Experimental assessment of tribological condition in metal … · Experimental assessment of...

Experimental assessment of tribological condition in metal cutting

1


Esmeraldo Miguel Cangundo

Abstract: The present work searches to establish adequate experimental methodologies that allow assessing the

tool-chip interaction through independent techniques. The experimental work were supported by an extended

study on the principal operating parameters, such as sliding velocity, applied load and the surrounding medium.

Orthogonal cutting conditions were employed in the present work, differing from the generality of the industrial

applications, in order to assure that the experimental work is conducted under similar controlled conditions.

Some traditional tribological characterization techniques were selected and experimental tests were conducted

with different tribo-pairs. Numerical simulation techniques were also applied for calculating the calibration

curves for some tribological characterization techniques. It was also demonstrated that, it is possible to obtain a

good average for the friction coefficient value on the pin-on-disc test, when the principal operating parameters

are controlled. In fact, the surface texture has proved to be one of the most important parameter that controls the

friction coefficient. The ring compression test also shows a good sensibility on the surface texture, but the test is

not appropriate for the typical values of the cutting operations, as the test saturates for a value of friction

coefficient about 0.4. It was also presented the analysis of the influence of the mechanical properties of the tribo-

pairs on the overall friction mechanism.

It is shown that pin-on-disc tests, when performed with an adequate control of surface texture, are capable of

providing a good estimate of the average value of the friction coefficient in mechanical processing applications

and, in certain cases, when the tribo-pairs have similar mechanical properties, the value of the friction coefficient

is independent of surface morphology.

Keywords: Friction, Pin-on-disc test, ring compression test, orthogonal cutting.

1. Introduction

The term „mechanical processing of materials‟ usually refers to a broad category of manufacturing processes

where the energy required to produce the desired shape of the parts is primarily due to plasticity and friction. In

case of forming processes, the changes in the shape of raw material are induced by forces applied by various

tools and dies whereas in case of cutting processes the changes are produced by removal of unwanted material

with the aid of a cutting tool.

A recent work published by Bil et al. (2004) where they compared the estimates provided by three different finite

element computer programs with experimental data and concluded that although individual parameters (such as

the cutting force, the thrust force and the shear angle) may be made to match experimental results, none of the

numerical simulation models was able to achieve a satisfactory correlation with all the measured process

parameters all of the time. Astakhov and Outeiro (2008) presented a comprehensive review of published research

work on finite element modelling of metal cutting and concluded that friction at the tool–chip and tool–

workpiece interfaces is not modelled properly.

Friction quantification can be executed via a benchmark test or by independent laboratory tests. Although the

accuracy and reliability of the friction coefficient determined in such benchmark cutting and forming conditions

is generally fair, but these tests require the availability of machine tools and are greatly influenced by operating

variables and fail to provide quantitative local information about the friction coefficient (Zemzemi et al., 2008).

The second approach for determining the friction coefficient consists of using simulative tests under laboratory

controlled conditions. The aim is to employ laboratory tests that are cheaper and easier to perform and provides

accurate control of the operating variables and simple quantitative measurement of friction without losing

relevance to real mechanical processing applications. In recent years, several authors have been arguing that pin-

on-disc testing is not adequate for analysing friction in metal cutting because it is not capable of reproducing the

contact pressure, temperature and material flow conditions that are commonly found in real metal cutting

applications (Grzesik et al., 2002). Several alternatives to pin-on-disc have been proposed by Olsson et al.

(1989), Hedenquist and Olsson (1991) and Zemzemi et al. (2007), among others.

Thus it can be concluded that pin-on-disc needs revisiting in order to identify what operating parameters need to

be carefully controlled in order to successfully employ this easy, quick and cheap simulative test for the

independent determination of the friction coefficient. This work proposes a new testing methodology that aims to

provide a new level of understanding for the influence of surface texture and roughness on the friction

coefficient. The comparison between the results obtained with the independent tribology characterization

techniques and those acquired in cutting tests specifically designed for assessment purposes that allows to

conclude the validity of the pin-on-disc simulative test for evaluating friction in mechanical processing


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applications. Different tribo-pairs are employed in order to investigate the influence of the mechanical properties

on the resulting surface morphology and friction coefficient.

