Journal of Materials Processing Technology 210 (2010) 212 218
Contents lists available at ScienceDirect
Journal of Materials Processing Technology
journal homepage: www.elsevier.com/locate/jmatprotec
Capability of high pressure cooling in the turning of surface hardened piston rods
D. Kramar", P. Krajnik, J. Kopac
Faculty of Mechanical Engineering, University of Ljubljana, Askerceva 6, 1000 Ljubljana, EU, Slovenia
a r t i c l e i n f o a b s t r a c t
Article history:
An experimental study was performed to investigate the capabilities of dry, conventional and high pres-
Received 16 June 2008
sure cooling (HPC) in the turning of surface hardened piston rods used in fluid power applications.
Received in revised form 3 February 2009
Machining experiments were performed using coated carbide tools at cutting speeds up to 160 m/min.
Accepted 4 September 2009
The cooling capabilities are compared by monitoring of chip breakability, process regions of operability,
cooling efficiency, tool wear, tool life and cutting forces. Test results showed that dry cutting could not
be performed due to long and ductile chips that were formed for all investigated cutting conditions. In
Keywords:
comparison to conventional cooling the significant increase of cutting speed and feed rate region of oper-
Fluid power products
ability was recorded when machining with HPC. Tool life analysis proved a five times increase in tool
Hard turning
life when machining with HPC. Furthermore HPC also improved chip breakability and reduced coolant
High pressure cooling
consumption.
Machinability
� 2009 Elsevier B.V. All rights reserved.
Tool wear
Chip breakability
1. Introduction carbide tools and cutting speeds in the range of 90 160 m/min can
be performed.
The research goal relates to an improvement of hard turning According to Klocke and Eisenbl�tter (1997) coolant has a direct
in order to increase its technological capability and to extend the influence on the manufacturing economics. Therefore, by abandon-
region of process operability. This can be achieved by applying HPC ing conventional cooling and using dry or HPC assisted machining,
that can reduce the coolant consumption in comparison with con- the cost related to the usage of coolant can be reduced. Weinert et al.
ventional cooling as well as improve the machinability of surface (2004) have shown that besides an improvement in the economic
hardened steel. efficiency of the machining process, dry machining principles can
End-users of fluid power products require surface hardened also contribute to the health of machine tool operators and envi-
components such as hydraulic cylinders and piston rods in order ronment concerns.
to improve their wear behaviour. In the manufacturing chain, the In this investigation the capabilities of dry, conventional and
inductive hardening process is followed by a finishing operation HPC in hard turning are compared. All machining experiments
that generates the component s final geometry. Traditionally, the are performed with coated carbide tools and cutting speeds
finishing operations are grinding processes, but within the last up to 160 m/min. The performances of different cooling condi-
years the performances of hard cutting operations have drastically tions are assessed on the basis of chip breakability, regions of
improved. The study of Klocke et al. (2005) has shown that hard operability, cooling efficiency, tool wear, tool life and cutting
cutting offer a higher flexibility, increased material removal rates forces.
and the possibility of machining with reduced coolant consump-
tion. As presented by Rech and Moisan (2003) hard turning usually
2. Analysis of existing work
employs high cutting speeds and advanced cutting tool materials
such as CBN, PCD and ceramics. Hard cutting with coated carbide
To achieve higher wear resistance, the piston rods are inductive
tools, low cutting speed and conventional cooling, usually results in
hardened before the finishing operation. In the hard turning of steel,
significant problems concerning extremely long chips and severe
the thermal influence can lead to high temperatures and structural
adhesion wear mechanisms. By applying HPC at flow rate 1.4 l/min,
alterations of the workpiece material, causing the change of piston
the friction and the heat induced in the tool chip interface can be
rods mechanical properties. The thermal impact mainly depends
reduced. It is expected that HPC assisted hard turning with coated
on the maximum temperature reached in the cutting zone as well
as the cooling capability.
Klocke and Eisenbl�tter (1997) have stated that many materi-
" als such as high-temperature alloys (titanium and nickel based),
Corresponding author. Tel.: [phone]; fax: [phone].
