

segment. As upsetting of the segment being formed progresses, the buildup of

stresses in the primary zone causes intense shear between this segment and the one

before it. The highly intense concentrated shear bands that are observed between

the segments at approximately 45° to the direction of cutting are actually formed

between the segment already formed and the one being formed. This phenomenon

repeats as the cutting process progresses.

The effects of cutting parameters and material hardness are found to be

interdependent variables governing saw-tooth chip formation, which affect the

transition from continuous to shear-localized chip formation in hard machining.

The formation of saw-tooth chips is affected by factors such as material properties

and tool geometry. Vyas and Shaw (1991), showed that high hardness of

workpiece, large negative rake angle and large undeformed chip thickness promote

the crack initiation and the consequent formation of saw-tooth chips in hard

machining, while cutting speed has only a modest effect. Hard turning of H13 tool

steel with a PCBN tool insert indicated that workpiece hardness and cutting speed

influence the transition from continuous to saw-tooth chip formation (Ng and

Aspinwall, 2002). At low speeds continuous chips were formed. However, saw-

tooth chips were produced at higher speeds (Zhang and Guo, 2009). Moreover, the

speed at which the transformation from continuous chip to saw-tooth chip occurred

depended on the workpiece hardness (Komanduri et al., 1982). Segmented chips

were obtained for workpiece with lower hardness when machining was done at

higher cutting speeds (Poulachon et al., 2001). As the rake angle becomes more

negative or the cutting speed increases, the chip morphology transition from saw-

tooth chip to individual segments takes place more rapidly (Guoa and David,

2004).

2.2.2 Cutting tools for hard turning

During hard turning, high hardness of workpieces, large cutting forces, and

high temperatures at the tool–workpiece interface impose extreme requirements

for tool rigidity and tool wear resistance. Cutting tools for turning hardened steels

9

must be made of materials which fulfill the following requirements (Koenig et al.,

1984):

1) High hardness at high temperatures

2) High transverse rupture strength

3) High toughness

4) High compression strength

5) High resistance to thermal shock and

6) High resistance to chemical reactions.

Research in cutting tool technology has led to the development of cutting

tool materials with improved performances, such as ultra-fine grain cemented

carbides, cermets, ceramics, cubic boron nitrides and diamond. 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). Tool coatings can improve the tool wear behaviour,

reduce the thermal load of the cutting tool by acting as thermal barrier and improve

the sliding behaviour on the flank and rake faces by acting as a solid lubricant.

Various studies have been conducted to investigate the performance of

coated carbide, ceramic and CBN tools during machining of various hard

materials. Cutting forces, tool wear and surface roughness are the major factors

considered while machining of ferrous alloys in their hardened state. Lima et at.

(2005) studied the machinability of hardened AISI 4340 and D2 grade steels at

different levels of hardness by using various cutting tool materials. During turning

of steel with a hardness of 42 HRC with the coated carbide insert, tool wear rate

increased smoothly. In the case of the steel with a hardness of 50 HRC, higher

wear rates were observed. While machining AISI D2 steel hardened to 58 HRC

using a mixed alumina-cutting tool, flank wear increased with cutting speed and

depth of cut, which resulted in tool failure by spalling.

The influence of cutting speed, feed rate, depth of cut and machining time

on machinability of AISI 4340 (48 HRC) steel with multilayer CVD coated

(TiN/MT TiCN/Al2O3) carbide tool was analysed by Suresh et al. (2012). The

10

result revealed that there was an increase in tool wear with increase in cutting

speed for all values of feed rates. The increase in tool wear at higher values of

cutting speed was due to the abrasion at the rake face of the tool as the machining

progresses. Combination of lower values of cutting speed, feed rate and depth of

cut were found to reduce tool wear.

More et al. (2006) compared the cutting performance of the CBN–TiN

coated carbide tool and commercially available PCBN tipped inserts during hard

turning of AISI 4340 alloy in terms of tool wear, surface roughness, and cutting

forces. Flank wear and crater wear were observed on both the tools. The flank

wear was mainly due to abrasive actions of the martensite present in the hardened

AISI 4340 alloy. The crater wear of the CBN–TiN coated inserts was less than that

of the PCBN inserts because of the lubricity of TiN capping layer on the CBN–

TiN coating. The CBN–TiN coated carbide inserts demonstrated a tool life of

approximately 18–20 min per cutting edge, whereas PCBN tools produced a tool

life of 32 min. A cost analysis, based on a single cutting edge, showed that the

CBN–TiN coated carbide tools are capable of reducing machining costs and can be

an important substitute to PCBN compact tools for hard turning applications.

Coelho et al. (2007) investigated the wear on PCBN inserts with different

coatings during turning hardened AISI 4340 steel with a hardness of 52HRC.

Three coatings namely, TiAlN-nano, TiAlN and AlCrN applied on a PCBN

substrate were tested during the investigation. Results revealed that the lowest tool

wear was obtained with TiAlN-nano coated tools followed by TiAlN, AlCrN and

uncoated PCBN tools. The wear mechanism was predominantly by abrasion of the

hard carbide particles in AISI 4340 microstructure. The higher hot hardness of the

TiAlN-nano coated tools delayed tool wear and lasted longer than TiAlN tools,

although temperature was high enough to reach the oxidizing range for both. The

performance of AlCrN tools was not as good as that of the other tools.

The wear behavior during turning of AISI 4340 hardened alloy steels by

CBN and ceramic tools was studied by Luo et al. (1999). Experimental results

showed that the main wear mechanism for the CBN tools was abrasion whereas

11

ceramic tools exhibited adhesive wear and abrasive wear. There was an increase in

the tool life for both the tools when the cutting speed was increased. This is

attributed to the formation of a protective layer on the chip–tool interface. The

protective layer was formed due to the dissolution of the binder on the tool with

the work material. It was observed that there was decrease on tool wear when the

hardness of the work piece was increased.

