Role of Fluids.pdf

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Two relatively recent strategies focused on reducing fluid use are Minimum Quantity

Lubrication (MQL) and dry machining.

There are four primary categories of cutting fluids that differ in terms of their thermophysical

properties, common process applications, and method of treatment. Straight oils are made up

entirely of mineral or vegetable oils, and are used primarily for operations where lubrication isrequired (Kalpakjian and Schmid, 2001). Despite being excellent lubricants, they exhibit very

poor heat transfer capabilities. Soluble oils are mixtures of oil and water and have increased

cooling capabilities over straight oils and offer some rust protection. Concerns with using

soluble oils center upon maintenance issues like evaporative losses and bacterial growth. Semi-synthetics are similar to soluble oils in performance characteristics, but differ in composition

because 30% or less of the total volume of the concentrate contains inorganic or other

compounds that dissolve in water. Semi-synthetics have better maintenance characteristics thansoluble oils, but do contaminate easily when exposed to other machine fluids and may pose a

dermatitis risk to workers. Synthetics are chemical fluids that contain inorganic or other

chemicals dissolved in large amounts of water and offer superior cooling performance.

Maintenance is also not a major issue with synthetics, however, cases of dermatitis are moreprevalent in workers and the lubrication functionality is weaker than with semi-synthetics

(IWRC, 1996).

Extensive use of cutting fluids in machining operations leads to a sizeable waste stream.

Responsible handling of used/waste fluid is needed to avoid the contamination of lakes, rivers,

and groundwater. Such handling may include the pre-treatment and treatment of cutting fluidwastes, but even the most disciplined manufacturer may still have fluid-related environmental

concerns associated with chip/workpiece fluid carry-off. It is worth noting that the cost of fluid

pre-treatment/treatment is sometimes higher than the purchase price of the fluid itself, and sincethe treatment is not always totally effective, disposal may lead to inadvertent water

contamination.

In addition to the environmental challenges of managing a used cutting fluid waste stream,

cutting fluids also introduce several health/safety concerns. The National Institute for

Occupational Safety and Health (NIOSH) estimates that 1.2 million workers involved in

machining, forming, and other metalworking operations are exposed to metalworking fluidsannually (NIOSH, 1998). Dermal exposure to these fluids represents a health concern, as does

the inhalation of airborne fluid particulate. The application of cutting fluids within a machining

operation often produces an airborne mist, and medical evidence has linked worker exposure tocutting fluid mist with respiratory ailments and several types of cancer (Mackerer, 1989; Thorne

et al., 1996). This makes the use of cutting fluids a health issue with the potential of both long-

and short-term consequences.

Over the past decade, cutting fluids have been studied extensively to characterize their relative

benefits and shortcomings in terms of their use within machining processes. Traditionally,

manufacturers have employed cutting fluids to serve the following functions: cooling, lubrication,corrosion inhibition, and chip flushing, and as a result, achieve such benefits as increased tool

life, improved workpiece quality, enhanced machine tool life, and effective chip management.

However, many issues surround the use of cutting fluids, including fluid system maintenance,


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fluid pre-treatment/treatment/disposal (Skerlos et al., 2000; Skerlos et al., 2001), and worker

health/safety concerns. This paper summarizes recent work performed to establish an improved

understanding of cutting fluid function and to describe the salient mechanisms associated withfluid mist formation/behavior. It also presents promising approaches that offer opportunities for

addressing cutting fluid related concerns while not compromising on process performance.

Traditional Role of Cutting Fluids

Cutting fluids have traditionally been used in machining operations to lubricate the chip-tool and

tool-workpiece interfaces, remove heat from the workpiece and cutting zone, flush away chips

from the cutting area, and inhibit corrosion (Shaw, 1942). While each of these four functionscan be employed as justification for cutting fluid usage, it is widely believed that the primary

functions of a cutting fluid are lubrication and cooling. Seminal contributions to the technical

literature in support of this belief are provided below.

LubricationAny study on the lubricating effects of a cutting fluid builds upon an understanding of the

mechanics and forces involved in a machining process. An early method proposed to analyze ametal cutting process was the orthogonal cutting model of Merchant (1941). This model is based

upon the assumptions that the cutting edge is perfectly sharp, deformation is plane strain, and

that the stresses on the shear plane are distributed evenly. Their model characterizes the

deformation geometry via the shear angle,, which describes the plane on which sheardeformation occurs. The forces acting on the chip at the rake face of the tool are balanced by the

force acting on the chip at the shear plane. This allows for the development of a system of force

equations that can be used to determine characteristics of the process.

Based upon the work of Merchant, Lee and Shaffer (1951) used plasticity theory, specificallyslip-line field theory, to develop a more sophisticated model to apply to the machining problem.Oxley and Hastings (1977) added strain hardening into the slip-line theory and successfully

applied it to predict cutting forces. The predictive abilities of this model were shown to be

extremely sensitive to the workpiece material. A major conclusion of slip-line field modeling is

that specification of rake angle and friction factor do not distinctively determine the shape of thechip. This is because more than one field can be constructed, each with a different chip

thickness and contact length with the tool.

Further studies sought to account for the complicating issues of material behavior, nonlinear

contact, high temperature, high strain rate, and large strain in metal cutting modeling/simulation.

A great many efforts have been made to use finite element methods to characterize the metalcutting process (Iwata et al., 1984; Strenkowski and Carroll, 1985; Lin and Pan, 1993; Marusich

and Ortiz, 1995). Over the last several decades, much work has concentrated on the

development of mechanistic models to predict cutting forces based upon the method proposed by

Sabberwal (1960). A reasonable amount of success has been achieved by simulating somemachining operations, but the method is process dependent, and material state quantities like

stress, strain, and cutting zone temperature are difficult to obtain.


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One of the principal challenges associated with the modeling of machining operations is the

complexity associated with the work-tool-chip interaction. The tool chip interface is

characterized by sliding contact between the tool and the workpiece at high normal pressure andtemperature. The energy that is consumed due to friction is mostly converted into heat on the

rake face, causing tool temperatures to be high. In order to counteract this extreme frictional

force, cutting fluids have been used as lubricants in some machining operations.

Shaw et al. (1951) experimentally observed that the cutting fluid does not lubricate at high

speeds. The possible explanations for this behavior included: chips are carrying cutting fluid

away too quickly for it to reach the cutting zone and serve as a fluid-film lubricant and the timeis too short for the fluid to chemically react with metal surfaces to form a solid-film lubricant.

Cassin and Boothroyd (1965) also found that no lubrication was evident at high cutting speeds.

They suggested that lubrication occurs at low speeds by diffusion through the workpiece or thatthe extreme pressure additives within the fluid react to form a boundary layer of solid-film

lubricant.

