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Journal of the Brazilian Society of Mechanical Sciences

Print version ISSN 0100-7386

J. Braz. Soc. Mech. Sci. vol.23 no.2 Rio de Janeiro  2001

http://dx.doi.org/10.1590/S0100-73862001000200009 

Application of Cutting Fluids in Machining Processes

 

Wisley Falco Sales
Pontifical Catholic University of Minas Gerais PUC Minas
Departament of Mechanical/Mechatronical Engineering
Rua Dom José Gaspar, 500 Coração Eucarístico
30535-610 Belo Horizonte, MG. Brazil
wisley@pucminas.br
Anselmo Eduardo Diniz
School of Mechanical Engineering (UNICAMP)
Department of Manufacturing Engineering
Rua Mendeleiev, S/N - Cidade Universitária "Zeferino Vaz"
Barão Geraldo
13.083-970 Campinas, SP. Brazil
anselmo@fem.unicamp.br
Álisson Rocha Machado
Federal University of Uberlândia – UFU
Mechanical Engineering Faculty
Av. João Naves de Ávila S/N
38400-902 Uberlândia, MG. Brazil
alissonm@mecanica.ufu.br

 

 

In the last decade a lot has been discussed about the suitability of using cutting fluid in abundance to cool and lubricate machining processes. The use of cutting fluid generally causes economy of tools and it becomes easier to keep tight tolerances and to maintain workpiece surface properties without damages. In the other hand, it brings also some problems, like fluid residuals and human diseases. Because of them some alternatives has been sought to minimise or even avoid the use of cutting fluid in machining operations. Some of these alternatives are dry cutting and cutting with minimum quantity of fluid (MQF). The main goal of this work is to discuss these tendencies. Therefore, topics like kinds and methods of applications of modern cutting fluids and what are new in this area will unavoidably be considered. MQF and dry cutting techniques, their applications and where it is not possible to apply them will also be focused. To exemplify the topics, this work will describe some of the researches been developed in two important Brazilian Universities: State University of Campinas (UNICAMP) and Federal University of Uberlândia (UFU).
Keywords: Cutting fluids, machining, MQF, dry cutting

 

 

Introduction

Since the beginning of the 20th century, when F.W. Taylor used water for the first time to cool the machining process and concluded it increased tool life, a large variety of cutting fluids has been used with this and other purposes. However, in the last decade a lot has been done aiming to restrict the use of cutting fluids in the production, due to the costs related to the fluids, ecological issues, human health and so on. (Heisel et al., 1998; Kalhofer, 1997; Klocke et al., 1997).

To minimize the use of cutting fluid, two techniques have been intensively experimented: cutting without any fluid (dry cutting, also known as ecological machining) and cutting with a minimum quantity of fluid (MQF), where a very low amount of fluid is pulverized in a flow of compressed air.

When MQF is used, the steam, the mist and the oil smoke are considered undesirable sub-products, since they cause an increase of air pollution. In Germany, the maximum concentration of pollutant in the air in mist form is 5mg/m3 and when the pollution is in steam oil form the limit is 20 mg/m3 (Heisel et al., 1998). This demands an efficient exhauster system that guarantees this compulsory air pollution control. But this could be far more advantageous than dealing with problems of discarding used cutting fluids.

In 1992, the volume of discarded soluble oil from the German industries were around 60% of the whole volume of lubricants used in manufacturing processes, about 1.151.312 ton. That represents a significant amount of money, varying from 7,5% to 17% of the manufacturing costs per part, even higher than the costs related with tooling (Heisel et al., 1998; Kalhofer, 1997). Therefore, even though the tooling costs may be increased by the use of either dry cutting or MQF, due to the increase of tool wear, the whole manufacturing cost may be lower when compared to the conventional process where cutting fluid is abundantly used. The new generation of cutting tools, particularly the ceramics and the ultrahard materials (PCBN and PCD), also encourage the change to ecological machining or to MQF. The properties of these tools are reaching such a level of qualities that can withstand the adversities imposed by those types of machining. Other advantages of using these techniques are: chips remain clean, with lower cost of reprocessing, easier cleaning and maintenance of shop floor and clean workpiece, cutting costs with degreasing (Granger, 1994).

