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Investigation into the Formation Mechanism and Distribution Characteristics of Suspended Microparticles in MQL Grinding Dongzhou Jia, Changhe Li*, Dongkun Zhang, Sheng Wang and Yali Hou School of Mechanical Engineering, Qingdao Technological University, 266033 Qingdao, P.R. China Received: October 31, 2013; Accepted: January 1, 2014; Revised: January 2, 2014
Abstract: A large number of patents have recently been devoted to the development of minimum quantity lubrication (MQL) grinding techniques that can significantly improve both grinding fluid alternatives in terms of environmental consciousness, energy saving, cost effectiveness, and sustainability. Among these patents, one is for a supply system for MQL grinding that turns the lubricant into pulse drops with fixed pressure, constant pulse frequency, and the same drop diameter. The drops are produced and injected into the grinding zone in the form of jet flow under high-pressure gas and air seal. With the growing demands of our environment, MQL has become widely used in the grinding and cutting machining processes. Based on the dangers of suspended air microparticles from machining to health and the environment, the sources, classification, and characteristics of suspended air microparticles were analyzed in this research. Taking account the features of microparticles, we explored the dangers posed by suspended microparticles to health and identified the common diseases attributed to these microparticles. With the use of the machining process, we analyzed the formation mechanism of suspended air microparticles and conducted research on the distribution characteristics of suspended air microparticles formed from the atomizing nozzle during MQL grinding. The relationship between jet parameters and microparticle size of sprays was explored. The results showed that atomized microparticle size has a linear relationship with nozzle caliber and gas-liquid flow ratio. Atomized microparticle size is directly proportional to the nozzle caliber but inversely proportional to gas-liquid flow ratio. We also analyzed the distribution rules of spray droplets and determined the amount of spray microparticles and volume distribution function.
Keywords: Grinding, jet parameters, minimal quantity lubrication (MQL), spray droplets, suspended microparticles. INTRODUCTION Traditional flood lubrication consumes a large amount of cutting fluids. In the U.S., the annual cutting fluid consumption is 100 million gallons, which is equivalent to 71 billion Yuan. In Japan, the annual cost of cutting fluid is 42 billion yen. In 1994, the German processing industry consumed 75,491 tons of cutting fluids, of which 28,415 tons are watersoluble cutting fluids [1-5]. The cost of cutting fluid accounts for 16% of total production costs. However, this percentage reaches 20% to 30% for some materials that are difficult to process. This percentage is significantly higher than the cost of tools, which only account for 2% to 4% of total production costs. The costs of cutting fluid not only refer to manufacturing costs, but also include routine maintenance, pretreatment, and handling costs. Although cutting fluids are discharged after treatment, they remain harmful to the environment, particularly to soil and water resources. Mineral oil is a major component of cutting fluid. The extremely poor biodegradability of mineral oil makes it difficult to degrade and enables it to remain for the long term once it flows in soils or lakes. According to the U.S. Environmental Protection Agency, oil has acute lethal toxicity to aquatic *Address correspondence to this author at the School of Mechanical Engineering, Qingdao Technological University, 266033 Qingdao, P.R. China; Tel: +86-532-85071757; Fax: +86-532-85071286; E-mail:
[email protected] 1874-477X/14 $100.00+.00
organisms, as well as long-term sub-acute lethal toxicity. When the oil content exceeds 10ppm in water, death of marine organisms may occur. When the content exceeds 30ppm, death of freshwater fishes may occur. Although waste fluid is discharged after rigorous mineral oil recycling, its long-term accumulation in water cannot be ignored. Additives in cutting fluids also generate environmental pollutions in various aspects. For example, extreme pressure agents are pollutants in seawater. Meanwhile, phosphate in antirust agents results in the eutrophication of lakes, rivers, and oceans, resulting in algae and red tides [6-11]. Using cutting fluid harms not only the environment, but also the workers in plants. During the machining process, high pressure and high temperature cause the vaporization and atomization of cutting fluids to form mists. The atomized microparticles of cutting fluid drift to the air and can be easily inhaled by workers, thus resulting in different types of lung diseases, including mild respiratory diseases, asthma, and even cancers. The influence of cutting fluid aerosol on the health of workers might extend from lung diseases to risks of esophageal, stomach, pancreatic, prostate, colon, and rectal cancers. To meet the requirements of environmental protection and to guarantee the health of workers, scholars have conducted a large number of exploratory studies on reducing and even eliminating the application of cutting fluid. Some © 2014 Bentham Science Publishers
Minimum Quantity Lubrication (MQL) Grinding
Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1