As noted above, while the primary functions of a cutting fluid are considered to be lubricatingand cooling, lubrication is the dominant function for only those machining operations that

employ low cutting speeds, e.g., drilling and tapping. It is to be expected that a fluid in these

operations would reduce the friction between the chip and the rake face. However, in drillingand tapping, a significant amount of friction between chip and tool occurs in locations other than

the rake and flank faces. Another source of friction results when the chips attempt to evacuate

through the flutes. The chips rub against the tool and hole wall, and in some cases the chips clogthe flutes, increasing torque and axial force, increasing tool temperature, and occasionally

marring the hole wall surface. In these cases, the presence of a cutting fluid can reduce the

friction between the chips and tool flutes, enabling the smooth evacuation of chips from the holeand avoiding chip clogging. Of course, the efficacy of the fluid as a lubricant is very dependent

on the success achieved in delivering it to the bottom of the hole. Furthermore, the character of

the chips produced in drilling and tapping play an important role in the chip cloggingphenomenon (Haan et al., 1997; Cao and Sutherland, 2002).

To characterize the clogging phenomenon in drilling requires an understanding of the chip

formation process. Kahng and Koegler (1976) gave an explanation of chip forming in twistdrilling. They proposed that chip curl is formed in the shear zone, influenced by the contact

between the chip and rake face of the tool. Chip breakage resulted when the chip impacts the

workpiece or the tool. Chip breaking methods were suggested to allow for easier chipevacuation. Nakayama and Ogawa (1978) studied the basic formation of chips, specifically

those formed in the drilling process. Continuous chips were found to have the rake face of the

tool as a tangential plane to the surface of the chip. Helical chips were described by their chipflow angle, radius of upcurl, and radius of sidecurl. The radii are functions of the cutting

velocity and the angular velocity of the chip. Haan et al. (1997) found that the significant

variables in determining chip size were the feed, workpiece material, and drill type.

In summary, for low cutting speeds such as found in operations like drilling and tapping, the

technical literature indicates that a cutting fluid can provide lubricating effects that serve to

reduce friction levels, and avoid such undesirable phenomena as chip clogging.


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Cooling and Heat Transfer

When a cutting fluid provides lubrication to a machining process, it serves to reduce frictionlevels and thus moderate increases in temperature (Merchant, 1958). For many machining

operations, however, the principal role of the cutting fluid is to remove heat during the process,

especially from the zones indicated in Figure 1. This is especially true for many modern-dayprocesses, in which potential lubricity benefits of a cutting fluid do not occur because of the high

speeds utilized during the process. The success of the cutting fluid in providing cooling can be

measured in different ways. A major focus has been on defining the temperature and its

distribution in the cutting zone. Early attempts to characterize temperature were done throughexperimental methods. They used a variety of techniques such as: i) thermocouple methods (e.g.,

tool-work thermocouple and embedded thermocouple) (Shore, 1925; Gottwein, 1925; Herbert,

1926; DeVries, 1968, Watanabe et al., 1977; Hirao, 1989; Agapiou and Stephenson, 1994), ii)infrared imaging (Schwerd, 1933; Boothroyd, 1961), and iii) microstructural change (Trent,

1984).

Fig. 1 Regions of heat generation in machining

Since experimental methods generally provide only limited information on complete temperaturedistribution, researchers have also employed models to establish temperature distributions. Shaw

et al. (1951) studied the resistance across the tool-chip interface and related this to temperature.

Chao and Trigger (1955) established models to predict the temperature on/near the shear planeusing the assumption that all of the mechanical energy associated with shearing was converted

into thermal energy, an assumption that is widely used in metal cutting. Childs et al. (1988)

compared finite element analysis results to experimental data in predicting the effects that acutting fluid has on the temperature distribution in machining. This study found that theeffectiveness of heat removal was dependent on the flow rate and application direction of the

fluid. Subramani et al. (1993) established a three-dimensional analytical model for predicting

the workpiece temperature distribution during dry cylinder boring. The model was verified bymeasuring the temperature in cast iron cylinder boring tests. Zheng et al. (2000) developed a

model to predict the temperature distribution in the workpiece during cylindrical boring of cast

aluminum alloys under wet conditions.


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Previous research has examined the perceived benefits of cutting fluids across a wide variety of

cases. It has also cast some doubts on the necessity of cutting fluid use in some machiningprocesses and under certain conditions. The most notable conclusion of the studies is that more

knowledge is required to further quantify the role that cutting fluids play. Furthermore, greater

information is needed to understand how cutting fluid mist, the source of many potential healthconcerns, is produced and behaves.

Cutting Fluids & Air Quality

The application of a cutting fluid stream to a rotating cylindrical workpiece, such as found in a

turning operation, is illustrated in Fig. 2. As is evident, the application of the fluid producesairborne particles of varying sizes. Cutting fluid mist (especially the small particulate that can be

inhaled and is too small to be seen in the figure) produced during machining operations may pose

a significant threat to worker health/safety. Safety/health regulations focus on the time weighted

average of the mass concentration of fluid mist to which a worker may be exposed for a given

work period. Common strategies to control the amount of mist exposure include the use ofenclosures, air filters, and mist collectors. However, these approaches prove to be costly both in

time, as access to machine tools may be restricted by enclosures thus increasing partloading/unloading time, and money, including the slowing of production and reduction in

process efficiency (Leith et al., 1996). In order to formulate a strategy to combat this problem,

there must be an understanding of i) air quality and its harmful effects and the impact of mist onair quality, ii) the mist formation process, and iii) mist behavior.

Fig. 2 Cutting fluid applied to a rotating cylindrical workpiece

Air Quality and Fluid Mist BasicsSeveral studies have shown statistically significant increases in cancer of the esophagus, stomach,

pancreas, colon, prostate, and rectum due to prolonged exposure to cutting fluid mist (Hands etal., 1996; Mackerer, 1989). Because of such health problems a number of government agencies

such as the National Institute of Occupational Safety and Health (NIOSH), the Environmental


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Protection Agency (EPA), and the Occupational Safety and Health Administration (OSHA) have

become involved in establishing standards and regulations for particulate exposure. Standards

set by industrial organizations and government agencies closely follow the U.S. NationalAmbient Air Quality Standards (NAAQS) established by the EPA. In 1987, these standards set a

maximum mass concentration for PM10, particulate matter less than 10 micrometers (also

referred to as thoracic particulate mass). This subset of particles represents the portion ofinhalable particles that pass the larynx and penetrate into the conducting airways and bronchial

regions of the lungs. Larger particles that enter this region can be evacuated from the body in a

short amount of time. In 1997, in response to a growing concern about smaller particles posing a

greater risk to human health, the standard was modified to address the risk associated withinhaling particles less than 2.5 micrometers in diameter (PM2.5). PM2.5 represents the fraction

of respirable particles that enter into the deepest part of the lungs, the non-ciliated alveoli.