Lim et al. (2001) studied the influence of work material on tool wear rates

using the wear map approach. Comparison of the flank wear characteristics of

TiC-coated cemented carbide tools during dry turning of two widely used steel

grades such as AISI 1045 and AISI 4340 steel was carried out. Severe flank wear

was observed when machining AISI 4340 steel compared to that of AISI 1040

steel.

The flank and crater wear characteristics of TiC-coated cemented carbide

tools during dry turning of a hot-rolled medium carbon steel (89 HRB) was

examined by Lim et al. (1999), under a wide range of machining conditions. At

high cutting speeds and feed rates, wear of the TiC layer on both flank and rake

faces was dominated by discrete plastic deformation which caused the coating to

wear up to the underlying carbide substrate. Wear also occurred as a result of

abrasion, cracking and attrition, with the latter leading to the wearing through of

the coating on the rake face under low speed conditions. When moderate speeds

and feeds were used, the coating remained intact throughout the duration of the

test.

It was observed that the wear mechanism of PCBN tool depends not only

on the chemical composition of the PCBN, and the nature of the binder phase, but

also on the hardness and microstructure of the work material. PCBN coated with

TiN improved the tool-life by reducing the diffusion between workpiece and tool

rake face (Poulachon et al., 2001).

Barry et al. (2001) investigated the wear mechanisms of CBN/TiC cutting

tools in the finish machining of BS 817M40 (AISI 4340) steel of 52 HRC. It was

12

observed that the dominant wear mechanism of CBN/TiC cutting tools was

chemical in nature.

2.2.3 Cutting fluid for hard turning

Hard turning process is associated with the high tool wear due to the high

cutting temperatures as the parts are turned in hardened state. High temperature

generated has adverse effect on dimensional accuracy, surface integrity and tool

life. In order to reduce the effect of high temperature during hard turning, use of

cutting fluid is a common practice in machining industries. Main function of

cutting fluid is to reduce the cutting temperature either through lubrication or

through cooling, or through a combination of both. Cutting fluids prevent the

overheating of workpiece, increase the tool life, improve surface finish, remove

chips from the cutting area, reduce the cutting forces and prevent corrosion of

workpiece and machine tool (Trend and Wright, 2000).

The use of cutting fluids provides technological benefits. But they give raise

to certain economical and environmental problems also. The cost of cutting fluid

ranges from 7% to 17% of the total machining cost where as the tool cost is only

2% to 4% (Klocke and Eisenblatter, 1997). Procurement and storage of cutting

fluid involves expenses and disposal of cutting fluid has to comply with

environmental legislation such as OSHA regulations (Sutherland et al., 2000)

which have become more stringent due to the recent awareness on the

environmental and occupational aspects on the shop floor. Cutting fluids used in

machining operations will vaporise due to high temperatures that exist in cutting

zones and form a mist. Vaporised cutting fluid particles suspended in atmospheric

air when inhaled by workers leads to different kinds of diseases. These may range

from minor skin irritation to respiratory problems, and even skin and other types of

cancer (Jarvholm and Lavenius, 1987). Because of this, there are more stringent

environmental legislations limiting the permissible exposure level of mist on the

shop floor. Therefore, elimination of the use of cutting fluids, if possible, can be a

significant economic incentive. Nowadays machining industries are being forced

13

to implement strategies to reduce the amount of cutting fluid they use in their

production lines.

Due to the technological innovations such as new tool materials, new tool

coating materials, and optimized tool geometry, machining without cutting fluid

called dry cutting, is being developed. Implementation of dry cutting requires

suitable measures to compensate for the primary functions of the fluid. Dry

machining eliminates all problems related to cutting fluids such as pollution and

health hazard. However in dry cutting operations, the friction and adhesion

between chip and tool tend to be on the higher side, which causes higher

temperatures, higher wear rates and, consequently, shorter tool lives (Klocke and

Eisenblatter, 1997). Thus tools used for dry machining must have high positive

rake angle and withstand high temperatures (Sreejith and Ngoi, 2000).

Procurement of cutting tools with the above features increases the overall

cost of machining. Since dry cutting is not possible for most applications and

cutting fluid is still harmful to the environment, the most reasonable step to

minimize the consequence of their use would be to reduce the consumption of

cutting fluids. Several techniques to apply a small quantity of cutting fluid have

been innovated and investigated during the last decade. The most widely used

methods found in literatures to alleviate the environmental and economical impacts

are Minimum Quantity Lubrication (MQL) and minimal fluid application.

Minimum quantity lubrication (MQL), also known as near dry machining

(NDM), refers to the use of low quantity of cutting fluid delivered in a compressed

air stream, directed at the cutting zone through an external supply nozzle (Machado

and Wallbank, 1997; Rahman et al., 2002). In MQL, lubrication is obtained via the

lubricant, while a minimum cooling action is achieved by the pressurized air that

reaches the tool-work interface. Further, MQL reduces induced thermal shock and

helps to increase the workpiece surface integrity in situations of high tool pressure

(Attanasio et al., 2006). In MQL application, heat transfer is predominantly in the

evaporative mode, which is more efficient than the convective heat transfer

prevalent in conventional wet turning.

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It is reported that the introduction of cutting fluid at the tool-chip interface

through specially designed cutting tools can bring forth better tool life, better

surface finish, low cutting force and better chip forms (Dhar et al., 2007).

However, the MQL technique has several limitations in its practical use. Applying

cutting fluid in the form of mist poses serious health hazards, as the mist or vapor

is toxic. Contact of mist with eye may cause irritation, and breathing of mist may

cause serious respiratory problems. In addition, oil mist also stains machine tool

and working space. In order to reduce the floating oil mist, a vacuum mist collector

is to be attached to each machine tool with a MQL system.