2. Theoretical Background

Every surface obtained by a technological process has its unique signature of the process and contamination.

When two surfaces are close enough, the initial contact occurs between the asperities, and with the increase of

the contact pressure, these asperities are deformed plastically. The phenomenon known as friction is due to a

number of mechanisms that probably act together but that may appear in different proportions under different

circumstances. Lenard (2002), suggested that the friction mechanism is mainly caused by: i) elastic and plastic

deformation of the asperities; ii) elastic and plastic deformation by ploughing; iii) adhesion and iv) the

deformation and/or fracture of the superficial oxides layers in contact (Figure 1).

Figure 1- Basic mechanisms of sliding friction: a) Adhesion; b) Ploughing; c) Asperities deformation (Holmberg et al., 2009).

Since the first studies in metal cutting, authors such as Ernst and Merchant (1941), Green (1952) or Finnie

(1956) suggested that the friction on the rake face is a result of many contributions, primarily from the contact

mechanics between the surfaces asperities (Figure 2). Since then, the friction modelling on metal cutting is based

essentially on the theoretical models of plastic deformation of the asperities.

ChipTool

Ar

Ar

Aa

pr

pr

s

s

n

Vchip

Workpiece

Figure 2- Schematic representation of tool/chip contact interface in orthogonal metal cutting.

The real contact area (Ar) between the chip/tool interface is always below the apparent contact area (Aa) and the

resulting friction force can be determine through the classical approximation of Coulomb (1875):

𝐹𝑓 = 𝜇𝐹𝑁 (1)

This approximation introduces a direct proportionality between the friction force Ff, on the opposite direction of

the movement and the force FN is applied on the body in cause. The constant of proportionality µ, also called

friction coefficient, can be obtained in terms of normal stress, n, and shear stress, , on the surface through the

Equation 2.

𝜏 = 𝜇.𝜎𝑛 (2)

When the stress on the interface is high the surface asperities will deform plastically. The plastic interaction

between the nearby asperities cannot be considered separately, the stress of the interface conducts to an increase

of the real contact area, so the real contact area becomes the apparent contact area. This situation leads to an

alternative model, also known as Prandtl friction law, where the shear stress is independent of the normal load on

the contact surface and proportional to k, the maximum value of the shear stress of the workpiece material.

(a) (b) (c)


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3. Experimental Background

This section starts by a brief description of the independent tribological calibration techniques and follows with

the presentation of a laboratory benchmark cutting apparatus that was specifically designed and fabricated for

assessing the friction coefficients determined by means of pin-on-disc and ring compression testing. The last part

of this section is focused on the experimental workplan.

3.1. Innovative pin-on-disc tests

Figure 3a shows a schematic representation of the innovative pin-on-disc tribometer that was designed and

fabricated by the author. The main components of the equipment can be divided into four different groups: (i)

basic structural parts, (ii) specific electrical and mechanical parts, (iii) control and measurement appliances and

(iv) grinding and polishing unit.

Personal computer

equipped with DAQ card

Signal conditioning and

amplifying unit

2D load cell

Pin

Disc

Precision ball screw

Frame with linear guides

Sand paper /

Polishing clothe

Disc

Grinding and

polishing unit

Pin

(a)

(b)

Figure 3– Pin-on-disc simulative tests. (a) Schematic representation of the innovative pin-on-disc tester and detail showing

the grinding/polishing unit, (b) Picture showing stainless steel discs prepared with different values of average surface

roughness (Ra).

The control and measurement appliances include a two-dimensional load cell, a multifunction data acquisition

system and a personal computer. The load cell manufactured specially for this work is fixed to the pin holder and

connected to a signal amplifier unit (Vishay 2100). A personal computer data logging system based on a data

acquisition (DAQ) card (National Instruments, PCI-6025E) combined with a special purpose LabView software

controls testing and acquires/stores the experimental values of the normal and tangential loads.