NIOSH recommends that exposure to thoracic particulate mass be limited to 0.4 mg/m3as a time

weighted average concentration for up to 10 hr/day for a 40 hr work week (NIOSH, 1984).

In an effort to understand the dominant factors in the production of potentially harmful cutting

fluid mist, much work has focused on identifying statistically significant variables of the process.For example, Gunter and Sutherland (1999) investigated the application of fluid to a rotating

cylindrical workpiece. This study found that the most significant variables in the production of

PM10 and PM2.5, respectively, were the spindle speed and workpiece diameter (bothcontributing to surface velocity). A study by Yue et al. (1996) reported similar findings, noting

that spindle speed plays the dominant role in influencing aerosol mass concentration for both

PM10 and PM2.5 mass concentration levels, and that significant submicrometer mist particleswere produced in the process. Increasing the spindle speed appeared to increase the amount of

submicrometer particles produced. Sutherland et al. (2000), in a study comparing wet and dry

machining, found that a significant amount of submicrometer size fluid mist particles wereproduced during the turning process, and increasing spindle speed increased the quantity of these

particles. The study also found that feed rate and depth of cut were significant factors in the

formation of both cast iron dust and fluid mist. They also contrasted air quality conditions,quantified by mass concentration, under wet and dry machining conditions. A randomized 2

3

factorial experiment with one replication was designed and conducted, varying cutting speed,

feed, and depth of cut. The experimental set was run once in the presence of a cutting fluid (5%

synthetic solution), as well as once in the absence of the fluid, yielding thirty-two total tests.Mass concentration was used as the response variable in an analysis of variance, and all main

effects were found to be significant in generating mist and dust. The amount of aerosol (mist for

the wet tests and dust for the dry tests) generated in the tests was found to be 12 to 80 timesgreater in wet machining than in dry machining.

To prevent or counteract the problems associated with the cutting fluid mist produced inmachining operations, there first must be an understanding of the mist formation process. As

illustrated in Fig. 3, two different mechanisms have been proposed as sources for cutting fluid

mist: atomization and vaporization/condensation. Both mechanisms of mist formation will be

discussed and their significance explored.


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Fig. 3 Cutting fluid mist generation mechanisms

Cutting Fluid Mist Formation

Some recent work has been focused on examining the formation and behavior of cutting fluid

mist. A predictive model has been established, with specific emphasis having been placed onmist formation in the turning process. A summary of this model development and its validation

follows.

Atomization, one of the mechanisms by which cutting fluid mist is produced, is the process bywhich a liquid jet or sheet disintegrates by the kinetic energy of the liquid itself, by exposure to

high-velocity air, or as a result of mechanical energy applied externally through a rotating or

vibrating device (Bayvel and Orzechowski, 1993). In the last half century, there have been manystudies conducted on liquid atomization due to interaction with a rotating element. Most

attempts to model the process have characterized the rotating element as a disk, as shown in Fig.

4. A liquid stream with a flow rate of Q is added at the center of the rotating disk. Carried byfriction, the liquid spreads as a film towards the outer edges. At the rim of the disk, the liquid

disintegrates into droplets. The liquid disintegration process is dependent on many variables.

Depending on characteristics like flow rate, angular speed, disk diameter, viscosity, and surfacetension of the fluid, it has been suggested that the fluid will disintegrate via three different modes:

drop mode, ligament mode, and film mode. These modes are depicted in Fig. 5.


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Fig. 4 Fluid application to a rotating disk

Of special interest in characterizing the atomization process for a rotating disk are the mean

diameter of the droplets produced by the different disintegration modes and also the transition

stages between modes. The drop mode occurs when flow rate is fairly low. Matsumoto et al.(1985) predicted the mean diameter of droplets formed in drop mode as follows:

RWeDd523.02.3 =

, (1)

where R is the radius of the disk and We is the Weber number. Based on the Weber analysis(1931) for the instability of a Newtonian jet, the mean droplet diameter formed from the ligament

mode is approximated as follows:

ligL dD 88.1= , (2)

where dlig is the diameter of the ligament. Matsumoto et al. (1985) also reported that the meandiameter of the droplets formed in film mode could be predicted by the following:

( )2.01.05.0Re1.11 FQF WeRD +

= , (3)

whereReis the Reynolds number and QFis the volumetric flow rate to form the film.


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Fig. 5 Three modes of atomization: (a) drop mode, (b) ligament mode, and (c) film mode

Making the assumption that transition between drop and ligament modes occurs as the drop

departure frequency from the disk edge exceeds a certain value, Matsumoto and Takashima(1978) developed the following semi-empirical relation to predict the flow rate at which

transition occurs:

15.195.02 Re2096.0 = WevRQLD , (4)

where, QLD is the critical flow rate when the transition occurs. Matsumoto and Takashima alsodeveloped an expression for the transition between film and ligament mode, which they assumed

to occur when the surface tension of the liquid was overcome by the inertial and centrifugal

forces beyond the edge of the disk. In order to define the transition between film and ligament

modes, QFL, the flow rate at this transition, is defined as:

883.0667.02 Re234.0 = WevRQFL . (5)

To better describe the behavior of the cutting fluid involved in the turning process, atomization is

characterized using a cylindrical workpiece. Given the geometry of the turning process, it isclear that the flow rates in the rotating disk theory are different from those in the rotating

cylinder theory. This means that the transitional flow rates defined in Eqs. 4 and 5 are not valid

for these conditions. In the case of a cylindrical workpiece, two rims of fluid form on the surfaceof the cylinder when the liquid is applied. While these rims behave similar to synchronous

rotating disks, they contain the potential for all three mechanisms of mist formation to occur

simultaneously as the flow rate changes around the circumference of the cylinder. Thissignificant difference requires the introduction of a new term, flow flux, to account for the

difference in conditions.

Flow flux is defined as the quantity of fluid purged from a unit length of the circumferenceduring one time unit.


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R

QF AB

=

, (6)

where, QABis the flow rate of the liquid atomized from an arc (AB) on the surface of the cylinder

and ! is the angle formed between points A, B, and the centerpoint of the cylinder. Fig. 6

illustrates the different mist formation mechanisms and the flow rates corresponding to thoseregions of the cross section.

Fig. 6 Droplet formation and corresponding flow rates on a cylindrical workpiece

Making critical assumptions regarding the fluid flow based upon the visual evidence of themodes shown in Fig. 6, the mean droplet size and particulate distribution (shown in Fig. 7) can

be predicted for each of the three formation mechanisms. These relations are

RWeDD523.02.3 = , (7)

( )7

2

23

7

1

3

2

7

2123.1

!

!

"

#

$

$

%

&

!

