7/26/2019 1-s2.0-S092401360700948X-main
1/11
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 3 ( 2 0 0 8 ) 221231
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
An experimental investigation on contact length during
minimum quantity lubrication (MQL) machining
B. Tasdelen a,, H. Thordenberg b, D. Olofsson a
a Materials and Manufacturing Technology Department, Chalmers University of Technology,
412 96, Gothenburg, Swedenb AB Sandvik Coromant, 811 81, Sandviken, Sweden
a r t i c l e i n f o
Article history:
Received 19 January 2007
Received in revised form
5 September 2007
Accepted 10 October 2007
Keywords:
MQL
Air
Dry
Contact length
Seizure
Sliding
a b s t r a c t
This paper describes experimental investigations on influence of different media such as
minimum quantity lubrication (MQL), compressed air and emulsion on toolchip contact
length. The results are compared with dry cutting in terms of toolchip contact and chip
morphology. The toolchip contact area was examined with scanning electron microscopy
(SEM), optical microscopy and white light interferometer. The orthogonal turning test series
were planned in such a way that the engagement time was altered from long to very short
(intermittent turning). The results showed that MQL and compressed air lowers the con-
tact length compared to dry cutting at short and longer engagement times. The contact
length is almost the same for MQL and compressed air assisted cutting, but the difference
is in sliding region with the shorter engagement times. Emulsion assisted cutting gave the
shortest contact length. The chips were also examined with optical and scanning electron
microscopy. Wider chips were observed with dry cutting which is a result of side flow. Dif-
ferent oil amount was also investigated with TiN coated inserts. The effect of oil and air
component of MQL on the contact length is understood that helps to clarify their role in
the whole process. It is concluded that MQL is a very suitable method for short engagement
time machining.
2007 Elsevier B.V. All rights reserved.
1. Introduction
The high percentageof cutting fluidscost in theoverall manu-
facturing cost have favored theuse of dry andnear-dry cutting
(Klocke et al., 2006; Braga et al., 2002; Dahr et al., 2006; Weinertet al., 2004; Schact et al., 2005; Tasdelen and Johanson, 2006).
Moreover, it is not only the cost that justifies dry machining
but also the environmental aspects (Klocke et al., 2006; Braga
et al., 2002; Dahr et al., 2006; Weinert et al., 2004).Minimum
quantity lubrication (MQL) is a method that enables reduc-
ing the amount of cutting fluids. MQL consists of a mixture of
pressurized air and oil microdroplets applied directly into the
Corresponding author. Tel.: +46 73 657 3924; fax: +46 31 772 1313.E-mail address:[email protected](B. Tasdelen).
interface between the tool and the chip. However, the ques-
tion of how thelubricants candecrease the friction under very
high temperatures and loads is still not answered especially
for long engagement times.
Over 120 years ago, Mallock wrote Lubricants seem to actby lessening the friction between the face of the tool and the
shaving, and the difficulty is to see how the lubricant gets
there. The simplest machining process, turning with creation
of a continuous chip, is themostsevere as faras friction is con-
cerned. Compared to milling, with its short cutting distance
per cutting edge engagement, in turning the large contact
stresses common to all machining processes are combined
0924-0136/$ see front matter 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmatprotec.2007.10.027
mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.jmatprotec.2007.10.027http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.jmatprotec.2007.10.027mailto:[email protected]7/26/2019 1-s2.0-S092401360700948X-main
2/11
222 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 3 ( 2 0 0 8 ) 221231
Fig. 1 Chiptool interface (Lijing, 2004).
with distances of continuous sliding that are enough to wipe
any initial contaminating lubricant films from the chiptoolcontact (Childs, 2002).
The lubricants act in the slightly loaded region where the
chip leaves the tool. There, they decrease the friction and
thereby cause the contact length and the chip thickness to
reduce, but even at very low speeds they never penetrate the
highest loaded area near the cutting edge; the toolchip con-
tact is as seen inFig. 1 (Childs, 2002; Lijing, 2004; Trent and
Wright, 2000; Childs et al., 2000).
