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Journal of Engineering Science and Technology Vol. 10, No. 7 (2015) 911 - 920 © School of Engineering, Taylor’s University
911
SURFACE ROUGHNESS AND CUTTING FORCES IN CRYOGENIC TURNING OF CARBON STEEL
T. C. YAP1,*, SIVARAOS
2, C. S. LIM
1, J. W. LEAU
3
1Faculty of Engineering and Technology, Multimedia University,
Jalan Ayer Keroh Lama, 75450 Melaka, Malaysia 2Faculty of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka,
Hang Tuah Jaya, 76100, Durian Tunggal, Melaka, Malaysia 3Microtherm Sdn Bhd, HICOM Industrial Area, 40000 Shah Alam, Selangor, Malaysia
*Corresponding Author: [email protected]
Abstract
The effect of cryogenic liquid nitrogen on surface roughness, cutting forces, and
friction coefficient of the machined surface when machining of carbon steel
S45C in wet, dry and cryogenic condition was studied through experiments.
The experimental results show that machining with liquid nitrogen increases the cutting forces, reduces the friction coefficient, and improves the chips
produced. Beside this, conventional machining with cutting fluid is still the
most suitable method to produce good surface in high speed machining of
carbon steel S45C whereas dry machining produced best surface roughness in
low speed machining. Cryogenic machining is not able to replace conventional
cutting fluid in turning carbon steel.
Keywords: Cryogenic machining, Cutting forces, Friction coefficient, Surface
roughness.
1. Introduction
Manufacturing sector is the second major contributors to Malaysia’s gross
domestic product in the year 2013 [1]. Machining or other material removal
processes are normally applied in manufacturing plant as part of the
manufacturing processes. Conventionally, cutting fluids are used to reduce the
cutting temperature and prolong the life of the machining tools. However, most
cutting fluids are harmful to environment, especially when they were disposed
improperly [2, 3]. Kovoor et al. studied the impact of effluent waste from
machining process on the quality of drain water. They found that effluent waste
increased the chemical oxygen demand, ammoniacal nitrogen, and total suspended
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solids in the drain water [3]. Beside this, conventional cutting fluids also
produces health problems [4-6]. Some of the additives added on cutting fluids
to provide the lubricating effects might cause a carcinogenic potential [4].
In addition, some additives such as barite is added as weighting agent in
drilling fluids can discharge heavy metal to the environment during drilling
process [7].
As such, new machining techniques/concepts such as dry machining and
cryogenic machining were proposed to replace conventional machining using
conventional cutting fluids [8]. Two types of cryogen are commonly used in
cryogenic machining. They are liquid carbon dioxide and liquid nitrogen. The
boiling points for both gases are –78.5oC and –196
oC. Both gases are abundant
and can be recycled; they can be compressed to liquid form, cool the cutting tools
and evaporate and become gas again.
Cryogenic machining was first explored by Bartle who used liquid carbon
dioxide as the coolant at 1953 [9]. Most of the recent study works on cryogenic
machining were focused on cutting forces, friction coefficient, tool wear, and
surface roughness. Improvements in cutting tool life or surface roughness under
cryogenic conditions have been reported by many researchers such as Biček et al.
[10], Hong et al. [11, 12], Wang et al. [8, 13], Dhar et al. [14] and Yap et al. [15].
In cryogenic machining, surface finish was improved due to reduction in tool
wear [8]. Beside this, Hong et al. [16] reported that cryogenic machining tends to
increase the cutting force because the titanium alloys become harder and stronger
at low temperature.
In this study, the cutting forces, friction coefficients and surface roughness
while machining carbon steel S45C were investigated under three conditions (dry
cutting, with cutting fluid and with cryogenic- liquid nitrogen cooling). Surface
roughness, cutting forces, and friction coefficient have been examined for
comparing the cryogenic machining to wet and dry turning.
2. Experimental Details
2.1. Work piece materials
The work materials used in this experiment was a medium carbon steel S45C. The
chemical composition and properties of carbon steel S45C are shown in Tables 1
and 2. The diameter and length of the carbon steel work piece are 36 mm and 500
mm respectively.