!

"

#

$

$

%

&=

RR

QRD L

NL l, (8)

( )2.01.05.0Re1.11 FQ

F WeRD +

= , (9)

where " is the cutting fluid density, QL is the volumetric flow rate to form ligaments, # is thecutting fluid surface tension, $is the angular velocity, and QFis the volumetric flow rate to form

film. The number of ligaments, Nl, is determined from the work reported by Kayano and

Kamiya (1978) that concentrated on rotating disk atomization:


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( ) ,1

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''(

)

**+

,

!!

"

#$$

%

&+++=

StNStNNWe

l

ll (10)

where Stis the stability number.

Fig. 7 Particulate Size Distribution due to formation and behavior

Runoff is a phenomenon that can occur in typical machining settings. Runoff usually occurs in

the film mode region when the fluid flow rate is very high. The fluid in the film region becomes

very unstable and drains off. Despite this occurrence, the drop and ligament modes that areoccurring are not affected by it and the model still holds for those modes.

Vaporization of cutting fluid under high temperatures, the second mist production mechanism,could present a significant problem due to the organic compounds contained in the fluid that

have been suggested to pose negative health effects. The fluid may vaporize or even boil during

the machining process since the temperature in regions well away from the cutting edge may be

in excess of 200C; surface temperatures close to the cutting edge can be as high as 650C(Childs et al., 1988). Concerns related to the relative importance of vaporization (and

subsequent condensation of fluid vapors to produce fluid mist) prompted a study to determine the

significance of vaporization in addressing concerns with cutting fluid use.

A 10% water diluted Chempakool 3121 concentrated cutting oil was examined under high

temperature and the vapor generated was condensed into liquid and analyzed through agravimetric analysis and a Gas Chromatography/Mass Spectrometry analysis. The results of the

analysis showed that approximately 99.87% of the cutting fluid vapor that was produced was

water vapor, thus not representing a health concern.


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Cutting Fluid Mist Behavior

Having developed a model for cutting mist formation, an explanation of behavior will completethe characterization. Once cutting fluid mist has formed, it undergoes three processes: settling,

evaporation, and diffusion. Settling is typically the result of two forces: gravity and resistance to

droplet motion by the surrounding gas. Stokes Law is often used to characterize the settlingvelocity. However, since much of the focus on cutting fluid mist is on particles produced below

10m in diameter, and these particles tend to fall faster than others because of a slip effect at

the surface of the particle, an adjustment must be made to Stokes Law as follows:

cd

s gCd

V

18

2

= , (11)

Where "d is the cutting fluid density of a particular droplet, d is the diameter of a particular

droplet,gis the acceleration due to gravity,

'(

)

*+

,

!"

#

$%

&

+

+=

8

8

106.655.0exp8.0514.2

106.6

1

d

dCc is the empirical slip correction factor, and

%is the small radial position within the fluid film.

Experiments have demonstrated that Eq. 11 extends the range of Stokes law to particles below

0.1m in diameter (which corresponds to a Reynolds number less than 1.0) (Hinds, 1982).

As a droplet is settling in the air, both evaporation and settling mechanisms are occurring

simultaneously and are influencing each other. The size of the droplet decreases due to the

evaporation, which leads to a smaller settling velocity. In turn, the slower relative motion of the

droplet will decrease the evaporation rate. The transient velocity of each droplet can be

calculated by Eqs. 11 and 12, while the transient mass is defined as:

M

G

Hm

Sc

Sh

dt

dm!"

#$%

&=!

"

#$%

&

3, (13)

where Sh is the Sherwood number, ScG is the Schmidt number of a surrounding gas, m is themass of the droplet, andHMis the specific driving potential for mass transfer. Also, the transient

temperature is given by:

( )

( ) m

m

C

L

e

TTNu

dt

dT

L

vG

G

&

!!

"

#$$

%

&+

!

"

#$

%

&=!

"

#$

%

&

1Pr3

1

, (14)

whereNuis the Nusselt number,Pris the Prandtl number, !is the ratio of surrounding gas heat

capacity to that of the liquid phase,Lvis the latent heat of evaporation, CLis the heat capacity of

the liquid, and dtdmm /=& is negative for evaporation. The subscripts on the variables denote

the gas phase away from the droplet surface (G), the vapor phase of the evaporate (v), and the

liquid phase (l). The particle time constant for Stokes flow is Gdd 18/2

= , and !is the non-

dimensional evaporation parameter.


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Cutting fluids are typically mixtures; the fluid composition examined by Yue et al. (2004) and

Sun et al. (2004) was 95% water combined with 5% soluble oil-based cutting fluid concentrate.

Yue et al. (2004) and Sun et al. (2004) established a mist behavior model that considered settlingand evaporation, and used this to predict the change in particle size within a work volume over

time, and also the particulate mass concentration within the volume. The validation data formass concentration, depicted in Fig. 8, shows a good agreement between the predicted behaviorand the actual data.

Fig. 8 Measured and predicted PM10 during settlingand evaporation [Yue et al., 2004; Sun et al., 2004]

Novel Approaches to Controlling Cutting Fluid Mist

In some operations, eliminating or reducing the amount of cutting fluid used is thought to be

practically impossible. In order to minimize health concerns, it may be advantageous to utilizethe knowledge of the operation to employ cutting fluid mist control strategies. The following

section discusses some of these strategies.

Kinematic CoagulationA novel approach for eliminating the cutting fluid mist is to capture the small airborne particles

with larger fluid droplets. The strategy of kinematic coagulation, the capture of smaller particles

by larger collector droplets, was explored by Kinare et al. (2004). These studies observed theeffects of the strategy on the airborne mist produced during a turning process. After a field of

mist particles has been created by the application of a cutting fluid during a turning process, an

atomizer then sprayed collector droplets of a controlled size into the field of motion of the


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process-generated mist particles. The particles from the atomizer collide with the other particles

and coalesce (Fig. 9), which created large particles that rapidly settled.

Fig. 9 Illustration of kinematic coagulation strategy

The behavior of the collector droplets is of particular importance while attempting to reduce thenumber of small particles in the air. When collision between a process generated particle and a

collector droplet occurs, four possible behaviors can result. The first possibility is that the two

particles could bounce off each other. They could also collide and permanently coalesce.Particle disruption could also occur, in which the particles temporarily coalesce, then break apartagain. Particle fragmentation could also possibly occur in the event that one particle rips past the

other and pulls some smaller particles with it. Of course, since it is desired to remove the

process generated mist particles from the breathable airspace, the goal of the kinematiccoagulation work is to identify conditions that promote collision and permanent coalescence.

