Tool Wear Characterization of Carbide Cutting Tool Insert Coated With Titanium Nitride TIN in a Single Point Turning Operation of Aisi D2 Steel - Muhammad Farouq b. Muhammad Faisal
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UNIVERSITI TE
I 1
KNIKAL MALAYSIA MELAKA
Tool Wear Characterization of Carbide Cutting
Tool Inserts coated with Titanium Nitride TiN)
in a Single Point Turning Operation of AISI
D
Steel
Report subm itted in accordance with the partial requirements of the
Universiti Teknikal Malaysia Melaka for the
Bachelor of Manufacturing Engineering Manufacturing Process)
Muham mad Farouq Bin Muham mad Faisal
Faculty of M anufacturing Engineering
March 2008
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BSTR CT
This study presents tool wear characterization of carbide cutting tool inserts coated
with titanium nitride TiN) in a single point turning opera tion of AISI D2 steel. A set
of experiments of 20 settings of cutting speed, depth of cut and feed rate were
performed on a CNC lathe without coolant. Surface roughness was measured by
Mitutoyo profilometer SJ-301) and flank wear was measured by Axioskop 40 and
the data was compiled into Design Expert software for analysis. From the result,
cutting speed was found to be the main factor to have significant effect on surface
roughness as well as flank wear. Depth of cut was also found to have interaction with
cutting speed to produce significant effect to surface roughness. No interaction
between the factors was found to give significant effect to flank wear. At the end of
this study, optimization was m ade by suggesting the most suitable sets of parameter
settings to produce minimum surface roughness and to produce minimum flank wear.
Suggestion on parameter settings to obtain both minimum surface roughness and
flank wear was also made.
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BSTR K
Kajian ini mendedahkan tentang sifat-sifat perkakas pemotongan sisipan karbaid
besalu t titanium nitraid TiN) melalui proses pem otongan satu titik putaran ke atas
keluli AIS I D2. Satu set eksperimen dengan 2 0 tetapan laju pemotongan, kedalaman
potongan dan kadar potongan dijalankan menggunakan mesin larik CNC tanpa
penggunaan cecair pelincir. Kekasaran permukaan diukur menggunakan Mitutoyo
profilometer SJ-301) dan hausan rusuk diukur menggunakan Axioskop 40 dan
maklumat-maklumat ini kemudian diisi ke dalam perisian Design Expert untuk
dianalisa. Daripada keputusan yang diperolehi, laju pemotongan didapati menjadi
faktor penting kepada penghasilan kekasaran permukaan dan juga hausan rusuk.
Kedalaman potongan pula didapati memberi pengaruh kepada laju pemotongan
terhadap penghasilan kekasaran permukaan. Tiada interaksi atau saling berpengaruh
anatara factor didapati memberi kesan kepada hausan rusuk. Di akhir kajian,
cadangan untuk setting parameter yang terbaik diberi bagi meminimumkan
kekasaran permukaan dan juga meminimum kan hausan rusuk. Cadangan bagi setting
parameter terbaik untuk mendapatkan gabungan peminimum an kekasaran permukaan
dan hausan rusuk juga dibuat.
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CH PTER
1
INTRODUCTION
1 ackground
Cutting tool is a very important field in the manufacturing industry.
A
lot of
discussions and studies have been made to improve the quality and usage of a cutting
tool. Cutting tools come in various types and shapes and various materials as well as
various coating materials. These different characteristics of the cutting tool serve
different types of applications. Cutting tool has to have certain aspect or criteria such
as hardness toughness and wear resistance. One of the most common cutting tool
materials is carbide.
When speaking of cutting tool we cannot avoid the word tool wear. Tool wear is an
area that researchers always wanted to eliminate or minimize. R educ tion of tool wear
will result in many beneficial outcomes like cost reduction and improvement in
machining quality. In order to improve the tool life study on the tool wear
charac teristics is needed so that ideas of wear preven tions can be found.
Lubrication is normally used in machining to prevent cutting at excessive high
temperature. Basically lower cutting temperature results in longer tool life of the
cutting tool. But in the case of cutting tool inserts with TiN coating the cutting tool
insert will work better at high temperature.
A
study on wear mechanisms and
performance done by Khrais and Lin
2007)
pointed out that dry cutting is better than
wet cutting for TiN coating inserts under high speed cutting.