In minimal fluid application technique, small quantity of cutting fluid was

applied in the form of a high-velocity, narrow pulsing slug (Varadarajan et al.,

2002). This method is free from the problems associated with mist (Aoyama et al.,

2008). Cutting performance during minimal cutting fluid application was superior

to that during dry turning and conventional wet turning on the basis of cutting

force, tool life, surface finish, cutting ratio, cutting temperature, and tool-chip

contact length (Varadarajan et al., 2002). The following section describes the

technique of machining with minimal fluid application in detail.

2.3 MINIMAL FLUID APPLICATION

Machining with minimal fluid application is a technique to minimise the

use of cutting fluid on the shop floor. In this technique, extremely small (2 to 5 ml)

quantities of proprietary cutting fluid are applied at the critical zones as a pulsing

slug. It is reported that the frictional forces between two sliding surfaces can be

reduced by rapidly fluctuating the width of the lubricant filled gap separating them

(Uzi Landman, 1998). Varadarajan et al. (2002) used this principle and developed

a minimal cutting fluid application system for minimizing the consumption of

cutting fluid during hard turning. They achieved the fluctuation of width of

lubrication that is filled in the gap between the tool rake face and the chip using a

high velocity narrow pulsing slug of cutting fluid. It is reported that (Attanasio et

al., 2006; Dhar et al., 2007; Philip et al., 2001), this new technique not only

15

reduced the usage of cutting fluid drastically but offered better cutting

performance as well when compared to conventional wet turning.

2.3.1 Hard turning with Minimal fluid application

Varadarajan et al. (2002) introduced minimal fluid application technique in

hard turning process as an alternative to conventional flooded application. They used a

specially formulated cutting fluid directed to the cutting zone in the form of thin

pulsing slug using a fluid application system developed for this purpose. The fluid

application system consisted of a fuel pump generally used for diesel fuel injection

in truck engines coupled to a variable speed electric drive. The test equipment

permitted the independent variation of the pressure at the fluid injector, the

frequency of injection and the rate of application of cutting fluid. During hard

turning of an AISI 4340 hardened steel of 46HRC, coolant-rich (60%) lubricant

fluid with additives was applied at tool-work interface at a rate of 2 ml/min. The

pulsing slug was applied at a pressure of 20 MPa maintained at the fluid

application nozzle with a high pulsing rate of 600 pulses/min. Cutting experiments

indicated that cutting force was lower with minimal fluid application when

compared to dry and conventional wet turning. The tool–chip contact length was

found to be the least during minimal fluid application. The cutting tool temperature

was lower in the case of minimal fluid application. The reduction in cutting force,

cutting temperature and tool-chip contact length during minimal application

brought forth better surface finish and improved tool life. In addition, it was

observed that tightly coiled chips were formed during minimal application, while

long snarled chips were prevalent during dry turning and loosely curled chips were

formed during conventional wet turning. It was observed that during minimal fluid

application, the quantity of cutting fluid was only 0.05% of that used during

conventional wet turning. The authors also suggested that this technique can be

implemented without major alterations in the existing facilities on the shop floor.

Further investigation on hard tuning with minimal fluid application was

carried out by Vikram Kumar et al. (2007). They compared the performance of

TiCN and ZrN coatings on carbide tools during turning of hardened AISI 4340

16

steel in conventional dry turning and wet turning with minimal fluid application

method by varying parameters such as speed and feed, maintaining constant depth

of cut. They also statistically analysed the influence of different cutting and fluid

application parameters for different coated tools on machining performance. It was

found that in all cutting conditions, minimal fluid application gave better

performance than dry turning and conventional wet turning on the basis of

parameters such as cutting force, temperature and surface finish. Among the fluid

application and cutting parameters, the exit pressure at the nozzle was found to be

the most significant factor influencing cutting force during machining with

minimal fluid application. The increase in the nozzle pressure causes an increase in

exit velocity of the cutting fluid. This allowed better penetration of cutting fluid in

to the tool-chip interface resulting in the reduction of friction. Authors finally

concluded that by carefully choosing the fluid application parameters it is possible

to produce high quality components with minimum fluid application.

Ram Kumar et al. (2008) investigated the effect of two pulsing jets of

cutting fluid during minimum fluid application. One high velocity pulsating jet

was applied at the tool-work interface and other was applied on the back side of

the chip. The pressure of the pulsing jet was kept at 1.2 bar. Additional pulsing jet

of cutting fluid on the back side of the chip promoted chip curl due to difference in

the top and bottom surface temperatures. The chip-tool contact length reduced

leading to the reduction in cutting force and improvement in tool life. In this

system of twin jet of cutting fluid in minimal fluid application, optimum cutting

performance was obtained when cutting fluid was applied at the rate of 5 ml/min

and with a pulsing rate of 300 pulses/min.

Minimal cutting fluid application in high speed slot milling of hardened

steel using coated carbide ball end mill was explored by Thepsonthi et al. (2009).

Cutting fluid was applied in the form of a high-velocity, narrow, pulsing slug at a

rate of 2 ml/min. The parameters of application were set at a pulsing rate of 400

pulse/min, pressure of 20 MPa, and delivery rate of 2 ml/min using a fluid

application system developed. The direction of fluid application was set against the

17

feed direction. The performance of machining with pulsing slug application was

compared with dry machining and machining with flood application. It was found

from the experimental results that the performance of pulsing slug application in

slot milling was superior to that of dry cutting and flood application in terms of

surface finish and tool wear. The lowest flank wear was obtained in pulsing slug

mode for most cutting conditions. This was attributed to the good lubricity created

by the pulsing slug mode of cutting fluid application. However, the pressure of

fluid injection (20 MPa) was high enough to move the chips away from the cutting

area but not high enough to flush them away from the machined surface.