Previous research observed that permanent coalescence is promoted when the ratio of collidingdroplet diameters was between ten and fifteen (Kinare et al., 2004). Smoluchowski (1917)

provided a relation that allows for estimating the number of droplets that need to be introduced in

order to accomplish the desired kinematic coagulation of the mist particles. A custom designed

atomizer system was developed to produce the desired size (100-200 micrometer) and number ofcollector droplets to interact with those produced by the turning process. To assist with the

atomizer design, a Malvern Series 2600 Particle Sizer was used to measure the droplet size

distribution produced by the atomizer. Kinare et al. (2004) noted that coagulation/coalescencewill only occur to a significant degree for particle concentrations in excess of 10

12/m

3, and that

collisions will occur more frequently as the concentration is increased. Air quality data collected

during a turning operation indicated particle concentrations in the range of 109-10

15/m

3, and this

observation was used to define additional atomizer characteristics.


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Once the atomizer was designed, experiments were performed to assess its efficacy (Kinare et al.,

2004). The addition of the atomizer droplets had a significant impact on the amount of fine

particles present in the air. Data from the Particle Sizer indicated that the atomizer was able toreduce mist levels by approximately 20%. These experiments concluded that of the variables

present within the process, the use of the atomizer was the most relevant to the massconcentration, count mean diameter (CMD), and number of particles. Furthermore, whenatomizer droplets were present, mass concentration depended heavily upon pump pressure and

nozzle geometry of the atomizer system. Most importantly, the work demonstrated the ability to

reduce airborne particulate matter through the application of the kinematic coagulationmechanism.

Innovative Machine Tool DesignIn spite of advances in machining technology, there will undoubtedly remain some processes that

will have to be performed wet in order to achieve the desired production rate and part quality.

For these processes, machine enclosures are widely employed to control air quality by containing

the mist and thereby protecting worker health. However, construction of an enclosure around themachine tool serves to only temporarily contain the machining generated mist. Machining mist

can escape, possibly in high mass concentrations, from openings in the enclosure. Also, opening

the enclosure access door upon completion of a machining process allows accumulatedmachining mist to enter the workers breathing zone. The effectiveness of a machine enclosure

is directly related to its ability to contain the cutting fluid mist produced during a wet machining

process to prevent deterioration of the air quality in the workers environment. Intelligentmachining system design, which considers the movement of machining generated mist

subsequent to its generation, can make machine enclosures more effective.

Air quality in the workplace is affected not only by the amount of mist that is generated during

the machining process, but also by its motion after it is created. While mist is produced in the

vicinity of the machine tool and workpiece, there are several mechanisms that disperse the mist

throughout the workspace. These mechanisms include air currents created by HVAC systems,mist exhaust systems, and/or the motion of the workpiece or the machine tool. In the presence of

a machine enclosure (without a mist exhaust feature) the influence of the HVAC system on mist

motion is eliminated and the movement of the mist inside the enclosure depends primarily on themovement of the machine tool or workpiece and the shape of the enclosed space. This

machining scenario can be numerically simulated to analyze the flow patterns that are produced

and the distribution of the machining generated mist within the machining enclosure.

To investigate the airflow effect on cutting fluid generated mist during a turning process a

computational fluid dynamic (CFD) study was conducted (Hii, 2005). To simulate the

movement of the mist within the machine enclosure an accurate representation of the enclosedspace was constructed, appropriate boundary conditions were applied to simulate the motion of

the workpiece, and the resulting airflow was simulated before cutting fluid droplets, representing

both ambient and machining generated mist, were incorporated into the model. Using this model,parametric studies were performed by varying the workpiece diameter and rotational speed to

investigate the effect on mist motion within the enclosure.


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The CFD model was constructed using Star-CD. The model geometry was a cross sectional

plane of the experimental enclosure around a lathe and was representative of the turning

experiments conducted by Kinare et al. (2004). The computational space is bounded by the

machine enclosure and includes the workpiece, the tool holder, and the chip/drip pan. Since thecutting fluid mist is generated primarily at the cutting zone and in its immediate vicinity, a two-

dimensional model of the machine enclosure was deemed sufficient for this preliminary study.

The first part of the preliminary study consisted of simulating the airflow inside the enclosure for

various combinations of workpiece diameter and rotational speed. The workpiece diameter was

varied between 2 inches and 4 inches and the workpiece rotational speed was varied between 400rpm and 2000 rpm. All simulations were conducted for 4 minutes of process operation with a

constant spindle speed. In all cases it was observed that the velocity field achieved a steady state

condition after approximately 1 minute. There is a rotating air stream around the workpiece anda recirculation zone directly above the workpiece. Outside of these two regions the airflow

velocity remains low.

Once the airflow velocity and pattern were obtained for the experimental enclosure underdifferent workpiece diameters and workpiece rotational speeds, fine droplets were introduced to

study the effect of airflow on the distribution of droplets and to observe any potential interaction.

Fine droplets having a diameter of 1 micrometer were introduced directly below, to the left, overthe top, and diagonally in the left upper quadrant of the model. Droplets representing suspended

ambient mist were assigned initial locations slightly away from the workpiece surface, while

machining process generated droplets were located in close proximity to the workpiece surface.In addition, machining process generated droplets were assigned an initial velocity based on the

workpiece rotational speed and suspended ambient droplets were assigned an initial velocity of

zero.

The air stream around the rotating workpiece effectively entrained the fine droplets that were

within close proximity to the workpiece, as can be seen in Figure 10, which displays some

representative results from the study. Fine droplets that were not entrained by the rotating airstream were carried by the recirculation zone air movement. The degree of entrainment was

related to the workpiece tangential velocity. A larger workpiece diameter or a higher workpiece

rotational speed resulted in a faster rotating air stream around the workpiece, which directlyincreases the level of droplet entrainment. Conversely, a smaller workpiece diameter or a lower

workpiece rotational speed allows more fine droplets to be carried around the model by the

recirculation zone air movement to become more evenly dispersed within the machine enclosure.It should be noted that the presence of fine droplets did not significantly affect the airflow

magnitude and pattern.


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Fig. 10 CFD Model of Airflow and Mist Behavior in a Machine Tool Enclosure

This preliminary study points to a promising and exciting strategy for dealing with machining

mist: design of the enclosure and machine tool elements. A CFD model such as that described

above, can be used to assess the impact of changes in the enclosure geometry, cross-slideposition and geometry, drip/chip pan position and geometry, etc. on air flow and mist behavior.

Airflow inlets and exhaust ports may also be configured to control mist and thus limit the amountof worker exposure.

Wet versus Dry Machining

In previous sections, the technical literature has been reviewed to characterize prior work thathas considered the role of cutting fluids in machining operations. Recent efforts that have

focused on describing the formation and behavior of cutting fluid mist have also been presented.

