Davim J. Paulo (editor). Machining. Fundamentals and Recent Advances

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210 V.P. Astakhov

in Figure 7.12. The AMQCL system employs a novel Coanda-coaxial injector and

spray applicator. The applicator employs a passive electrostatic charging mechan-

ism to enhance droplet uniformity, spray force and machined surface deposition.

Alternatively, an active electrostatic charging system may be employed to provide

combination spray charging capability. The AMQCL system is interfaced both

mechanically and electronically with a machining operation.

AMQCL technology exploits the Coanda effect for both additive injection and

spray flow pattern control. A mixture of propellant gas and additive move along

the Coanda profile as a thin film, powered by both propellant gas and atmospheric

pressure shear stresses (Figure 7.13). The higher-pressure (and warmer) propel-

lant-additive film sheds into and mixes with the lower-pressure (and cooler) coax-

Additives

High voltage power

supply (HVPS)

GAS

Coolant

Advanced

MQCL

Sprays

1. CO

coolant generator (CoolPulse

)

2. Lubrication additives (gases, liquids, solids)

3. Propellant gases (clean air, clean dry air,

3. gaseous nitrogen, carbon dioxide)

4. Active ES charging unit (Optional)

5. Triaxial (CO

/Additive/Propellant) delivery line

6. Triaxial-to-Coanda stream separator

7. Coanda propellant-additive stream flow

8. Coanda-induced airstream flow

9. Coaxial coolant stream flow

10. Injection, cooling, charging and spray formation

Coanda-Coaxial Flow

Active

electrophoretic sensi-

tivity control(ESC)

AMQCL

Control System

Machine Tool/

Control System

INTERFACE

Pulser

MMV

Coanda-Coaxial Spray

Nozzle

Figure 7.12. Schematic of the AMQCL system (Courtesy of Mr. D. Jackson, ToolCool Co.)

Electrosol

Spray

Atmosphere

Additive

Film

Gas

Film

Coanda

Profile

Figure 7.13. Coanda effect (Courtesy of Mr. D. Jackson, ToolCool Co.)

Ecological Machining: Near-dry Machining 211

ial CO

stream. This is analogous to a waterfall, resulting in rapid vortex mixing

and electrostatic charging of the cooling lubricant aerosol (referred to as an Elec-

trosol by ToolCool Co.). Moreover, the Coanda effect creates an atmospheric-

pressure tunnel which extends for a long distance from the nozzle tip towards the

machining operation. The Coanda tunnel confines and shields the charged cooling

lubrication spray as it moves from the nozzle to the cutting zone.

AMQCL aerosol chemistries are formed and delivered on the fly by combining

one or more chemical and physical cooling lubricant ingredients. The composition

may include liquids, extreme pressure solid additives, and reactive gases which are

combined with a propellant gas and injected into a metered flow of charged CO

gas–solid aerosol (Figure 7.14). Each ingredient provides or controls a physical

and/or chemical dimension of the resulting machining fluid chemistry,including

cooling capacity, penetration power, boundary layer reactivity, lubricity, viscosity,

spray particle size and uniformity. AMQCL sprays have variable geometry, in-

cluding adjustable cooling lubricant characteristic from dry to wet composition,

room temperature to cryogenic temperature, and spray pressures ranging from

0.07 to 1 MPa, or much higher if desired.

The combinational use of renewable bio-based feedstocks such as CO

and

vegetable lubricants (SlickSnow

) provides an environmentally sound and

worker-friendly alternative to petroleum-based cooling lubricants. Vegetable-

based lubricants offer significant advantages such as superior tool lubricity, better

surface finish, higher feed rates and safer chemistry. Using a bio-based lubricant

in a minimum quantity application eliminates the costs associated with treatment,

Figure 7.14. AMQCL chemistries

212 V.P. Astakhov

environmental compliance and disposal, among other operational cost factors.

Moreover, vegetable oils such as soybean oil exhibit natural rust inhibition on

metal surfaces, a very important characteristic for ferrous machining applications.

Compared to petroleum-based fluids, bio-based lubricating fluids perform as

well as or better when machining steel and aluminium, cooling and lubricating the

cutting surface as they remove small metal chips and enabling faster, more accu-

rate machining. Bio-based lubricating fluids are biodegradable, non-toxic, have

low volatile organic compounds (VOCs) emissions, high flash point and no offen-

sive odour.

