Effects of Minimum Quantity Lubrication (MQL) on

Effects of Minimum Quantity Lubrication (MQL) on Chip Formation Mode, Tool Wear, and Surface Roughness in Turning AISI-1040 Steel with Uncoated Carbide Tool Mithu, M. A. H . \ Khan, Μ. M. A.1 and Dhar, N. R.2 Department of Industrial and Production Engineering, Shah Jalal University of Science and Technology, Sylhet-3114, Bangladesh. 2 Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh. Email: [email protected] 1

ABSTRACT

life.

The

magnitude

of

this

cutting

temperature

increases, though in different degree, with the increase This research work was carried out with the view of investigating

quantity

high production machining is constrained by rise in

lubrication (MQL) on the cutting performance of AISI

temperature. This problem increases further with the

1040 steel, in terms of chip formation mode, tool wear,

increase in strength and hardness of the work material.

and

the

machined

effects

surface

of

minimum

of cutting velocity, feed and depth of cut. As a result,

to

It was observed III that, in machining ductile metals

completely dry and wet machining. In this study, the

roughness

as

compared

producing long chips, the chip-tool contact length has a

minimum quantity lubrication was provided with a

direct influence on the cutting temperature and thermo-

spray of air and cutting oil. However, compared to the

chemical wear of cutting tools. The cutting temperature

dry and wet machining, M Q L machining performed

becomes maximum on the rake face of the tool at a

much better, mainly due to substantial reduction in

certain distance from the cutting edge where cratering

cutting

zone

temperature

enabling

favorable

chip

formation and chip-tool interaction. It also provides substantial reduction in tool wear, which enhanced the tool

life,

and

surface

roughness.

Furthermore,

occurs. Such high rake face temperature can also raise the temperature at the flank of the tool. In addition to usual flank wear and crater wear the

it

cutting tools often attain notching on the flanks and

provides environment friendliness and improves the

grooving on the rake surface at the outer ends of the

machinability characteristics.

engaged portions of the cutting edges. On the major

Keywords:

end of the depth of cut and is characterized by deeper

cutting edge, the grooving wear occurs at the extreme MQL;

Turning;

Tool

wear;

Surface

abrasion of the tool edge. On the end cutting edge, the

roughness; Steel; Carbide tools.

grooving wear is characterized

by smaller

multiple

notches. Several mechanisms have been proposed 111 to explain grooving wear, such as: i) development of a

1. INTRODUCTION

work-hardened/abrasive oxide layer on the cut surface, Machining is inherently characterized by generation

ii) formation of thermal cracks due to steep temperature

of heat and high cutting temperature. At such elevated

gradient, iii) presence of side-spread material at the

temperatures the cutting tools, if not sufficiently hot and

edges of a newly cut surface, and iv) fatigue of tool

hard, may lose their form stability quickly or wear out

material due to cutting force fluctuations at the free

rapidly

surface caused by lateral motions of the edges of the

resulting

in

increased

cutting

forces,

dimensional inaccuracy of the product and shorter tool

chip.

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107

Effect of MQL on Chip Formation in Turning AISI-1040 Steel

Vol. 9, Nos. 1-2, 2008

Trent 131 also reported that in m a c h i n i n g ductile metals, the chip contact length plays a significant role on

the chip

and

tool

temperature

which

becomes

temperature by more effective and efficient cooling has become extremely essential. Generally,

suitable cutting fluid is employed

to

m a x i m u m almost at the centre o f the chip-tool contact

reduce this problem through cooling and lubrication at

surface

the cutting zone. But it has been experienced /13/ that

where

crater

wear

begins

and

grooves

intensively.

lubrication

is

effective at

low

speeds

when

it

is

Kosa et al. IM suggested that in m a c h i n i n g ductile

accomplished by diffusion through the workpiece and

metals, heat and temperature developed due to plastic

by forming solid boundary layers from the extreme

deformation and rubbing of the chips with tool m a y

pressure additives, but at high speeds no sufficient

cause

which

lubrication effect is evident. The ineffectiveness of

affects machining operation. Austenitic stainless steels

lubrication o f the cutting fluid at high speed machining

continuous

built-up

of welded

debris

are generally considered difficult-to-machine because of

is attributed 111 to the inability of the cutting fluid to

high

ductility.

reach the actual cutting zone and particularly at the

Therefore, tools will be subjected to high frictional heat,

chip-tool interface due to bulk or plastic contact at high

and chips will have a tendency to stick and cause severe

cutting speed.

work-hardening

rate,

toughness

and

built-up edge formation.