scholars probed into dry cutting, finding that this process can achieve satisfactory results under specific processing conditions. Under most processing conditions, sound lubrication and cooling of the cutting area cannot be accomplished, resulting in the increase in cutting force and temperature in cutting areas. Therefore, the life service of tools and workpiece surface quality are reduced, sometimes even causing workpiece surface burns (especially during grinding). With this academic background, Baheti [12] proposed Minimal Quantity Lubrication (MQL) technology. MQL refers to the minimum quantity of lubricants that enters the hightemperature grinding zone after being mixed in highpressure gas and atomized with high-pressure draft (4.0 bar to 6.5 bar). The traditional flood cooling feed liquid of grinding fluid is 60L/h for a unit of the grinding wheel width, whereas the consumption of MQL grinding fluid is 30mL/h to 100mL/h for a unit of the grinding wheel width [13-18]. The high-pressure draft contributes to cooling and chip removal. Lubricants attach onto the finished surface of a workpiece, forming a layer of protective film and serving as lubrication. This technology integrates the advantages of flood cooling grinding and dry grinding, presenting lubrication effects similar to those of traditional flood cooling grinding. Lubricants adopt vegetable oil as the alkyl ester of base oil, which is characterized by excellent biodegradability, good lubricating properties, high viscosity index, low volatility, recyclability, short production cycle, and insignificant environmental diffusion. Although MQL grinding achieves satisfactory results, the atomizing nozzle will produce a large amount of suspended air particles. Without efficient control, these particles will harm workers and the environment. Based on these considerations, we should further explore the sizes and component characteristics of suspended microparticles in air. CATEGORIZATION AND PENDED AIR PARTICLES
FEATURES
OF
SUS-
American National Standards Institute defined Dust, Fumes and Mists in Industrial Health Engineering published in 1947. Aerosol Technology by Hinds defined aerosols of other types, such as smoke and sprays. Table 1 describes the causes and typical sizes of aerosols [2-6]. The most commonly found aerosols in industrial environments are defined as follows: Dusts refer to solid microparticles generated by the grinding of organic or inorganic substances, such as rocks, Table 1.
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metals, and grains. Dusts cannot easily attach to other materials without static electric force. Gravity and its influence make dusts unlikely to spread in air. Fumes are solid microparticles formed by the agglomeration of volatilized molten metal, which often presents chemical reactions, such as oxidation reaction. Microparticles in fumes usually form flocs. Mists are microparticles of suspended droplets formed by the spraying, bubbling, and splashing of liquids or formed in the agglomeration from gas to liquid. Smokes are aerosols attributed to incomplete combustion. Microparticles may be liquid or solid [7-11]. Sprays are formed by the mechanical crushing of liquids. Homogeneous aerosol refers to the aerosol formed by the same chemical compositions. Monodisperse aerosol contains microparticles with basically the same size. Polydisperse aerosol contains microparticles with different sizes, the sizes of which should be characterized by using statistical methods. Microparticle size is essential to determining the behavioral and toxicological characteristics of aerosol. Microparticle size determines the kinematics of aerosols in air. Microparticle size refers to the diameter of particles, and the aerodynamic diameter da is used to describe the effective diameter of irregular (non-spherical) particles. da is defined as follows: the diameter of a sphere with the unit density of 1g/cm moving with a low Reynolds number in still air that has the same precipitation velocity as actual microparticles. Precipitation velocity refers to the velocity when the air buoyancy (resistance) of microparticles is equal to the gravity of microparticles. Table 2 shows the precipitation time of microparticles with different aerodynamic diameters. In some cases, Stokes diameter is used to define the sphere diameter that has the same density and precipitation velocity as actual microparticles. With reduced microparticle size, gravity is significantly reduced when the dynamics behavior of microparticles is mainly diffusion. In this case, the liquidity equivalent diameter is often used. The U.S. Environmental Protection Agency has classified the microparticle sizes, as shown in Table 3. The component characteristics of suspended air microparticles during machining are diversified. The compositions of suspended microparticles are mainly determined by the compositions of raw materials. If suspended microparticles of aerosol formed during processing undergo chemical reactions, such as oxidation reaction, aerosol becomes known as secondary aerosol. Once secondary aerosol is produced, composition analysis will become complicated. Devices used to identify the composition of microparticles in-
Causes and Size Range of Microparticles. Name
Cause
Size Range (m)
Dust
Mechanical disruption
>2
Fumes
Condensation
10
Large microparticles
2.5 < da < 10
Fine microparticles
0.1 < da < 2.5
Super-fine microparticles
< 0.1
Relationship Between Microparticle Size and its Sedimentary Location in the Human Body. Microparticle Size (m)
Sedimentary Location in Human Body
> 10
Nasal cavity
5-10
Nasopharynx
2-5
Mucociliary clearance in air tube and bronchia areas
0.01-2
Lungs and pulmonary alveoli
< 0.01
Expiratory clearance