The main goal of this work is to discuss these tendencies. Therefore, topics like kinds and methods of applications of modern cutting fluids and what are new in this area will unavoidably be considered. MQF and dry cutting techniques, their applications and where it is not possible to apply them will also be focused. To exemplify the topics, this work will describe some of the researches been developed in two important Brazilian Universities: State University of Campinas (UNICAMP) and Federal University of Uberlândia (UFU).

 

Cutting Fluids Classification

There are several ways of classifying cutting fluids and there is no standardization to establish one of them within the industries. May be the most popular classification gathers the products like the following classification:

I. Air

II. Water Based Cutting Fluids:

a) – water;
b) – emulsions (soluble oil);
c) – chemical solutions (or synthetic fluids);

III. Neat Oils:

a) – mineral oils;
b) – fatty oils;
c) – composed oils;
d) – extreme pressure oils (EP);
e) – multiple use oils.

Compressed air can be used aiming to cool the cutting region, through either a pure air jet or mixed with another fluid. It must be directed to the interface, against the under surface of the chip and may have good performances. Water, due to its high corrosion ability in ferrous materials, it is practically ignored as cutting fluid.

Emulsions

Soluble Oils

Soluble oils as the emulsions are popularly known, are bi-phase composites of mineral oils added to water in proportion that varies from 1:10 to 1:100. It contains additives (emulsifiers) to allow the mixture of oil particles and water. These additives decrease the surface tension forming a stable monomolecular layer in the oil–water interface. Therefore these additives provide the formation of small particles of oil, which can result in transparent emulsions.

The stability of the emulsions are related to the development of an electrical layer in the oil-water interface. Repulsive forces among particles of the same charge avoid their coalescence. To avoid the bad effects of the water of these emulsions, anticorrosive additives, like sodium nitrite, are used. Biocides are also in the formulation to avoid bacteria growing. They must not be toxic and prejudicial to the human skin. The EP and antiwear additives that increase the lubrication properties are the same used in neat oils. However, the use of chlorine in cutting fluids is being restricted all over the world, due the harm it causes to the environment and to the human health. For this reason they have been replaced by sulfur and calcium based additives. Both, animal and vegetal grease can also be used to enhance lubrication properties.

Semi Synthetic Fluids (Microemulsions)

The semi synthetic fluids have 5% to 50% of mineral oil plus additives and chemical composites which dissolve in water forming individual molecules of microemulsions. The presence of a large amount of emulsifiers, compared to soluble oil, provides a more transparent appearance to the fluid. The lower amount of mineral oil and the presence of biocides, increase the fluid life and reduce health risks, compared to the emulsions.

EP and anticorrosion additives are used like in the soluble oils. Additives which guarantee a more acceptable color for the fluids are also used.

Solutions

Solutions are monophase composites of oils completely dissolved in the water. There is no need on emulsifiers, because the composites react chemically, forming a monophase. Synthetic fluids (without mineral oils) belong to this type of cutting fluid.

Synthetic Fluids

As already said, this kind of cutting fluids do not have mineral oil in its composition. They are based on chemical substances which form a solution with water. They are made of organic and inorganic salts, lubricant additives, biocides, lubricant additives, among others, added to water. They have a longer life than other fluids, because they are not attacked by bacteria and, thus, the number of replacement in the machine tank is reduced. They form transparent solutions, what cause a good visibility in the machining process and have additives which provide high wettability and, therefore, high cooling ability. The most common synthetic oils also provide good corrosion protection. The most complex ones are of general use and, besides, good cooling ability, they also have good lubrication ability. When the synthetic fluids have only anticorrosion additives and the EP properties are not necessary, they are called either chemical fluids or true solutions and present good cooling properties.

Neat Oils

Vegetal and animal oils were the first lubricant used as pure oil in metal cutting. However, their use became impossible due to the high costs and quick deterioration, but they are still used as additives in mineral fluids, aiming to increase the lubrication properties.