The cooling and lubricating effects by cutting fluid

T h e heat generated during m a c h i n i n g 15/ also raises

/14, 15/ influence each other and diminish with increase

the temperature o f the cutting tool tips and the work-

in cutting velocity.

surface

enter

near

the

cutting

zone.

Due

to

such

temperature and pressure the cutting edge plastically

and

wears

rapidly,

which

high

deforms leads

to

dimensional inaccuracy, increase in cutting forces and

the

Since the cutting fluid does not

chip-tool

interface

during

high

speed

machining, the cutting fluid action is limited to bulk heat removal only. T h e machining temperature could be reduced

to

premature tool failure. On the other hand, the cutting

some

temperature, if it is high and is not controlled, worsens

characteristics

the surface topography and impairs the surface integrity

optimizing the tool geometry and by proper selection of

by oxidation and introducing residual stresses, micro-

the process parameters /16-18/. S o m e recent techniques

cracks and structural changes.

have

Past research has been focused on the temperature and its distribution in the cutting zone because it is believed that it has a direct impact on tool life 161. The

extent

enabled

by

improving

of the work

partial

the

material

control

of

machinability metallurgically,

the

machining

temperature by using heat resistance tools like coated carbides, C B N etc. But C B N tools are very expensive. Moreover,

like

other

manufacturing

activities,

primary function of cutting fluids is to reduce this

machining also leads to environmental pollution /19, 20/

cutting temperature

life 111. The

mainly because of the use of cutting fluids. These fluids

cutting fluids are believed to reduce cutting temperature

often contain sulfur (S), phosphorus (P), chlorine (CI) or

either by removing heat as a coolant or reducing the

other

heat generation as a lubricant. In addition, the cutting

lubricating performance. These chemicals present health

and

increase tool

extreme-pressure

additives

to

improve

the

fluid has a practical function as a chip-handling medium

hazards. Furthermore, the cost of treating the waste

/8/. Cutting fluids also help in machining o f ductile

liquid is high and the treatment itself is a source of air

materials by reducing or preventing formation of a built-

pollution.

up edge (BUE), which degrades the surface finish 191. However, the high cutting temperature is controlled

It has been estimated /21/ that about one million workers are exposed to cutting fluids in the United

by profuse cooling /10-12/. But such p r o f u s e cooling

States

with conventional cutting fluids is not able to solve

composition,

these problems fully even when e m p l o y e d in the form

constituents and may be irritant or allergenic. Also, both

of jet or mist.

the

may

be

fluids are more

toxic

complex than

in

their

bacteria and fungi can effectively colonize the cutting fluids and serve as source of microbial toxins. Hence

for precision

significant negative effects, in terms of environmental.

the

of some

they

cutting

modern

machining,

advent

Since

machining process and harder materials and for demand

108

With

alone.

control

of

machining

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M.A.H. Mithu, M.M.A. Khan andN.R. Dhar

International Journal for Manufacturing Science & Production

health, and safety consequences, are associated with the

referred to as "near dry lubrication" /24/ or "micro

use of cutting fluids. The effects of exposure to the

lubrication" /25/, has been suggested since a decade ago

fluids on health have been studied for over 50 years;

as a means of addressing the issues of environmental

beginning with the concern that cutting fluid (oil) is a

intrusiveness and occupational hazards associated with

potential etiologic factor for occupational skin cancer

the airborne cutting fluid particles on factory shop

(Epidemiological

long-term

floors. The minimization of cutting fluid also leads to

exposure to metalworking fluids can lead to increased

economical benefits by way of saving lubricant costs

incidence of several types of cancer). The International

and workpiece/tool/ machine cleaning cycle time.