clude X-ray fluorescence spectrometry (XRF), protoninduced X-ray emission spectroscopy (PIXE), X-ray diffraction analyzer (XRD), optical emission spectrometer, and mass spectrometer. Suspended particle sizes during machining also vary. Different microparticle sizes cause the variance of precipitation locations in the human body, thus imposing different degrees of influences on human health, as shown in Table 4. Therefore, to obtain specific information on size, sampling detection of aerosol and counting statistics were conducted. According to the statistics data, a columnar diagram can be established to present the amount of microparticles and microparticle size. The same method can be used to describe the relationship between quality distribution and microparticle size. The distribution parameters of microparticle size can be further identified [12-17]. These parameters include the mean, median, and mode. Mean is obtained based on the sum of microparticle sizes divided by the amount of microparticles. Median refers to the value in the middle of microparticle sizes in ascending order. Mode is the value the most frequently appeared among all microparticle sizes. According to reports, inflammatory activities of lowtoxicity particles are related to large surface area, indicating that microparticle size, followed by chemical composition, is
the most influential factor in the toxicological response of microparticles. Scholars have explored the size and composition of suspended microparticles. Bayvel and Orzechowski analyzed the sizes of suspended particles after the condensation following the atomization of cutting fluid. Park [3] studied the 3D morphology of oil mist microparticles during grinding and obtained the mist volume microwave conversion. Liu and Li [911] conducted experiments and analyzed the atomization characteristics of cutting fluid under the condition of MQL, acquiring size distribution with different parameters. Hou and Yu sampled and tested mists of cutting fluid, as well as analyzed the composition of mist. However, under specific grinding conditions, the amount of drifting microparticles, the composition of drifting microparticles, the drifting tracks, and the concentration of suspended microparticles in the machining environment after drifting were unexplored. EFFECT OF SUSPENDED AIR PARTICLES ON HEALTH Over the past few decades, based on the advantages and disadvantages of the coolant in machining, many studies
Minimum Quantity Lubrication (MQL) Grinding
have been conducted on coolant. Coolant is used for the purposes of cooling, lubrication, corrosion, and debris cleaning. The use of coolant can improve the service life of tools, the quality of the workpiece, and the effectiveness of cleaning debris, thus improving the processing conditions significantly. However, a large amount of cooling liquid will produce considerable waste streams. To avoid the contamination of rivers, lakes, and lands because of the use and disposal of coolant, cooling pretreatment and processing are often needed, which will increase the costs of cooling liquid waste. Part of coolant attaches to debris and the workpiece and inevitably influences the environment, even with rationally disposed cooling liquid waste. Notably, the costs of cooling pretreatment and processing are higher than those for manufacturing, and treatment is not always effective, thus causing water pollution. Waste streams of the coolant will not only result in environmental problems but also give rise to health and safety issues. According to the U.S. Bureau of Labor, in 2004, the employment rate of the manufacturing industry was less than 14%, causing a rate of non-fatal occupational diseases of 42% [19-23]. These occupational ailments include lung diseases, which can be attributed to unacceptable operating ambient air quality. Concentrations of suspended microparticles in the operating ambient air that exceed the standard will seriously endanger the respiratory health of workers. According to related statistics, in 2000, the deposits of suspended air microparticles in the respiratory system and the human body caused 2960 deaths. Approximately 20% to 30%, adult patients experienced asthma attacks because of overproof suspended microparticles in the working environment. The fourth largest cause of death in the U.S. is chronic obstructive pulmonary disease, 15% of which is related to the overproof suspended microparticles in the working environment [12-17]. These data demonstrate the harm caused by suspended microparticles in the working air of the manufacturing industry to worker health. The human respiratory system absorbs air from the atmosphere, and the air flows through the nose, throat, epiglottis, trachea, bronchi, and bronchium and then enters the alveoli. Gas exchange occurs in the alveoli. In the gas exchange process, oxygen enters the blood system, while carbon dioxide enters the alveoli. Through respiration, carbon dioxide is excreted. Suspended microparticles enter the human body and are deposited in different locations of the respiratory system. Microparticle size is the major factor affecting precipitation locations. According to the different precipitation locations of microparticles in the respiratory system, suspended microparticles can be divided into three categories. (1) Respirable part: all suspended air microparticles available during inhalation through the nose and mouth (2) Part that enters the thoracic cavity: all suspended air microparticles available during inhalation through throat (3) Part that enters the respiratory region: suspended air microparticles available during inhalation and reaching the gas exchange region of lungs The sizes of suspended air microparticles of these three parts size are 100, 10, and 4m, respectively. Notably, sus-
