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Plasma arc cutting (PAC) is an unconventional process widely used in manufacturing of heavy plate products. This work reports on the research results of machining quality of the workpiece in the plasma arc cutting on the low carbon low alloy steel. An experimental investigation of the characteristics of machining accuracy and surface integrity was carried out for basic machining parameters (cutting speed, arc current, arc voltage, plasma gas pressure, stand-off distance and nozzle diameter). The kerf geometry was determined with three accuracy parameters (top kerf width, bottom kerf width and kerf taper angle). The parameters of deviation present due to plasma curvature were defined by drag and pitch of drag line. The surface roughness was determined with two main roughness parameters through scanning the surface topography (roughness average and maximum height of the profile). The surface properties were determined over microstructure in heat affected zone (HAZ). The results show an acceptable machining quality of the PAC, so that this process is an excellent choice for fast and efficient material removal. However, the plasma arc cutting is not suitable for the final machining because of the metallurgical variations in the HAZ.
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ADVANCED TECHNOLOGIES AND MATERIALS VOL. 45, NO. 1 (2020)
DOI: 10.24867/ATM-2020-1-001
ADVANCED TECHNOLOGIES &
MATERIALS
http://journal-atm.org
* Corresponding author's.e-mail: maring@uns.ac.rs Received: 1 June 2020; Accepted: 25 June 2020
AT M
JOURNAL
An Experimental Analysis of Cutting Quality in Plasma Arc Machining
Marin Gostimirović*, Dragan Rodić, Milenko Sekulić, Andjelko Aleksić
University of Novi Sad, Faculty of Technical Sciences, Department of Production Engineering, Trg D. Obradovica 6, 21000 Novi Sad, Serbia
A B S T R A C T
Plasma arc cutting (PAC) is an unconventional process widely used in manufacturing of heavy
plate products. This work reports on the research results of machining quality of the workpiece in
the plasma arc cutting on the low carbon low alloy steel. An experimental investigation of the
characteristics of machining accuracy and surface integrity was carried out for basic machining
parameters (cutting speed, arc current, arc voltage, plasma gas pressure, stand-off distance and
nozzle diameter). The kerf geometry was determined with three accuracy parameters (top kerf
width, bottom kerf width and kerf taper angle). The parameters of deviation present due to plasma
curvature were defined by drag and pitch of drag line. The surface roughness was determined with
two main roughness parameters through scanning the surface topography (roughness average and
maximum height of the profile). The surface properties were determined over microstructure in
heat affected zone (HAZ). The results show an acceptable machining quality of the PAC, so that
this process is an excellent choice for fast and efficient material removal. However, the plasma arc
cutting is not suitable for the final machining because of the metallurgical variations in the HAZ.
Key words: plasma arc cutting; machining accuracy; surface topography; heat affected zone
1. INTRODUCTION
Modern industrial production is characterized by the need
to satisfy consumers in terms of range, quality, prices and
delivery times of products or services. This means that
just attractive products and services can survive on the
global market, which provides consumers with more than
expected. In this context, the industrial systems of the
future must be able to adapt to the demands of the market.
In the manufacturing industries, cutting process is one of
the first stages in production of a particular component or
product [1-3]. The cutting process entails the use of
different techniques. The choice of technique depends on
the type of material to be cut, as well as size, shape and
the required level of precision of the worked part. There
are two basic cutting methods in use.
Examples of classic cutting techniques are the use of
sawing and flame cutting [4]. Sawing is a mechanical
cutting process of workpiece with circular and band
sawing machines. This method is used for smaller
sections and more precise cutting operation, but it is very
slow production method. On the other hand, Flame
Cutting (FC) is oxy-fuel gas cutting thermal process
which is used during the hard and rough work. Depending
on the application, this method is often the preferred
process because it’s much faster than mechanical
machining. The flame cutting also is very portable and
has low equipment costs. However, due to the flame heat,
the edges being cut can often form a highly defective
surface layer [5], known as a heat affected zone (HAZ).
In recent times, modern manufacturing largely conducts
cutting operations which are supported by unconventional
technologies. These processes are defined as a group of
cutting methods that remove the material by various non-
traditional techniques, not using direct contact between
the tool and the workpiece. Examples of unconventional
cutting methods are the use of water jet cutting, laser
beam cutting and plasma arc cutting. The main
advantages of unconventional technologies material
cutting processes are quality cuts and cost effective with
the ability to cope with different types materials and
geometrical configurations.