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There are other various reasons to eliminate the usage of coolan t. Lubrication in the
machine shops contribu tes about 15 percent of the production cost Schneider 2001).
In o ther words, if use of lubrication is elim inated, the production cost can be reduced
up to 15 percent. Lubrication also contribu tes to pollution. The waste cu tting fluid
cannot be disposed off without treatment. This will even increase the cost of
production. Other than water pollution, a ir pollution also takes place when coolant is
used. During machining, air-borne mist is produced. There is a ce rtain limit fixed by
Occupational Safety and Health Administration OSH A) to the mist produced by the
coolant into the air. In conclusion, eliminating coolant by implementing dry cutting
brings many benefits to the industry.
1 2 roblem Statement
Tool wear is inherent in machining. There are many steps and measures taken to
reduce the effect of tool wear on cutting tools. One of the steps is applying surface
treatment on the base cutting tool material. A popular method is applying coating
onto the base cutting tool material by the use of physical vapor deposition PVD)
method. By studying the behaviour of the tool wear with respect to machining
parameters, tool life can be optimized by choosing the right machining parameters.
The optimum tool is not necessarily the least expensive or the most expensive, and it
is not always the same tool that was used for the job last time. The best tool is the
one that has been carefully chosen to get the job done quickly, efficiently and
economically Schneider 2001). Because of this, it is necessary to characterize
specific cutting tool, coating material and w ork piece combination to understand the
interaction between machining parameters and tool wear performance. In this study
the combination of TiN coated carbide tool and D2 work piece was evaluated using
single point turning under dry mach ining condition .
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1 3
bjective
The objectives of this study are:
a) To study the influence of machining param eters to the surface roughness of
AISI
D2
steel.
b) To study the influence of machin ing param eters to the tool wear of carbide
cutting tool inserts coated with titanium nitride TiN).
c) To define optimum process parameter settings to minimize tool wear and
surface roughness using response surface methodology @SM) for single
point turning of carbide cutting tool inserts coated with titanium nitride TiN )
on AISI D 2 steel work m aterial.
1 4
Scope
This study focuses on tool wear characterization of carbide cutting tool inserts coated
with TiN. The types of tool wear to be analyzed are crater wear and
fl nk
wear. The
machining process done is a single point turning operation with no coolant involved
dry cutting). The machining parameters evaluated are cutting speed, feed rate and
depth of cut. The w ork specim en or material machined is AISI D 2 steel. The surface
finish of the machined work is also being taken into account. All of the
experimentation will be done using design of experiment DOE) approach.
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CH PTER 2
LITERATURE
REVIEW
2 1 verview
In this chapter, related published works by other researchers like journals are
reviewed. These works or studies are the ones involving machining and cutting tools.
Other related information mainly obtained from books and articles are also included.
This review covers turning operation, machining parameters and cutting conditions,
cutting tool insert carbide coated with titanium nitride), tool wear in the turning
operation, work piece material AISI
D 2
steel) and the selected design of experiment
method.
2 2 Turning peration
Turning operation means that when the machining process is running, the work
material moves in a rotational direction. The machine tool used for this operation is
the lathe machine. This is shown in Figure 2 1 The cutting tool is the section that
will be moving. In other words, the cutting tool will feed onto the rotating work
material. The material removal process is shown in Figure 2 2 The material is
removed layer by layer depends on the depth of cut until the desired dimension is
reached.
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igure2 1: Lathe turning
igure
2 2:
Turning operation
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2 2 1 achining Parameters
In any
m chining
operation , the param eters involved needs to be determined prior to
the operation. In turning operation, the three most comm on machining parameters are
cutting speed, feed rate, and depth of cut. These cutting parameters differ depending
on the cutting tool and work material used and the type of operation to be done
Schneider 2001). For example, work material with high hardness should not have
high depth of cut that will result in shorter tool life. According to I S 0 3 6 8 5 :1 9 9 3 0 ,
for cutting speed, the setting is determined from the surface to be machined and not
on the diameter. Coated cutting to01 like TiN coating should not be cut at slow speed
or the tool will not perform well and the tool life will a lso be shorter. The cutting tool
insert and its coating will be explained in the next chapter.
Cutting forces greatly influence the outcom e of the process where the cutting forces
generally influenced by variation of the machining parameters. All cutting forces
increase with the increase of the flank wear surface area. Increasing flank wear area
results in an increasing area of contact between the tool tip and the work piece.