Neat oils are basically either pure mineral oils or mixed with additives, generally of extreme pressure type. The use of these oils as cutting fluids have decreased due to the high cost, fire risks, inefficiency in high cutting speed, low cooling ability, smoke formation and high risk to the human health when compared to water based cutting fluids. The additives may be either chlorine or sulfur based or the mixture of these two substances, what result in EP properties for the fluid. Phosphorous and fatty additives are also used and act as antiwear elements. Mineral oils are hydrocarbons obtained from the petrol refining of crude oil. Their properties depends on the chain length, structure and refining level.

 

Cutting Fluid Functions

The main functions of cutting fluids are:

·    Lubrication at low cutting speeds;
·    Cooling at high cutting speeds;

And less important:

·    To help the chip removal of the cutting zone;
·    To protect the machine tool and workpiece against corrosion.

At low cutting speeds, cooling is not very important, while lubrication is important to reduce friction and avoid the formation of built-up-edge. In this case, an oil based fluid must be used. At high cutting speeds, the conditions are not favorable to fluid penetration, to reach the interface and work as a lubricant. In these conditions cooling becomes more important and a water based fluid must be used.

As lubricant, the cutting fluid works to reduce the contact area between chip and tool and its efficiency depends on the ability of penetrating in the chip-tool interface and to create a thin layer in the short available time. This layer is created by either chemical reaction or physical adsorption and must have a shearing resistance lower than the resistance of the material in the interface. In this way it will also act indirectly as a coolant because it reduces heat generation and therefore cutting temperature.

It is neither completely clear how cutting fluid reaches this interface, nor how deep it can go. Trent (1967, 1991) says that the lubricant have no access to the seizure zone on the tool rake face. Childs and Rowe (1973) also affirms this theory and comments that further studies must be done in the chip-tool interface besides the difficulties encountered to access the seizure zone. Postinikov (1967) suggested that the lubricant penetrates against the metal flow, reaching the tool nose, through a capillary action, assuming that the contact in the interface is not total (sliding conditions). Williams (1977) supports this point of view. Some experiments with transparent sapphire tools (Horne, et al., 1978), demonstrated that the cutting fluid flow reaches the interface by the lateral parts of the contact, instead of moving against the chip flow. No matter the penetration method, cutting fluid, once in the interface, must form the lubricant layer with shearing resistance lower than the material resistance. It may also restrict the chip welding on the tool rake face, if suitable additives are used. The lubrication efficiency will depend on the fluid properties, such as: wettability characteristics, viscosity and layer resistance. These properties may be obtained with a suitable mixture of additives.

As coolers, cutting fluids decrease cutting temperature through the heat dissipation (cooling) When water based fluids are used cooling is more important than lubrication. It was experimentally proved (Shaw, et al., 1951) that the cutting fluid efficiency in reducing temperature decreases with the increase of cutting speed and depth of cut.

The cutting fluid ability of sweeping the chips away from the cutting zone depends on its viscosity and its volume flow, besides, of course, the kind of machining operation and chip type formed. In some machining operations such as drilling and sawing, this function is very important, because it may avoid chip obstruction and, consequently, tool breakage.

 

Determining of the Cooling Ability of Cutting Fluids

Aiming to classify the main cutting fluids based on their cooling ability, Sales (1999) developed a methodology which consisted in heating a standard workpiece and monitoring the cooling curve of it. This workpiece was fixed to the clutch of a lathe jigs and rotated at 150 rpm and its temperature was measured using an infrared sensor. The data acquisition started when the workpiece temperature reached 300°C and the measurement continued up to room temperature. Emulsions and synthetic fluids were applied using a concentration of 5%. Synthetic fluids are based on poliglicol, containing water and additives. The synthetic oil 1 is different from synthetic oil 2 due to small variations in their formulas. Figure 1 shows the results of this experiment.

 

 

The cooling ability in crescent order is: dry cutting, neat, oil emulsion, synthetic-2, water and synthetic-1. The fact that synthetic oil 1 presented a cooling ability greater than water, which theoretically has a greater convection ability, was a surprise. A deeper analysis of the curves behavior in high temperature showed that water presented lower cooling ability even than synthetic oil 2 and neat oil.

