Journal of Materials Processing Technology 210 (2010) 212–218

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Journal of Materials Processing Technology

j o u r n a l h o m e p a g e :

w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c

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

Article history:
Received 16 June 2008
Received in revised form 3 February 2009
Accepted 4 September 2009

Keywords:
Fluid power products
Hard turning
High pressure cooling
Machinability
Tool wear
Chip breakability

a b s t r a c t

An experimental study was performed to investigate the capabilities of dry, conventional and high pres-
sure cooling (HPC) in the turning of surface hardened piston rods used in fluid power applications.
Machining experiments were performed using coated carbide tools at cutting speeds up to 160 m/min.
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
comparison to conventional cooling the significant increase of cutting speed and feed rate region of oper-
ability was recorded when machining with HPC. Tool life analysis proved a five times increase in tool
life when machining with HPC. Furthermore HPC also improved chip breakability and reduced coolant
consumption.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The research goal relates to an improvement of hard turning

in order to increase its technological capability and to extend the
region of process operability. This can be achieved by applying HPC
that can reduce the coolant consumption in comparison with con-
ventional cooling as well as improve the machinability of surface
hardened steel.

End-users of fluid power products require surface hardened

components such as hydraulic cylinders and piston rods in order
to improve their wear behaviour. In the manufacturing chain, the
inductive hardening process is followed by a finishing operation
that generates the component’s final geometry. Traditionally, the
finishing operations are grinding processes, but within the last
years the performances of hard cutting operations have drastically
improved. The study of

Klocke et al. (2005)

has shown that hard

cutting offer a higher flexibility, increased material removal rates
and the possibility of machining with reduced coolant consump-
tion. As presented by

Rech and Moisan (2003)

hard turning usually

employs high cutting speeds and advanced cutting tool materials
such as CBN, PCD and ceramics. Hard cutting with coated carbide
tools, low cutting speed and conventional cooling, usually results in
significant problems concerning extremely long chips and severe
adhesion wear mechanisms. By applying HPC at flow rate 1.4 l/min,
the friction and the heat induced in the tool–chip interface can be
reduced. It is expected that HPC assisted hard turning with coated

∗ Corresponding author. Tel.: [phone]; fax: [phone].

E-mail address:

[e-mail]

(D. Kramar).

carbide tools and cutting speeds in the range of 90–160 m/min can
be performed.

According to

Klocke and Eisenblätter (1997)

coolant has a direct

influence on the manufacturing economics. Therefore, by abandon-
ing conventional cooling and using dry or HPC assisted machining,
the cost related to the usage of coolant can be reduced.

Weinert et al.

(2004)

have shown that besides an improvement in the economic

efficiency of the machining process, dry machining principles can
also contribute to the health of machine tool operators and envi-
ronment concerns.

In this investigation the capabilities of dry, conventional and

HPC in hard turning are compared. All machining experiments
are performed with coated carbide tools and cutting speeds
up to 160 m/min. The performances of different cooling condi-
tions are assessed on the basis of chip breakability, regions of
operability, cooling efficiency, tool wear, tool life and cutting
forces.

2. Analysis of existing work

To achieve higher wear resistance, the piston rods are inductive

hardened before the finishing operation. In the hard turning of steel,
the thermal influence can lead to high temperatures and structural
alterations of the workpiece material, causing the change of piston
rods mechanical properties. The thermal impact mainly depends
on the maximum temperature reached in the cutting zone as well
as the cooling capability.

Klocke and Eisenblätter (1997)

have stated that many materi-

als such as high-temperature alloys (titanium and nickel based),
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,

Öjmertz 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
reported benefit was referred to the decrease of tool–chip contact
length, which contributed to the decrease of temperature. In the
next study

Ezugwu et al. (2005a)

have assessed the whisker rein-

forced ceramic tool life during the machining of Inconel 718 at
different cutting speeds and under different coolant pressures. It
was shown that at all cutting speeds, tool life increased when the
coolant pressure of up to 15 MPa was employed. However, when
pressure was increased from 15 to 20.3 MPa, tool life decreased
rapidly due to excessive notching at the depth of cut region. The
authors attributed notch wear to the erosion of the ceramic tool,
caused by the HPC.

Ezugwu et al. (2005b)

have also assessed

the tool life of uncoated carbide and CBN tools when turning
Ti–6Al–4V alloy using conventional and HPC. When using CBN and
uncoated carbide tools, tool life was increasing with coolant pres-
sure throughout the pressure region of operability up to 15 MPa.
With further increase in pressure the opposite trend was observed.
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
boiling and cutting down the heat transfer. They also stated that
the optimum coolant pressure appears to be in relation to the total
heat generated during machining. In the latest study

Ezugwu et

al. (2007)

have analysed the surface generated when machining

Ti–6Al–4V alloy with PCD tools using conventional and HPC for fin-
ish turning. Surface roughness and micro-structure together with
tool life were investigated. HPC proved longer tool life and absence
of workpiece surface hardening.