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Water Jet Cutting (WJC) is a mechanical process, with or
without abrasive particles [6]. This process uses a high-
power kinetic energy of the water flow to the cutting of
the workpiece material. The process produces a precise
cut on all types of materials (metal, stone, glass, plastic,
composites, ceramic etc.), but is especially suitable for the
cutting of very hard and difficult-to-machine materials
with complex contours. In the WJC process there is no
change in mechanical and morphological properties of the
workpiece material. Therefore, this process is especially
appropriate for heat sensitive materials, because there is
no heat affected zone. A significant advantage of water jet
cutting is environmental aspect, since it does not create
hazardous waste in the form of dust or gas. Basic
disadvantage of water jet cutting is long processing time.
Laser Beam Cutting (LBC) is a thermal process involving
photon energy as a heat source [7]. The high light energy
is focusing on the surface of the workpiece, which is then
heating, melting and vaporizing. This process removes
almost all metallic or non-metallic material. The LBC
process is best suited for sheet cutting, complex profile
cutting, brittle materials machining and drilling and
perforating very small holes. The laser cutting doesn't
take long and has a high level of efficiency and accuracy.
In case of laser beam cutting, the heat affected zone and
deformation in the part are very small. Disadvantages of
the LBC are high energy consumption and maintenance
cost, the gases released during the process can be toxic
and reflective metals can't be cut using this technology.
Plasma Arc Cutting (PAC) is a thermal non-traditional
material removal process [8-10]. The PAC process uses a
focused jet of high velocity and temperature ionized gas
to melt, vaporize and remove material from the
workpiece. The plasma arc cutting is the most common
materials cutting technique for larger and more
demanding plates, profiles and pipes. Advantages of
plasma arc cutting are that any metal can be cut, process
offers fast cutting speeds, slag-free cuts and operation
with more burners. Disadvantages of the PAC process are
low quality cut and a slightly wider kerf, relatively high
power and gas consumption as well as generation of
smoke and noise during process.
The basic cutting techniques and their machining
characteristics are illustrated in Fig. 1.
In this work is presented investigation of the plasma arc
cutting from the standpoint machining characteristics of
the process. It is known that plasma arc cutting offers
very high productivity but low cut quality. In order to
enhance the cutting quality of PAC, since its
commercialization, researchers are constantly examining
ways to improve this process.
Pawar and Inamdar [11] systematized the parameters
affecting quality of plasma arc cutting from the standpoint
of dimensional accuracy and surface quality without any
further operations. The paper highlights recent results
obtained by using plasma cutting process based on
experiments and various methods that have been used for
optimisation of PAC process.
Fig. 1 The review of the basic cutting techniques
Salonitis and Vatousianos [12] conducted an experimental
research of the plasma arc cutting for assessing the quality
of the cut. The cutting quality has been monitored by
measuring the kerf width, kerf taper angle, surface
roughness and heat affected zone. The quality
characteristics were assessed by varying the processing
parameters, such as the cutting speed, workpiece
thickness, plasma power and gas pressure.
Gariboldi and Previtali [13] conducted experiment to
improve the cutting quality performed by high tolerance
plasma arc cutting. The experiment was investigated
under different process conditions like using several
cutting speed with the adoption of oxygen or nitrogen as
working and shielding gases. They found that if oxygen
was used as the plasma gas, a higher cutting speed and
better quality of kerf geometry parameters were achieved
due to the oxidation reaction.
Bini, Colosimo, Kutlu and Monno [14] revealed that
cutting speed and arc voltage affect the kerf geometry
formation mechanism. They also concluded that waviness
can be reduced by reducing the cutting speed.
Radovanovic and Madic [15] modeled plasma arc cutting
process using artificial neural networking Using this
model the cutting speed and arc current were selected
which correspond to the cut with minimal surface
roughness.
Since it is still the main task of the plasma cutting to
achieve the high productivity with as good quality as
possible, in this paper special attention is directed on the
effect of machining conditions on the accuracy and
change of surface integrity. If the machining conditions
are poorly chosen, it can substantially diminish
exploitation features of the plasma arc cutting. Therefore,
in order to enable machining with a high performance, it
is necessary to investigate the effect of the PAC process
on the cut quality of the workpiece. Specifically, in this
paper specifics of the plasma arc cutting were
experimentally investigated and their influence on
material removal rate, surface topography, heat affected
zone and kerf geometry accuracy.