E-mail address: [e-mail] (D. Kramar). hardened steels and other hard to machine materials cannot be
0924-0136/$  see front matter � 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmatprotec.2009.09.002
D. Kramar et al. / Journal of Materials Processing Technology 210 (2010) 212 218 213
effectively machined without cooling even with latest tool coat-
ings. Despite the high cost of coolants, the most common cooling
method in machining still refers to flooding the cutting area with a
large quantity of coolant.
Pigott (1953) was the first author to discuss the use of HPC in
steel turning with high speed steel tools. In this research coolant
was delivered at a pressure of 2.76 MPa directly at the clearance of
the tool. More recently, �jmertz and Oskarson (1999) have carried
out machining experiments on Inconel 718 with a HPC in the region
of 80 380 MPa. The high pressure jet was applied directly into the
tool chip interface. It was found that cooling introduced by the
HPC enhanced the surface finish quality and reduced burr. At high
pressure, the HPC jet penetrated deeper into the tool chip inter-
face, which reduced the fracture toughness of the chip material that
resulted in effective chip breaking. The test results however indi-
cated an accelerated notch wear rate on SiC-whiskers reinforced
ceramic tools.
Significant reduction of temperature in the cutting zone and sur-
face roughness due to the use of HPC was reported by Kaminski
and Alvelid (2000). Dahlman (2002) has concluded that material
properties such as hardness and ductility determine whether high
pressure or high flow have to be used to get enhanced cooling effect
in the turning of soft stage steels with carbide tools.
Ezugwu and Bonney (2004) have confirmed the feasibility of
using HPC in the rough turning of Inconel 718 with coated car-
bide tools. The investigation showed that the HPC could reduce the
temperature in chip tool and workpiece tool interfaces. Another
Fig. 1. Different HPC delivery: (a) between the rake face and the chip; (b) into the
reported benefit was referred to the decrease of tool chip contact
clearance; (c) towards the rake side through the tool.
length, which contributed to the decrease of temperature. In the
next study Ezugwu et al. (2005a) have assessed the whisker rein-
carbide tools with cutting speeds in the range of 90 160 m/min is
forced ceramic tool life during the machining of Inconel 718 at
filling this gap. As shown later this machining operation is attain-
different cutting speeds and under different coolant pressures. It
able by supplying a vegetable oil-based emulsion as a coolant at
was shown that at all cutting speeds, tool life increased when the
flow rates higher than 1.1 l/min and pressures larger than 70 MPa.
coolant pressure of up to 15 MPa was employed. However, when
pressure was increased from 15 to 20.3 MPa, tool life decreased
3. High pressure jet assisted turning
rapidly due to excessive notching at the depth of cut region. The
authors attributed notch wear to the erosion of the ceramic tool,
In turning, the chip formation is largely influenced by the heat
caused by the HPC. Ezugwu et al. (2005b) have also assessed
and friction generated in the contact zone between the rake face of
the tool life of uncoated carbide and CBN tools when turning
the tool and the machined surface material. Conventional cooling
Ti 6Al 4V alloy using conventional and HPC. When using CBN and
uncoated carbide tools, tool life was increasing with coolant pres- is not efficient to prevent extreme thermal loading in the cutting
zone. Compared to the conventional cooling, the idea of HPC is to
sure throughout the pressure region of operability up to 15 MPa.
deliver a high pressure jet of emulsion in the cutting zone. The jet
With further increase in pressure the opposite trend was observed.
can be applied in two ways:
The authors attributed this decrease to the critical boiling action of
the coolant at the tool edge, since it was possible to sweep the tool
"
surface faster with the higher jet speed, thus lowering the rate of With an external nozzle:
boiling and cutting down the heat transfer. They also stated that The coolant is delivered directly between the rake face and
the optimum coolant pressure appears to be in relation to the total the chip (Fig. 1a) or to the gap between the flank face and the
heat generated during machining. In the latest study Ezugwu et workpiece (Fig. 1b).
"
al. (2007) have analysed the surface generated when machining Through internal channels:
Ti 6Al 4V alloy with PCD tools using conventional and HPC for fin- The coolant is delivered through the tool using small holes in
ish turning. Surface roughness and micro-structure together with the insert (Fig. 1c).
tool life were investigated. HPC proved longer tool life and absence
of workpiece surface hardening.