Nomenclatures
Ra Average roughness
Rmax Maximum roughness depth
Rq Root mean square roughness
Abbreviations
TiN Titanium nitride
CVD Chemical vapor deposition
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Table 1. Chemical Composition of Carbon Steel [17].
Grade C
(%)
Si
(%)
Mn
(%)
P(%)
max.
S(%)
max.
Cr(%)
max.
Ni(%)
max.
Cu(%)
max.
S45C 0.42-
0.50
0.17-
0.37
0.50-
0.80 0.035 0.035 0.25 0.25 0.25
Table 2. Selected Properties of Carbon Steel S45C [17].
Tensile
strength,
(MPa)
Yield
strength,
(MPa)
Elongation
ratio, (%)
Reduction of
area, (%)
Degree of
hardness
≥600 ≥355 ≥16 ≥40 ≤229HB
2.2. Cutting tool
The cutting tool used in this experiment was TiN CVD coated carbide Mitsubishi
TNMG 160408R -ES US735. Tool holder MTJNR-2020K-16 was used in this
experiment. A new cutting edge was used for each condition.
2.3. Cutting conditions
The experiments were conducted at constant depth of cut of 0.3 mm and feed rate
of 0.121 mm/rev. Three different cutting speeds were used; they are 175.93,
201.06, and 226.19 m/min. Three environments (Cryogenic liquid nitrogen, dry
and with cutting fluid) were studied. The cutting conditions are shown in Table 3.
Table 3. Cutting Conditions.
Cutting speed (m/min) Depth of cut (mm) Feed rate (mm/rev)
175.93, 201.06, 226.19 0.3 0.121
2.4. Experimental setup and procedure
All machining tests were carried out on a SunMax Machine LA-430 conventional
lathe. The cutting force components (feed force, radial force, cutting force) were
observed by a 3-component dynamometer (Kistler, Type 9265 B). The surface
roughness of the machined surfaces was measured using a surface roughness
tester (Mahr Perthometer S2). The standard cut-off length of 0.08 mm was used
for surface roughness measurement. The turning tests were conducted in three
environments. For cryogenic condition, liquid nitrogen jet was injected to the
interface between the carbon steel and cutting insert by internal pressure of the
liquid nitrogen tank. The internal pressure of the liquid nitrogen tank is 0.0345
MPa. The set-up of cryogenic machining is shown in Fig. 1. For machining with
cutting fluid, water-based emulsion (AEROIL Kool cut C-300, 5% oil and 95%
water) was selected.
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Fig. 1. Set-up of Cryogenic Machining.
3. Results and Discussion
3.1. Surface roughness
The surface roughness was measured and presented in average roughness (Ra),
root mean square roughness (Rq), and maximum roughness depth (Rmax). Figure 2
shows the surface roughness of carbon steel work piece under different cutting
conditions. In general, dry machining is able to generate good surface finished at
lowest cutting speed (175.93 m/min). Turning at lowest cutting speed (175.93
m/min) generated good surface roughness, and the difference due to environment
(dry, cutting fluid, cryogenic) is not significant. However, at highest cutting speed
studied, 226.19 m/min, conventional cutting fluid outperformed dry machining
and cryogenic machining (in terms of Ra, Rq, and also Rmax).
Machining without cutting fluid or with liquid nitrogen produced higher
roughness depth, Fig. 2(c). Beside this, Figs. 2(a)-(c) indicates that cutting with
conventional cutting fluid provided a stable surface roughness within the range of
speed studied (175.93 – 226.19 m/min). Cryogenic machining with low pressure
is not able to replace conventional cutting fluid if good surface roughness is
required. Current results are different from results reported by previous studies
[18, 19]. This might due to different tool work materials selected.
(a)
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(b)
(c)
Fig. 2. Surface Roughness of Carbon Steel
under Three Different Cutting Environments.