External cooling is always applicable to external turning. How-

ever, external cooling may be difficult to apply in internal turning.

Wertheim et al. (1992)

have investigated HPC through the tool rake

face, shown in

Fig. 1

c.

Schoenig et al. (1993)

have used the same

cooling principle for the turning of titanium with uncoated carbide
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-
pressive chip loads were about 276 MPa. The high pressure water
jet application worked both as a source of cooling and as a hydraulic
chip breaker. The reported tool life was increased by five times.

The analysis of existing work in the discussed machining area

revealed a technological gap. The hard turning of steels with coated

Fig. 1. Different HPC delivery: (a) between the rake face and the chip; (b) into the
clearance; (c) towards the rake side through the tool.

carbide tools with cutting speeds in the range of 90–160 m/min is
filling this gap. As shown later this machining operation is attain-
able by supplying a vegetable oil-based emulsion as a coolant at
flow rates higher than 1.1 l/min and pressures larger than 70 MPa.

3. High pressure jet assisted turning

In turning, the chip formation is largely influenced by the heat

and friction generated in the contact zone between the rake face of
the tool and the machined surface material. Conventional cooling
is not efficient to prevent extreme thermal loading in the cutting
zone. Compared to the conventional cooling, the idea of HPC is to
deliver a high pressure jet of emulsion in the cutting zone. The jet
can be applied in two ways:

• With an external nozzle:

The coolant is delivered directly between the rake face and

the chip (

Fig. 1

a) or to the gap between the flank face and the

workpiece (

Fig. 1

b).

• Through internal channels:

The coolant is delivered through the tool using small holes in

the insert (

Fig. 1

c).

The HPC between the rake face and the chip decreases the con-

tact length. On the other hand, the cutting zone can be reached by
delivering the coolant below the flank face of the tool. The follow-
ing procedure is the most efficient to reduce the temperature in
the cutting zone. In this investigation the coolant delivery shown
in

Fig. 1

a has been used.

4. Experimental work

4.1. Experimental setup and equipment

All machining experiments were carried out on surface induc-

tive hardened steel Ck45 (W.Nr.:1.1191 or AISI 1045). The depth

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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 Al

2

O

3

-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 5

◦

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
type instrument Mitutoyo - Surftest SJ-301. The experimental setup
is shown in

Fig. 2

.

4.2. Experimental sequence

Experiments were conducted in dry, conventional and HPC con-

ditions. The experimental sequence consisted of three steps:

1. In the first step, initial experiments were conducted in order to

determine coolant pressures that yield adequate chip breaka-
bility and cooling capability. Within this step the influence of
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
the boundaries of the process cutting speed and feed rate. The
methodology involved measurements of the cutting forces and
an analysis of the generated chips and was based on the French
national standard NF E 66-520-6

AFNOR (1994)

: tool–material

pair (TMP). This step was required because no machining data
was available for the hard turning of steel with coated carbide

Fig. 2. Machine tool used in experimental work.

tools. During these experiments the depth of cut and the coolant
condition were kept constant.

3. In the third step, experiments were performed with the cut-

ting speed and the feed rate that belong to the cross-section
of overlapped regions of operability for particular cooling con-
dition determined in the previous step. By measuring the tool
wear, the assessment and comparison of cooling capability was
conducted. During these experiments the depth of cut was also
kept constant.

5. Results and discussion

5.1. Initial experiments

Within the initial experiments different pressures were

applied while the cutting speed,

v

c

= 98.5 mm/min, the feed rate,

f = 0.25 mm/rev, and the depth of cut, a

p

= 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
of chips was observed. However, the lack of cooling was noticed
because the chips were significantly burned as can be seen in

Fig. 3

. Insufficient cooling is related to a low coolant flow, with

the amounts of 0.4 l/min at 10 MPa and 0.7 l/min at 30 MPa.
At pressure higher than 70 MPa, which yields coolant flow of
1.1 l/min, good breakability of chips as well as suitable cooling was
observed.

Further, the influence of the coolant pressure on the cutting

force components was analysed. The feed and radial force decrease
as soon as the HPC is applied but no significant trend can be
noticed with the increase of the pressure. In the case of the cut-
ting force it is more difficult to identify a trend, whereas the
small variations observed can be considered to be within the mar-
gin of measurement error. The values presented in

Fig. 4

are the

mean values of three consecutive measurements of feed and radial
forces.

For the subsequent steps in the experimental sequence, the

pressure of 110 MPa was chosen. This pressure yields a flow of
approximately 1.4 l/min.

5.2. Regions of operability

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
feed rate), then successively varying each parameter over its region
with the other parameter held constant at the baseline level.

5.2.1. Dry cutting

In dry cutting, long and ductile chips were formed regardless of

the cutting parameters.