The grinding and polishing unit is a custom built piece of equipment that serves the purpose of producing and

regenerating the desired texture and roughness in the surface of the discs after the completion of each test. As

seen in the detail of Figure 3a, the unit combines the rotational velocity of the sand paper or polishing clothes

with the rotation of the disc in order to ensure average values of the surface roughness (Ra) in the range 0.007 to

0.8 µm. These values are measured with a contact roughness tester (from 0.2 to 0.8 µm) and with an atomic

force microscope (below 0.2 µm) along the direction perpendicular to the rotation of the discs. Resort to the

grinding and polishing unit at the end of each test followed by roughness measurements guarantees repeatability

and an analogy of the surface topography of the discs irrespective of the friction test case.

3.2. Benchmark ring compression test

The ring compression test was executed in the Instron 1200 KN universal testing machine, where the ring

specimen was compressed in several stages. In the end of each stage the values of its height, inner and outer

diameter were measured accordingly, and applied latter to a calibration abacus. These tests were developed in

room temperature at „quasi-static‟ conditions.

The surface finish of the compression plates was controlled through grinding and polishing methods, obtaining a

radial texture as presented on the Figure 4 and the average values of surface roughness (Ra) ranges from 0.04 to

Ra = 0.007 µm Ra = 0.2 µm Ra = 0.3 µm Ra = 0.4 µm Ra = 0.5µm Ra = 0.8 µm


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0.65 μm. These values were measured by a contact roughness tester along the direction of the material flow

during the compression of the specimens.

Specimen

h0

di

de

Plate (a) (b)

Figure 4– Ring compression tests. (a) Schematic representation of the surface texture of the plate, (b) Picture showing

stainless steel and plates prepared with different values of average surface roughness (Ra).

3.3. Benchmark mechanical processing testing apparatus

In the present investigation the estimates of the friction coefficient provided by different friction calibration

techniques were compared against measurements performed in benchmark cutting experiments. The assessment

is performed under orthogonal cutting condition, that is, the cutting edge of the tool was straight and

perpendicular to the direction of motion. The benchmark experimental set-up was installed in a CNC milling

machine (Figure 5a) and is essentially composed by a cutting tool, a cutting specimen, a three-dimensional

piezoelectric dynamometer and a self grinding and polishing unit (refer to detail in Figure 5a).

The cutting tool has a rake face angle α = 0º and a clearance angle = 5º and was manufactured from the same

materials as the rotating discs of the pin-on-disc machine. The cutting tools clearance surface are maintained

constant with a Ra = 0.05 um. The cutting specimen is made of the same material as the pin of the pin-on-disc

machine and is fixed directly on the three-dimensional piezoelectric dynamometer. The three-dimensional

piezoelectric dynamometer (Kistler 9257B) is attached to a signal amplifier (Kistler 5011B) and allows

measuring the cutting forces during testing. The system is linear across its entire range, measures forces with an

accuracy of 1% and its resolution allows measuring almost any dynamic changes in the forces of great

amplitude. A personal computer data logging system based on a multifunction data acquisition card (National

Instruments, PCI-6025E) combined with a special purpose LabView software controls testing, acquires and

stores the experimental data.

The self grinding and polishing unit removes pick-up material and produces a texture in the cutting tool that

closely matches the rotating disc under comparative evaluation. Figure 5a shows a schematic detail of the self

grinding and polishing unit together with pictures of the cutting tools made from AISI 316L stainless steel that

were self-prepared with different types of textures and surface roughness.

Personal computer

equipped with DAQ card

3-axis CNC milling machine

Piezoelectric

dynamometer

Cutting tool

Specimen

Signal Amplifiers

FzFx

Sand paper /

Polishing clothe

Cutting Tool

(a)

(b)

Figure 5 – Benchmark orthogonal metal cutting. (a) Schematic representation of the apparatus and detail showing the self

grinding/polishing unit, (b) Picture showing the cutting tools that were prepared with different values of surface roughness.