A
previous study indicated that the greater the value of the flank wear area, the higher
the friction of the tool on the work piece and the higher heat generation occured, this
ultimately caused higher value of cutting force Sikdar Chen 2002).
2 3 Cutting Tool
From metalworking point of view, a cutting tool can generally be stated as a tool
used to remove material from a work piece work material) usually by the use of
abrasive cutting and shear deformation . Schneider 2001) stated that for a cutting tool
to be able to do its job efficiently, the cutting tool needs to have certain
characteristics of which three of them are listed below.
a)
ardness Hardness and strength of the cutting tool must be maintained
at elevate temperatures also called hot hardness.
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b
Toughness Toughness of cutting tool is needed so that tools do not chip
or fracture, especially during interrupted cutting operations.
Wear esistance
Wear resistance means the attainment of acceptable
tool life before tools need to be replaced.
2 3 1 uttingTool nsert
The term insert is applied when a cutting tool is screwed or clamped onto a tool
holder to be fixed on a machine tool; where in this particular study, the machine tool
would be a
N
lathe machine. Most of high-performance cutting tools use the
insert method. Inserts are normally m ade sym metrically so that when the first cutting
edge is dull they can be rotated , presenting a fresh cutting edge. This will effective ly
increase the life of the tool insert. Figure 2.3 shows some different shapes of cutting
tool insert.
Figure 2.3: Insert shapes
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2 3 2
utting Tool Insert Material
Carbide tools are one of the most com mon cutting tool material used in the industry.
~f
to carbon steels and high speed steels carbide tools can machine much
faster at higher speed. Carbide tools can also work at higher working temperature.
Figure 2 4 show s a set of T iN-coated carbide cutting tool inserts and Figure 2 5 show
a cutting too1 insert fixed to a holder and lathe machine.
Figure 2 4: TiN-coated carbide cutting tool inserts
Figure 2 5: Cutting tool insert fixed to a holder and lathe machine
8
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2 3 3 utting Tool oating Material
Coating of a cutting tool is done to achieve some improving factors on the cutting
tool insert Kalpakjian Schrnid 2001). There are many type of coating material
used for specific applications. One of the methods to coat a cu tting tool is using the
physical vapor deposition PVD ) method. Another technique is chemical vapor
disposition CVD). CVD coa ting requires higher temperature that makes it unsu itable
for coating tool steels. PVD technique to applying titanium nitride (TIN) can be done
at a much lower temperature at 400°C Ostwald Munoz 1997). PVD also allows
sharper corners and low er coefficient of friction.
Titanium nitride TiN) coating is one of the many coatings applied using the PVD
method. TiN has been one of the most extensively investigated hard coatings in
laboratory as well as in real life applications Mo et al. 2007). study by Park and
Baik 2007) also pointed out that TiN coating has been extensively used for surface
coating because of its superior wear and oxidation resistance as compared to binary
metal nitride coatings TiN coating have been found effective in cutting stainless
steel.
TiN coatings are known to be high in oxidation resistance which makes them
favorable in dry cutting cond itions Smith et al. 1996). On the aspect of tool wear,
TiN coatings can have common wear behaviour. study on wear mechanisms and
tool performance of TiN coated inserts during machining done by Khrais and Lin
2007) showed that TiN coated inserts have three wear stages, namely, running into
wear, semi-steady state or gradual wear, and catastroph ic wear. This behaviour is
also pointed out by Fang 1994) and Chubb and Billingham 1980)
in
their research
papers.
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Coating of cutting tool has to have some characteristics of which some are shown
below.
ardness
high surface hardness from your coating is one of the best
ways to increase tool life. Generally speaking, the harder the material or
surface, the longer the tool will last.
b)
Wear R esistance This is the ability of the coating to protect against
abrasion. Although a material may not be hard, elements and processes
added during production may aid in the breakdown of cutting edges.
c)
Surface Lubricity high coefficient of friction causes increased heat,
leading to a shorter coating life or coating failure. However, a lower
coefficient of friction can greatly increase tool life.
d)
Oxidation Temperature
This is the point at which the treatment starts to
break down. A higher oxidation temperature rating improves success in
high heat applications. Although the titanium nitride TiN) coatings may
not be as hard as titanium carbonitride TiCN) at room temperature, it
proves to be much more effective in applications where heat is generated.