5.2.2. Conventional cooling
5.2.2.1. Cutting speed region of operability. For these experiments
the feed rate, f = 0.25 mm/rev, and the depth of cut, a

p

= 2 mm,

were kept constant according to the TMP methodology. The mini-
mum cutting speed is reached when significant changes in specific
cutting force and/or surface finish were observed. The maximum
cutting speed was determined by monitoring the surface finish
and the shape of the chips. At cutting speeds higher than

v

c

=

115 m/min the surface roughness began to increase and the chips
were getting undesired shapes. The region of operability for cutting
speed for this TMP in conventional cooling conditions is between

v

c

= 90 m/min and

v

c

= 115 m/min.

5.2.2.2. Feed rate region of operability. For these experiments the
cutting speed,

v

c

= 98.5 mm/min, and the depth of cut, a

p

= 2 mm,

were kept constant. During experiments no significant alterations
in specific cutting force could be observed. Therefore, the shape of
the chips was the region of operability selection criterion. At feed
rates higher than f = 0.27 mm/rev, the chips got undesired shapes,
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.

At conventional cooling the cutting fluid flow rate was approx-

imately 6 l/min.

5.2.3. High pressure cooling (HPC)
5.2.3.1. Cutting speed region of operability. As in the case of conven-
tional cooling, for all experiments the feed rate, f = 0.25 mm/rev, and
the depth of cut, a

p

= 2 mm, were kept constant. The pressure was

set to 110 MPa. In HPC the minimum cutting speed,

v

c

= 90 m/min,

has been clearly determined by the evolution of a specific cutting
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.

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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,

v

c

= 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,

v

c

= 98.5 m/min, and

depth of cut, a

p

= 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.

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

for TMP for conventional cooling and HPC conditions.

5.3. Cooling capability

The capability of cooling is characterized by tool life and tool

wear.

Ueda et al. (1999)

have shown that crater wear changes the

effective rake face angle and leads to a changing cutting behaviour.
Excessive crater wear weakens the tool just behind the cutting edge
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
width of flank wear, referred to as the flank wear land width VB,
is considered a good indicator of the state of the cutting tool, as
demonstrated by

Poulachon et al. (2004)

. An increase in VB leads

Fig. 7. Regions of operability for TMP in HPC and conventional cooling.

to increased friction between the cutting tool and the workpiece
and hence a higher specific cutting force and more generated heat.

Poulachon et al. (2004)

have found out that a flank wear land width

of VB = 0.1 mm does not lead to workpiece damage.

In order to assess and compare the capability of cooling

conditions, experiments were carried out within the common
cross-section of overlapped regions of operability for a particular
cooling condition as shown in

Fig. 7

. The following cutting condi-

tions were employed in this experimental step:

• Depth of cut a

p

= 2 mm.

• Feed rate f = 0.25 mm/rev.
• Cutting speed

v

c

= 98.5 m/min.

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 KB

max

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

shows the tool flank face wear plot for both conven-

tional and HPC conditions. For the selected criteria VB = 0.1 mm,
tool life in the case of HPC was about 10 min, which is approxi-
mately five times longer than in the case of conventional cooling. It
should be pointed out that the consumption of coolant in the case
of HPC is more than four times lower as in the case of conventional
cooling.

Fig. 10. Tool wear development for conventional and HPC.

The effect of HPC on the tool rake face wear can also be observed

in

Fig. 11

where both tools for conventional and HPC conditions at

VB = 0.1 mm are compared. The contact length in HPC conditions is
approximately one third shorter than in the case of conventional
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

The presented research is based on experimental compari-

son of three hard turning operations, where the main difference
is in the utilized cooling. In the first case dry cutting is per-
formed. In the second and the third case conventional and HPC
conditions are employed. The experimental work has proved that
the hard turning of steels with coated carbide tools and cutting
speeds in the range of 90–160 m/min is not possible in dry cut-
ting conditions with a flat faced tool. The machining capabilities of
conventional and HPC methods are compared with respect to chip
breakability, tool wear, tool life and cutting forces. Both cooling
conditions are characterized with process regions of operability for
the employed tool–material pair. The major concluding remarks
regarding the HPC advantages over conventional cooling in the
hard turning of steel with flat faced coated carbide tools are as
follows:

• Extension of the region of operability for a given tool–material

pair. An approximately 45% increase in the maximum achievable
cutting speed and a 25% increase in the maximum achievable feed
rate were proven.

• A five times increase in tool life (VB = 0.1 mm), from 2 to 10 min

was achieved.

• Significant increase in chip breakability.
• All HPC advantages described above were achieved with reduced

coolant consumption by four times.

Acknowledgment

This research was conducted within the PROHIPP integrated

project, under the 6th Framework Programme for Research and
Technological Development, financed by the European Commis-
sion.

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