Ra = 0.65 µm Ra = 0.35 µm Ra = 0.20 µm Ra = 0.12 µm Ra = 0.04 µm Ra = 0.06 µm

Ra = 0.75 µm Ra = 0.36 µm Ra = 0.32 µm

Ra = 0.015 µm

Ra = 0.12 µm

Ra = 0.052 µm

Cutting edge

Rake face


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3.3. Experimental workplan

Different tribo-pairs for the experimental tests of the present work were chosen with care, in order to evaluate

quantitatively the influence of the differences in mechanical strength of the tribo-pairs on the interaction of

surface roughnesses and resulting coefficient of friction. The rotating discs were manufactured from AISI 316L

stainless steel and polyvinylchloride (PVC).

The pin is a cylinder with 8 mm diameter made of technically-pure lead (99.9%). The mechanical behaviour of

this material is very close to the ideal rigid-plastic material model which is assumed by the generality of the

analytical models of mechanical processing because of the almost absence of strain hardening. In addition, the

choice of technically-pure lead makes possible to simulate at room temperature and at low strain-rates the

material flow behaviour of conventional steels at the usual forming and machining temperatures (Altan et al.,

1970). The pins were also manufactured with cooper, a material that presents mechanical properties close to the

AISI 316L stainless steel.

Table 1 presents the experimental workplan for the pin-on-disc simulative tests. The experiments were designed

in order to isolate the influence of the process parameters that are considered more relevant for the frictional

behaviour; (i) the applied forces, (ii) the velocity, (iii) the texture and surface roughness of the tools and (iv) the

difference in the relative mechanical strengths of the tribo-pairs. Different types of surrounding media were also

applied: atmospheric air, oxygen and nitrogen, and liquid lubricant (Mobilgear 627). The discs were degreased at

the beginning of each test in order to ensure clean surface conditions in all the cases. The forces were measured

by the two-dimensional load cell with a frequency rate of 300 Hz.

Case Ra [µm] Specimen Disc Velocity [m/s] Lubrication Atmosphere

1 <=0.1

Lead-AISI 316L

and

Lead-PVC

AISI 316L and

PVC

0.072, 0.216, 0.432

and 0.72 Yes/No

Atmospheric

air, Oxygen

and Nitrogen

2 0.2

3 0.3

4 0.4

5 0.5

6 0.8

Table 1– Experimental workplan for pin-on-disc simulative tests.

For the ring compression test, early theoretical studies related with the specimens shape and dimensions used to

determined the friction factor (Male, 1964) concluded that the specimens having the geometrical ratio de:di:h

(Figure 4a) of 6: 3: 2, are best suited for the purpose of evaluating the friction factor. In this experimental work,

the specimens had the following dimensions 24: 12: 8 mm, manufactured with cooper and lead.

Table 2 presents the experimental workplan for the ring compression tests. Lubricated and dry conditions were

also tested, reproducing the conditions of the pin-on-disc simulative tests.

In order to avoid deposition of the specimen material on the plate, compromising the overall results, the plates

are cleaned with ethanol between stages of compression stages. After each test, the plate surface is reconditioned

through grinding and polishing procedures and the surface roughness re-measured.

Case Plates Ra [µm] specimen plate Lubrication

1-2 0.04

Lead-AISI 316L

and Cooper-AISI 316L AISI 316L Yes/No

3-4 0.06

5-6 0.12

7-8 0.20

9-10 0.35

11-12 0.65

Table 2 – Experimental workplan for ring compression tests.

The workplan for the benchmark metal cutting tests was designed to match the pin-on-disc experiments with the

lead-stainless steel tribo-pair in order to ensure that the comparison between the two types of tests was

performed in the same conditions. The elimination of lubrication (dry cutting conditions), temperature and

strain-hardening from the experimental workplan is crucial for reducing the number of parameters that influence

friction. The same applies to the undeformed chip thickness that was kept constant and equal to 0.2 mm for all

the testing conditions, and all the tests were executed with quasi-static cutting velocity. Otherwise, the number of

possible combinations of variables would become quite large. The experiments were done in a random order and

at least two replicates were produced for each test configuration in order to provide statistical meaning.