The HPC between the rake face and the chip decreases the con-
External cooling is always applicable to external turning. How-
tact length. On the other hand, the cutting zone can be reached by
ever, external cooling may be difficult to apply in internal turning.
delivering the coolant below the flank face of the tool. The follow-
Wertheim et al. (1992) have investigated HPC through the tool rake
ing procedure is the most efficient to reduce the temperature in
face, shown in Fig. 1c. Schoenig et al. (1993) have used the same
the cutting zone. In this investigation the coolant delivery shown
cooling principle for the turning of titanium with uncoated carbide
in Fig. 1a has been used.
tools. The applied coolant pressure was up to 345 MPa, and the cut-
ting insert orifice was located near the cutting edge, where the com-
4. Experimental work
pressive chip loads were about 276 MPa. The high pressure water
jet application worked both as a source of cooling and as a hydraulic
4.1. Experimental setup and equipment
chip breaker. The reported tool life was increased by five times.
The analysis of existing work in the discussed machining area All machining experiments were carried out on surface induc-
revealed a technological gap. The hard turning of steels with coated tive hardened steel Ck45 (W.Nr.:1.1191 or AISI 1045). The depth
214 D. Kramar et al. / Journal of Materials Processing Technology 210 (2010) 212 218
of the hardened surface layer was between 1.5 and 1.8 mm with
a hardness of 58 HRc. The piston rods with a diameter of 30 mm
and a length of 400 mm were turned on a lathe of high stiffness,
equipped with a high pressure plunger pump of 250 MPa pressure
and 3 l/min flow capacity. The cutting tool inserts used in the exper-
iments were Al2O3-coated carbide cutting tools SNMA 120408 KR
432. The cutting inserts had a 0.8 mm nose radius and had no chip
breaking geometry on the rake face. A standard sapphire orifice
of 0.3 mm diameter, commonly used in water jet cutting applica-
tions, is mounted with a custom made tool clamping device that
enables accurate coolant jet adjustments. The coolant jet is directed
to the cutting edge at a low angle of 5ć% with the rake face at the
distance of 22 mm. The coolant was a 5.5% vegetable oil-based
emulsion without the presence of chlorine. The cutting tool was
mounted on the static dynamometer (Kistler 9259A). The measure-
ment chain further includes a charge amplifier (Kistler 5001), a
spectrum analyzer (HP 3567A) and a PC for data acquisition and
analysis. Tool wear measurements and images were acquired with
a CCD camera mounted on a Mitutoyo TM microscope aided with
imaging software. Surface roughness was measured with a stylus
Fig. 2. Machine tool used in experimental work.
type instrument Mitutoyo - Surftest SJ-301. The experimental setup
is shown in Fig. 2.
tools. During these experiments the depth of cut and the coolant
4.2. Experimental sequence
condition were kept constant.
3. In the third step, experiments were performed with the cut-
Experiments were conducted in dry, conventional and HPC con-
ting speed and the feed rate that belong to the cross-section
ditions. The experimental sequence consisted of three steps:
of overlapped regions of operability for particular cooling con-
dition determined in the previous step. By measuring the tool
1. In the first step, initial experiments were conducted in order to
wear, the assessment and comparison of cooling capability was
determine coolant pressures that yield adequate chip breaka-
conducted. During these experiments the depth of cut was also
bility and cooling capability. Within this step the influence of
kept constant.
coolant pressure on the cutting forces was analysed.
2. In the second step, regions of operability for all three cooling con-
ditions were determined. The particular region of operability sets 5. Results and discussion
the boundaries of the process cutting speed and feed rate. The
methodology involved measurements of the cutting forces and 5.1. Initial experiments
an analysis of the generated chips and was based on the French
national standard NF E 66-520-6 AFNOR (1994): tool material Within the initial experiments different pressures were
pair (TMP). This step was required because no machining data applied while the cutting speed, vc = 98.5 mm/min, the feed rate,
was available for the hard turning of steel with coated carbide f = 0.25 mm/rev, and the depth of cut, ap = 2 mm, were kept con-
Fig. 3. Chip forms regarding the cooling conditions and coolant pressure.