Evident from the discussions to this point, it may be desirable to greatly reduce or even eliminatecutting fluids from machining operations. Certainly, depending on the tolerances associated withthe desired output of the machining process, different processes are more suitable for reduction

or elimination of cutting fluid. Because of its accessible cutting zone, turning operations have

the potential to be performed dry. Sawing and milling also provide excellent opportunities toeliminate cutting fluid use because their interrupted nature ensures short breaking chips, good

chip clearance, and cooling of the cutting edges (Diniz and Micaroni, 2002). Hole-making

processes like drilling and tapping are often hard to accomplish without some fluid lubricant,


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since chip removal is a key to process efficiency. With these thoughts in mind, some recent

contributions focused on dry versus wet applications are provided below.

Open Faced OperationsAn open faced machining operation is one where the interaction surfaces between the tool and

the workpiece are readily accessible for easy chip removal and cutting fluid application.Examples of open faced operations are milling, sawing, cylinder boring, and turning. Theseoperations are those that tend to utilize cutting fluids primarily for their cooling and heat transfer

capabilities. Transferring the heat produced by a machining operation away from the cutting

zone, tool, and workpiece may be important for at least two reasons: i) thermal distortions canlead to poor machined component accuracy, and ii) elevated temperatures can lead to higher tool

wear rates. Therefore, in considering dry (or nearly dry) versus wet machining scenarios, these

reasons have dominated recent discussions in the literature.

One of the challenges of dry machining is the concern about increased wear rates in the absence

of a cutting fluid (Weinert et al., 2004), and researchers continue to explore techniques to

attenuate these elevated wear rates, one of which is improved cutting tool materials. Cementedcarbides are, by far, the most widely used tool material. As a rule of thumb, as the grain size

becomes finer, the more wear resistant the material becomes (Dreyer et al., 1997). Reducing the

grain size of tungsten-carbide powders below 0.8 m equates to a set of conditions in which evensmall tools can be produced to have good cutting edge stability and dry machining of high

strength materials is plausible (Byrne et al., 2003). Some applications require characteristics

other than those provided by the cemented carbides at elevated temperatures. In these casescermets can be used as cutting materials. Cermets have a higher hot hardness in comparison to

cemented carbides, and thus make it possible to cut at higher speeds. Cermets also have

excellent chemical stability against oxidation and tribochemical wear and a reduced affinity for

diffusion, due to the ceramic component (Porat and Ber, 1990). To withstand machining ofharder materials like gray cast iron and hardened steels, while enduring high temperatures and

providing a longer tool life, ceramic materials are applied. This class of cutting materials is

known for high hot hardness and reduced resistance to thermal shock (making it possible to cutwithout the aid of a coolant) but suffers from a lack of toughness. Turning and milling tests

performed on tools with SiC reinforcements and a very fine aluminum powder by Narutaki et al.

(1997) indicate that the new material permits significantly higher limits on feed rate and lowerwear rates. Like ceramics, cubic boron nitride (CBN) tools are generally used in machining of

harder materials like cast iron and hardened steel because of its excellent high hardness and

chemical wear resistance characteristics (Tnshoff et al., 1995). Polycrystalline diamond (PCD)is the hardest tool material available, and it is often used to machine light metals like aluminum

and magnesium. Despite high strength, low coefficient of friction, and other positive qualities,

its application is limited due to the graphitization phenomenon that occurs above 600C (Byrne

et al., 2003). Tooling characteristics can play a large role in determining if a process should berun in wet or dry conditions.

Tool coatings can reduce the rate of abrasive and adhesive wear by acting as a barrier betweenthe cutting tool substrate and the workpiece material. A wide variety of different coating

materials and coating strategies exist. Multilayer coatings combine the favorable characteristics

of each coating and distribute stress better than monolayer coatings. They also relieve crack


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energy by deflection and branching, thus delaying tool failure. Technological advancements

have recently allowed for the use of coatings on the order of nanometers (Ducros et al., 2003;

Cselle et al., 2003). Since more layers are desirable and the cutting edge radius is dependent on

the total thickness of the coating, it is easy to see the advantages of these very thin coatings. Ahigh performance class of coatings called supernitrides has also been shown to exhibit improved

wear behavior in a set of dry milling tests on 42CrMo4V (Erkens et al., 2003). The use of veryhard coatings like CBN for machining ferrous materials and CVD (chemical vapor deposition)diamond for machining of non-ferrous metal alloys helps to reduce wear given a demanding set

of circumstances.

One of the oft-quoted benefits of a cutting fluid in an open faced machining operation is its

ability to transfer heat. Heat transfer can be of substantial benefit in the reduction of surface

error, a measure of the deviation of the machined surface from that of a surface produced underideal conditions. In particular, a lack of poor machined cylindricity in engine bores can produce

poor performance due to increased oil consumption, frictional loss, and excessive wear of piston

rings. With this in mind, Cozzens et al. (1995) examined the difference between wet and dry

machining of Al 308 and Al 390 die cast cylinders. Temperature along the length of the bore(measured by thermocouple probes positioned along the workpiece axis), forces, and coolant

temperature were measured for each test. In the presence of a cutting fluid, the temperature

initially spiked at each position, but at a significantly smaller magnitude than during dry cutting.The peak and average temperature was reduced by as much as 50% when comparing the wet and

dry machining tests.

In another study of boring 308 die cast aluminum cylinders on a vertical milling machine, Zheng

et al. (2000) studied the effects of cutting fluid use via inverse heat transfer and finite element

methods to determine estimates of the effective convection coefficient that the fluid provided.The purpose of the work was to determine the effect that cutting fluid presence had on

temperature and surface error produced by the process. The study also successfully developed

an analytical model to predict temperature distribution, and observations regarding performance

measures of the process were made. Fig. 11 depicts the two dimensional governing equation ofthe heat transfer in a cylindrical bore. The equation, which considers a longitudinally moving

heat source and heat losses through the inside and outside of the bore walls, is

t

w

k

tzgH

dz

dw

=+

),(2

2

2

, (15)

where, !is the difference between the wall and ambient temperatures, "is the thermal diffusivity

of the material, k is the thermal conductivity, H is the ratio of heat convection coefficient to

thermal conductivity (h/k), and g(z,t) is the heat source strength. Equation 15 is subject to thefollowing boundary and initial conditions:

0=+

H

zatz= 0, andLrespectively

!= 0 at t= 0, for all z. (16)


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where, the heat source strength at cutting tool position zs in the boring operation may be

estimated by:

)(),( sss zzgtzg = . (17)

Fig. 11 Two dimensional heat transfer in a cylinder bore

The term zsmay be further expressed in relation to feed and the initial position of the tool. The

proposed model was consistent with the measured data. Furthermore, it was clear that thethermal expansion, rather than the elastic deflection from the cutting forces, was the dominant

factor in influencing surface error. Also, the peak magnitude of the surface error was smaller in

the presence of a cutting fluid. The introduction of a cutting fluid reduced the surface error fromthat produced in dry boring by approximately one half.