3.2. Cutting forces
In this study the first 10 s of machining were used for measurement of cutting
forces. In general, higher cutting forces (main cutting force, feed force and
radial force) were obtained under application of cryogenic liquid nitrogen. This
results is similar to Hong et al. [16] where this phenomenon can be attributed to
hardening of carbon steel by cryogenic liquid nitrogen. The details of feed
force, radial force and main cutting force at two different cutting speeds are
presented in Figs. 3 and 4. In cutting speed of 201.06 m/min, conventional
cutting oil showed the lowest cutting forces when compare to dry machining
and cryogenic machining. However, when the cutting speed increased to 226.19
m/min, dry machining produced the lowest cutting forces. The reduction of
cutting forces with cutting speed in dry machining can be due to the hot
softening at the tool work interface.
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Fig. 3. Measured Cutting Forces at 201.06 m/min.
Fig. 4. Measured Cutting Forces at 226.19 m/min.
3.3. Friction coefficients
The feed force, radial force and main cutting force were converted into
component of force along the tool face (tangential force) and component of force
normal to the tool face (normal force) based on equations discussed in [15, 20].
The friction coefficients while machining carbon steel in dry, wet, and cryogenic
condition were then calculated by dividing the tangential force by the normal
force. Figure 5 shows the friction coefficient increases as the cutting speed
increases and friction coefficient in cryogenic machining is lower than friction
coefficient in dry and wet machining of carbon steel. In dry machining, dry
sliding motion of tool-work generates heat. High interface temperature soften the
surface asperity and this further causes asperity easier to deform [15]. When
asperities deform, it enlarges the actual contact area of the tribo pairs and then
increases the friction force. In addition, Fig. 5 also shows that cryogenic liquid
nitrogen reduces the friction coefficient between cutting tool and carbon steel for
about 32% for cutting speed of 201.06 m/min and 24% for cutting speed of
Surface Roughness and Cutting Forces in Cryogenic Turning of . . . . 917
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226.19 m/min. This is because cryogenic liquid nitrogen reduces the interface
temperature and then reduces the actual contact area and then the adhesive
friction force.
Fig. 5. Friction Coefficient while Machining Carbon Steel.
3.4. Macro morphology of chips produced
The macro morphology images of the cutting chips obtained by dry and cryogenic
machining are shown in Figs. 6-9. At cutting speed of 175.93 m/min, cutting
chips collected from dry machining were continuous (Fig. 6) while chips
collected from cryogenic machining were broken into smaller pieces, (Fig. 7).
When the cutting speed increased from 175.93 m/min to 201.06 m/min, the
cutting chips obtained from dry machining were still in continuous and snarled
shape (Fig. 8) while cryogenics turning produced more short cutting chips
(Fig. 9). Generally, long/snarled chips collected from dry machining are
unfavourable in machining [20]. In contrast, smaller and shorter chips are
favourable in machining; smaller chips allowed liquid nitrogen to penetrate into
the elastic zone of the tool-work interface, similar to finding of Nandy et al. [21].
In term of chips produced, cryogenic machining is better than dry machining.
Fig. 6. Cutting Chips Obtained from Dry Turning, at 175.93 m/min.
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Fig. 7. Cutting Chips Obtained from Cryogenic Turning, at 175.93 m/min.
Fig. 8. Cutting Chips Obtained from Dry Turning, at 201.06 m/min.
Fig. 9. Cutting Chips Obtained from Cryogenic Turning, at 201.06 m/min.
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4. Conclusions
An experiment was conducted to study dry, wet and cryogenic turning of carbon
steel S45C. The experimental result shows that cryogenic liquid nitrogen jet
reduces friction coefficient in turning the carbon steel and also improves the chips
produced. However, turning with liquid nitrogen deteriorates the surface
roughness of the machined surface. Dry machining is able to produce best surface
roughness in low speed machining but it also produces unfavourable long chip
and high friction coefficient. Therefore, the conventional wet machining is still
the best method to produce good machined surface of carbon steel at higher
machining speed.
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