A model of lubricant penetration has been introduced by
Williams (Childs, 2002)and is developed further in (Childs et
al., 2000).It supposes that the existence of surface roughness
in the lightly loaded region where the chips leave the tool
results in the real area of contact there being less than thenominal area. Lubrication is possible when the penetration
length fraction (the ratio of contact length (t) to the length of
the channels (lp)) approaches 1 and is ineffective for values
less than 0.1, see Fig. 2. Thus, a lubricant does not have to pen-
etratethe whole contact: byattacking at the edge,it canreduce
the total contact length. Moreover, cooling of the chip results
in increased up-curling of the chip and consequently reduc-
tion of the contact length (Escursell, 2003).Therefore, contact
length which is the common factor affecting both tool life and
chip form (Sadik and Lindstrom,1995) is the mainscopeof this
work. The aim of this work was to understand the following
phenomena:
Does MQL lower the contact lengthat different engagement
times of insert? Is it compressed air that cools and curls up the chip that
results in lower contact length?
What is the effect of amounts of oil droplets on the contact
length?
How does the coating and oil amount affect the response of
MQL?
In order to see the effect of oil droplets and air on the con-
tactlength,the whole engagement timewas dividedinto small
engagement intervals. This enables to clarify the lubrication
and cooling effects of MQL on the contact length.
2. Experimental set-up
The tests were performed with uncoated and TiN coated
inserts. In order to understand the role of air and oil con-
stituents of MQL, the tests were performed dry, with MQL
(24 ml/h oil and 125 l/min air) and with only compressed air
(125 l/min). The air is supplied from an external compressor
and directly fed into air drier and then either to the MQL unit
or to the external nozzle, seeFig. 3.
In order to perform orthogonal turning tests, a special
insert geometry was used. The inserts have 0 rake angle and
4 clearance angle. A special nozzle holder was mounted on
the tool holder in order to direct the aerosol flow to the cut-ting zone. As the thickness of the work pieces was 2.5 mm, it
was possible to use the cutting edge for two different tests,
for example dry and MQL, seeFig. 4.The cutting data for both
coated and uncoated inserts is shown inTable 1.
The main goal of the tests was to clarify the role of com-
pressed air and oil droplets for different time of engagements
in intermittent turning. Therefore, the work piece mate-
Fig. 2 Lubricant penetration model (Childs et al., 2000).
7/26/2019 1-s2.0-S092401360700948X-main
3/11
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 3 ( 2 0 0 8 ) 221231 223
Fig. 3 Test set-up.
Table 1 Machining data
Work piece Vc (m/min) f(mm/rev) ac (mm) Cuttingtime (sec)
100Cr6 steel 200 0.1 2.5 5.4287
rial (100Cr6) was turned into 2.5 mm thickness cylinder and
machined with 1.5mm circular saw on the longitudinal axis
to have different number of grooves for different tests. By
increasing and decreasing the number of grooves, the engage-
ment time of the section (between two grooves) was altered.
The engagement time in the section was 0.06786s for two-
Table 2 Engagement times between two neighboringgrooves
# Grooves Engagement timein the section (s)
Total engagementtime (s)
Intermittent
2 0.0678 5.4287
6 0.0226 5.428712 0.0113 5.4287
24 0.0056 5.4287
grooved work piece and 0.02262 s for six-grooved work piece,
as shown in Fig. 5 and Table 2. Work piece with 12 grooves was
also testedfor uncoatedinsert and24-groovedwork piece was
tested for TiN coated insert.
The grooves were machined with circular saw to a maxi-
mum depth of 40 mm. However, work pieces with 12 and 24
grooves were not machined in the whole length. In this case,
the depth of the grooves was machined to a maximum depth
of 8 mm in order to keep high stability of each section.