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4. Results and discussion

The following section is structured in three parts. The first part is focused on the frictional response of the

independent tribological characterization techniques; the second part is centred in assessing the friction

coefficient determined by the pin-on-disc and the ring compression test with data collected from benchmark

metal cutting experiments, with special emphasis placed on the influence of surface texture, roughness and the

mechanical properties of the tribo-pairs on the friction coefficient; the last part of this section is focused

essentially on the impact of the surrounding media on the overall friction behaviour.

4.1 Friction in the pin-on-disc simulative tests

The pin-on-disc simulative tests took full advantage of the innovative features that were incorporated in the

design of the equipment (section 3.1) which allow tests to be performed under variable loading conditions.

Figure 6a shows a typical plot of the normal and tangential (friction) forces as a function of the test time

retrieved from case 3 of Table 1 using a PVC disc.

(a) (b)

Figure 6– Results from pin-on-disc tests for case 3 of Table 1 using a rotating disc of polyvinylchloride (PVC). (a) Normal

and tangential forces as a function of time for a test velocity v=0.072 m/s, (b) Typical friction coefficient graphic showing the

tangential force as a function of the normal force for different test velocities.

By extending the analysis to the entire set of velocities included in the experimental workplan it is possible

obtain the result depicted in Figure 6b. This figure allows two different types of conclusions to be drawn. Firstly,

it shows the friction force to be directly proportional to the normal force applied on the pin and the friction

coefficient not being affected by the contact pressure. Secondly, it reveals friction not to be significantly

influenced by the sliding velocity. In fact, the deviation of the friction coefficient observed in the whole range of

velocity testing conditions is very small.

The first conclusion is in close agreement with the fundamental laws of friction and had been previously drawn

by Bowers et al. (1953) in case of polyethylene, polytetrafluorethylene and halogenated derivatives. The second

conclusion may be surprising when analyzed in the light of previous works in tribology but falls right in-line

with the experimental results published by Shooter and Thomas (1952) in case of steel-polymer tribo-pairs and

more recently by Sedlacek et al. (2009) in case of Al2O3 pins with 100Cr6 steel discs. It may, however, be

argued that the range of velocities employed in the pin-on-disc tests is not sufficient enough to support the

aforementioned conclusion but, for the testing conditions under investigation, it seems that the surface texture,

the roughness and the relative mechanical strengths of the pins and discs play the key role in the frictional

behaviour of the tribo-pairs.

4.2 Friction and surface roughness

The calibration curves were determined by a reverse identification process based in the FEA (Figure 7). The

numerical simulation was been performed with the software IFORM2, developed at the Unit of Manufacturing

and Industrial Management in Instituto Superior Técnico (IST). For the case of stainless steel/cooper, the lowest

friction value is slightly lower than the stainless steel/lead case. We can also observe that the friction coefficient,

for the cooper case, rapidly reaches the maximum value (𝜇 = 1), almost independent on the surface roughness of

the plates. In the lubricated condition, it shows that the inner diameter reduces with the roughness of the

compression plates, and the specimen geometry does not saturate with the surface roughness, like under dry

condition. For higher Ra values, the lubrication shows improved results, as the difference between the dry and

0

50

100

150

200

250

300

0 0.2 0.4 0.6

Fo

rce

[N]

Time [s]

Normal Force

Tangencial Force

0

10

20

30

40

50

60

70

80

0 100 200 300 400

Tan

gen

cial

Fo

rcel

[N

]

Normal Force [N]

0.072m/s

0.43m/s

0.72m/s


7

lubricated curve, as for the smoother plates, the differences are not so evident, the friction coefficient is slightly

lower for the lubricated case.

Figure 7 – Ring compression tests results and calibration curves for the lead specimens.

Figure 8 shows the variation of the friction coefficient with the surface roughness obtained from the pin-on-disc

tests that were performed with the two different tribo-pairs that are described in section 2.3 (Table 1). In case of

the lead-stainless steel tribo-tests, the analysis of Figure 8 allows the identification of three different regions; (i)

a leftmost region (Ra<0.1) where the friction coefficient is constant and takes the smallest value among all the

test cases, (ii) a rightmost region (Ra>0.5) where the friction coefficient is constant and reaches the largest value

among all the test cases and (iii) a region in between where the friction coefficient progressively grows from the

smallest to the largest measured values.