studies

indicate

that

Significant progress has been made in dry and semi-

Agency for Research on Cancer has concluded that there is "sufficient evidence" that mineral oils used in

dry

machining

recently,

and

minimum

quantity

the workplace are carcinogenic 1221. Basically, workers

lubrication (MQL) machining in particular has been

are exposed to metal cutting fluids via three routes 1231;

accepted as a successful semi-dry application because of its environmentally friendly characteristics. Some good

skin exposure, aerial exposure and ingestion. Skin exposure is the dominant route of exposure, and

it

is believed

that

about

80

percent

of

all

occupational diseases are caused by skin contact with fluids 1221. Cutting fluids are

Some

researchers /31/

used

this

technique

in

the

reaming process of gray cast iron (GG25) and aluminum

of

alloy (A1SI12) with coated carbide tools and concluded

involve

that it caused a reduction of tool wear when compared

important causes

occupational contact dermatitis, which may

results have been obtained with this technique /26-30/.

mixed

to the completely dry process and, consequently, an

fluids generally determine irritant contact dermatitis and

improvement in the surface quality of the holes. The

either irritant or allergic mechanisms. Water

allergic contact dermatitis when they are in touch with

drilling of aluminum-silicon alloys is a process where

workers skin. Non-water-miscible fluids usually cause

dry cutting is impossible 1221, due to the high ductility

skin disorders such as folliculitis, oil acne, keratoses and

of

Pollution-free manufacturing is increasingly gaining due

workpiece

material.

Without

cooling

and

lubrication, the chip sticks to the tool and breaks it in a

carcinomas. interest

the

to

recent

pollution-

Machado and Wallbank /33/ conducted experiments

prevention legislation, European initiatives on product

on turning medium carbon steel (AISI1040) using a

take-back

export

Venturi to mix compressed air (the air pressure was of

or recycling,

development which

of

very short time during cutting.

affect many

industries in the US, and a growing consumer, demand

2.3 bar) with small quantities of a liquid lubricant, water

for green products and production processes. Concern

or soluble oil (the mean flow rate was between 3 and 5

for the environment, health and safety of the operators,

ml/min). The mixture was directed onto the rake face of

as well as the requirements to enforce the environmental

a carbide tool against the chip flow direction. The

protection laws and occupational

safety and

health

regulations are compelling the industry to consider a

application of a mixture of air and soluble oil was able to reduce the consumption

of cutting fluid, but it

MQL machining process as one of the viable alternative

promoted a mist in the environment with problems of

instead of using conventional cutting fluids.

odors,

bacteria and

fungi growth

of the

overhead

The modern industries are, therefore, looking for

flooding system. For this reason, the mixture of air and

possible means of dry (near dry), clean, neat and

water was preferred. However, even if the obtained

pollution-free

Minimum

results were encouraging, the system still needed some

Quantity Lubrication ( M Q L ) refers to the use of cutting

development to achieve the required effects in terms of

fluids of only a minute amount - typically of a flow rate

cutting forces, temperatures, tool life and surface finish.

machining

and

grinding.

of 50 to 500 ml/hour - which is about three to four

Varadarajan,

Philip

and

Ramamoorthy,

/34/

orders of magnitude lower than the amount commonly

developed alternative test equipment for injecting the

used in flood cooling conditions, where, for example, up

fluid and used it with success in hard turning for which

to 10 liters of fluid can be dispensed per minute. The

a large supply of cutting fluid is the normal practice.

concept of minimum quantity lubrication, sometimes

The test equipment consisted of a fuel pump generally

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109

Effect of MQL on Chip Formation in Turning AISI-1040 Steel

Vol. 9, Νos. 1-2, 2008

used for diesel fuel injection in truck engines coupled to

steel in a powerful and rigid lathe (BMTF Lathe,

a variable electric drive. A high-speed electrical mixing

Bangladesh, 15 hp) at different cutting velocities (Vc)

chamber facilitated thorough emulsification. The test

and feeds (S„) under dry and MQL conditions to study

equipment permitted the independent variation of the

the role of MQL on the machinability characteristics of

injection pressure, the frequency of injection and the

that work material mainly in respect of pattern and chip

rate of injection. The investigations performed by the

formation mode, tool wear, and surface roughness. The

authors revealed that a coolant-rich (60%) lubricant

experimental conditions are given in Table 2.1. The

fluid with minimal additives was the ideal formulation.