A new near dry cutting system called direct oil drop supply technique

(DOS) was proposed by Aoyama et al. (2008). The DOS system consist of a gear

pump which supply oil to a discharge unit at 0.4 MPa. The discharge unit

generates the intermittent oil pressure pulse to the DOS nozzle through a thin

stainless steel pipe. This pipe elastically expands and shrinks in response to the

pressure pulse application, and a tiny high-speed small oil drop is discharged from

the nozzle in phase with the rapid shrinkage of the pipe. Compressed air was

supplied at the cutting zone for cooling and chip cleaning. The discharge speed of

an oil drop from the nozzle was about 30 m/s. In order to supply small oil drops

against the peripheral air flow generated by the tool rotation, it was supplied with

high speed. The performance of the DOS technique was evaluated by the milling

processes. The proposed DOS lubrication technique considerably reduced the

amount of oil mist floating in the workspace compared to the existing MQL mist

supply technique.

2.4 DESIGN OF EXPERIMENTS

Design of experiments (DOE), also called experimental design, is a

structured and organized way of conducting and analyzing controlled tests to

evaluate the factors that are affecting a response variable. Design of experiments

was invented by Ronald A.Fisher in the 1920s. The three principles of

experimental design such as randomization, replication and blocking can be

utilized in industrial experiments to improve the efficiency of experimentation

18

(Jiju Antony, 2003). These principles of experimental design are applied to reduce

or even remove experimental bias.

While designing industrial experiments, there are factors, such as power

surges, operator errors, fluctuations in ambient temperature and humidity, raw

material variations, etc. which may influence the process output performance.

Such factors can adversely affect the experimental results and therefore must be

either minimized or removed from the experiment. Randomization is one of the

methods experimenters often rely on to reduce the effect of experimental bias. By

properly randomizing the experiment, it is possible to average out the effects of

noise factors that may be present in the process. In other words, randomization can

ensure that all levels of a factor have an equal chance of being affected by noise

factors.

Replication means repetitions of an entire experiment or a portion of it,

under more than one condition. Replication has two important properties. The first

property is that it allows the experimenter to obtain an estimate of the experimental

error. The second property is that it permits the experimenter to obtain a more

precise estimate of the factor/interaction effect. If the number of replicates is equal

to one or unity, it is not possible to make satisfactory conclusions about the effect

of either factors or interactions. Replication can result in a substantial increase in

the time to conduct an experiment. Moreover, if the material is expensive,

replication may lead to exorbitant material costs. Any bias or experimental error

associated with setup changes will be evenly distributed across the experimental

runs or trials using replication. The use of replication in real life must be justified

in terms of time and cost.

Blocking is a method of eliminating the effects of extraneous variation due

to noise factors and thereby improving the efficiency of experimental design. The

main objective is to eliminate unwanted sources of variability such as batch to-

batch, day-to-day, shift-to-shift, etc. The idea is to arrange similar experimental

runs into blocks (or groups). Generally, a block is a set of relatively homogeneous

experimental conditions. The blocks can be batches of raw materials, different

19

operators, different vendors, etc. Observations collected under the same

experimental conditions (i.e. same day, same shift, etc.) are said to be in the same

block. Variability between blocks must be eliminated from the experimental error,

which leads to an increase in the precision of the experiment.

The methodology of DOE is fundamentally divided into four phases,

namely the planning the phase, the designing phase, the conducting phase and the

analyzing phase. The planning phase is made up of problem recognition and

formulation, selection of response or quality characteristic, selection of process

variables or design parameters, classification of process variables, determining the

levels of process variables and listing all the interactions of interest. Most

appropriate design for the experiment is selected in the designing phase. The size

of the experiment is dependent on the number of factors and/or interactions to be

studied, the number of levels of each factor, budget and resources allocated for

carrying out the experiments. In conducting phase, planned experiment is carried

out and the results are evaluated. Having performed the experiment, the next phase

is to analyse and interpret the results so that valid and sound conclusions can be

derived. DOE techniques help in the following:

To determine the design parameters or process variables that affects

the mean process performance.

To determine the design parameters or process variables that

influence performance variability.

To determine the design parameter levels those yield the optimum

performance.

To determine whether further improvement is possible.

Various tools are used in DOE for the analysis of experimental results.

These include the main effects plot, the interactions plot, the cube plots, pareto plot

of factor effects, the normal probability plot of factor effects, the normal

probability plot of residuals, the response surface plots and regression models

The main uses of design of experiments are

• Discovering interactions among factors

20

• Screening many factors

• Establishing and maintaining quality control

• Optimizing a process, including evolutionary operations (EVOP)

• Designing robust products

Dr. Genichi Taguchi developed methods for experimentation that were

adopted by many engineers. These methods and other related tools are now known

as robust design, robust engineering, and Taguchi Methods.

2.4.1 Design of experiments using the Taguchi approach

Dr. Taguchi developed a new methodology for designing experiments. His

concept brought about a unique quality improvement technique that differs from

traditional methods of DOE. This methodology has taken the design of

experiments from the exclusive world of the statisticians and brought it more fully

into the world of manufacturing technologist. The Taguchi approach has been

successfully applied in several industrial organizations and completely changed

their outlook on quality control.

Taguchi approach in experimental design has the objective of designing

products/processes so as to be robust to environmental conditions and component

variation (Ross, 1989). To achieve desirable product quality by robust design,

Dr.Taguchi proposed a three-stage approach, i.e., system design, parameter design,

and tolerance design.

In system design, the engineer applies his scientific and engineering

knowledge to produce a basic functional prototype design. In the product design

stage, the selection of materials, components, tentative product parameter values,

etc., are involved. In the process design stage, the analysis of processing

sequences, the selections of production equipment, tentative process parameter

values, etc., are involved. Since system design is an initial functional design, it

may be far from optimum in terms of quality and cost.