According to CoolTech Co, bio-based oils have been demonstrated to provide

superior metal machining characteristics due to their somewhat polar nature.

Unlike non-polar lubricating films, polar boundary layer films have a natural af-

finity for the workpiece surface, aligning in a highly ordered orientation (like

magnets) with respect to the surface. This type of boundary layer lubrication is

much stronger (i.e., higher shear), producing better finishes, reducing tool chatter

and increasing the longevity of cutting tool edge surfaces.

Used in combination with CO

, spray mixtures produced are non-flammable

and non-corrosive even at extremely high temperatures with sparking. Moreover,

articles being processed are blanketed in a fire-quenching media (CO

), which

prevents chips from igniting or burning and oil smoking.

Test results provided by CoolTech Co show that, unlike simple gases such as

nitrogen and air, CO

exhibits very strong hydrocarbon solubility (i.e., CO

gas

exhibits

600% higher solubility in oils compared with compressed air). Due to

this unique physico-chemical properties and cohesion energy, the CO

gas modi-

fies the lubricant and coolant additive properties to produce mixtures with lower

surface tension and lower viscosity, which aids in penetration into chip–tool capil-

lary interfaces. Moreover, CO

itself behaves as a reactive boundary layer lubri-

cant, forming carboxylic acid functional groups during tribochemical reactions.

AMQCL technology provides widely adjustable and fractional cooling–

lubricant compositions of CO

coolant (Fc), propellant gases (Fp), and minimum

quantities of lubrication additives (Fa). Adjustable spray pressure, temperature,

coolant particle size and lubricant additive concentration allow a machinist to

customize a cooling lubricant composition for any application. One or more indi-

vidually controllable and flexible Coanda spray applicators may be employed to

provide an optimum cooling, lubricating, and cleaning pattern. Moreover, the CO

coolant spray may be pulsed rapidly (CoolPulse

) to provide enhanced spray

penetration and cooling in the cutting zone in certain applications, but without

interrupting or compromising lubrication delivery.

7.5.3 Why NDM Works

7.5.3.1 Some Reported Results

To understand why NDM machining works, a great body of the reported results on

NDM have to be classified in a systemic fashion and analyzed using the funda-

mentals of metal cutting tribology. The lack of information on the experimental

conditions, including the parameters of the aerosol, prevents any reasonable sys-

Ecological Machining: Near-dry Machining 213

tematization of the work done. Wu and Chien evaluated NDM of three different

steel materials [24]. They found that the machining performance is affected by the

lubrication type and its flow rate, the nozzle design, the distance between nozzle

and tool tip, and the workpiece material. All these parameters are found to be

dependent on the work material and process conditions. They concluded that only

when the appropriate oil quantity and appropriate distance between the nozzle and

tool tip are selected properly does NDM provides the optimum process condition.

Unfortunately, many of these important parameters are not reported in many re-

search documents and papers on NDM. As a result, a process or manufacturing

engineer who wants to implement NDM is not equipped with sufficient recom-

mendations to make proper choices of the equipment, parameters and regimes of

NDM for a particular machining operation. Although professional magazines have

published a number of articles on NDM (for example [25–29]), these articles do

not present complete and systematic information on NDM even at the application

level, limiting their scope to rather promotional aspects.

Some research work aimed to answer the question about how good NDM is

compared with dry cutting. Ueda et al. [30] found that temperature reduction in

oil-mist turning is approximately 5%, while in oil-mist end milling it is 10–15%

and in oil-mist drilling it is 20–25% compared to the temperature in dry cutting.

Khan and Dhar [31] found that NDM with vegetable oil reduced the cutting forces

by about 5–15%. The axial force decreased more predominantly than the power

force. They attributed this reduction as well as the improved tool life and finish of

the machined surface to reduction of the cutting zone temperature as the major

reason for the improved performance of machining operations. Similar results

were obtained in machining 1040 steel [32]. Li and Liang [33] found cutting

forces in machining 1045 steel lower in NDM compared with dry cutting. They

also attributed this reduction to the cutting temperature difference.