The explanation of these results may be found on the phenomenon occurring when a fluid like water, with low ebullition point (100°C), starts contacting a body in high temperatures. At this moment the quick heat transfer causes the liquid evaporation. This process reduces a little the hot body temperature, but the vapour formes a barrier preventing fresh volume of liquid, from reaching its surface and, therefore, decreases the heat transfer efficiency. Another important factor is the fluid wettability, which is regularly higher for cutting fluids than for water. The higher wettability of the cutting fluid implies in less splashing action and therefore a greater chance for heat exchange.

Experiments with emulsions and synthetic fluids at 10% concentration presented similar results, what confirmed the higher cooling ability of synthetic oil 1 compared with the other fluids experimented.

Based on the cooling curves, the convection coefficients of the fluids, h, were calculated and shown on Figure 2.

 

 

In spite of the fact that this classification is an effective indication of the cooling ability of the fluids, it does not mean that the fluid that has the highest convection coefficient will provide the lowest temperature in the chip-tool interface. The results obtained by Sales et al. (1999), after machining experiments on AISI 8640 steel with the same cutting fluids and measuring the process temperature using the tool-workpiece thermocouple method, proved it. Figure 3 shows their results.

 

 

When the fluids with the highest cooling ability were used, the chip-tool interface temperature during the process were the highest, with one exception: dry cutting presented higher temperature than cutting with the neat oil. The lowest interface temperatures were obtained when using the integral neat oil and for the dry cut.

The comparative analysis of the chip-tool interface temperature values therefore does not indicate a higher or lower cooling ability of a cutting fluid. In machining processes, a fluid that removes a larger amount of heat, may promote a reduction in the softening effect of the workpiece material caused by the heat. With this, the metal keeps its resistance at higher levels than when a cutting fluid with lower cooling ability is used. In such cases, more energy is consumed by shearing, what causes higher temperatures in the chip-tool interface. Figure 3 shows that the higher the temperatures (or cutting speeds) more pronounced is this effect. For cutting speed of 244 m/min, the temperatures when the fluids with the highest cooling ability are used (synthetic 1 and water) are much higher than the temperatures given by using the others fluids.

 

Determining the Lubricity of the Cutting Fluids

In order to determine the lubricity of cutting fluids scratch test techniques was used by Sales (1999). As the Upsala’s pendulum test has a dynamic characteristic this technique was considered because it can be easily correlated to machining. An AISI 8640 steel workpiece and a cemented carbide indentor were used, both specifically design for this experiment. Figure 4 illustrates schematically the apparatus used. The indentor is fixed in the tip of a pendulum having a predetermined mass and released from a known height. The indentor will scratch the steel workpiece and pass over it up to a determined height. With this height and the loss of mass of the workpiece the scratch specific energy can be calculated. With the parameters used in these tests the scratch velocity was 246 m/min which confirms that it is very close related to the figures used in conventional machining operations.

 

 

The cutting fluids were individually covering the workpiece surface in such a way that the scratch was made submerse. Figure 5 shows details of this.

 

 

After testing various cutting fluids under several indentor depth (depth of cut) the specific energy against loss of mass curves were obtained. Figure 6 shows these curves after adjustment in a power series.

 

 

The specific energy drops rapidly with increasing of the scratch depth. This was attributed to size effect (Franco, 1989) and to the chip formation mechanism (Bryggman et al., 1985).

In this investigation the lower the loss of mass the higher the medium lubricity. With the results shown in Figure 6 the decreasing order of lubricity was: Integral neat oil, emulsion (soluble oil), dry condition, synthetic – 2, synthetic – 1 and pure water.

Analysis of these results must also consider the coolant ability of the medium because the softening effect caused by the shearing processes is influenced by it. The results showed that medium with higher coolant ability presented the higher scratch specific energy. Those mediums with higher lubricity presented the lower scratch specific energy.

 

Applications

When a cutting fluid is applied, it may cause benefits, do not interfere or even be negative to harm the processes, depending on the cutting conditions, workpiece and tool material.