D. Kramar et al. / Journal of Materials Processing Technology 210 (2010) 212 218 215
Fig. 4. The influence of the coolant pressure on the feed and radial force.
stant. At pressures 10 and 30 MPa a relatively good breakability 5.2.2. Conventional cooling
of chips was observed. However, the lack of cooling was noticed 5.2.2.1. Cutting speed region of operability. For these experiments
because the chips were significantly burned as can be seen in the feed rate, f = 0.25 mm/rev, and the depth of cut, ap =2mm,
Fig. 3. Insufficient cooling is related to a low coolant flow, with were kept constant according to the TMP methodology. The mini-
the amounts of 0.4 l/min at 10 MPa and 0.7 l/min at 30 MPa. mum cutting speed is reached when significant changes in specific
At pressure higher than 70 MPa, which yields coolant flow of cutting force and/or surface finish were observed. The maximum
1.1 l/min, good breakability of chips as well as suitable cooling was cutting speed was determined by monitoring the surface finish
observed. and the shape of the chips. At cutting speeds higher than vc =
Further, the influence of the coolant pressure on the cutting 115 m/min the surface roughness began to increase and the chips
force components was analysed. The feed and radial force decrease were getting undesired shapes. The region of operability for cutting
as soon as the HPC is applied but no significant trend can be speed for this TMP in conventional cooling conditions is between
noticed with the increase of the pressure. In the case of the cut- vc = 90 m/min and vc = 115 m/min.
ting force it is more difficult to identify a trend, whereas the
small variations observed can be considered to be within the mar-
5.2.2.2. Feed rate region of operability. For these experiments the
gin of measurement error. The values presented in Fig. 4 are the
cutting speed, vc = 98.5 mm/min, and the depth of cut, ap = 2 mm,
mean values of three consecutive measurements of feed and radial
were kept constant. During experiments no significant alterations
forces.
in specific cutting force could be observed. Therefore, the shape of
For the subsequent steps in the experimental sequence, the
the chips was the region of operability selection criterion. At feed
pressure of 110 MPa was chosen. This pressure yields a flow of
rates higher than f = 0.27 mm/rev, the chips got undesired shapes,
approximately 1.4 l/min.
as shown in Fig. 5. The region of operability for feed rates for this
TMP is between f = 0.224 mm/rev and f = 0.265 mm/rev.
5.2. Regions of operability
At conventional cooling the cutting fluid flow rate was approx-
imately 6 l/min.
The determination of regions of operability is based on one-
factor-at-a-time experimental approach. This method consists of
selecting a starting point for each parameter (cutting speed and
5.2.3. High pressure cooling (HPC)
feed rate), then successively varying each parameter over its region
5.2.3.1. Cutting speed region of operability. As in the case of conven-
with the other parameter held constant at the baseline level.
tional cooling, for all experiments the feed rate, f = 0.25 mm/rev, and
the depth of cut, ap = 2 mm, were kept constant. The pressure was
5.2.1. Dry cutting
set to 110 MPa. In HPC the minimum cutting speed, vc = 90 m/min,
In dry cutting, long and ductile chips were formed regardless of
has been clearly determined by the evolution of a specific cutting
the cutting parameters.
force at a low cutting speed, which is the result of a built-up edge
Fig. 5. Chip forms in conventional cooling for different cutting parameters.
216 D. Kramar et al. / Journal of Materials Processing Technology 210 (2010) 212 218
Fig. 6. Chip forms in HPC for different feed rates.
(BUE). The maximum cutting speed, vc = 158 m/min, was chosen
in a way that the experiments could be run safely. The generated
chips had a desired shape.
5.2.3.2. Feed rate region of operability. As in the case of conventional
cooling, for all experiments the cutting speed, vc = 98.5 m/min, and
depth of cut, ap = 2 mm, were kept constant. The pressure was set
to 110 MPa. The lower limit for feed rate is determined by the size
of the chips. At low feed rates, f = 0.16 mm/rev, chips were too short
and could damage the slides of the lathe during the operation, as
seen in Fig. 6. At feed rates higher than f = 0.36 mm/rev, vibrations
and long chips were generated. During the experiments the specific
cutting force was just slightly higher than its theoretical value.