In an effort to better understand the role of cutting fluids as coolants, Shen et al. (2001)

developed an analytical model for predicting the workpiece temperature in peripheral milling.First, a set of dry milling tests were performed. Assuming heat transfer by means of natural

convection during dry milling and using an inverse heat transfer method, the heat source strength

was calculated. Next, using the cutting power data, the fraction of heat transferred to theworkpiece in the operation could be estimated. Then, the heat fraction for the set of wet milling

tests is assumed to be the same as the counterpart in the dry tests. This allows for the heat source

strength to be estimated again, and now using the cutting power and the heat fraction data, theconvection coefficient of the cutting fluid can be estimated based on the temperature data. Table

1 depicts some of the quantitative estimates of the cutting fluid convection coefficients.


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Table 1 Estimated convection coefficients in peripheral

milling (shaded tests employed a cutting fluid)

These investigations yielded some important conclusions. Increases in the speed, feed, and depth

of cut yield an increase in the heat source strength. Heat transferred to the workpiece alsoincreases. Furthermore, the estimated convection coefficient for wet milling was approximately

three orders of magnitude larger than that during dry milling, indicating the ability of the fluid to

function as a coolant during this process.

Closed Faced OperationsClosed face operations are those that have a fairly inaccessible cutting zone and interactionsurfaces that are not easily accessible for chip removal and cutting fluid application. Examples

of closed face operations are drilling, tapping, and reaming. These are the operations that

primarily utilize cutting fluids for their lubricating properties. Studies on both drilling and

tapping indicate that the presence of a cutting fluid impacts the machining forces or spindletorque required for these operations. These studies also established that the presence of a cutting

fluid significantly improves surface finish.

Haan et al., (1997) performed a series of eight test plans to measure a group of drilling outputs,

including torque and thrust force, as well as performance outputs like surface finish and hole

quality, to determine the most significant effects given different configurations. Standardfactorial design methods were employed, and normal probability plots were used to determine

statistically significant effects. Presence of a cutting fluid did affect the torque data. In all but

one test plan, the average torque was reduced when a cutting fluid was present. Two key pointsshould be noted. First, feed was consistently found to be a more significant variable than the

presence of a cutting fluid. Secondly, no significant effect was found when comparing results

between the use of 2% and 8% concentration cutting fluid (water soluble oil). An even more

significant observation in the drilling experiments was the effect the cutting fluid had on surface


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finish and hole quality results. By inspection, it was quite clear that the hole created without the

aid of a cutting fluid had a significantly larger average surface finish, as well as a significantly

larger variation.

Cao and Sutherland (2002) found that similar improvements were made in the tapping process

when cutting fluid was present. Initial tests found that there was considerable inconsistency intapping torques and axial forces even under the same machining conditions. The torque data wasfound to consist of two parts. The first part is the base load which constituted the contribution

due to chip formation and overall tool/workpiece friction. They established a model for the

process that considered this effect, with the friction behavior dependent on measured fluid-basedproperties. The second part is termed the chip packing load, and is due to the chips clogging the

flutes of the tap and causing excessive torque.

From Fig. 12, it appears that the model fairly accurately predicts the base load. Since the chip

clogging phenomenon was not taken into account in this model, the random loading spikes are

not predicted by the model. While a tapping oil provided a noticeable reduction in torque and

axial force, soluble and straight oils either provided no benefit to the operation or actuallyincreased the torque and force.

Fig. 12 Modeled and measured tapping loads due to

different lubrication conditions

The tapping experiments revealed that the use of cutting fluid can reduce the friction in the

process (assuming the appropriate fluid is selected). The coefficient of friction significantly

changes in the presence of a cutting fluid; friction was found to be approximately four timeslarger for dry cutting as opposed to wet cutting. The experiments also showed that chip packing

load was a significant factor in terms of such measures as thread quality and tap breakage.


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In a study of cutting fluid conditions in the boring of die cast aluminum alloy tubes, the presence

of a fluid, regardless of concentration, did not significantly affect the machining forces (Cozzens

et al., 1999). Furthermore, normal probability plots showed that the cutting fluid had no effecton built up edge (BUE) or the surface finish of the machined part. This study further lends

support to the assumption that the use of a cutting fluid only plays a significant role in machining

operations where the primary need is lubricity.

In the absence of a cutting fluid to provide lubrication in closed face operations, other methods

must be explored to provide this function. One promising dry machining enabling technology is

advanced cutting tool coatings. As noted previously, coatings can retard the process of tool wear;but, they can also be employed to enhance tribological behavior. Some soft coatings are

considered self-lubricating, which reduce the friction between the tool and the workpiece and

therefore reduce the cutting forces and heat generated (Byrne et al., 2003; Derflinger et al., 1999).

Novel Approaches to Reduction of Cutting Fluid Use

As discussed, eliminating or reducing the amount of cutting fluid in some processes could be

done without compromising performance measures. In these cases, operations can be performedto satisfaction in a more economically feasible and environmentally responsible manner. The

following section discusses some of these strategies.

Minimum Quantity LubricationMinimum quantity lubrication (MQL) is a strategy that can offer technological and economic

advantages over traditional fluid applications (Weinert et al., 2004; Klocke et al., 1996). As the

name implies, MQL seeks to reduce the amount of cutting fluid used in an operation. In terms oftechnological advancement, MQL is to dry machining as a hybrid car is to vehicle powertrains -

a step in the right direction. An MQL process can be performed with or without a transportmedium, such as air, and a pump supplies the tool with the fluid - generally straight oil - as arapid succession of precisely metered droplets. Quantitatively, MQL is associated with the use

of between 10 and 50 mL of cutting fluid per machine hour. Emulsions and water are usually

only used when it is essential to cool the tool more efficiently than is possible with straight oil.In contrast to the lubricating function, minimum quantity cooling (MQC) has been largely

unexplored, but offers promise in some situations (Klocke et al., 2003).


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Fig. 13 Various MQL Systems [Weinert et al., 2004]

MQL ApplicationThe method by which a fluid is added to the machining system, especially under minimum

quantity lubrication conditions, can greatly affect the efficiency with which cutting fluid

functions are performed. As illustrated in Fig. 13, the fluid can be applied in two manners:

externally, through the use of separately secured nozzles, and internally, through channels builtinto the tool. Each application method has advantages and disadvantages (Klocke et al., 2003;

Karino, 2002). A short discussion will highlight key features of both.

The external supply method is used in sawing, end and face milling, and turning operations. Inthe drilling, reaming, and tapping operations, this method is only appropriate when the ratio of

l/d is less than three. If the operation parameters exceed that limit, then the tool may have to bewithdrawn several times to be wetted again. Problems also exist in external supply when more

than one tool must be used (Suzuki, 2002).