 

Applications Where Cutting Fluid Offers Benefits

Cutting with low strength tools, like high speed steels, demands the use of cutting fluid. This is due to the fact that the heat generated during cutting increases a lot the tool temperature, reducing its mechanical strength and, thus, making easier the occurrence of plastic deformation and complete failure. In this case, cutting fluids reduce the temperature, not allowing the tool to loose its strength and making possible the use of relatively high cutting speeds. Drilling, broaching, milling, threading with high speed steel tools are typical examples of these operations where the use of cutting fluids is essential.

Another important application of cutting fluid is in operations where low surface roughness and/or tight dimensional tolerances are required. In these cases, the lubricant guarantees a good surface finish and the cooling fluid guarantees the tight tolerances, because it avoid thermal expansion of the workpiece.

When drilling materials that generate discontinuous chips, like grey cast iron, cutting fluid application becomes fundamental, mainly in deep drilling. In this case, the main cutting fluid function is to carry the chips away from the cutting zone, what other wise could cause chip jamming and, consequently, a possible tool breakage.

Usually continuous cutting (turning, boring, etc.) of any metal (and also several non metallic materials) with carbide tools (with or without coating) is carried out with application of a cutting fluid. In such cases, the fluid increase tool life and, therefore it may reduce costs. Machado et al. (1997) showed how cutting fluids are important in turning AISI 8640 steel with cutting speeds up to 400 m/min and P35 carbide tools coated with three layers of TiC, Al2O3, TiN. They experimented several kinds of cutting fluids. Figure 7 shows the tool life results obtained by them. It can be seen in this figure that when dry cutting is used tool life is much shorter than when any kind of cutting fluid is used.

 

 

Applications Where Cutting Fluid Does Not Interfere in the Process

Actually cutting fluids always, in some way, interfere in the process. They may either pollute the work environment or impregnate workpiece and machine components, what may cause the washing of the machined parts necessary. However, in terms of tool life, there are some applications where cutting fluid either do not contribute or contribute just marginally for the efficiency of the process. Typical examples are the machining of grey cast iron (exception is deep drilling), magnesium and aluminum alloys. Other examples are the machining of plastic materials or resins. The machining of these kinds of materials depends strongly on how abrasive the material is, and therefore, it is impossible to affirm whether cutting fluids interfere or not in the process.

In machining of grey cast iron, cutting fluid may increase tool life, mainly in the cases where diffusion is the dominant wear mechanism. However, Trent (1991) proved that this gain is not substantial and, generally, the application of a cutting fluid generates a negative because the cost of purchasing and maintenance of the product is higher than the gains with the benefits.

When machining magnesium and aluminum alloys, dry cutting is also very common. These materials have high machinability, because they have low melting point (650°C and 659°C, respectively). The exceptions are some aluminum-silicon alloys. In the hypereutetics alloys (Si above 11%) Si is in the form of hard particles (> 400 HV), abrasive and with high melting point (~1420°C) in the aluminum matrix. These large particles of Si (average diameter up to 70 m m) generate high tension and temperature on the tool faces causing rapid tool wear. In such cases, the flood application of an emulsion or synthetic fluid is fundamental to reduce the wear. Another alternative that will be treated in detail later is the application of minimum quantity of fluid (MQF). This may be sufficient to prevent accelerated tool wear.

Another exception to the use of dry cutting when machining aluminum alloy is in drilling operations. In this case, the chips tend to stick on the tool and make difficult the evacuation of them, what can cause drill breakage. Therefore, in this case an abundant volume of cutting fluid or even MQF must be used. In other operations, in general, dry cutting is recommended, unless tight dimensional tolerances and low surface roughness are required. Due to the high ductility of the material, it tends to stick on the tool, producing poor surface roughness. They also have high thermal expansion coefficient, causing the obtaintion of high tolerances difficult. In these cases application of cutting fluids acting both as a lubricant and as a coolant will contribute to reduce the inherent problems.

When machining magnesium, more serious problems may occur when water based fluids are applied, because water reacts with the chips, releasing hydrogen, which may cause ignition and fire hazards.