3. Results and discussions
The tests series started with the comparison of MQL and dry
for different engagement times (two- and six-grooved work
pieces) for uncoated inserts. Then MQL and compressed air
assisted cutting were compared in terms of contact length
and how different contact zones look like. Afterwards, the
Fig. 4 (A) Tool holder and nozzle fixture, (B) uncoated inserts, (C) schematic cutting 2D and (D) MQL nozzle direction and
schematic cutting (3D).
7/26/2019 1-s2.0-S092401360700948X-main
4/11
224 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 3 ( 2 0 0 8 ) 221231
Fig. 5 The work pieces with (A) two grooves and (B) six grooves.
comparison between MQL and compressed air was contin-
ued with even shorter engagement time both with coatedand uncoated inserts. The chip morphology and the effect
of oil amount on contact length were also evaluated in the
work.
3.1. Contact length and chip analysis with uncoated
inserts
MQL decreased the contact length compared to dry cutting for
both short and long engagement time of the section at inter-
mittent turning. When the chip starts to slide on the sliding
region, the material starts hardening due to high deformation
ratio in the shear zone. Due to the high temperature and fric-tion it clads on the insert especially in this sliding region of
the natural contact. The difference in contact length is mainly
seen in the sliding region. The clad material in the sliding
region is thicker for dry cutting especially when cutting the
work piece with six grooves, see Figs. 6 and 7.The surface
topographyevaluation of the inserts by verticalscanning inter-
ferometry (VSI) shows the natural contact length (Lc) and thesliding length (Ls) for MQL and dry, as shown inFigs. 6 and 7.
Comparison tests were performed between MQL and com-
pressed air for short and long engagement times. It was seen
that MQL and compressed air give the same total contact
length when cutting two and six-grooved work pieces. The
earlier studies have shown that MQL and only compressed air
have the same cooling effect (Tasdelen and Johanson, 2006).
Thesame cooling effectresults in same chip up-curling radius
for MQL and compressed air that may have resulted in the
same contact length. However, the difference occurs in sliding
region. This difference is observed as clad material in sliding
region as shown inFig. 8.The difference is obvious when cut-
ting six-grooved work piece, as shown in Fig. 9. Otherwise bothcontact length and the thickness of the clad material are the
same for longer engagement time tests (work piece with two
grooves).
To summarize, MQL lowers the contact length compared
to dry cutting and the effect is mainly due to cooling effect
Fig. 6 Contact length comparison between MQL and dry cutting. (A) Optical microscopy image of insert that cut work piece
with two grooves. (B) Optical microscopy image of insert that cut work piece with six grooves. (C) SEM image of insert that
cut with MQL (work piece with two grooves). (D) SEM image of the insert that cut dry (work piece with two grooves). (E) SEM
image of the insert that cut with MQL (work piece with six grooves). (F) SEM image of the insert that cut dry (work piece with
six grooves).
7/26/2019 1-s2.0-S092401360700948X-main
5/11
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 3 ( 2 0 0 8 ) 221231 225
Fig. 7 Tool-contact surface topography for the inserts from the tests with six-grooved work piece. (A) MQL and (B) dry.
Fig. 8 Contact length comparison between MQL and compressed air assisted cutting. (A) Optical microscopy image of
insert that cut work piece with two grooves. (B) Optical microscopy image of insert that cut work piece with six grooves. (C)
SEM image of insert that cut with compressed air (work piece with two grooves). (D) SEM image of the insert that cut with
MQL (work piece with two grooves). (E) SEM image of the insert that cut with MQL (work piece with six grooves). (F) SEM
image of the insert that cut with compressed air (work piece with six grooves).
from air constituent in the aerosol flow. This was proven by
having the same contact length for MQL and compressed
air. However, when the engagement time of the section is
short, the lubrication of oil droplets at the grooves is obvi-
ous. The lubrication effect, which is inversely proportional to
the thickness of clad material in the sliding region, increases
with the increase in the number of grooves, see Fig. 9.The
summary of the measurements is given in Fig. 10.All these
tests were performed two times; however, the test results
that were performed on the same insert are presented on
the same plot in order to decrease the variation due to insert
quality.