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pended air particles matters (PM) have different chemical compositions and sizes [1-4]. As shown in Table 1, suspended air microparticle size influences the penetration degree into the respiratory system and penetration locations. Human lung function has evolved over thousands of years in the natural environment with suspended microparticles. Clearing mechanisms of suspended microparticles in the bronchi and alveoli are useful for clearing excessive residues of microparticles. However, overexposure to an overproof environment of suspended microparticles will exceed the capability of microparticle clearing mechanisms. When the human body inhales more suspended microparticles than the clearing mechanism capability, suspended microparticles will be deposited in the human body and exerting negative biological effects. The deposited microparticles will influence the first organ they come in contact with and even other organs. The effects of suspended microparticles include allergies, irritation, and carcinogenic effects, which may result in pneumonia, respiratory diseases, and lung tumors. The effect of suspended microparticles on health is subject to different factors, including the duration and frequency of exposure, workers’ own body sensitivity, and types of inhaled chemical substances of aerosol. Under short-term or long-term exposure, symptoms may be observed. According to the toxicity of inhaled substances, short-term exposure may cause acute poisoning, whereas long-term or frequent exposure may result in chronic poisoning. The effects of short-term exposure on health include irritation of the nose, chest, and eye, bronchitis, cough, edema, pneumonia, nausea, and vomiting. Diseases from long-term exposure include asthma, emphysema, silicosis, lung cancer, laryngeal cancer, and urinary tract cancer. Since its foundation, the U.S. Environmental Protection Agency began to build standards related to the effect of suspended air microparticles on human health and the environment. In 1987, the Environmental Protection Agency established the microparticle size standard, which required that the size of discharged microparticles should not be smaller than 10m. In 1997, the Agency presented a criterion, indicating that the size of discharged microparticles should not be smaller than 2.5m. Microparticles with size smaller than 2.5m can reach the gas exchange area of the lungs through the respiratory system and even enter other organs, such as the olfactory nerve center. The China National Ministry of Environmental Protection announced on January 16, 2012 that PM2.5 and ozone (8h concentration) were included into the routine air quality assessment. The standard limits of PM10 and nitric oxide were also elevated. Some scholars verified that the indoor PM2.5 concentration of households was over five times that of PM2.5 in the atmospheric environment. The PM2.5 concentration of workshops was over 30 time that of households [24-27]. Working while exposed to such a concentration cause workers to face serious health problems. Moreover, cutting fluids are an ideal environment for the growth of bacteria and fungi. Studies show that when cutting fluid pH exceeds 10, some extremophiles can still survive and grow in extreme environments. Bacteria in cutting fluid will split the emulgator and reduce the lubricating capability of the fluids. Bacteria can also alter the pH value of cutting fluid, which increases the risks of tool and workpiece corro-
56 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1
sion. Moreover, cutting fluid with bacteria and fungi is risky for staff in the operating environment. To control the growth of bacteria in the cutting fluid, various types of fungicides, wetting agents, and sterilization agents are used in the industry. However, even when fungicides are used, some bacteria, such as aeruginosa, can still survive in the cutting fluid. To control the growth of bacteria in the cutting fluid, chemical additives are often used. Chemical additives are extremely harmful to the health of workers and the environment. If cutting fluid contains chemical additives, its own natural degradation capacity will be weakened. Some countries have banned the discharge of wastes with fungicides in the sewage system. Spinosad and fungicides can maintain the function of cutting fluid but also place the health of workers in danger. Fungicides used in many factories will release methanol, a potential carcinogen. According to the International Agency for Research on Cancer, mineral oils used in metalworking are carcinogenic substances. Exposure to air full of suspended mineral oil microparticles will result in occupational skin cancer. Even fungicides that do not release methanol are dangerous to human health and are highly corrosive to skin. In addition to fungicides, cutting fluid contains many other chemical substances, which are also very harmful to health and the environment. Extreme-pressure (EP) cutting fluids contain inactive sulfur and chlorine compound additives, which will react with the metal surface and form a protective film with low friction on the smooth surface. Although these chemical substances reduce the friction between tools and the workpiece surface, they are harmful to the health of workers and the environment [28-35]. During machining, chlorcosane in EP cutting fluid will produce dioxin under the condition of heating and pressure. Dioxin is known to be an extremely harmful substance. SUSPENDED AIR MICROPARTICLES PRODUCED DURING MQL GRINDING As previously mentioned above, to reduce processing costs and the negative effects of coolant on workers and the environment, scholars have proposed dry cutting technology. However, the insufficient processing capacity of cooling and lubrication cause dry cutting technology to reduce the service life of tools and surface quality of workpiece significantly, even resulting in workpiece surface burns. This condition is particularly notable during the grinding process,
Fig. (1). MQL jetting diagram.
