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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.
7
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
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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.
28
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.
Recommended