In general, the performance of TiN coatings is superior than others in their class and
the current state of the art of TiN may be further enhanced in the future Smith et al.
1996).
2 4 oolWear
Tool wear can generally be described as the gradual failure of cutting tools due to
regular operation. There are many type of tool wear that includes edge wear, nose
wear plastic deformation, mechanical breakage, crater wear and flank wear. There
are many reasons to how these wear happen. For instance, mechanical breakage can
be caused by excessive force that causes immediate failure or thermal forces that
could also cause the wear. The machining parameter settings of the operation are
known to be one of the most common factors that cause tool wear. From a study on
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rnrface finish and flank wear using multiple regression models and neural network
models done using turning as the machining operation, it is suggested that the best
tool life was obtained in lowest feed rate and lowest cutting speed combination Ozel
et al. 2007).
2 4 1 Crater Wear
The m ost comm on tool wear in machining operation is crater wear and flank wear.
These two types of wear are the ones to be studied in this study. According to
Boothroyd and Knight 2006), under very high speed cutting cond itions, crater wear
is often the factor that determines the life of the cutting tool; crater wear becomes so
severe that the tool edge is weakened and eventually fractures but when tools are
used under economical conditions, the wear of the tool on its flank, which is the
flank wear, is usually the con trolling factor.
Crater wear occurs on the rake face of the tool Figure 2.6). It is a crater-like wear
occurred on the surface parallel to the cutting edge. Kalpakjian and Schrnid 2001)
suggested that the most significant factors that influence crater wear are tem perature
at the tool-chip interface and the chemical affinity between the tool and work piece
materials. Additionally, the factors influencing flank wear see next sub-topic) also
influence c rater wear.
crater \\ ear
Figure 2.6: Crater wear
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study by M anoj Kurnar et al. 2006) on crater wear mechanisms of TiCN-Ni-wc
cermets during dry machining said that with the increase in speed and feed rate,
cutting becomes steady with a consequ ent reduction in the cutting force.
2 4 2 lank Wear
Flank wear occurs on the relief face of the tool and is generally caused by the
rubbing of the tool along the machined surface Figure 2.7) that causes adhesive
and/or abrasive wear or from very high temperature machining that affects the tool
material properties as well the work piece surface Kalpakjian Schrnid 2001).
Figure 2.7: Flank wear
From a study on tool wear prediction in turning by C houdhury and Srinivas 2004), it
is suggested that the cutting velocity and the index of diffusion coefficient have the
most significant effect, followed by the feed rate and the depth of cut.
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2 4 3 Tool Wear Measurement
When measuring tool wear, there are certain criteria that need to be considered.
There are different types of i~ st ru m en ts an be used to measure tool wear. No matter
what instruments used for measuring, the most important thing in tool wear
measuring is the geom etry of the tool wear it self. Figure 2.8 below describes these
geometries.
flank wear
t nd
K crater width
KH rater centre distance
T
cr ter
depth
Figure 2.8: Tool wear on turning tools.
Source IS
3685:1993 E)
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TO measure the tool wear, the m ajor cutting edges is divide into four zones.
Zone is the curved part of the cutting edge at the tool corner.
Zone B is the remaining straight part of the cutting edge between zone
and zone
A.
Zone is the quarter of the worn cutting edge length b farthest away from
the tool corner.
Zone
N
extends beyond the area of mutual contact between the tool and
workpiece for approximately lm m to 2mm along the major cutting edge. T he
wear is of notch wear.
The width of the flank wear land
VBB
shall be measured within zone B
in
the tool
cutting edge plane
P
perpendicular to the major cutting edge and from the position
of the original major cutting edge. The crater depth KT shall be measured as the
maximum distance between the cra ter bottom and the original face in zone B
2 5
Work
Material
According to American Iron and Stee l Institute AISI), D2 steel is a high-carbon,
high-chromium tool steel alloyed with molybdenum and vanadium characterized by.
High wear resistance
High compressive strength
Good through-hardening properties
High stability in hardening
Good resistance to tempering-back
From the characteristics of the steel listed above, it can be suggested that AISI D2
steel is not one of the easy to machine work material. This is good for the study
because this study focuses on tool wear that resulted from turning operation. Using
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N s I D2 steel tool wear can be generated on the inserts with minimal machin ing
time. Figure 2 9 shows AISI D2 steel work m aterial.