Jia et al.
when temperatures in the grinding zone are high because of the substantial amount of specific energy consumed by material removal. To address this problem, scholars have proposed MQL technology, which ensures good lubrication performance and reduces the use of cutting fluid. However, the cooling performance of MQL is unsatisfactory. According to the theory of heat transfer enhancement, scholars proposed the nanoparticle jet MQL, which mixes minimal quantities of lubricant, solid nanoparticles, and compressed air to form three-phase flow by using an atomizing nozzle. The threephase flow is then jetted to the cutting area in a vaporous state, as shown in Fig. (1). Shen onducted experiments and analyses on the cooling performance of nanoparticle jet MQL, highlighting that although nanoparticles have excellent heat transfer property, heat convection and boiling heat transfer properties serve a more important function during nanoparticle jet MQL grinding. The three-phase flow is then jetted to the cutting area in a vaporous state, such that different nozzle structures will produce different microparticle sizes in the jetting process. When the cutting fluid is sprayed into the cutting area, it will intensely collide with the workpiece or tools in highspeed rotation or evaporate at high temperatures after reaching the cutting area. These conditions result in the complex formation of ultimate suspended microparticles, during which mechanical, physical, and chemical factors intertwine and react. In traditional pouring lubrication, two mechanisms occur to form suspended microparticles. One is that the high temperature in the cutting area gasifies the cutting fluid, which is condensed and forms suspended microparticles in air. The other is that the intense collision between the cutting fluid and the workpiece or tools during high-speed rotation facilitates the atomization of cutting fluid into smaller suspended microparticles. Chen et al. provided a formation model of suspended microparticles in traditional pouring lubrication and used a rotating disk for the circumference atomization of the model. In the model, drop formation was divided into three stages, namely, liquid film formation stage, liquid strip formation stage, and drop formation stage [35-40]. They presented the computation equations of microparticle size after atomization. However, the deficiency of cutting fluids during dry grinding causes a large amount of solid dust to be produced during the grinding of metal materials. These dusts drift in the air and form suspended microparticles. In nanoparticle jet MQL grinding, the formation of suspended microparticles is deter-
Minimum Quantity Lubrication (MQL) Grinding
Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1
57
condensation
gasification
suspended particles the gas barrier layer
atomization floating
grinding wheel
feed direction
workpiece ᐕԦ
Fig. (2). Generation of suspended microparticles in grinding.
d b
c
a Fig. (3). Experimental setup. (a) CNC grinding machine, (b) Conventional flood grinding, (c) MQL grinding, (d) MQL supply equipment.
mined by three mechanisms: atomization, evaporation, and drifting, as shown in Fig. (2) [28]. The atomization mechanism transfers mechanical energy to the surface energy of drop. The three-phase flow jet intensely collides with the grinding wheel during high-speed rotation, such that droplets in the three-phase flow are further fragmented into smaller droplets. During evaporation, the grinding zone produces abundant heat, and grinding fluid evaporates in air at high temperatures. The evaporated fluid is then condensed into smaller droplets. During drifting, the small drop diameter jetted by the atomizing nozzle and the existence of nanoparticles in compressed air cause some microparticles to be blown in air to form suspended microparticles. Microparticle sizes under three mechanisms are very small, such that they tend to drift and stay in air for a long time. These suspended microparticles form harmful aerosols, endangering the health of workers. The formation mechanism of suspended microparticles indicates that unlike traditional pouring lubrication, nanoparticle jet MQL will produce a portion of drifting suspended microparticles. In sum, when using MQL for metal cutting, three main channels produce suspended microparticles: 1) oil mist and
refined nanoparticles jetted by the atomizing nozzle drift in air to form suspended microparticles; 2) the intense collision between the three-phase fluid and the workpiece or tools in high-speed rotation cause the cutting fluid to be further atomized to form smaller microparticles, which drift and stay in air to form suspended microparticles; and 3) the heat in the cutting area (especially in the grinding zone) causes the smoke from evaporation or burning to be gasified and to drift in air to form suspended microparticles. These suspended microparticles stay in air for a long time, causing the suspended microparticle content in air to exceed the standard significantly, which is extremely harmful to workers. These properties of suspended nanoparticles seriously endanger human health once these nanoparticles are inhaled. Zhang [29] determined the effect of nozzle caliber, compressed air pressure, cutting fluid flow rate, and gas-liquid flow ratio on atomized microparticle sizes in MQL grinding. Straight-surface grinding experiments (with no cross feed) were conducted on an instrumented surface grinding machine (KP-36), as shown in Fig. (3a). An external fluid delivery system (precision dispenser-AMCOL Corp.) was used for MQL grinding, as shown in Fig. (3b). The surface grind-
58 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1
Jia et al.
SDUWLFOHGLDPHWHUP
particle diameter / m
nozzle exit diameter / mm
DLU SUHVVXUH 03D
Flow rate 3g/s, air pressure 0.3 MPa, gas-liquid flow ratio 0.2 (a) Effect of nozzle caliber on microparticle diameter.