 

Applications Where Cutting Fluid Is Negative to the Process

There are typical examples where cutting fluid application harm the harms process and, therefore, it must not be used.

Generally, machining with ceramic tools must be performed without fluid, because it may promote thermal shocks and, eventually, cause tool breakage. Some ceramic tools, mainly those based on Si3N4 and the "whiskers", because they have higher toughness and thermal shock resistance, can avoid this kind of failure and, so, allow some advantages when cutting fluid is applied.

Other examples of dry machining are interrupted cuttings (like milling) with carbide tools, where the main kind of wear are cracks of thermal origin that leads to the formation comb of cracks (Ferraresi, 1977). In such cases, cracks of thermal origin, transversal to the tool cutting edge appear just after a few minutes of cut. They look like those seen on Figure 8a. They are originated by the cyclic variation of the temperature, due to the interrupted nature of cutting. The cutting edge is heated during the cutting period and cooled during the idle period. These cracks, as cutting goes on, will increase and propagate, leading to the formation of comb cracking type of wear (De Melo et al., 2000). Figure 8b illustrates the comb cracks in a worn tool at the end of its tool life.

 

 

When this kind of wear is dominant, cutting fluid application will increase even more the temperature variation and accelerate the process of crack generation, decreasing tool lives. Vieira et al. (1997) showed this negative effect of cutting fluids after comparing the machinability of AISI 8640 steel in dry cut and using several types of cutting fluids (emulsion, synthetic and semi-synthetic fluids) using P45 carbide tools triple coated with TiN, TiC and TiN. In all experiments dry cutting presented better performances than wet cuttings as can be seen on Figure 9.

 

 

Machining of hardened materials is another typical example where cutting fluid can be detrimental to the process. Cutting fluid should work just as a coolant for the tool, but the regular process of application makes the fluid to reach all chip formation zones, cooling also the workpiece. Therefore, the softening effect caused by the large amount of heat generated is not substantial, requiring higher amount of energy to shear the material and to form the chip, demanding high cutting forces and generating high temperatures in the chip-tool interface. Due to the high hardness of this kind of materials (usually higher than 30 HRc), softening caused by the process of heat generation is fundamental to increase the performance of the process. Cutting fluid hinders this and will be negative to the process. Teixeira et al. (2000) turned 52100 steel with 60 HRc of hardness using PCBN tools under several cutting speeds and using three types of cooling system: dry cut, minimum quantity of fluid (10 ml/h) and flood application of a soluble oil. The main conclusions of their work are: a) dry cutting presented the best performance related too tool life; b) flood application, besides presenting a shorter tool life, caused a poorer workpiece surface roughness; c) MQF condition presented an intermediary performance between dry cutting and cutting with emulsion.

 

Application of Minimun Quantity of Fluid (MQF )

The choice of a cutting fluid and its method of application depend on important points such as cost (not just costs of acquisition, but also costs of recycling and maintenance), environmental effects and influence on human health. These points are becoming more and more important as the law restrictions on environmental issues become more strong. An alternative for the use of flood of cutting fluid is the application of a mist of oil or minimum quantity of fluid (MQF), as is being coined among the scientists. Actually this technique consists of a mixture of drops of cutting fluids (neat oils or emulsions) in a flow of compressed air, generating an "spray" which is directed to the cutting region to work as lubricant and coolant. Machado and Wallbank (1997), Heisel et al. (1998) and Waingaertner et al. (2000), among other researchers investigated this technique in several processes and proved that it could be a feasible alternative, depending on cutting conditions, pulverization parameters and kind of cutting fluid used.

The MQF technique decreases feed and cutting forces when machining medium carbon steel with low cutting speeds, mainly for feeds higher than 0,25 mm/rev, as can be seen on Figure 10 (Machado and Wallbank, 1997). In these conditions the values of forces obtained with the mist system were even lower than those obtained with the application of an emulsion using conventional method (overhead flood). For these experiments, a venturi was designed and both water and soluble oil was mixed with air at flows of 294 ml/h and 196 ml/h respectively.