The objective of the parameter design is to optimize the settings of the

process parameter values for improving performance characteristics and to identify

the product parameter values under the optimal process parameter values. In

21

addition, it is expected that the optimal process parameter values obtained from the

parameter design are insensitive to the variation of environmental conditions and

other noise factors. Therefore, the parameter design is the key step in the Taguchi

method to achieving high quality without increasing the cost. Traditionally, a large

number of experiments have to be carried out when the number of the process

parameters increases. To solve this task, the Taguchi method uses a special design

of orthogonal arrays to study the entire parameter space with a small number of

experiments only. Tolerance design is a way to fine-tune the results of the

parameter design by tightening the tolerance of factors with significant influence

on the product.

A loss function is then defined to calculate the deviation between the

experimental value and the desired value. Dr. Taguchi recommended the use of the

loss function to measure the performance characteristic deviating from the desired

value. The value of the loss function is further transformed into a Signal-to-Noise

(S/N) ratio. Usually, there are three categories of the performance characteristic in

the analysis of the S/N ratio, namely, the lower-the-better, the higher-the-better,

and the nominal-the-better. They are calculated as:

Nominal the better:

2log10/

y

Ts

yNS

Larger the better (maximize):

n

i i

Lyn

NS1

2

11log10/

Smaller the better (minimize):

n

i

iS yn

NS1

21log10/

where ȳ, is the average of observed data, 2

ys is the variance of y , n is the number of

observations and y is the observed data.

The S/N ratio for each level of process parameters is computed based on the

S/N analysis. Regardless of the category of the performance characteristic, the

larger S/N ratio corresponds to the better performance characteristic. Therefore,

the optimal level of the process parameters is the level with the highest S/N ratio.

Furthermore, a statistical analysis of variance (ANOVA) is performed to

see which process parameters are statistically significant. With the S/N and

22

ANOVA analyses, the optimal combination of the process parameters can be

predicted. Finally, a confirmation experiment is conducted to verify the optimal

process parameters obtained from the parameter design.

Following are the steps are involved in the parameter design phase of

Taguchi method (Shaji et al., 2003):

1. Identification of the objective of the experiment;

2. Identification of quality characteristic (performance measure) and its

measurement systems;

3. Identification of factors that may influence the quality characteristic,

their levels and possible interactions;

4. Selection of appropriate Orthogonal Array (OA) and assign the

factors at their levels to the Orthogonal Array (OA);

5. Conduct the experiments described by the trials in the Orthogonal

Array (OA);

6. Analysis of the experimental data using the S/N ratio, factor effects

and the ANOVA to see which factors are statistically significant and

find the optimum levels of factors;

7. Verification of the optimal design parameters through confirmation

experiment.

Qualitek-4 software (Nutek Inc., USA) can be used for automatic design of

experiments using Taguchi method. It is used by researchers worldwide for

automatic design and analysis of engineering experiments. It has the provision to

automatically design experiments based on user-indicated factors and levels. The

program selects the array and assigns the factors to the appropriate column. For

more complex experiments, there is a manual design option. The program also

performs the three basic steps in analysis: main effect, analysis-of-variance, and

optimum studies. Analysis can be performed using standard or signal-to-noise

ratios of results for smaller, bigger, nominal, or dynamic characteristics. Results

can be displayed using pie charts, bar graphs, or trial-data-range graphs. In

addition to analysis of DOE results, the software also has a large number of

23

capabilities dealing with the Taguchi Loss Function and its relationships with other

population performance measures. Qulitek-4 software was widely used in the

design of experiments and the analysis of results in the present investigation.

2.5 PROMOTION OF CHIP CURL

Tool-chip contact length and contact area is an important parameter

influencing heat generation and friction in metal cutting process. According to De

Chiffre et al. (1982), tool chip contact length is an index of the main cutting force.

Low tool chip contact length can lead to lower cutting force, lower tool wear and

better surface finish. Hence any mechanism that will lead to reduction in tool chip

contact length can bring forth better cutting performance.

Tool-chip contact length changes according to the contact phenomena in the

tool-chip interface zone, which is predominantly affected by the cutting speed

(Abukhshim et al., 2004). Tool chip contact is found to increase with cutting speed.

Application of cutting fluid reduced the tool chip contact length by increasing the

chip curl (Seah and Li, 1997). Other mechanisms that can reduce tool chip contact

length are contamination of tool rake face, promotion of plastic flow at the

backside of the chip and reduction of cutting temperature (Varadarajan et al.,

2002).

Astakhov (2010) explained embrittlement action of the cutting fluid that

reduces the strain at facture of the work material which is known as Rebinder

effect. Rebinder showed that the absorbed films prevent closing of micro cracks.

These unhealed micro cracks at the machining zone serve as a stress concentrators

and as a result the energy required for machining gets reduced.

Tasdelen et al. (2008) investigated the effect of MQL and air on tool-chip

contact length. Application of MQL in accompaniment with compressed air

reduced the tool-chip contact length due to the cooling effect of air that results in

chip up-curling.

Bermingham et al. (2012) studied the effect of high pressure cooling

technique on tool chip contact length during turning of Ti–6Al–4V. It was found

24

that the presence of high pressure emulsion reduced tool chip contact length and

brought about better tool life.

Promotion of chip curl can be achieved by cooling the top side of the chip

and making it to bend away from the tool, which may lead to the reduction of tool-

chip contact length. In the present investigation an auxiliary high velocity minimal

pulsing jet was applied on the top side of the chip to find out whether it is possible

to reduce the tool-chip contact length.