2. PLASMA ARC MACHINING
Plasma Arc Machining (PAM) is one of the relatively
recent nonconventional manufacturing processes. This
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technique was patented by R. Gate in 1957. Initially it
was used for cutting difficult-to-machine materials and
later its application was extended to a large number of
other production operations that require high
concentrations of thermal energy: welding, sintering,
coating, depositing, heat treatment, etc [16, 17].
In physics, the term plasma describes a gas state of matter
which has been heated to a sufficiently high temperature
to become partially or completely ionized gas. Thereby,
ionized gas represents a mixture of positive and negative
ions, as well as neutral free electrons, atoms, molecules,
photons and radicals. Because of the high density of ions
and electrons, plasma is highly electrically conductive.
Temperatures in the plasma range from 10,000 to
30,000 °C.
2.1 Working principle of PAM process
The plasma arc machining is based on the high thermal
and kinetic energy of ionized gas, directed to the surface
of the workpiece. These energies lead to the development
of a very high temperature in the region of plasma impact
on the material. The temperature at the machining zone
reaches the value of 4,000 to 10,000 °C. Such a high
temperature causes intense melting and vaporizing
(combustion) of any type of workpiece material.
Afterwards, the molten material together with the eroded
particles is removed by the explosive activity of the
internal dynamic forces of the plasma channel, as well as
by a fast jet of the plasma source. Thereby, an extremely
high concentration of thermal energy leads to a number of
physical-chemical processes and phenomena in the wider
area of the workpiece surface.
How the hot gas intensively comes out of a nozzle, there is
a chance of overheating of plasma torch. A water flow is
used to surround the plasma torch to avoid its overheating.
For the process of plasma arc machining, the mechanism of
the ionization of working gas is mainly realized by heating
the gas to a very high temperature, as a result of which
ionized gas is formed. This type of ionization involves the
passage under pressure of a mixture of gases (working and
shielding gas) through the space between the anode and the
cathode, where, with power supply (DC) a strong electric
arc is maintained. In the result of this is formed an ionized
gas flowing from the nozzle in the form of a high heat
plasma. The working gases used to create plasma are argon,
helium, nitrogen, carbon dioxide or air, as well as a mixture
of these gases. Hydrogen or oxygen is used as a shielding
gas in the PAM.
The electric arc can be performed directly in a plasma
torch between the electrodes (anode and cathode) or
between the workpiece (anode) and the central electrode
(cathode). The first variant can be used for processing all
types of materials, but the intensity of the plasma is
slightly lower. In the second variant large thermal energy
is generated, which is important when machining
difficult-to-machine materials of large thickness, but it is
possible to process only electrically conductive materials.
Shown in Fig. 2 is principle of PAM process, input
parameters (workpiece, machine, plasma torch and
plasma gas) and output performance (productivity,
accuracy and surface integrity).
Fig. 2 PAM process with machining characteristics
2.2 Quality of plasma cutting
Similar to other machining processes, productivity,
accuracy and quality are the most important performance
of plasma arc cutting. In that context, it is very important
to have a good knowledge of the characteristics and
properties of the PAC process to get the best performance.
Basic characteristics of the PAC are: velocity of plasma
jet; arc current, voltage and power; gas pressure and flow
rate; and nozzle diameter and stand-off distance.
The productivity of PAC process is expressed by the
material removal rate and in this process it is defined as
the machining speed. The accuracy is defined by the
tolerances applied to the dimensions and form of the
workpiece. The quality is expressed through the surface
topography and surface properties. Thereby, the
effectiveness of the machining process determines the
productivity and the product functionality defines the
accuracy and quality.
The quality of a part is particularly significant
characteristic of the plasma arc cutting. It is generally
defined by the following properties: machining accuracy
and surface integrity. Fig. 3 is showing the PAC process
mechanism with the quality indicators.
Machining accuracy is determined by the degree of
concordance of the actual dimension and shape of the part
with the nominal geometry. The plasma arc cutting
accuracy is typically defined by kerf geometry (ISO
9013). The kerf geometry is normally expressed by top
kerf width Kt, bottom kerf width Kb and kerf taper angle
[7, 14].