Other groups of researches have compared NDM with wet machining. Dhar

et al. studied the effect of NDM in tuning of 4340 steel using external nozzle and

aerosol supply to the tool [34]. They found that the temperature at the tool–chip

interface reduced by 5–10% (depending upon the particular combination of the

cutting speed and feed) in NDM compared to wet machining. As a result, tool life

and finish of the machined surface improved by 15–20%. Interestingly, the authors

found that the tool life is the same for dry and wet machining, which is in direct

contradiction with common shop practice. Filipovic and Stephenson [35] did not

find any difference in tool life in gundrilling and cross-hole drilling of crankshafts

between wet machining and NDM. Yoshimura et al. [23] found that, in machining

of aluminium, the cutting force is lower and the surface finish is better with OoW

NDM compared with dry, traditional NDM and wet machining.

Obikawa et al. [36] evaluated the performance of NDM in high-speed grooving

of a 0.45%C carbon steel with a carbide tool coated with TiC/TiCN/TiN triple

coating layers. Studying tool life in grooving, they found that a vegetable oil sup-

plied at constant rate of 7 ml/h reduced the corner and flank wears more effec-

tively than water-soluble oil at high cutting speeds of 4 and 5 m/s.

Using a minimum quantity of lubricant (MQL) and a diamond-coated tool in

the drilling of aluminium–silicon alloys, Braga et al. [37] showed that the per-

formance of the NDM process (in terms of forces, tool wear and quality of ma-

214 V.P. Astakhov

chined holes) was very similar to that obtained when using a large amount of wa-

ter-soluble oil, with both coated and uncoated drills.

Studying turning of brasses, Davim et al. [38] concluded that, with proper se-

lection of the NDM system, results similar to flood lubricant condition can be

achieved.

The result of a comprehensive and well-grounded study of turning of AISI

1045 steel of HB 163 hardness by Li and Liang [39] showed that the cutting force

and temperatures are always lower for wet machining with a water-soluble oil

compared with NDM and dry machining.

7.5.3.2 Explanations Provided

Surprisingly, very few explanations are provided to clarify why NDM works. The

known explanations of the efficiency of NDM came from the oil mist principle

that was developed by a bearing manufacturer in Europe during the 1930s. Today,

this method of lubricating is applied to all types of other machine tools, web and

sheet processing equipment, belt and chain conveyors rolling mills, vibrators,

crushers, centrifuges, kilns, pulverizers, ball mills, dryers and liquid processing

pumps. Boundary lubricating layer of extreme-pressure (EP) additives and accu-

rate delivery of the lubricant to the contact surface just before they engaged into a

contact plush cooling by oil droplets evaporation were borrowed to explain the

effectiveness of NDM in metal cutting.

Obikawa et al., trying to explain the effectiveness of NDM, suggested that the

air supply plays an important role in transporting the oil mist to the interface be-

tween the flank wear land and machined surface (the tool–workpiece interface)

[26], i.e., it was assumed that the oil can penetrate the tool–workpiece interface

due to the high pressure of the compressed air that serves as a vehicle for the oil

droplets. No comparison between the contact pressure at this interface and that of

compressed air was made.

Liao and Lin [40] argued that MQL can provide extra oxygen to promote the

formation of a protective oxide layer that that might form between the chip and the

rake face over the tool–chip interface. This layer is basically quaternary compound

oxides of Fe, Mn, Si and Al, and has proved to act effectively as a diffusion bar-

rier. Hence, the strength and wear resistance of a cutting tool can be retained,

which leads to a significant improvement in tool life. It was found that there exists

an optimal cutting speed at which a stable protective oxide layer can be formed.

Unfortunately, no direct conclusive evidence of the existence of such a layer were

provided.

López de Lacalle et al. [41] suggested that the flow of air with oil drops acts in

three different ways:

• It removes the heat generated during cutting by the convection of the in-

jected air and the evaporation of part of the oil.

• It decreases friction between chip and the rake face of the tool, since oil

drops are small enough to get into the chip–tool interface.

• It removes the chip of the working area due to the action of compressed

air.