The
Table
Figure 2 9: AISI D2 steel work material
3mposition of the AISI
D2
steel is shown in T able
2 1
below.
2 1:
AISI
D2
steel composition
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There are other standards for the code of the AISI D2 steel used around the world.
The D2 code is based on AISI standard. Even with different code names, they are
still the same steel. For example, in United Kingdom, they use the code B.S.
D ~
while in Germany it is DIN 1.2379.
ISI D2 steel is one of the most widely used steel in the industry. AISI D2 steel is
recommended for tools requiring very high wear resistance, combined with m oderate
toughness (shock-resistance). It can also be supplied in various finishes, including
the hot-rolled, pre-machined and fine machined condition. Some of the many
applications of ATSI D2 still can be seen in the list below.
a) Coining Dies
b) Cold Extrusion Dies
c) Thread-Rolling Dies
d) Crushing Hammers
e) Gauges and Measuring Tools
f
Knurling Tools
g)
Dies for molding of ceramics, bricks, tiles, grinding wheels and abrasive
plastic.
2 6 Surface Roughness
Surface roughness can generally be described as the geometric features of the
surface. Surface roughness is related closely to surface integrity. Surface integrity
referred more to the properties of the surface like fatigue life and corrosion resistance
which are strongly influenced by the type of surface produced. According to
Kalpakjian and Schrnid (2001), the main factors that influence surface roughness are
as follows.
a) temperatures generated during processing
b residual stresses
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c) metallurgical transformation
d)
surface plastic deformation, tearing and cracking
study done using response surface methodology RSM) on a turning process stated
that the feed rate was found out to be the dominant factor on the surface roughness,
but it decreased with decreasing cutting speed, feed rate and depth of cut for these
tools. Sahin Motorcu 2007).
2 6 1 Surface Roughness Measurement
There are two most common surface roughness measurement that are arithmetic
mean value R,) and root-mean-square average Rg ) , both can be calculated using
Equation 2.1 and 2.2 below respectively.
The datum line is placed at the center of the graph so that the sum of the area above
and below the line is equal Figure 2.10.
Digitized data
I
urface profile enter datum) line
Figure 2.10: Coordinates for surface roughness measurement
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Surface roughness measuring instrum ents used are called surface profilometers. The
instrument will measure and recode the surface roughness. The most co-only used
are the ones using diamond as the stylus. The stylus will travel in a straight line on
contact with the surface to be measured Figure 2.11.
Figure 2.11: a) measuring with stylus. @ path of the stylus
2 7
esign
of
Experiment
Design of Experiment DOE) can be defined as a structured, organized method for
determining the relationship between factors affecting a process and the output of
that process NIST 2006). DOE can also be described as a methods and tools to plan
and run a series of experiments. The objective of DOE is to determine the variables
in a process that are the critical param eters and their target values Besterfield 2004).
It is a method by which purposeful changes is made to the input factors of the
process in order to observe the effects on the output.
DOE uses statistical methods to analyze the data achieved to come up with a result.
By using formal experimental techniques, the effect of many variables can be studied
at one time Besterfield 2004). There are several me thods can be used as design of
experiments namely Taguchi methods, Full Factorial, Fractional Factorial, Failure
Mode and Effect Analysis FMEA ) and Response Surface Methods RSM).
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2.7.1 Response Surface ethodology
Response Surface Methodology RSM ) is a set of techniques designed to find the
best value of a designed response. There are times when using methods other than
RSM,
the best value of the values of the response cannot be obtained. This is when
RSM comes into play. n important aspect of RSM is the Central Composite Design
CCD). CCD is very useful for building a second order or quadratic model for the
response variable without needing to use a complete three-level factorial experiment
NIST 2006).
Design E xpert software is used to define experimental matrix. The experiment will
be done using predetermined settings and improvement and optimization will be
done afterwards. For every input parameter for the experiment, the minimum and
maximum value is determined on the software and the software will produce a series
of matrix for the experiment. The experim ent will be done based on this matrix.
Linear regression is used in RSM to predict experimental input and output
relationship. Three major analyses to be carried out are the main factor, interaction
and ANOVA.