In the leftmost region of the graphics the surface roughness of the stainless steel discs is very small and, for that

reason, the sliding between the pin and the discs is smoother. The basic source of friction is adhesion (µµadh)

and the friction force resulting from the relative movement between the pin and the disc should be roughly equal

to the force that is needed for shearing the junctions formed by localized pressure welding (cold welding) at the

asperities.

Figure 8 – Friction coefficient as a function of surface roughness for pin-on-discs tests dry condition.

On the contrary, in the rightmost region of the Figure 7 and 8 (where the surface roughness of the discs is very

large) there is a more pronounced interaction between the asperities. The tips of the asperities on the discs

(harder material) may penetrate and plough into the surfaces of the pins (softer material) and produce loose

debris from microcutting in the asperity level (Figure 9). This results in stronger resistance to sliding (Bowden

-60%

-40%

-20%

0%

20%

40%

60%

80%

0% 10% 20% 30% 40% 50% 60%

(Do

-Di)

/Do

(h0-hi)/h0

Ra = 0.04 um dry Ra = 0.06 um dry



Ra = 0.04 um lubricated Ra = 0.12 um lubricated

Ra = 0.35 um lubricated Ra = 0.65 um lubricated

u = 0.00 u = 0.05

u = 0.10 u = 0.18

u = 0.20 u = 0.25

u = 0.30 u = 0.40

u = 0.50 u = 1.00

0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Fri

ctio

n C

oef

fici

ent

Ra [um]

Pin-on-disc Pb/AISI 316L

Pin-on-disc Pb/PVC

Pin-on-disc Cu/AISI 316L

Fitting Pin-on-disc Pb/AISI 316L

Fitting Pin-on-disc Pb/PVC

Fitting Pin -on-disc Cu/AISI 316L

lubricated

dry


8

and Tabor, 1964) and explains the observed increase in the friction coefficient under high levels of surface

roughness.

Ra < 0.1 µm 0.1 µm < Ra < 0.5 µm Ra > 0.5 µm Independent from Ra Figure 9 – Illustration showing the interaction between the pin and the disc for different values of surface roughness.

Figure 8 also discloses significant differences between the variation of the friction coefficient as a function of

surface roughness for lead-stainless steel and lead-PVC tribo-pairs. In fact, the pin-on-disc tests performed with

the PVC discs provide a friction coefficient 𝜇 ≅ 0.24 that is approximately constant and independent from the

surface roughness. This may be explained by the fact that PVC discs have much closer mechanical properties to

lead pins than stainless steel discs. In other words, contact surfaces with similar mechanical strength and

hardness are able to considerably reduce (or, eliminate) the interaction between asperities because the amount of

penetration and ploughing of the asperities on the discs into the surfaces of the pins, or vice-versa, is generally

smaller. As a result of this, resistance to sliding under dry frictional conditions will be independent of the

original surface roughness of the pins and discs of the tribo-pair.

4.3 Friction and the surrounding medium

The interaction between surfaces is a point of great interest in any tribological studies and it can take place in

different media. In the pin-on-disc simulative tests the pin interface material is constantly removed from the disc

during the test, by introducing a proper controlled atmosphere, the impact of the surrounding medium can be

evaluated. It can be verified in Figure 10, the presence of a nitrogen atmosphere, the results are similar to the

atmospheric air, which is basically composed by nitrogen. The introduction of an atmosphere that promotes the

formation of oxides, only composed by oxygen, the value of the friction coefficient raises almost 10% from the

original value.

Figure 10 – Influence of the atmosphere of the surroundings medium on the friction coefficient.

The application of a liquid lubricant (Figure 11) results in a significant reduction on the overall friction

coefficient value for both materials: lead and cooper. Due to the absorbent nature of the PVC, the tests utilizing

the PVC discs were executed only in dry conditions. It was also observed that depending on the tribo-pair, the

lubrication has a different influence on other parameters: for cooper-AISI 316L, the friction coefficient remains

constant, independent of the sliding velocity; a different result was achieved by using the lead-AISI 316L, it

turns out to be very sensitive for low sliding velocities (below 0.3 m/s), and becomes independent and constant,

for higher sliding velocities.