Oneof the most important parameters that canchange the
contact length is the feed rate (Sadik and Lindstrom, 1995).
However, increasing the feed rate would cause higher forces
or decreasing the feed rate would increase the compressive
stress on the inserts. On the other hand, testing different cut-
ting speeds will result in different chip speed, contact length
and different amount of heat involved in the cutting zone.
Therefore, a comparison test series were performed to see
Fig. 9 Tool-contact surface topography of the inserts from the tests with six-grooved work piece. (A) MQL and (B)
compressed air.
7/26/2019 1-s2.0-S092401360700948X-main
6/11
226 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 3 ( 2 0 0 8 ) 221231
Fig. 10 Total contact length measurements.
the response of MQL and compressed air at different cutting
speeds. As shown in Fig. 11, the contact length difference
is almost the same for MQL and compressed air with lower
and higher speeds. Moreover, the difference between MQL
and compressed air in the sliding region is still the same in
terms of clad material, see Fig. 11.The insert that cut with
compressed air has thicker clad material that shows a higherfriction at both speeds compared to MQL cutting. However, it
is also observed that the thickness of the clad material in slid-
ing region for MQL starts to increase with increasing speed.
This can be due to the higher heat that may burn the oil
before it lubricates, or the time in the grooves is not enough
forthe lubrication with higherspeed (300 m/min)compared to
200m/min. A combination of these two explanations is also
possible. Fig. 12 is the summary of the contact length mea-
surement comparison.
The next topic for the investigation was what would be the
difference if the engagement time is decreased even further
(shorter engagement time in the section by shorter cutting
distance of the section) additionally by turning a work piece
Fig. 12 The contact length values for MQL and
compressed air at different speeds (six grooves).
with 12 grooves. As seen inFig. 13,not only the clad mate-
rial in sliding region but also the total contact length is lower
for MQL compared to compressed air. A summary of con-
tact length measurements shows that when the engagement
Fig. 11 Comparison between MQL and compressed air for different speeds (six grooves).
7/26/2019 1-s2.0-S092401360700948X-main
7/11
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 3 ( 2 0 0 8 ) 221231 227
Fig. 13 Comparison of MQL and compressed air (12 grooves). (A) Optical microscopy view. (B) SEM image of the same insert.
time becomes smaller in the sections of work piece, the insert
passes much timeat grooves(withoutengagement) in thetotal
cutting distance. Thus, the total time at the grooves (without
engagement) increases for the insert which makes the lubri-
cation become dominant compared to the cooling effect. The
length of the engagement decreases when a work piece withmore grooves is machined and this increases the effect of
lubrication on the overall cutting. The summary of result is
given inFig. 14.
Since the real difference in sliding region starts when the
tests were performed with six-grooved workpiece, a reference
test was made with emulsion. The shortestcontactlength was
obtained with emulsion assisted cutting, as shown inFig. 15.
This may also prove theeffect of cooling on thecontactlength.
Regarding the chips from the tests that were made on
work piece with six grooves, it was observed that the chips
Fig. 14 The contact length values for MQL and
compressed air at different engagement times.
Fig. 15 The contact length values for emulsion and MQL (six grooves).
7/26/2019 1-s2.0-S092401360700948X-main
8/11
228 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 3 ( 2 0 0 8 ) 221231
Fig. 16 Chip morphology difference (after cutting a work piece with six grooves).
are more up-curled for emulsion. The small radius of cur-
vature is also an indicator of the short contact length. The
chips fromMQL and compressed air assistedtests have almost
the same radius of curvature. The largest contact length that
was observed on the rake face was also observed as the
largest radius of curvature of the chips after dry cutting. The
other interesting observation is that chips produced in dry
cutting have side curl radius due to difference in speed onthe inner and outer diameter of work piece, as shown in
Fig. 16.The same phenomenon was not seen for other tests
meaning that the speed is almost the same throughout the
cutting edge during cutting. The chips produced in dry cut-
ting are wider than the chips produced in cutting with MQL,
compressed air and emulsion. The wider chips for dry cut-
ting is a result of side flow in the shear plane that was also
observed in the earlier works of the authors with conventional
inserts.