Fig. 7. Regions of operability for TMP in HPC and conventional cooling.
According to the spindle speed limitation, cutting speeds higher
than 200 m/min have not been tested and the upper limit was fixed
to 158 m/min for safety reasons. Fig. 7 shows region of operability
to increased friction between the cutting tool and the workpiece
for TMP for conventional cooling and HPC conditions.
and hence a higher specific cutting force and more generated heat.
Poulachon et al. (2004) have found out that a flank wear land width
5.3. Cooling capability
of VB = 0.1 mm does not lead to workpiece damage.
In order to assess and compare the capability of cooling
The capability of cooling is characterized by tool life and tool
conditions, experiments were carried out within the common
wear. Ueda et al. (1999) have shown that crater wear changes the
cross-section of overlapped regions of operability for a particular
effective rake face angle and leads to a changing cutting behaviour.
cooling condition as shown in Fig. 7. The following cutting condi-
Excessive crater wear weakens the tool just behind the cutting edge
tions were employed in this experimental step:
and can cause a sudden break down of the cutting edge. The flank
wear occurs as a flattened area on the cutting tool flank face. The
" Depth of cut ap = 2 mm.
width of flank wear, referred to as the flank wear land width VB,
" Feed rate f = 0.25 mm/rev.
is considered a good indicator of the state of the cutting tool, as
" Cutting speed vc = 98.5 m/min.
demonstrated by Poulachon et al. (2004). An increase in VB leads
Fig. 8. Tool wear for conventional cooling conditions.
D. Kramar et al. / Journal of Materials Processing Technology 210 (2010) 212 218 217
Fig. 9. Tool wear for HPC conditions.
5.3.1. Conventional cooling
The distribution of the flank wear VB proved very uniform. It
can be seen that besides flank wear also wear on the rake face has
occurred in the shape of crater wear KBmax as shown in Fig. 8. The
increase of the tool wear can be correlated to the increase of the
specific cutting force calculated after each experiment.
The flank wear of VB = 0.1 mm has occurred in less then 2 min.
5.3.2. High pressure cooling (HPC)
The distribution of the wear along the flank face was rather uni-
form, however some marks of notch wear at the depth of the cut line
can also be noticed. Crater wear was also present on the rake face.
This can be seen in Fig. 9. The changes in the specific cutting force
and roughness were noticed as a consequence of the increasing tool
wear.
Fig. 10. Tool wear development for conventional and HPC.
Fig. 10 shows the tool flank face wear plot for both conven-
tional and HPC conditions. For the selected criteria VB = 0.1 mm,
The effect of HPC on the tool rake face wear can also be observed
tool life in the case of HPC was about 10 min, which is approxi-
in Fig. 11 where both tools for conventional and HPC conditions at
mately five times longer than in the case of conventional cooling. It
VB = 0.1 mm are compared. The contact length in HPC conditions is
should be pointed out that the consumption of coolant in the case
approximately one third shorter than in the case of conventional
of HPC is more than four times lower as in the case of conventional
cooling.
cooling.
Fig. 11. Comparison of flank wear at VB = 0.1 mm and contact length for conventional and HPC.
218 D. Kramar et al. / Journal of Materials Processing Technology 210 (2010) 212 218
6. Conclusions References
AFNOR Standart NF E 66-520, Couple outil matiŁre, 1994. Domaine de fonction-
The presented research is based on experimental compari-
nement des outils coupants. vols. 1 6.
son of three hard turning operations, where the main difference
Dahlman, P., 2002. Comparing the temperature reduction in high-pressure jet-
assisted turning using high pressure versus high flow. Journal of Engineering
is in the utilized cooling. In the first case dry cutting is per-
Manufacture 216, 467 473.
formed. In the second and the third case conventional and HPC
Ezugwu, E.O., Bonney, J., 2004. Effect of high-pressure coolant supply when machin-
conditions are employed. The experimental work has proved that
ing nickel-base, Inconel 718, alloy with coated carbide tools. Journal of Materials
Processing Technology 153 154, 1045 1050.
the hard turning of steels with coated carbide tools and cutting
Ezugwu, E.O., Bonney, J., Fadare, D.A., Sales, W.F., 2005a. Machining nickel-base,
speeds in the range of 90 160 m/min is not possible in dry cut-
Inconel 718, alloy with ceramic tools under conditions with various coolant
ting conditions with a flat faced tool. The machining capabilities of
supply pressures. Journal of Materials Processing Technology 162 163, 68 73.
