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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]

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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).

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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).

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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).

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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.

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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).

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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).

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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.

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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.

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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-

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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.

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Tasdelen, B., Johanson, S., 2006. Minimum quantity lubrication(MQL)a step to understanding of the process. Annals CIRP,329335.

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