Comparing the application of the same lubricant on different calibration techniques (Figure 11b), we can observe

that, the same influence of the surface texture on the friction coefficient evolution. But for the ring compression

tests tends to underestimate the friction coefficient value, for both tribo-pairs, differing from the pin-on-disc test

results.

0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Fri

ctio

n C

oef

fici

ent

Ra [um]

Pin-on-disc Pb/AISI 316L ar

Pin-on-disc Pb/AISI 316L O2

Pin-on-disc Pb/AISI 316L N2

Fitting Pin-on-disc Pb/AISI ar

Fitting Pin-on-disc Pb/AISI 316L O2

Fitting Pin-on-disc Pb/AISI 316L N2


9

(a) (b)

Figure 11 –friction coefficient under lubrication condition, (a) Influence of sliding velocity, (b) Influence of roughness

surface.

4.4 Assessment of friction evaluation techniques

In case of the cutting tools are required not only to ensure similar values of surface roughness but also to make

certain that predominant patterns of surface texture were aligned in the same working direction as that employed

in the pin-on-disc tests. This is very important because the friction coefficient depends on the texture of the

harder counter surface as recently shown by Menezes et al. (2009). The comparison between pin-on-disc and

metal cutting measurements of the friction coefficient for lead-stainless steel tests shows a general good

agreement (Figure 12). Major differences are found at the extreme regions of the graphics. For the cases with a

low Ra value (Ra < 0.25) the friction coefficient obtained in the cutting test is superior, where the size effect has a

greater influence. Due to the technical difficulties to isolate the effects of the measured forces that act on the

cutting edge and the clearance surface, the values registered on the piezoelectric dynamometer are greatly

influenced by these components. On contrary, at the rightmost zone of the Figure 12, the results obtained by the

pin-on-disc simulative tests present a higher value than the cutting test, can be explained by the formation of

oxides on the pin/disc interface is higher than in cutting, where the newly formed surfaces are not oxided.

Figure 12 – Friction coefficient as a function of surface roughness for different friction calibration techniques in dry

condition.

In the case of the ring compressive tests, the sudden increase of the friction coefficient to the maximum value,

can be explain by the geometric sensibility of the specimen on the friction coefficient, with the increase of the

friction, it can be observed in the Figure 7, that the calibration curves from µ = 0.5 and 1 are very close to each

other. The common ring shape specimen geometry is not suited for calibrating high friction value.

5. Conclusions

The present work concluded that pin-on-disc is feasible to evaluate frictional conditions in metal cutting

applications when combined with experimental procedures to guarantee an adequate control of the texture and

the surface roughness and to allow acquisition of data to be performed in a single turn in order to avoid any

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8

Fri

ctio

n C

oef

fici

ent

Sliding velocity [m/s]

Pb/AISI 316L

Cu/AISI 316L

Fitting Pb/AISI 316L

Fitting Cu/AISI 316L

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

Fri

ctio

n C

oef

fici

ent

Ra [um]


Ring Pb/AISI 316L


Ring Cu/AISI 316L



Fitting Pin-on-disc Cu/AISI 316L

Fitting Ringl Cu/AISI 316L

0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Fri

ctio

n C

oef

fici

ent

Ra [um]


Ring Pb/AISI 316L

Orthogonal Cutting Pb/AISI 316L


Ring Cu/AISI 316L


Fitting Ring Pb/AISI 316L

Fitting Orthogonal Cutting Pb/AISI 316L

Fitting Pin -on-disc Cu/AISI 316L

Fitting Ring Cu/AISI 316L


10

rubbing of the pin on the same track of the disc. Results also allow concluding that significant differences in the

mechanical strength of the materials of pins and discs lead to important variations of the friction coefficient with

roughness whereas pins and discs with similar mechanical strength and hardness will generally lead to friction

coefficients that are independent from surface roughness, under dry lubrication conditions. The frictional

behaviour is also affected by the surrounding media, where the interdependence of the some frictional

parameters is revealed with the application of different types of media.

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