Ezugwu, E.O., Da Silva, R.B., Bonney, J., Machado, �.R., 2005b. Evaluation of the per-
conventional and HPC methods are compared with respect to chip
formance of CBN tools when turning Ti 6Al 4V alloy with high pressure coolant
breakability, tool wear, tool life and cutting forces. Both cooling
supplies. Intenational Journal of Machine & Tool Manufacturing 45, 1009 1014.
conditions are characterized with process regions of operability for
Ezugwu, E.O., Bonney, J., Da Silva, R.B., Cakir, O., 2007. Surface integrity of finished
the employed tool material pair. The major concluding remarks turned Ti 6Al 4V alloy with PCD tools using conventional and high pressure
coolant supplies. Intenational Journal of Machine & Tool Manufacturing 47,
regarding the HPC advantages over conventional cooling in the
884 891.
hard turning of steel with flat faced coated carbide tools are as
Kaminski, J., Alvelid, B., 2000. Temperature reduction in the cutting zone in water-jet
follows: assisted turning. Journal of Materials Processing Technology 106, 68 73.
Klocke, F., Eisenbl�tter, G., 1997. Dry cutting. Annals of the CIRP 46, 519 526.
Klocke, F., Brinksmeier, E., Weinert, K., 2005. Capability profile of hard cutting and
" Extension of the region of operability for a given tool material
grinding processes. Annals of the CIRP 54, 557 580.
�jmertz, K.M.C., Oskarson, H.B., 1999. Wear on SiC-whiskers reinforced ceramic
pair. An approximately 45% increase in the maximum achievable
inserts when cutting Inconel with waterjet assistance. Tribology Transactions,
cutting speed and a 25% increase in the maximum achievable feed
http://www.informaworld.com/smpp/title%7Econtent=t713669620%7Edb=
rate were proven.
all%7Etab=issueslist%7Ebranches=42 - v4242, 471-478.
" A five times increase in tool life (VB = 0.1 mm), from 2 to 10 min
Poulachon, G., Bandyopadhyay, B.P., Jawahir, I.S., Pheulpin, S., Seguin, E., 2004. Wear
behavior of CBN tools while turning various hardened steels. Wear 256, 302 310.
was achieved.
Pigott, R.J.S., 1953. Method of applying cutting liquids. US Patent 2,653,517.
" Significant increase in chip breakability.
Rech, J., Moisan, A., 2003. Surface integrity in finish hard turning of case-hardened
" All HPC advantages described above were achieved with reduced steels. International Journal of Machine Tools & Manufacture 43, 543 550.
Schoenig, F.C., Khan, A.K., Atherton, A., Lindeke, R., 1993. Machining of titanium
coolant consumption by four times.
using water jet assistance through the insert. In: Proceedings of 7th American
Water-Jet Conference, Seattle, Washington, pp. 801 812.
Acknowledgment Ueda, T., Al Huda, M., Yamada, K., Nakayama, K., 1999. Temperature measurement
of cBN tool in turning of high hardness steel. Annals of the CIRP 49, 63 66.
Wertheim, R., Rotberg, J., Ber, A., 1992. Influence of high-pressure flushing through
This research was conducted within the PROHIPP integrated
the rake face of the cutting tool. Annals of the CIRP 41, 101 106.
project, under the 6th Framework Programme for Research and Weinert, K., Inasaki, I., Sutherland, J.W., Wakabayashi, T., 2004. Dry machining and
minimum quantity lubrication. Annals of the CIRP 53, 511 537.
Technological Development, financed by the European Commis-
sion.