Ecological Machining: Near-dry Machining 215

Unfortunately, no direct supporting evidence was given for these suggestions, the

validity of which was justified by the reduced cutting force and tool wear. The

boundary conditions selected for their finite element analysis as well as the pa-

rameters of the jet of compressed air and parameters of the coolant were not based

on the known experimental data. This results in a conclusion that machining with

any kind of nozzle arrangement of their NDM resulted in lower tool flank wear

compared with that found in wet machining with a water-soluble oil as the cool-

ant. Although this computational result is in direct contradiction with everyday

machining practice, no explanation for this was provided.

7.5.3.3 Feasible Explanation of the Effectiveness of NDM

To understand the subject, one should ask himself a few simple questions: Why

does NDM work at all? Why does it show somehow better results than flood or

even high-pressure MWF supply? In other words, how is this physically possible,

as the flood and high-pressure MWF approaches definitely remove much more

thermal energy? Why does an aerosol containing an oil and air mixture have (ac-

cording to all NDM research and promotional papers) greater cooling ability than

water-soluble MWFs even through the heat capacity of water is ten times greater

than that of oil and much greater than that of air, bearing in mind that heat removal

due to the evaporation of oil droplets is negligible small because of the inherent

properties of oil and tiny oil flow rate (30–60 ml/h)?

A thorough consideration of the possible action of the aerosol in NDM [1] al-

lows the following conclusions to be drawn:

• Aerosols do not act as lubricants or boundary lubricants in metal cutting. It

has been conclusively proven that, even when cutting fluids are applied,

a high contact pressure exists between the chip and tool rake face, particu-

larly along the plastic part of the tool–chip contact length, which combined

with high temperature along the actual contact area and surrounding zone,

precludes any fluid access to the tool–chip and tool–workpiece interfaces

[1]. This is the case even if a jet of MWF of extremely high pressure (15

MPa as maximum available on commercial high-pressure MWF supply

equipment) is used. As the maximum pressure of the aerosol cannot exceed

that of compressed air (normally available at a maximum of 0.7 MPa and

then reduced due to hydraulic losses to 0.5 MPa), the penetration ability of

the aerosol is far too low compared even with MWF supplied through the

tool at a high pressure.

• Aerosols used in NDM always have lower cooling ability than a properly

selected water-soluble MWF. There are two possible cooling actions of the

aerosol in NDM, namely cooling due to droplet evaporation and due to

forced convection. As the oil flow rate in NDM is very small (normally

30–60 ml/h), the cooling action due to droplet evaporation is also small.

Astakhov introduced a parameter called the cooling intensity

to ac-

count for forced convection of the MWF in metal cutting [1]

065 067 033 033 032

γν

−

.. . . .

h cf cf P cf cf cf

KvkC (7.1)

216 V.P. Astakhov

As can be seen, the cooling ability of a cooling medium depends upon the

velocity of this medium,

v , its thermal conductivity

k , its specific heat

−Pcf

C , its specific gravity

and its viscosity

. A simple comparison of

the listed parameters in the case of a high-pressure water-soluble MWF with

those in NDM shows that the former has much greater cooling ability [1].

• Aerosols cannot change the shape of the forming chip due to their direct

mechanical action regardless of the nozzle location. To comprehend this,

one should consider their momentum. For a moving body, the momentum

is just the product of the mass of the body and its speed, Momentum is tra-

ditionally labeled by the letter p, and the definition of momentum is there-

fore p

mv for a body with mass m moving at speed v. As the mass of the

aerosol is small compared to that of a jet of a high-pressure water-soluble

MWF, its action on the forming chip is negligibly small compared with

that of a high-pressure water-soluble MWF.

Having read and analyzed these conclusions, one may ask a logical question: Why

does NDM work as reported by many researchers and practitioners? As the proper

answer to this question is not yet known, trial-and-error methods are used to de-

termine the parameters of NDM.