ranges of the cutting velocity (Vc) and feed rate (S„)

During hard turning of an AISI 4340 hardened steel of

were

46HRC (460 HV), the optimum levels for the fluid

recommendation and industrial practices. Depth of cut,

selected

based

on

the

tool

manufacturer's

delivery parameters were: a rate of 2 ml/min, a pressure

being less significant parameter, was kept fixed.

of 20 MPa and a high pulsing rate of 600 pulses/min. In comparison, for the same cutting conditions, with dry

Table 2.1

cutting and wet cutting, the minimum quantity of

Experimental conditions

cutting fluid method has led to lower cutting forces, temperatures, better surface finish, longer tool life. In

Machine Tool

addition, it was observed that tightly coiled chips were

Work Material : AISI 1040 steel

formed

Size

: Φ 1 1 0 X 6 5 0 mm

Cutting Tool

: Uncoated Carbide (SNMM 120408-

during

wet

turning

and

during

minimal

application, while long snarled chips were prevalent during dry turning. It must be noted that during minimal

: BMTF Lathe, Bangladesh, 15 hp

PM)

application, the rate of fluid was only 0.05% of that used during wet turning. The major part of the fluid used

during

minimal

quantity

application

was

evaporated; the remnant was carried out by work and chips and was too low in volume to cause contamination of the environment.

SNMM 120408 Tool Holder

The review of the literature suggests that the concept

Working

: PSBNR 2525 M12

Tool: -6, -6, 6, 6, 15, 75, 0.8 (mm)

of minimum quantity lubrication (MQL) presents itself

Geometry

as a possible solution for machining in achieving slow

Process

tool wears while maintaining cutting forces/power at

Parameters

reasonable levels, provided that the MQL parameters

Cutting

can be strategically tuned. The main objective of the

velocity, V c

present work is to experimentally investigate the roles

Feed rate, S 0

of minimum

Depth of cut, t : 1 . 5 mm

quantity

lubrication

(MQL)

on

chip

formation mode, tool wear and surface roughness in

MQL supply

turning alloy steel (AISI 1040 steel) by the industrially used uncoated carbide tool (SNMM 120408 TTS) at

: 72, 94, 139 and 164 m/min : 0.10, 0.13, 0.16 and 0.20 mm/rev : Air: 3.5 bar, Lubricant: 200ml/h through external nozzle

Environment

: Dry, Wet and MQL

different cutting velocities and feeds combinations as compared to dry and wet machining.

The MQL needs to be supplied at high pressure and impinged at high speed through the nozzle at the cutting zone. Considering

the

conditions

required

for the

2. EXPERIMENTAL CONDITION

present research work and uninterrupted supply of MQL

AND PROCEDURE

at constant pressure over a reasonably long cut, a MQL delivery system has been designed, fabricated and used.

Experiments have been carried out by plain turning a

The thin but high velocity stream of MQL was projected

110 mm diameter and 650 mm long rod of AISI-1040

along the cutting edge of the insert, as indicated in a

110

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Μ.Α.Η. Mithu, Μ.Μ.Α. Khan andN.R.