Surface integrity is described by the properties of the
workpiece material, above and below the surface, after
being modified by machining or other surface generation
process. The surface integrity refers to the external
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topography aspects of the surface (surface morphology
and texture surface) and the internal physical and
mechanical metallurgy aspects of the subsurface (surface
layer modification).
Fig. 3 Quality indicators of PAC process
The surface topography in plasma arc cutting is defined
by micro-geometry deviation, surface roughness and
waviness (ISO 4287). The micro-geometry deviation is
present due to plasma curvature, and defined by drag n
and pitch of drag line f. It is specificity of PAM
mechanism, because during interaction with the material
the plasma jet loses its kinetic energy and changes its
shape. Full and straight cuts are transformed into curved,
deformed or unfinished [6]. The surface roughness is
represented by different profile amplitude parameters.
Roughness average Ra, which is the arithmetic average of
the absolute values of the profile heights over the
evaluation length ln, is the primary parameter of the
surface roughness. Waviness is defined by surface
irregularities on a larger scale than the roughness. It is
determined by the waviness spacing Sw and waviness
height Wt.
The surface metallurgy in plasma arc cutting includes the
properties of the material layer, respectively study of the
nature of the microstructure, microhardness, residual
stress, crack, burn, etc [5, 18]. The metallurgical
characteristics of the material are caused by the thermal
energy, and this is called the heat affected zone (HAZ).
3. EXPERIMENTAL SETUP
Experimental investigations were carried out on the 3-axis
plasma arc cutting machine type CNC Omnicut 4000 by a
manufacturer MGM from Czech Republic, Fig. 4/a. Main
technical data of the PAC machine are as follows: table
area 12000x3000 mm, working current 0 to 130 A, arc
voltage 50 to 150 V, total power consumption 1.5 kVA
max, cutting speed 20-30000 mm/min, max thickness of
the cut sheet 200 mm, positioning accuracy 0.3 mm. This
configuration provides excellent dynamic and static
properties, high stiffness of the machine carriage, high
cutting accuracy and high productivity.
Fig. 4 PAC machine (a) and RSt 37-2 steel plate after cutting (b)
In the paper, first the basic experimental tests were carried
out to assess the quality indicators for a reference level of
process parameters in plasma arc cutting. The reference
level of parameters selected as representatives are: arc
current I=80 A, arc voltage U=127 V, plasma gas pressure
p=0.5 MPa, nozzle diameter d=1.75 mm, stand-off
distance z=2 mm and impact angle
=90. The range of
the cutting speed was v=700 to 2500 mm/min. Water
cooled nozzle and air plasma / air shield combination was
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used for plasma cutting operation.
Then, in the extended experiment some reference
parameters were further varied. The arc current was
chosen from the interval 50 to 100 A and the arc voltage
in the interval 121 to 133 V. The plasma gas pressure was
0.5 and 0.7 MPa, the nozzle diameter was 1.5 and 1.75
mm and the stand-off distance was 1 to 3 mm. While a
certain value was varied, the other values were held
constant at the reference level.
The basic work material used in the experiment was DIN
RSt 37-2 carbon low alloy steel (0.17% C, 1.4 % Mn,
0.05% S, and 0.05% P), tensile strength Rm=420 MPa.
RSt 37-2 is steel which contains ferrite and pearlite
microstructure, where the ferrite is much higher with
respect to the pearlite, Fig. 5. Thickness of the workpiece
was 10 mm. The cut workpiece is shown in Fig. 4/b.
The samples were cut by the PAC under certain
machining conditions. First, the parameters of the kerf
geometry (top kerf width Kt, bottom kerf width Kb and
kerf taper angle
) were measured. Then, the parameters
of the plasma jet deviation (drag n and pitch of drag line f)
were measured. The measurements were conducted using
digital caliper with accuracy to 0.001 mm, as well as by
reading the values at the photo samples recorded using
USB digital microscope with 200x magnification.
Fig. 5 Microstructure of workpiece material at magnification of 200x
Surface roughness of the workpiece in the PAC process
was estimated by measuring a set of surface parameters.