One of the feasible and physics-based explanations is the so-called embrittle-

ment action of the cutting fluid, which reduces the strain at facture of the work

material. This action is based on the Rebinder effect [1]. Rebinder’s studies were

directly concerned with the metal cutting process. Conducting a great number of

cutting tests under different cutting conditions and with different cutting fluids, he

observed the formation and healing of microcracks. The latter was particularly

pronounced when machining ductile materials where large plastic deformation of

the layer being removed was observed. The results of Rebinder’s study showed

that absorbed films prevent microcracks from closing (healing due to plastic de-

formation of the work material). Because each microcrack in the machining zone

serves as a stress concentrator, a lower energy was required for cutting. Pursuing

this direction, Epifanov [1] found that the penetration of the foreign atoms (from

the decomposition of the cutting fluid) produced an embrittlement effect in a man-

ner similar to hydrogen embrittlement. He concluded that plasticity of the work

materials reduces. Our current understanding of the Rebinder effect is as the alter-

nation of the mechanical and physical properties of materials due to the influence

of various physiochemical processes on the surface energy [1].

As shown in Chapter 1, the work of plastic deformation of the layer being re-

moved in metal cutting is the greatest, so it is the prime cause of tool failure due to

various mechanisms of wear. Once this work is done, it is too late to mitigrate its

consequences using inferior (in terms of cooling ability) NDM as it cannot com-

pete against the cooling ability of a flood or a high-pressure though water-soluble

tool MWFs. The only feasible way for NDM to work in metal cutting is to in-

crease the embittlement of the layer being removed and thus to reduce the work of

plastic deformation done in the transformation of the layer being removed into the

chip. Available information about the practice of NDM suggests that an atomized

oil possesses great ability to enhance the Rebinder effect. In the author’s opinion,

Ecological Machining: Near-dry Machining 217

this explains the effectiveness of NDM. As the lowering of the surface energy of

a solid to alter its mechanical properties can be achieved by adsorption, chemi-

sorption, surface electrical polarization and surface chemical reactions [1], the

research, selection of designs and parameters and testing of the effectiveness of

NDM should be driven in this direction.

7.5.4 Consideration of the NDM System Components

Experience of implementing of NDM shows that this technology can be successful

if and only if the whole NDM system of components is considered and special

attention is paid to each and every component individually while maintaining the

coherence of the system. Therefore, a 360°

analysis of all components listed in

Figure 7.15 is a prerequisite before any practical implementation of NDM. Unfor-

tunately, this is not the case in practice, which in the author’s opinion is the major

hurdle in implementing this technology.

Regardless of its scope, this analysis should always start from the NDM setup.

At this stage, a particular NDM type (see above), aerosol components and parame-

ters should be selected and tested in terms of their suitability for the considered

project. In other word, the feasibility of NDM should be verified and data for the

efficiency analysis should be collected for critical machining operations, tools and

work materials to be involved with NDM.

Workpiece

location

Chip handling

Rigidity

Accuracy and

repeatability

External

mist formation

Internal

mist formation

Droplet size

Medium

Discharge and

flow rate

Geometry

Design

Chip breaker

Coolant passages

and nozzles

Versatility

Re-setting

Design for

dry/nearly

machining

Chip handling

Self-cleaning

Safety

Fire protection

Stock to be

removed

Hole and slot

designs

Spatial geometry

Mass, Quality

requirements

Reliable,

Enviromentally

Friendly, Cost

Efficient

Dry/Near Dry

Machining

Hot hardness

Thermal properties

Adhesion and

Diffusion resistance

Plastic lowering

(high temperature

creep)

Internal

External

Continuous

Interrupted

Required

productivity

Cost per unit

Hardness

Strength

Strain at fracture

Thermoconductivity

Thermoexpansion

coefficient (TEC)

Metallurgical and

structural

modification

Figure 7.15. Components of the NDM system

218 V.P. Astakhov

A detailed cost analysis and the entire scope of the manufacturing system have

to be studied prior to implementation of NDM. As such, the depth and extent of

analysis depend on the scope of the project, starting with implementing NDM for

a single stand-alone machine and finishing with a NDM manufacturing system.

The wider the scope, the larger the system that should be included in such an

analysis.

7.5.4.1 Cutting Tool

The next important issue to be considered is the cutting tool. The implementation

of NDM requires specially designed cutting tools if the real advantages of NDM

are to be achieved.

Unfortunately, a common notion is that with external NDM (NDM 1.1 and

NDM 1.2) the same cutting tool used for flood coolant will work [27]. Although

this is true in some particular cases, the maximum advantage of NDM is achieved

if the cutting tool is specially designed for NDM, including specific tool geometry

and tool material (including coating). Cutting tools and tool holders for NDM 2

should or modified be designed specially for NDM.