Dhar

International Journal for Manufacturing Science & Production

frame within Fig 2.1, so that the coolant reaches as close

value of chip reduction coefficient, ζ (ratio of chip

to the chip-tool and the work-tool interfaces as possible.

thickness after and b e f o r e cut). T h e cutting insert w a s

The schematic view o f the experimental set-up is shown

withdrawn at regular intervals to study the pattern and

in Fig 2.1. T h e M Q L j e t has been used mainly to target

extent of wear on main and auxiliary flanks for all the

the rake and flank surface and to protect the auxiliary

trials. The average width o f the principal flank wear, VB

flank to enable better dimensional accuracy.

and auxiliary flank wear,

The effectiveness, e f f i c i e n c y and overall e c o n o m y of

metallurgical

Vs were measured

microscope

(Carl

Zesis,

using

351396,

machining any work material by given tool depend

G e r m a n y ) fitted with micrometer o f least count Ιμηι.

largely only on the machinability characteristics of the

T h e surface roughness of the machined surface after

tool-work material under the r e c o m m e n d e d condition.

each cut was measured by a Talysurf (Surtronic 3+

Machinability

Roughness

is

usually

judged

by

(i)

cutting

temperature which affects product quality and cutting tool

performance

(ii)

pattern

and

mode

of

Checker,

Taylor

Hobson,

UK)

using

a

sampling length of 0 . 8 m m .

chip

formation (iii) magnitude of the cutting forces which affects power requirement, dimensional accuracy and

3.

EXPERIMENTAL RESULTS

vibration (iv) surface finish and (v) tool wear and tool

AND DISCUSSION

life. In the present work, chip formation mode, tool wear, and surface roughness are considered for studying the role of minimum quantity lubrication.

During machining any ductile material(s), heat is generated at the (a) primary deformation zone due to shear and plastic deformation (b) chip-tool interface due to secondary deformation and sliding (c) work-tool interfaces due to rubbing. All such heat sources produce maximum temperature at the chip-tool interface, which substantially influence the chip formation mode, cutting forces and tool life.

Therefore,

detrimental

attempts

cutting

are

temperature.

made

to

reduce

Conventional

this

cutting

fluid application may, to some extent, cool the tool and the j o b in bulk but cannot cool and lubricate expectedly effectively at the chip-tool interface where the temperature is high. This is mainly because the flowing chips make mainly bulk contact with the tool rake surface and may be followed by elastic contact just before leaving the contact with the tool. Bulk contact does not allow the cutting fluid to penetrate in the interface. Elastic contact allows slight Compressor

penetration of the cutting fluid only over a small region by capillary action. The cutting fluid action becomes more

Fig. 2.1:The

schematic

view

of the

experimental

set-up

and more ineffective at the interface with the increase in Vc when the chip-tool contact becomes almost fully plastic

The form, colour and thickness of the chips directly

or bulk.

and indirectly indicate the nature of chip-tool interaction

The form (shape and colour) and thickness of the

influenced by the m a c h i n i n g environment. T h e chip

chips directly and indirectly indicate the nature of chip-

samples were collected during short run and long run

tool

machining for the Vc-S„ combinations under dry, wet

environment. The pattern o f chips in machining ductile

and M Q L conditions. T h e form and colour of all those

metals

chips were noted down. T h e thickness of the chips was

properties

repeatedly measured by a slide caliper to determine the

particularly rakes angle, levels of V, and S„, nature of

interaction are

found of

the

influenced to

depend

work

by upon

material,

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the the tool

machining mechanical geometry

Effect of MQL on Chip Formation in Turning AISI-1040 Steel

Vol. 9, Nos. 1-2, 2008

chip-tool

interaction

and

cutting

environment.

In

of these ductile chips did not change appreciably but

absence of chip breaker, length and uniformity of chips

their

increase with the increase in ductility and softness of the

smoother. This indicates that the amount of reduction of

work material, tool rake angle and cutting velocity

temperature and presence of MQL application enabled

unless the chip-tool

favourable chip-tool interaction and elimination of even

interaction

is adverse

causing

intensive friction and built-up edge formation.

back

surface

appeared

much

brighter

and

trace of built-up edge formation.