The measured parameters were roughness average Ra
(arithmetic mean deviation of the profile) and maximum
height of the profile Rt (vertical distance between the
highest and lowest points of the profile). These parameters
are the most widely used in surface roughness
measurements. The surface roughness measurements were
performed using the profilometer Mitutoyo SJ-301, Japan.
Surface properties of the specimens were assessed by
investigation of surface layer modification of the
workpiece after the plasma arc cutting. In this testing a
metallographic examination of the microstructure was
implemented, i.e. the heat affected zone (HAZ) was
evaluated. The surface metallographic identification of
the workpiece was performed with an Olympus
BHM/BH2 microscope with 1000× magnification from
Japan. For metallographic preparation was used the
equipment of the Struers, Denmark.
4. R
ESULTS AND ANALYSIS
4.1 Basic experiment
The basic experimental investigation considered the quality
of the workpiece in the plasma arc cutting. The speed
cutting was varied for the set reference parameters.
Thereby, the machining accuracy and surface integrity
parameters of the PAC process were measured. The test on
every sample indicated a very small measuring error, which
indicates a good repeatability of the measurement results.
Fig. 6 shows the results of experimental investigation of
geometry kerf for the selected plasma cutting conditions,
specifically top kerf width Kt, bottom kerf width Kb and
kerf taper angle of both sides
R/
L. The chosen images,
presented in the same view, show the prepared samples for
low, medium and high cutting speed. The results show that
the increase of cutting speed leads to a higher kerf taper
angle and smaller kerf width. Besides, it is evident that the
increase of cutting speed is limited by the incomplete
cutting of the material and/or the appearance of dross.
Fig. 6 Influence of the cutting speed on kerf geometry parameters
On the other hand, as shown in Fig. 7 is obtained
dependence of factors of the plasma jet deviation (drag n
and pitch of drag line f) and surface roughness parameters
(roughness average Ra and maximum height of the profile
Rt) on cutting speed in PAC process, for a reference level
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of parameters. In the same figure, are shown the
experiment samples for three characteristic cutting
speeds. It can be seen that the values of the micro-
geometry deviation and surface roughness are higher
when the cutting speed increases. It is evident that the
increase of the cutting speed leads to very poor surface
topography and roughness.
Fig. 7 Influence of the cutting speed on plasma jet deviation
In this study, metallographic investigations of the surface
properties of the material show that there are heat affected
zone (HAZ) at all machining conditions of PAC.
However, the metallographic examinations showed the
uniform size of the HAZ during various cutting speeds.
Fig. 8 shows a photomicrograph of the surface layer of
the investigated RSt 37-2 carbon steel after the plasma arc
cutting, i.e. its microstructure transformation compared to
the bulk material. The analysis of the microstructure of
material revealed three characteristic layers which are: the
resolidified layer, the modified surface layer and the
interlayer. The resolidified layer is a thin zone of the
deposition of residual molten material on the surface. The
modified layer consists of the ferrite-pearlite
transformation to martensite. The interlayer shows
gradual transition of modified layer into bulk material. It
is noted that observed thickness of the total heat affected
zone is in the range of about 1mm.
Fig. 8 Photomicrograph of surface layer of carbon steel cut by PAC at
magnification of 100x
4.2 Extended experiment
In the extended experiment some additional tests were
conducted in order to provide a more complete view of
plasma arc cutting quality. The main reference parameters
were further varied, namely, arc current, arc voltage,
plasma gas pressure, stand-off distance and nozzle
diameter. Thereby, only the macro geometrical
parameters of quality were measured.
The concept of deviation of the measured value from the
nominal one is used for an evaluation of the accuracy of
the kerf geometry. Delamination factor DF is the main
geometric characteristic of the damages of the plasma arc
cutting process. The delamination factor was defined as
the ratio of top kerf width Kt to bottom kerf width Kb
during the cutting of the workpiece material (DF=Kt/Kb).
Fig. 9 shows the influence of the cutting speed on
delamination factor for two arc currents in PAC process.
The diagram shows that the increase of cutting speed
results in increased delamination factor, especially at high
speeds. Thereby, a larger arc current increases the
delamination factor too.
Fig. 9 Dependence of the delamination factor on cutting speed
Shown in Figure 10 is obtained dependence of the
delamination factor on arc voltage during two plasma gas
pressure. The diagram shows that there exists an optimal
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arc voltage which results in minimum delamination
factor. It is also seen that the larger plasma gas pressure
decreases the delamination factor.