According to Wang and Kou (1997), cost of cutting fluid during machining

with minimal fluid application can be further reduced by replacing the emulsified

cutting fluid by high pressure water jet. Experimental studies suggested that the

use of high pressure water as cutting fluid at the cutting zones is an efficient

method for improving the cutting performance as this reduces the tool chip contact

length (An et al., 2011), provides better cooling, reduces tool-chip interface

friction and eliminates the adhesion of hot chips to the cutting edge (Habak et al.,

2011). Use of water vapour as coolant and lubricant was also studied by some

researchers (Liu et al., 2005; Liu et al., 2007) for improving the cutting

performance.

It is evident from all the above research works that the use of water as

coolant and lubricant is a new cooling and lubricating technology which can

alleviate pollution and ensures a green environment at the shop floor. In the

present investigation an attempt was made to make use of a pulsing slug of water

on the top side of chip to exploit the cooling effect of the water to bend the chip

away from the tool so as to reduce the tool chip contact length.

2.6 APPLICATION OF SEMISOLID LUBRICANTS IN METAL CUTTING

Cooling and lubrication are very critical to ensure better cutting

performance during hard turning on account of the high friction and intense heat

generation involved in the process. Cutting fluids have been traditionally used to

deal with this problem. But, the application of conventional cutting fluids increases

the cost of manufacturing and creates environmental pollution and health related

problems to operators. All these factors give motivation to investigations aimed at

25

minimizing or eliminating cutting fluid during hard turning. But any attempt made

to reduce the quantity of cutting fluid must compensate for the functional

requirement of cutting fluid by some other means. Friction and the associated heat

generation can be reduced by providing better lubrication at the at the tool-work

interface. Advancement in modern tribology has identified many solid lubricants,

which are proven to provide lubricity over a wide range of cutting conditions. Few

research works are reported in the field of metal machining that make use of solid

and semi solid lubricants.

Vamsi Krishna et al. (2008) studied the effect of solid lubricant mixture like

Graphite in SAE 40 oil and boric acid in SAE 40 oil during turning of EN8 steel.

Experiments were carried out with graphite and boric acid (particle size 50 mm)

mixed with SAE 40 in proportions of 5, 10, 20, 30 and 40% by weight. Machining

performance was affected by the type and amount of solid lubricant in SAE 40 oil.

It was observed that there was improvement in cutting performance when solid

lubricant was used as a mixture with SAE 40 oil. Among the lubricants, 20% boric

acid in SAE 40 oil provided the best performance. The improvement on

performance of solid lubricants is attributed to its layered lattice structure that

allows it to act as an effective lubricant film.

The influence of boric acid and graphite particle size on cutting

performance during turning of EN8 steel was investigated by Nageswara Rao et al.

(2008). Machining was carried out with graphite and boric acid. The size of

graphite particles varied from 50 to 200 µm. It was found that better performance

was obtained for a particle size of 50 µm.

Suresh Kumar Reddy et al. (2006) studied the effect of solid lubricant

assisted end milling of AISI 1045 steel with graphite and molybdenum disulphide

as lubricants and reported that that there was a considerable improvement in the

cutting performance when compared to that during machining with conventional

cutting fluids. It was found that the application of molybdenum disulphide during

machining offered better cutting performance.

26

Alberts et al. (2009) investigated the effect of graphite nanoplatelets as solid

lubricants during surface grinding. It was observed that larger diameter (15µm)

graphite platelets dispersed in isopropyl alcohol (1% concentrate by weight) when

applied as coating, reduced grinding forces and specific energy during surface

grinding, and improved the surface finish.

The use of graphite as solid lubricant in grinding of medium carbon steel

and bearing steel for eliminating cutting fluid was investigated by Shaji et al.

(2002), with a newly developed experimental set-up. Cutting performance of

graphite assisted grinding was compared with that during dry and conventional wet

grinding. Results showed a remarkable reduction of tangential force, specific

energy and cutting temperature when graphite was used as solid lubricant. Better

lubrication provided by graphite resulted in the reduction of frictional forces at the

wheel-workpiece interface.

Shaji et al. (2003) further analysed the effect of the process parameters such

as speed, feed, infeed and mode of dressing the wheel on the force components and

surface finish developed based on Taguchi’s methods. The quantity of infeed was

found to be a prominent factor influencing the normal and tangential force

components. The mode of dressing was found to be the next prominent factor

influencing the force components.

Suresh Kumar Reddy et al. (2010) explored the possibility of application of

graphite as a lubricating medium during drilling of AISI 4340 steel, as an attempt

to develop an alternative to conventional wet drilling. An electrostatic solid

lubrication applicator was developed to supply predefined amounts of solid

lubricant mixture as a high velocity jet and with an extremely low flow rate to the

machining zone. Lubricating oil SAE 40 was chosen as the mixing medium for

graphite. The results indicated that the use of a solid lubricant mixed with SAE 40

oil can improve cutting performcance.

Vamsi Krishna et al. (2010) investigated the use of nanosolid lubricant

suspensions in lubricating oil during turning of AISI 1040 steel with carbide tools.

Boric acid particles of 50 nm size were used as a suspension in SAE-40 and

27

coconut oil. Machining was carried out with varying proportions of solid lubricant

suspensions. Influence of solid lubricant to oil proportion on cutting temperature,

tool flank wear and surface roughness were studied. Cutting temperature, tool

flank wear and surface roughness decreased significantly with inclusion of

nanolubricants. It was observed that, coconut oil based nanoparticle suspensions

showed better performance compared to SAE-40 based lubricant.

Use of solid lubricants such as graphite and molybdenum disulphide during

turning of bearing steel using ceramic inserts was investigated by Dilbag et al.