Table 3.1 shows that the steel when machined by the

The colour of the chips also became much lighter,

pattern type SNMG insert under both dry and wet

i.e. metallic from burnt blue depending upon V c and S 0

condition produced both half-turn and spiral type chips

due to reduction in cutting temperature by minimum

at lower feed rates and more or less spiral type chips at

quantity lubrication.

higher feed rates. When machined with MQL the form

Table 3.1 Shape and colour of chips at different cutting velocities and feeds Feed rate,

Vc(ml

S0 mm/rev

min)

0.10

0.13

0.16

0.20

Π) Half turn

112

Environment Dry

Wet

MQL

Shape

Colour

Shape

Colour

Shape

Colour

72

Half turn

Burnt blue

Half turn

Metallic

Half turn

Metallic

94

Spiral

Blue

Half turn

Metallic

Half turn

Metallic

139

Spiral

Golden

Spiral

Metallic

Spiral

Metallic

164

Spiral

Golden

Spiral

Metallic

Spiral

Metallic

72

Spiral

Burnt blue

Half turn

Metallic

Spiral

Metallic

94

Half turn

Burnt blue

Half turn

Metallic

Spiral

Metallic

139

Spiral

Burnt blue

Spiral

Metallic

Half turn

Metallic

164

Half turn

Burnt blue

Spiral

Metallic

Half turn

Metallic

72

Half turn

Burnt blue

Half turn

Golden

Half turn

Metallic

94

Half turn

Burnt blue

Spiral

Golden

Half turn

Metallic

139

Half turn

Burnt blue

Spiral

Golden

Half turn

Metallic

164

Half turn

Burnt blue

Spiral

Golden

Half turn

Metallic

72

Spiral

Burnt blue

Spiral

Golden

Half turn

Metallic

94

Spiral

Burnt blue

Spiral

Golden

Half turn

Metallic

139

Spiral

Burnt blue

Spiral

Metallic

Half turn

Metallic

164

Spiral

Burnt blue

Spiral

Metallic

Half turn

Metallic

φ.

• QJ i 'n>

Tubular

Spiral

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1 Ribbon

!

M.A.H. Mithu, M.M.A. Khan andN.R.

International Journal for Manufacturing Science & Production

Dhar

The cutting temperature generally increases with the

T h e chip reduction

coefficient, ζ (ratio of chip

increase in V c and S 0 , though in different degree, due to

thickness after and b e f o r e cut), one of the important

increased energy input and it could be expected that

machinability indices, also plays a sizeable role on

M Q L would be more effective at higher values of V c

cutting forces and hence on cutting energy requirements

and S 0 . The average chip-tool interface temperature

and cutting temperature. Fig. 3.3 and Fig. 3.4 clearly

could be effectively measured under dry, wet and M Q L

s h o w that throughout the present experimental domain

condition very reliably throughout the

experimental

the value of ζ gradually decreased with the increase in

domain. However, the distribution of temperature within

V c though in different degree for the different tool-work

the tool, work and chip cannot be determined effectively

combinations, under both dry, wet and M Q L conditions.

using experimental techniques. T h e evaluated role of

The value of ζ usually decreases with the increase in

in

Vc particularly at its lower range due to plasticization

turning the steel by the carbide insert at different V c and

and shrinkage of the shear zone for reduction in friction

S 0 under dry, wet and M Q L conditions is shown in Fig.

and built-up edge formation at the chip-tool interface

3.1 and Fig. 3.2, respectively.

due to increase in temperature and sliding velocity. In

M Q L on average chip-tool interface temperature

machining steel by carbide tool, usually the possibility of built-up edge formation and size and strength of the

1000

built-up edge, if f o r m e d gradually increase with the βυ

950

increase in temperature due to increase in Vc and also Su

I900 ι

and then decrease with the further increase in Vc due to

«

too much softening of the chip material and its removal

850

by high sliding speed. It is also noted in this figure that ζ 8

decreased

all

along

also

with

the

increase

in

S„

4. 750

ΐt>

expectedly due to increase in average rake angle with

If 700

increase in uncut chip thickness. 650

600 60

70

80

90

100

110

120

130

140

150

160

170

Cutting velocity. Vc, m/min

Fig 3.1: Variation in chip tool interface temperature with cutting velocity at feed rate 0.10 and 0.13

^

7

-

c (1) Ö

mm/rev under dry, wet and M Q L conditions c

ο •δ

1000

u

950 Q.

S

I 900