Side flow is the flow of material to the sides in the shear
plane. The higher the temperature in the shear plane the less
viscous is the material and the more side flow is seen. The
material that flows to the side makes the chips wider thandepth of cut and generally sticks on the hills of feed marks
and makes the surface finish worse.
The difference between MQL and compressed air that
resulted in different contact length was also observed in the
chip shape. Theoil dropletsaffect thecontact in thebeginning
of chip formation that results in smaller radius of curvature in
the head of the chips, as shown inFig. 17.
3.2. Contact length and chip analysis with TiN coated
inserts
The tests were performed with the same set-up that was used
for uncoated inserts. Since the edges were treated and thus
have more wear resistance, a work piece with 24 grooves was
also tested. The contact length of the inserts that were tested
on work pieces without grooves, with two, six grooves had
no apparent difference for MQL and compressed air cutting.
With 12- and 24-grooved work pieces the difference in sliding
region was more obvious, as shown inFigs. 18 and 19.Due to
the friction in this sliding region, the chip has a rubbing effect
on the insert that results in sticking of chip material on thisregion of insert. Therefore, the thickness of this clad material
in this sliding region may be connected to friction. The clad
material is thicker and due to fast cooling, cracks on that clad
material were observed at sliding region for compressed air
assisted cutting. For MQL assisted turning no cracking in the
clad material was observed since the thickness and length of
that region is shorter compared to inserts that cut with com-
pressed air assistance. The summary of how contact length
Fig. 17 Chip up-curl, smaller radius of curvature at the
beginning of chip formation (12 grooves) for: (A)
compressed air and (B) MQL.
7/26/2019 1-s2.0-S092401360700948X-main
9/11
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 3 ( 2 0 0 8 ) 221231 229
Fig. 18 Comparison of insert rake face that cut 24-grooved work piece showing a thicker clad material with cracks in air
assisted cutting. (A) SEM image of the insert that cut with MQL. (B) SEM image of the insert that cut with compressed air
assistance. (C) Cracks on the clad material in sliding region of the insert.
Fig. 19 Toolchip contact surface topography of the coated inserts (24-grooved work piece). (A) MQL and (B) compressed air.
Fig. 20 Contact length measurements for MQL and
compressed air cutting with TiN coated inserts.
changes with engagement time in the section is shown in
Fig. 20.
The other test series were made with two, six and 24
grooves with two different amounts of oil. The lower amount
is 24 ml/h and the higher amount was approximately 70 ml/h.
There was not an obvious difference for two- and six-
grooved work pieces. However, the difference was seen in
sliding zone when the engagement time of the section is
lowered to 0.0056s with 24 grooves. The clad material is
thicker and more apparent when the oil amount is less
proving a higher friction with less oil amount, as shown in
Figs. 2123.
7/26/2019 1-s2.0-S092401360700948X-main
10/11
230 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 3 ( 2 0 0 8 ) 221231
Fig. 21 Comparison of MQL with low and higher amount of oil (work piece with 24 grooves).
Fig. 22 Toolchip contact topography of the coated inserts (24-grooved work piece). (A) MQL with 24 ml/h and (B) MQL with
70 ml/h.
Fig. 23 Contact length measurements for lower (24 ml/h)
and higher (70 ml/h) amount of oil.
4. Conclusions
The effect of oil and air at minimum quantity lubrication was
evaluated in this work. The followings arethe summary of this
study:
MQL and compressed air lower the contact length compared
to dry machining and the difference is basically at sliding
region. However, shortest contact length was observed at
emulsion assisted cutting.