Flow rate 3g/s, gas-liquid flow ratio 0.2, nozzle caliber 1mm (b) Effect of compressed air pressure on microparticle diameter. SDUWLFOHGLDPHWHUP
SDUWLFOHGLDPHWHUP
JULQGLQJIOXLGIORZUDWH JV
DLUOLTXLGIORZUDWH
Flow rate 3g/s, air pressure 0.3MPa, nozzle caliber 1mm Gas-liquid flow ratio 0.2, nozzle caliber 1mm, air pressure 0.3MPa (d) Impact of gas-liquid flow ratio on microparticle diameter. (c) Effect of cutting fluid flow rate on microparticle diameter. Fig. (4). Effect of various parameters on atomized microparticle diameter.
ing experiment with the use of 40CrNiMo steel adopted the following surface grinding parameters: velocity of grinding wheel: 30m/s; feed speed of the workpiece: 0.1m/s; grinding depth: 10m; width of the grinding wheel: 7.2mm; and MQL jet flow rate: 2.5mL/min. The confocal laser scanning microscopy was used to scan and detect the droplet distribution. Figures 4a-4d presents the respective relationships between atomized microparticle sizes and the above parameters. Figures 4a-4d show the relationship of nozzle and working condition parameters with atomized microparticle diameter. As shown in the figures, atomized microparticle size basically has a linear relationship with nozzle caliber and gas-liquid flow ratio. Atomized microparticle size is directly proportional to the nozzle caliber, but inversely proportional to gas-liquid flow ratio. Although atomized microparticle size does not have a linear relationship with compressed air pressure and cutting fluid flow rate, an optimal atomization parameter can be found. In Figs. (4b & 4c), when compressed air pressure was 0.4MPa and cutting fluid flow rate was 5g/s, the atomized microparticle size was the smallest.
Atomized microparticle size is heterogeneous, such that the average size is generally used to represent the atomization degree of microparticles. For the expression of average size, the following two approaches are often used [1-4]: (1) Mass Middle Diameter (MMD) MMD dm is an assumed value. The total masses of droplets larger than this diameter are equal to the total masses of droplets smaller than this diameter, that is,
M
d dm
= Md d
(1) m
We can clearly observe a smaller dm results in a smaller atomized microparticle size. (2) Sauter Mean Diameter We assume that sprays are formed by microparticles with the same diameter (dSMD), and the total surface area and total volume of microparticles are the same as those of the actual jet mist, that is,
V=
N d SMD 3 = N i di3 6 6
(2)
Minimum Quantity Lubrication (MQL) Grinding
Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1
2 = N i di2 S = N d SDM
d SMD =
i
i
showed sound fitting with the Rosin-Ramller distribution curve. Ma et al. [31] conducted substantial experiments and verified that the droplets showed satisfactory fitting with logarithmic normal distribution in number distribution and exhibited satisfactory fitting with Rosin-Ramller distribution in volume distribution. The logarithmic normal distribution can be expressed by the following equation:
(3)
N d N d
3 i 2 i
(4)
where N is the number of spray microparticles, and Ni is the number of microparticles with a diameter of di. According to the definition, we obtain;
d SMD
6V = S
( )
f d =
(5)
(
ln d ln d G exp 2 2 ln S 2 d ln SG G 1
m dG = exp f i ln di i=1
The use of only mean size to describe the atomization characteristics of sprays is inaccurate. Scholars seek to determine the microparticle distribution expressions to express microparticle size, as well as the number and volume of microparticles with different sizes. However, the current expressions of microparticle distributions are empirical equations, which exhibit a deficiency in theoretical expression. Rosin-Ramller equation is commonly used to express the volume distribution of droplets:
ln SG =
m
f ( ln d i
i
ln dG
)
2
(7)
(9)
To investigate further the size and distribution of suspended microparticles in the MQL machining, Park et al. conducted experiments and theoretical analyses. In the experiment, a silicon wafer was installed on a computer numerical control grinding machine, and MQL nozzle was used to jet minimal quantities of lubricant to the silicon wafer. The experiment selected different compressed pressure P, nozzle distance D, and minimal quantities of lubricant flow velocity V. With the use of confocal laser scanning microscopy, we found that 2D and 3D shapes of small droplets
After the segmentation of drop microparticles according to sizes, m is the total number of droplets at the i-th end, di is the drop microparticle diameter at i-th section, and fi is the frequency of drop microparticle number at the i-th section. Figure 5 shows the curves of lognormal distribution and Rosin-Ramller distribution.
where di is the microparticle diameter; n is the index number of the homogeneous degree of microparticles, normally between 1.8 and 4; and R is the ratio of microparticles with size larger than di in all spray microparticles. d is the dimension constant of microparticles. Ji [30] conducted a theoretical analysis of the sizes and distributions of the spray atomized droplets from a Type Y nozzle and verified the conclusion through experiments. The experiment data
2
(8)
i=1
(6)
)
where f(d) is the probability density function, SG is the amount related to drop diameter deviation (dimensionless), and dG is the diameter characteristic value related to drop diameter (m). SG and dG can be calculated using the maximum likelihood method.