Internal supply is advantageous in the operations of drilling, reaming, and tapping, where l/d

ratios tend to be large. This ensures that the cutting fluid is constantly available close to the

cutting edge, and eliminates the concerns regarding nozzle positioning errors and geometric

clearance issues inherent to supply pipes and nozzles. There are one- and two-channel supplysystems available. In the two-channel system air and oil are fed separately through the spindle,

and then are combined to form an aerosol just ahead of the cutting edge. Each supply system haslimits on the amount of fluid it can supply to the cutting zone; system selection will dependheavily on process needs.

MQL Fluid TypeCutting fluid selection is also a major consideration when evaluating the entire machining system

(Weinert et al., 2004; Suda et al., 2002). Typically fluids are selected based on their ability to

influence performance, as reported in some of the previously mentioned studies. Due to low


26/32

consumption rates in MQL operations, secondary characteristics such as biodegradability,

oxidation stability, and storage stability are more important because of environmental

compatibility and chemical stability concerns.

Environmental compatibility is most heavily dependent upon biodegradability (McCabe and

Ostaraff, 2001). Because of their advantageous biodegradability characteristics, vegetable oilshave typically been used in MQL machining, while synthetic and polyol esters are starting to beconsidered more frequently. Storage is also a major consideration. Lubricant used in an MQL

system must remain stable for long periods of time and under high temperatures.

MQL fluids fall into two primary groups: synthetic esters and fatty alcohols. Synthetic esters

(generally vegetable oils) are commonly used because of their good lubrication properties,

resistance to corrosion, high flash, and boiling points. However, fatty alcohols do achieve betterheat removal and when vaporized, produce little in terms of residue as compared to synthetic

esters. Synthetic esters generally are used in operations where lubrication is the primary need for

a cutting fluid, whereas fatty alcohols are used in MQL applications that require the cutting fluid

for heat removal (N.N., 1996; Suda et al., 2001).

MQL Performance

A number of MQL performance studies have begun to appear in the literature, and Weinert et al.(2004) discuss a number of these. Peripheral milling tests were recently performed to examine

the effects of fluid application strategy (dry, MQL, and fluid flood), axial depth of cut, flow rate,

and air pressure (Ju et al., 2005). The measured responses from these experiments were cuttingforce, workpiece temperature, machined surface error, and air quality. A synthetic fluid, mixed

at a 5% concentration with water, was used in the tests in which cutting fluid was applied. The

experiments in which the MQL fluid application was employed used an external applicationsystem in which the fluid and air were mixed in the process before being projected through the

nozzle and to the cutting zone.

The work of Ju et al. (2005) concluded that while MQL application was not as successful asflood application in reducing the workpiece temperature, it did provide a sizeable improvement

over dry machining. On the other hand, the measured forces were nearly identical across the

three fluid strategies: flood, MQL, and dry. Not surprisingly, increases in fluid flow rate andair pressure were found to reduce temperature and improve surface finish.

Modulated DrillingAnother machining application that has seen an increased concentration of effort for

environmentally responsible manufacturing is drilling. As discussed earlier, drilling is one of the

few machining operations that require a cutting fluid mainly for its lubricating properties. The

fluid is required in typical drilling operations to provide lubricity between the chip and the tool.Chip clogging is often the cause of drilling process failure, and given the nature of drill flutes

and the continuous behavior of chips in this process, dry drilling has some significant obstacles

to overcome.

Some success has been reported with an innovative dry drilling approach (Ackroyd et al., 1998;

Toews et al., 1998). The work employed low frequency axial modulations of the spindle


27/32

produced by a linear motor. This movement interrupted the built-up edge (BUE) formation in an

attempt to achieve dry drilling. This method is credited with the ability to control the chip

shape/size and produce chips that can be easily channeled out of the drill flute. These smallerchips may pass through the flutes much easier than the long chips that typically cause adhesion

and clogging. McCabe (2002) reported on work to employ a similar method using high

frequency modulations instead of low frequency modulations. The limited success of this effortmay have been due to the inability of the piezo-based actuator to produce high enough oscillationamplitudes.

The work of Filipovic and Sutherland (2005) concentrated on addressing the limitations thatwere reported by Toews et al. (1998) and McCabe (2002). A new magnetostrictive-actuator and

tool holder assembly was used, as shown in Fig. 14, to initiate high frequency modulations

during the drilling process. As the magnetic field is rapidly adjusted, the magnetostrictive driverod elastically deforms to produce oscillations. This sinusoidal motion of the tool results in the

production of discrete chips, especially for oscillation frequencies in the range of 25-50 Hz.

Fig. 14 Modulated drilling tool holder assembly and setup

To verify the improved performance characteristics of the process due to the inclusion of the new

assembly, a 24-1

factorial experiment was conducted. Four variables of interest were examined:

spindle speed, feed, frequency ratio (oscillation frequency/spindle rotation frequency), and

workpiece material. As well as the beneficial effects observed due to modulation of chip size,this experiment clearly demonstrated that the axial drill oscillation can reduce the process torque

and thrust force. The initial success of tool holder/actuator assembly in providing axial drill

modulations appears to offer promise for the dry drilling of aluminum.


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Summary and Conclusions

An extensive examination of the functions of cutting fluids and interactions with othermachining system components has led to a general understanding of the roles that cutting fluids

play in a metal cutting process. A model for the formation and behavior of cutting fluid mist has

been presented. Health issues associated with exposure to cutting fluid mist have been discussed.Comparisons between wet and dry machining for several machining operations have been made

using both analytical and experimental investigations, and findings have been presented to

suggest that intelligent strategies may allow for the reduction or even the elimination of cuttingfluids from certain machining processes. Novel approaches have been identified that can be

utilized to eliminate or greatly reduce the amount of fluid that is needed for a machining

operation. Novel approaches have also been proposed to eliminate or control cutting fluid mistfor those situations in which, for the short term, a cutting fluid is deemed to be a necessary

process requirement.

Acknowledgements

This work was supported, in part, by the Ford Motor Company, Chrysan Industries, UNIST Inc.,the NSF-ARPA Machine Tool Agile Manufacturing Research Institute (MT-AMRI), and the

National Science Foundation under Grant Nos. DMI-9502109, DMI-9628984, and DMI-

0070088. The authors gratefully acknowledge the research contributions of Steven Batzer,Tengyun Cao, Aleks Filipovic, Kenneth Gunter, Deborah Haan, Chuanxi Ju, Lucas Keranen, Sid

Kinare, Walter Olson, Ge Shen, Jichao Sun, Yan Yue, and Yuliu Zheng in support of this effort.

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