Fig. 10 Influence of the arc voltage on delamination factor
Fig. 11 shows the influence of the arc current on
delamination factor for two nozzle diameters. The
diagram shows that the increase of arc current results in
significantly reduced delamination factor, and this for
both nozzle diameters. Thereby, a smaller nozzle diameter
makes somewhat lower delamination factor.
Fig. 11 Dependence of the delamination factor on arc current
Shown in Figure 12 is obtained dependence of the
delamination factor on nozzle stand-off distance during
two cutting speeds. The diagram shows that there exists a
slight increase of the delamination factor with increase of
cutting speed. Of course, a lower cutting speed is more
acceptable.
4.3 Discussion
Analysis of previous experimental results revealed that
most impact on the plasma arc cutting quality has the
cutting speed. Increase of the cutting speed increases the
macro and micro geometrical parameters of quality
(parameters of the kerf geometry, parameters of the
plasma jet deviation and surface roughness parameters).
Also, the arc current has a significant effect on the
machining quality. On the other hand, the arc voltage,
plasma gas pressure, stand-off distance and nozzle
diameter have somewhat less impact on the quality.
Besides, the mentioned machining parameters have
mostly the opposite effect on the quality, which
significantly complicates control design of the plasma arc
cutting process.
Fig. 12 Influence of the stand-off distance on delamination factor
At the same time, it is important to note that there is heat
affected zone at any machining conditions of the plasma
arc cutting. The thickness of the HAZ is related to
machining conditions, but above all with the material
thickness and its thermal conductivity. Thereby, influence
of PAC machining parameters on the HAZ is almost
identical.
As previously mentioned, it is evident that the machining
quality in PAC process depends on a number of
machining parameters. Figure 13 shows the level of
variation of the most important parameters of the quality,
i.e. degree of their effect on the PAC process. The level of
variation of the plasma arc cutting parameter was
determined by using advanced statistical analysis.
Fig. 13 The level of variation of the machining parameters on the
quality of PAC process
5. CONCLUSIONS
Plasma arc cutting is a non-traditional highly productive
process widely used in plate processing made from
difficult-to-machine materials. Compared to other cutting
techniques, the PAC process achieves the fastest cutting
speed with satisfactory accuracy and surface finish.
However, the plasma arc cutting leads to intense
concentration of heat in the cutting area, which is why
heat affected zone is always present.
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On the ground of experimentally measured values it can
be concluded that increasing of the cutting speed and arc
current leads to a significant deterioration of machining
quality of the PAC process. It is primarily the cut
geometry deviation, presence of the jet lag effect present
due to plasma curvature, increase of the surface roughness
and the occurrence of dross. However, the other
machining parameters have different impact on the
cutting quality, which additionally complicates setting
and guidance of the plasma arc cutting process.
The investigations show that there is a major heat affected
zone at defined machining conditions in the PAC process.
The heat affected zone is related to above all with the type
and thickness of material. The value of the HAZ is greater
for thicker non-conductive material that is cut at lower
cutting speed and larger arc current.
ACKNOWLEDGEMENTS
This paper presents a part of researching at the Project
financed by Ministry of Education, Science and
Technological Development of the Republic of Serbia.
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jme.2010.075
... A key feature of the process of material removal is his ability to produce vital components of complex geometry, exceptional surface finish, high dimensional accuracy, and usually from difficult-to-machine materials. It covers a wide range of material removal operations such as turning, drilling, milling, grinding and other processes [1][2][3][4]. ...
... Since the main problem of material removal process is to achieve a high-quality finish with the highest possible productivity level, special attention is focused on the optimization of the processing conditions. There is a series of works which deal with optimization of machining strategy using various techniques [3,4,27,[31][32][33]. However, although heat and temperature are parameters which directly affect the process performance, their use for the optimization is quite complex in nature. ...
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... While this hardness value creates a negative situation for machining processes on the surfaces, the machining process of a part prepared by laser cutting is carried out relatively easily. Drilling/editing a plasma-cut hole later with a drill is particularly demanding [14]. Considering the hardness distribution, high hardness in plasma arc cutting is seen in the region of 750-950 μm. ...
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