The above equation shows that a smaller Sauter diameter of spray microparticles results in a larger surface area of microparticles, which is conducive to the vaporization and evaporation of sprays. This condition can significantly improve the cooling effect of sprays. Meanwhile, a larger surface area of microparticles imposes greater harm on human health.
d n R = 100 exp i % d
59
Fig. (5). Curves of lognormal distribution and Rosin-Ramller distribution.
60 Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1
Jia et al.
formed by jetted minimal quantities of lubricant were obtained. According to the 3D shapes, that is, information provided by three dimensional images, the volume equation of mist jet microparticles was proposed as follows: M
N
(
Vdroplet = Vi, j = xy hi, j hzero i=1 j =1
)
(10)
where Vi, j is the discrete volume at Pi, j(Xi, j Yi, j Zi, j), h is the height of droplets at p, hzero is the edge height of droplets, the total volume of droplet Vdroplet is the sum of discrete volumes V, and h can be obtained from the decoding of HEI image. M and N should be consistent with the mist microparticle size obtained from the HEI image. Once the droplet volume is calculated, its diameter Deq in air can be calculated as follows:
3Vdroplet Deq = 2 4
1
3
(11)
Although the mist microparticle size refers to the size of microparticle that drifted in air, further atomization and gasification, as well as other processes, of droplets are not considered. In fact, the size of suspended microparticles in air is in one order of magnitude. This diameter can be used to describe the size of suspended microparticles. The author also provided the size estimation equation of super-fine microparticles according to the difference value.
D3D = 0.0012D22D + 0.1997D2 D 0.0987
(12)
where D2D represents the 2D diameter of droplets on silicon wafer, and D3D is the diameter of droplets in the air. The 2D and 3D diameters should be acquired from the same silicon wafer with the same composition in one experiment. Moreover, the research presented the general condition of microparticle size distribution, as shown in Table 4. where D2D represents the 2D diameter of droplets on silicon wafer, and D3D is the diameter of droplets in the air. The 2D and 3D diameters should be acquired from the same silicon wafer with the same composition in one experiment. The research presented the general condition of microparticle size distribution, as shown in Table 5. This experiment did not consider the effect of specific grinding parameters on size, motion curve, and concentrations of suspended microparticles. The preliminary exploration has laid a foundation for current studies on suspended microparticles in MQL.
Table 5.
CURRENT & FUTURE DEVELOPMENTS Three-phase flow of nanoparticle MQL can be mainly divided into two parts. One part enters the grinding zone for efficient lubrication and cooling effects. The other part drifts in air through various means to form suspended microparticles, which endangers human health. Some scholars explored the distribution of MQL droplets and preliminarily analyzed the effect of the flow rate, compressed air pressure, nozzle caliber, and gas-liquid flow ratio of cutting fluid on MQL. With the use of confocal laser scanning microscopy, scholars scanned and detected the droplet distribution. Microwave conversion method was used to calculate the volume of droplets. In previous research, the negative effects of MQL on human health were determined. Although, the optimal MQL parameters that can be used to reduce the amount of suspended microparticles were determined, a fundamental solution was not presented. The major research direction has become the investigation of controllable MQL jet that can be maintained in the cutting area to the maximal degree for cooling and lubrication. Scattering and drifting should be reduced as much as possible. This area is also a focus of research on recovery processing and recycling after drifting. Recent patents have been proposed for this purpose. Li et al. [41] invented a supply system for the grinding fluid in nanoparticle jet MQL. This system produced MQL lubricant by adding solid nanoparticles to degradable grinding fluid. The MQL supply device turns the lubricant into pulse drops with fixed pressure, unchanged pulse frequency, and constant drop diameter. The drops will be produced and injected in the grinding zone in the form of jet flow under highpressure gas and air seal. This device includes all advantages of MQL while possessing better cooling performance and excellent tribological properties. This device effectively solves the pitfall of grinding burn, improves the surface quality, and achieves clean production with high efficiency, low consumption, environment friendliness, and resource conservation. However, this patent did not involve the organic relations between the nanometer MQL and the surface morphology of the workpiece, i.e., micro-bulges, and failed to establish the internal relations between the formation of lubricating oil film and the surface morphology of the workpiece or the oil film mechanism of nanoparticle MQL grinding on the rough surface of the workpiece. Satoshi et al. [42] invented a method that provides an oil composition for cutting and grinding by using a minimum quantity lubrication system. According to the invention, the oil composition for cutting and grinding by a minimum quantity lubrication system can achieve excellent balance between misting property and floating mist inhibition. This oil composition can ensure that an adequate amount reaches
Microparticle Size Distribution of Oil Mists. Microparticle Diameter
Amount of Microparticles
D < 20m
123
20m D < 40m
17
D 40m
1
Minimum Quantity Lubrication (MQL) Grinding