(2008). Results indicated that the use of solid lubricants improved cutting

performance in terms of surface finish and tool wear. There was about 15%

reduction in the value of surface roughness when compared to that during dry

turning when molybdenum disulphide was employed as a solid lubricant. But

when graphite was used the reduction in surface roughness was only 8%.

Nageswara Rao et al. (2008) used boric acid as solid lubricant during

turning of EN8 steel using HSS and carbide cutting tools. Cutting performance

during turning under the lubricating action of boric acid was compared with that

during dry and wet turning. The results revealed that the use of boric acid can

bring forth a cutting performance better than that is possible during conventional

wet turning.

During turning with minimal fluid application, since only a very small

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

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

performance. From the review of literature, it was found that the presence of semi

solid lubricants can bring forth improvement in cutting performance. Moreover, all

these literatures investigated only the effect of boric acid and graphite on cutting

performance. No research work has been reported with silicon grease as solid

lubricant which is basically a good industrial lubricant. In the light of this, it was

decided to investigate the use of grease as a semisolid lubricants in pure form as

well as impregnated with graphite as a means to enhance lubrication during hard

turning with minimal fluid application in the present research work.

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2.7 APPLICATION OF HEAT PIPE IN MACHINING

Application of heat pipe as an alternative to conventional method of

removing heat from the cutting zone is an emerging area of interest among

researchers. Chiou et al. (2007) investigated the performance of a cutting tool

embedded with a flat heat pipe during turning using Finite Element Analysis. The

finite element analysis of heat transfer behavior showed that the temperature near

the cutting edge dropped significantly by the presence of an embedded heat pipe.

Cutting experiments were conducted to validate the predictions of the finite

element model. Predictions of the finite element model matched well with the

experimental results.

Noorul Hag et al. (2006) investigated the effect of parameters such as

diameter of heat pipe, length of heat pipe, magnitude of vacuum in the heat pipe

and the material used for making heat pipe on cutting performance. Heat transfer

efficiency of heat pipe during hard turning of engine crank pin material using

mixed alumina insert was studied. A set of heat pipe parameters for optimum

performance were arrived at by performing a nine run experiment. There was

considerable improvement in tool life when a 400 mmHg vacuum was maintained

in a heat pipe made of brass having length 40mm and diameter 7mm was used.

Liang et al. (2011) studied the effect of heat pipe in reducing the tool–chip

interface temperature of the cutter with a flat heat pipe attached on the rake face of

insert during dry turning. The results showed that the tool–chip interface

temperature could be reduced effectively with heat pipe cooling and the reduction

in temperature is found to be more at the higher cutting speed.

Zhu et al. (2012) experimentally verified the feasibility and effectiveness of

heat-pipe cooling in end-milling operations. The results demonstrated that use of

heat pipe cooling reduced tool wear and prolonged the tool life of end mill cutter.

Zhu et al. (2013) made a numerical study in order to investigate the effect of heat

pipe cooling during drilling operations by predicting the thermal, structural static

and dynamic characteristics of the tool. The numerical simulation indicated that

29

heat pipe assisted drilling reduced the peak temperature and stress on the tool tip

when compared to dry drilling.

Review of literature indicated that cutting performance can be improved by

introducing heat pipes for removal of heat from the cutting tools. Heat pipe

assisted cooling system can reduce or eliminate the need for cutting fluids and the

associated pollution and contamination of the environment. However the effect

heat pipe during minimal fluid application was not investigated so for. In the

present research work an attempt was made to investigate the applicability of heat

pipes in cooling the cutting tool during hard turning with minimal fluid

application.

2.8 USE OF VEGETABLE OILS AS CUTTING FLUID

The increasing awareness on environmental and health aspects of industrial

activities and governmental regulation are forcing industrialists to reduce the use

of mineral oil-based metalworking fluids as cutting fluids. As cutting fluids are

complex in their composition, they may be harmful to operator’s health and it is

very difficult to dispose off. The growing demand for environmental friendly

cutting fluid has opened an avenue for using vegetable oils as an alternative to

petroleum based cutting fluids in machining operations (Shashidhara and Jayaram,

2010; Lawal et al. (2012)).

Vegetable oils are tri-esters of straight-chained, mostly unsaturated fatty

acids with glycerol and have higher levels of biodegradability and much lower

toxicity than conventional mineral or synthetic oils (Adhvaryu et al., 2004). In

addition, vegetable oils have others advantages such as very low volatility, good

lubricity and high viscosity index, as well as lower cost than synthetic oils.

Vegetable oils in their natural form has limited use as industrial fluids due to their

low thermal and oxidative stabilities, narrow viscosity range and higher pour

points than both mineral and synthetic oil-based lubricants.

Kuram et al. (2011) formulated crude and refined sunflower oil based

cutting fluids and used these vegetable based cutting fluids to evaluate the thrust

force and surface roughness during drilling. Refined sunflower oil based cutting

30

fluid gave lower surface roughness than crude sunflower oil based cutting fluid,

while crude sunflower based cutting fluid showed lower thrust force than refined

sunflower based cutting fluid.

Cetin et al. (2011) compared the performances of sunflower oil based

cutting fluid and canola oil based cutting fluid with mineral oil based cutting fluid

during turning of AISI 304L austenitic stainless steel with carbide inserts. It was

observed that, sunflower oils based cutting fluid and canola oils based cutting fluid

offered better cutting performance in terms of surface finish and tool wear when

compared to mineral oil based cutting fluid. Khan et al. (2009) investigated the

applicability of vegetable oil based cutting fluid in MQL application. Cutting tests

were conducted with AISI 9310 as the work and uncoated carbide as the cutting

tool during turning operation and it was observed that vegetable oil based cutting

fluids offered better cutting performance in terms of cutting temperature, tool wear

and surface roughness. Rahim and Sasahara (2011) studied the use of palm oil as a

lubricant during high speed drilling of Ti-6Al-4V and compared the performance

with that when a synthetic ester was used. Palm oil exhibited lower tool wear rate.