Round-ended cutting tools such as drills and reamers should be designed with spe-

cial channels having a maximum allowable cross-sectional area and extremely

smooth configurations without dead ends. A dedicated line should go to every contact

(cutting, rubbing, locating, etc) area. Void and trapped areas in the aerosol flow path

should be eliminated. Exit holes and nozzles must be positioned to provide aerosol

directly to the chip-forming zones and special consideration should be give to chang-

ing position of these holes with tool re-sharpenings. In multi-stage reamers, the

cross-sectional area of these holes should be selected to provide balanced aerosol

supply to all stages or, better, balanced in proportion to the amount of work material

cut by various stages. An example of channel design is shown in Figure 7.16.

The design of non-round tools requires special attention. The flow of aerosol

coming through the centre of the spindle and a central hole in the tool holder has

to change direction to reach the machining zone. The best the designer can do is to

make the bend of the aerosol channel as gradual as possible, avoiding sharp turns

Figure 7.16. MQL internal channel design for a reamer (courtesy of Mapal USA Co.)

Supply for any

cutting/rubbing

area/zone

Double channel supply

of oil and air

Ejector chamber – oil-air mixing

Smooth mist channel

guiding

Ecological Machining: Near-dry Machining 219

at any cost. This present a great challenge for small-sized tools. A number of de-

sign concepts should be considered and discussed before making the final decision

on the proper tool design, as tool manufactures have very little experience with the

design of tools for NDM.

The foregoing consideration implies that standard inexpensive tools cannot be

used in NDM, as tools for NDM should substantially re-designed or, at least,

modified, which makes them special. Therefore, one should be prepared to bear

greater initial tooling costs and these costs should be a part of the analysis made

prior to considering an NDM application. As these new tools are not off-the-shelf

products stored by manufacturers and distributors, the lead time to deliver, modify

and repair this tool is much longer than with standard tools.

New procedures in tool setting should also be introduced. One of the most criti-

cal of these is the verification of the aerosol flow behaviour after each setting or

re-setting of the cutting tool [42]. To do this, special equipment that matches the

production machinery (the same aerosol system, oil, oil flow rate, operating pa-

rameters, etc.) should be available for spray pattern testing of the tool–tool holder

assembly. Naturally, this adds time and thus cost that should also be included in

the cost analysis.

An additional important tooling issue that should be addressed in NDM is the

modification of standard tool holders to prevent any clearance in the line that in-

troduces aerosol to the tool holder and any cavity in the tool holder [42].

7.5.4.2 Chip Management

MWFs perform two important technological functions: (a) cooling the workpiece so

that it does not distort excessively and does not get excessively hot. As a result, its

proper in-process and post-process gauging as well as interoperation handling (load-

ing/unloading and transportation) do not present problems, (b) flushing away chips

so the tool, the workpiece, the fixture and the machine are not damaged. The chal-

lenge for NDM is to provide substitutes for these critical functions. Although since

the late 1990s a number of research programs and pilot projects have been conducted

to establish and verify the fundamentals of dry cutting [10], it is not yet obvious

whether such substitutions can be achieved economically in industrial applications.

Smooth, continuous chip evacuation is a key feature in any high-productive

machining operation, particularly when high-penetration-rate tools, i.e., in the

automotive industry in machining of aluminium alloys, are considered. High-

pressure MWF is effective in ejecting chips from the drills, reamers, taps etc. In

NDM, air replaces the liquid, which presents a problem as air cannot produce the

ejection force that a liquid at a high flow rate can exert. A feasible solution should

be sought in the location of aerosol exit holes, increased and optimized chip flutes

and adjusted speeds and feeds.

Even if the chips are removed from the hole being machined, they must still be

removed from the tool and fixtures into a collection hopper to prevent damage to

the tool, workpiece and machine tool. To do this, special vacuum hoods have to be

designed and installed. Such a hood can be a part of the machine, as shown in

Figure 7.17. It is obvious that these hoods make tool change, tool maintenance and

setting more difficult and time consuming.