According to NISTISEMATECH, the advantages of RSM are as follows.
a)
Simplified equation representing a complex system
b) Sensitivities are easily obtained
c) Optimization is easily obtained
d) Rapid and efficient
e)
The use of RSM allows for bringing more knowledge earlier in the design
process
f
Enabling technique for Advanced Design approaches
g) Instantaneous evaluations
The use of RSM and DOE along with the methods for running the whole study is
discussed on the next chapter.
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CHAPTER
3
METHO OLOGY
2 1 Overview
This chapter covers the methods for running and com pleting the whole project Every
detail on the project and how to run the experiment is included also From the type of
material and obtain ing the result to analyzing and finalizing the report Every step to
be gone through is also featured in this chapter The flow and sequence of the whole
project can be seen in Figure 3 1
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We Sele
Litc
srature
Machin
ng
Measure
. . .
IQCIZ
rouahn
/
lank.
nput Dal
3ta Anal
rs s
-it Sumn
nary
ectionode1 sell
Significant .... . .
Lack of Fit
1 N-iit significant Lack of Fit
int main
interactic
factors
3ns
and
timizatio
4
a 2 ,
igure
3 1:
Project flowchart
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3 2 Literature Review
Literature review requires reviewing of other previous published works that could be
related to the study or that have useful information to be used in the study. It includes
gathering of information from books and resources from the internet. In literature
review, the information and theory regarding the experiment and the machining
process which is single point turning is defined. The inputs for the experiment is the
three machining parameter that are cutting speed, feed rate and depth of cut. As for
the output responses from the machining process, there are two responses to be
considered that are tool wear and surface roughness. To be able to obtain the result
from experiment, it is essential
to know how to measure them. There are certain
equipments to be used for measuring them that will be discussed later on in this
chapter. Only then the analysis from the result can be m ade using R esponse Surface
Methodology RSM).
3 3 Experiment
3 3 1
Machining Param eters Settings
The experiment will carried out using the RSM approach. The experimental matrix is
defined using Design Expert 7 This software requires range of experiment input to
be keyed into it and then the software will suggest the best experiment design to be
carried out. The inputs for this experiment are cutting speed, feed rate and depth of
cut. The parameters chosen are based on previous study by Mohd Razali et al.
2002). The ranges for each of them are as follows.
Cutting speed
Depth of cut
Feed rate
200 00 m/min
0.2
0 5
rn
0.3
0 5
rnm/rev
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33 2 Experimental Design
The experimental design Was based On Cen tral Composite Design CCD). The
factors for the experiment which are the parameters are keyed into Design Expert
and the experiment matrix will be obtained as in Table 3 1 The experiment is
consists of 8 factorial points, 6 axial points and 6 center points.
Table 3 1: Experimen t matrix extractedfrom esignExpert 7
There are two responses that are collected from the experiment that are as follows.
Response Flank wear pm
Response Surface Roughness pm
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3 3 3 Tool and Work Material
The experiment was conducted in accordance to I S 0 3685:1993(E). The cutting tool
inserts to be used for the machining process are HSS insert (IS 0 SPGN 120308) with
TiN-coatings and manufactured by Sumitom0 Electric Hardmetal. The tool holder
used is (FTllR-44A) which is manufactured by Sumitomo Hardmetal. The work
piece used for machining is AlSI D2 X155CrVMo 12 1 The specification and
composition can be seen in Table 3.2 and 3.3.
Table 3.2: Material specifications
Table 3.3: W ork material chemical com position
Material
Insert: HSS IS0 SPGN 120308
Coating: PVD TiN-coating
Manufacturer: Kennametal
Work material: KRUPP
2379 XI55 CrVMo 12 1
Element Com osition
Mo 0.1
Specifications
cutting edge length = 12.7mm
0 d
=
12.7mm
thickness s
=
3.1 8mm
AlSl 0 2 steel dia.
=
100mm
length
=
250mm
3 3 3 quipments
Single point turning was selected as the machining operation and the machining
process was done using HAAS CN C lathe machine. Figure 3.2 shows the machining
process running. The model number for the machine is S L 20T. All experiment was
done using this machine. The machining process was done without the use of coolant
(dry machining). For the response from the experiment, there were two measuring
equipments used. The tool wear w as measured using Toolmaker s Microscope,
model number Axioskop 40 and manufactured by Ze iss Carl. For surface roughness,
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