MQL and compressed air lower the total natural contact
length due to the cooling effect of air that results in chip
up-curling that decreases the contact length. The decrease
of contact length for compressed air and MQL is the same
for long engagement times. The oil droplets decrease the
friction at sliding region which is observed as thinner clad
material at sliding regionfor MQL. At thevery short engage-
ment times the decrease of friction in the sliding region
starts to affect the whole contact length and lubricationeffect overcomes the cooling effect.
Theamount of oilinfluences the contact lengthat very short
engagement times.
The chips for dry cutting are wider and have side curl due
to speed difference at outer and inner diameter of the work
piece. The wider chips are result of side flow that can make
the surface finish worse for dry cutting. Thus, MQL can be
a potential candidate for parting-off, grooving and drilling
where thenarrow chipsof MQL andcompressed air assisted
cutting may result in better surface finish.
TiN coating that results in a different friction and temper-
ature distribution in the cutting zone changes the effect of
MQL and compressed air on contact length and chip mor-
7/26/2019 1-s2.0-S092401360700948X-main
11/11
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 3 ( 2 0 0 8 ) 221231 231
phology. The effect of oil droplets was seen at even further
shorter engagement times compared to uncoated inserts.
Since MQL andcompressedair decrease thenatural contact
length withshorterengagement times, newinsertswith dif-
ferent geometry (grooves and chipbreakers) maybe usedfor
the optimization of the process.
A change from dry to MQL can result in benefits due to
shorter contact length but a change from emulsion to MQLshould be evaluated in terms of many other parameters
such as tool life, surface finish, forces, etc.
Analysis of thetoolchip contact area with white light verti-
cal scanninginterferometer is a potential methodfor future
works.
r e f e r e n c e s
Braga, D., Diniz, U., Miranda, A.E., Coppini, W.A., 2002. Using aminimum quantity lubricant (MQL) and a diamond coatedtool in the drilling of aluminiumsilicon alloys. J. Mater.
Process. Technol. 122, 127138.Childs, T.H.C., 2002. Friction modeling in metal cutting. Wear,
WEA-97762.Childs, T.H.C., Maekawa, K., Obikawa, T., Yamane, Y., 2000. Metal
Machining, Theory and Applications. ISBN 0 340 69159, pp.7374.
Dahr, N., Kamruzzaman, M., Ahmed, M., 2006. Effect of minimumquantity lubrication (MQL) on tool wear and surfaceroughness in turning AISI- 4340 steel. J. Mater. Process.Technol. 172, 299304.
Escursell, M., 2003. Applications of High-Pressure Jet-AssistedTurning, Licenciate, Thesis work, ISSN 1651-0984, p. 8.
Klocke, F., Gerschwiler, K., Fritsch, R., Lung, D., 2006. PVD-coatedtools and native ester an advanced system for
environmentally friendly machining. Surf. Coat. Technol. 201,43894394.
Lijing, X., 2004. Estimation of 2-dimension Tool Wear Based onFinite Element Method, Ph.D. Thesis, ISSN 0724-4967, p. 9.
Sadik, I.M., Lindstrom, B., 1995. The effect of restricted contactlength on tool performance. J. Mater. Process. Technol. 48,275282.
Schact, M., Spie, G., Stockhammer R., 2005. Metal-cuttingmanufacturing, 4th issue of the compendium. Willy Vogel AG,Berlin.
Tasdelen, B., Johanson, S., 2006. Minimum quantity lubrication(MQL)a step to understanding of the process. Annals CIRP,329335.
Trent, E.M., Wright, P.K., 2000. Metal Cutting, fourth edition, p.322.
Weinert, K., Inasaki, I., Sutherland, J., Wakabayashi, W.T., 2004.Dry machining and minimum quantity lubrication. AnnalsCIRP 53, 511537.
Recommended