2.8.1 Coconut oil as cutting fluid

Coconut oil belongs to the unique group of vegetable oils called lauric oils.

The fatty acids present in coconut oil are presented in Table 2.1. More than 90% of

the fatty acids present in coconut oil are saturated. The saturated nature of coconut

oil imparts strong oxidation stability to coconut oil based lubricants. It remains as a

white crystalline solid at temperatures below 20 ˚C. The physical properties of

coconut oil are summarised in Table 2.2. Being a vegetable oil having a typical

triacylglycerol structure, it shares most of the important properties of other

vegetable oils such as high viscosity index, good lubricity, high flash point and

low evaporative loss. Though coconut oil also shares the disadvantage of poor low

temperature properties of other vegetable oils, it shows much better thermal and

oxidative stability because of the high percentage of saturated fatty acids present in

it. Table 2.3 presents a comparison of the properties of coconut oil with other

vegetable oils.

31

Table 2.1 Fatty acids present in coconut oil

Name of fatty acid Carbon Chain Percentage

Caprylic Acid C 8:0 7%

Capric Acid C 10:0 5.4%

Lauric Acid C 12:0 48.9%

Myristic Acid C 14:0 20.2%

Palmitic Acid C 16:0 8.4%

Stearic Acid C 18:0 2.5%

Oleic Acid C 18:1 6.2%

Linoleic Acid C 18:2 1.4%

Table 2.2 Physical properties of coconut oil

Properties Value

Density (g/cm3) 0.926

Cetane Number 37

Flash Point (oC) 225

Viscosity index 165

Jayadas et al. (2007) investigated the influence of an antiwear and the

extreme pressure additive on the tribological performance of coconut oil. It was

observed that coconut oil, though a good boundary lubricant as far as the

coefficient of friction is concerned, showed poor resistance to wear compared to

commercial lubricants. The antiwear and extreme pressure additives are to be

added to improve the tribological properties of coconut oil.

Anthony and Adithan (2009) studied the influence of coconut oil on tool

wear and the attainable surface finish during turning of AISI 304 steel with carbide

tools. They compared the performance of coconut oil with two more cutting fluids

namely an emulsion and a neat cutting oil. The results indicated that coconut oil

performed better than the other two cutting fluids in reducing the tool wear and

improving the surface finish.

32

Jayadas and Prabhakaran Nair (2006) compared the onset temperature of

thermal degradation and oxidative degradation of coconut oil with sesame oil,

sunflower oil and a mineral oil (Grade 2T oil). It was observed that the onset

temperature of thermal degradation of coconut oil was found to be lower than that

of sunflower oil and sesame oil whereas the onset temperatures of oxidative

degradation were comparable.

Table 2.3 Comparison of the properties of commonly used vegetable oils

PROPERTIES COCONUT

OIL

CASTER

OIL

RAPE

SEED OIL

SOYABEAN

OIL

JATROPHA

OIL

Kinematic

Viscosity

@40ºC (cst)

27.6 220.6 45.6 32.93 47.48

Kinematic

Viscosity

@100ºC (cst)

5.9 19.72 10.07 8.08 8.04

Viscosity Index 165 220 216 219 208

Density(g\cm3)

@ 15 ºC 0.926 0.9666 0.9456 0.928 0.923

Flash point(°C) 225 250 240 240 240

Pour point(°c) 20 -27 -12 -9 0

Vamsi Krishna et al. (2010) compared the performance of cutting fluids

consisting of nano boric acid suspensions in SAE-40 and that in coconut oil during

turning of AISI 1040 steel with cemented carbide tool with a specification

SNMG120408. Boric acid particles of 50 nm particle size were used in the

suspensions. The percentage of suspension was varied at three levels namely

0.25%, 0.5% and 1% by weight. It was observed that the flank wear and the

surface roughness reduced considerably with the increase in the percentage of

suspensions in cutting fluid. It was also observed that cutting fluid consisting of

nano suspension in coconut oil performed better than the one made of nano

particles suspension in SAE-40 and a suspension consisting 5% by weight in

33

coconut oil gave a better cutting performance in terms of cutting temperature, tool

wear and surface roughness.

Since vegetable oils were found to be promising alternative to mineral

based oils due to their environmental friendly characteristics, it was decided to

formulate a cutting fluid with a vegetable oil as the base and study it’s

performance during hard turning with minimal fluid application. Coconut oil was

selected as the vegetable oil considering its availability, physical properties and

lubricating ability.

2.9 SUMMARY

The review started with an analysis on the mechanism of chip formation

and the role of cutting fluid during hard turning. The applicability of minimal fluid

application techniques during hard turning was analysed, which included the

technological, economical and environmental advantages of the minimal fluid

application. Literature on techniques and methodology of design of experiments

were reviewed and the reports on experiments based on Taguchi techniques were

analysed in depth. Reports on recent techniques used for reducing tool chip contact

length were reviewed. Review of literature revealed the suitability of semi solid

lubricants in promoting rake face lubrication. A comprehensive review was made

on literature connected with the heat pipe as a means to cool the cutting tool. A

study on the application of vegetable oils as cutting fluid revealed that coconut oil

has a very good potentiality to be used as base for making cutting fluid and may be

thought of as an alternative to petroleum based cutting fluids.

In the light of the review of literature, it was decided to develop schemes to

promote chip curl by introducing an auxiliary pulsing slug of cutting fluid on the

back side of the chip, to improve rake face lubrication by introducing semi solid

lubricants at the critical zones, to effect better cooling of the cutting tool by

introducing heat pipes near the tool inserts and finally to improve the environment

friendliness of the minimal fluid application scheme by formulating a coconut oil

based cutting fluid.