the working section for cutting and grinding by a minimum quantity lubrication system. Li and Wang [43] developed a method and device for the prediction of the grinding surface roughness under nanoparticle jet MQL. The device has a sensor lever, the left end of which is equipped with a stylus. The stylus is in contact with the wheel surface. The right end of the sensor lever is connected to an inductor displacement sensor. Meanwhile, the pivot of the lever is hinged with the measuring equipment. Other connections can be found between the inductive displacement sensor and AC power, between data output end of inductive displacement sensor and the filter amplifier, between the filter amplifier and the calculator and the oscillometer, as well as between the calculator and the memorizer. The wheel morphology is characterized by a matrix. According to the formation mechanism of surface topography in the workpiece grinding and processing, the surface roughness of the workpiece was predicted. Li and Han [44] created a three-phase flow supply system of nanoparticle jet MQL grinding with the following features: nanofluids are transferred to the nozzle through the fluid channel. Meanwhile, the high-pressure gas enters the nozzle through the fluid channel, and the gas is fully mixed and pulverized with nanofluids in the mixing chamber of the nozzle. The gas is then led to the swirl chamber after acceleration in the accelerating chamber. Meanwhile, the compressed gas obtains access to the venthole in the accelerating chamber, such that the three-phase flow is further rotationally blended and accelerated. The three-phase flow is then injected to the grinding zone in the form of atomized droplets through the nozzle. Li and Jia [45] studied and developed the technology and device for the formation of micro-bulge oil film on the workpiece surface under nanoparticle jet flow. This device delivers nanofluids to the nozzle under the coupling of nanoparticle jet MQL and micro-bulges in the grinding zone. Nanofluids are injected to the grinding zone under compressed air with a relatively high velocity to form the microbulge oil film, which achieves the maximum cooling and lubrication in the grinding zone. This system has all advantages of MQL, considers the morphological characteristics of the workpiece surface, and forms the micro-bulge oil film. When nanoparticles are attached to the rough surface of the workpiece, good cooling performance and excellent tribological properties can be obtained, which effectively resolve the grinding burn and improve surface quality. Li et al. [46] developed a technique called carbon nanotube (CNT) jet for the grinding of nickel-base alloy. CNT nanoparticles and the grinding medium are mixed to produce nanofluids, which are injected in the form of jet flow into the grinding zone. CNT nanoparticles take away substantial heat of grinding from the zone, thus enhancing the cooling performance of the grinding medium. This feature mitigates the limitation of MQL in terms of cooling performance. Meanwhile, CNT has excellent tenacity with its tube wall formed by the hexagonal structure of graphitic layers, thus ensuring satisfactory performance of lubrication and maintaining the properties and advantages of MQL. Moreover, the lubrication area of the grinding liquid was enlarged, which effectively reduced the friction between the wheel and the work-
Recent Patents on Mechanical Engineering 2014, Vol. 7, No. 1
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piece, as well as between the wheel and cuttings. Therefore, the service life of the wheel was prolonged, and the completeness of the workpiece surface was improved. Li et al. [47] invented a grinding process and equipment with nanofluids. The nanofluid grinding technique provides solid nanopowders to the grinding zone as the cooling and lubrication medium during grinding. This device involves the setting of lubricant container, water container, nanopowder container, and high-pressure air pump on the rack. Different mediums and high-pressure air are converged and led to the spray chamber for mixing and atomization. The mixed medium then passes through the flexible pipe and is sprayed by the nozzle to the area. With a high heat conductivity coefficient of nanopowders, nanofluids will take away substantial grinding heat while passing through the high-temperature area. This condition enhances the cooling performance of the grinding medium and improves the cooling performance in MQL. Meanwhile, owing to their light weight, nanopowders retain the properties and advantages of MQL, while achieving the desired effects of high grinding quality, low costs, and zero pollution. This approach is the fourth generation of cooling and lubrication techniques, following wet grinding, dry grinding, and MQL. Li et al. [48] invented a measuring equipment for the thermal characteristics of nanofluids. The equipment is characterized as follows: the assay of heat conductivity coefficient of nanofluids and that of convective heat transfer coefficient can be accomplished on one device. In addition, with the hydraulic pump simulation of a grinding supply system, nichrome resistance wire was used to heat nanofluids to obtain the heat flux boundary condition, which is the same as the grinding condition. This system achieves excellent equipment integration rate, utilization efficiency, accuracy, and reliability. This system successfully optimized the situation in which the assays of heat conductivity coefficient of nanofluids and convective heat transfer coefficient were conducted on different devices. Shibata et al. [49] invented a cutting/grinding processing method by using a minimal quantity lubrication system, comprising a step of supplying the oil composition of the present invention in the form of mist along with a compressed fluid to the processing sites of a workpiece. CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest. ACKNOWLEDGEMENTS This research was financially supported by the National Natural Science Foundation of China (50875138; 51175276), the Shandong Provincial Natural Science Foundation of China (Z2008F11; ZR2009FZ007). REFERENCES [1]
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