 

 

In these experiments the authors also found little reduction on surface roughness parameter (Ra ) and on chip thickness when MQF was used, compared with dry cutting and flood of cutting fluid. See Figure 11.

 

 

With cutting speeds higher than 100 m/min the results were not similar. There was no or just little cutting fluid influence on the machining forces. These results suggest that even with MQF it is difficult for the fluid to penetrate in the chip-tool interface in order to work as a lubricant (to reduce the contact area between chip and tool), when the dynamics of the process are too high. However, it was observed that the amplitude of variation of the force components was reduced with MQF, proving that the "spray" guarantees a better penetration in the sliding zone of the chip-tool contact area. This fact was proved by the reduction of the standard deviation of the force components average values.

The same authors (Machado et al., 2000) also tested MQF using additives for cutting fluids with different lubrication characteristics, using the same methodology. Product "A" having addition of 10% sulfur and a flow of just 25 ml/h mixed with compressed air and product "B" having no sulfur at all and with a flow of 39 ml/h were used. With these products a higher lubrication action was observed, even under higher cutting speed (200 m/min). This fact was demonstrated by the considerable reduction of the feed force, as can be seen on Figure 12.

 

 

Braga et al. (1999a and b) tested the use of several cooling/lubrication systems in drilling of an aluminum-silicon alloy (7.5% of silicon), using solid carbide drills with and without PCD coating. These systems were: dry cutting, cooling just with dry compressed air, MQF with oil flow of 10, 30 e 60 ml/h and overhead flood of soluble oil.

The main conclusions of the work were:

a) it is impossible to carry out the operation with either dry cutting or pure dry compressed air, because the chip sticks to the spiral channels of the drill, causing its breakage after few holes. The use of MQF makes the operation feasible, and the increase of oil flow in the mixture (from 10 to 60 ml/h) does not make the process performance better;

b) comparing MQF with 10 ml/h and flood of soluble oil, it can be verified that both cooling systems generate holes with similar qualities (roughness, roundness, diameter accuracy and cylindricity). Drilling with both systems also presented, in the majority of the experiments, similar values of tool life and cutting forces;

c) based on these results, the authors concluded that it is not necessary too much cooling (which can be achieved when high amount of cutting fluid is used) for drilling this alloy and to efficiently lubricate the process it is not necessary a large volume of oil.

On the other hand, Scandiffio (2000) tested the use of MQF in turning at high cutting speeds 1045 steel with coated carbide tools ISO P15, comparing it with dry cut and overhead flood of soluble oil. In this case MQF did not present a better performance than dry cutting neither in terms of workpiece surface roughness, nor in terms of tool life and cutting forces in the several cutting conditions tested. Therefore, the use of MQF in this situation is not recommended. Dry cut presented, generally, a worse performance than overhead flood of oil in terms of tool life and similar performance in terms of surface roughness and cutting forces. Figure 13 shows the tool life results obtained in these experiments (tool life criteria – medium flank wear, VBB = 0.3 mm). As can be calculated from this figure, at 360 m/min, tool life using dry cut was around 67% of that obtained with abundant flood, about the same at 445 m/min and 70% of the tool life when cutting at 530 m/min. Therefore, the choice between both must be done based on an economic and environmental analysis. As already mentioned, some times it is better to spend more with tools to save with absence of oil and, avoiding recycling and all the damages the oil can bring.

 

 

To contribute further with the discussion of using or not MQF, some points must be raised. They are:

Environmental Pollution – The substitution of abundant oil by MQF is based, among other factors, on environmental issues. But it must be remembered that even MQF causes pollution, because the pulverization of oil in the air flow causes the suspension of a lot of oil particles on the air, which also demands some requirements of the system, like a machine completely encapsulated with protection guards and a good exhaustion system with particle control. According to Heisel et al., (1998) the size and kind of particles (steam, mist or oil smoke) are important information for this particle control.

Consumption – The application of mist frequently is made with total loss of the cutting fluid used. Even with low oil flow (< 50 ml/h) the fluid consumption must be calculated and considered. Just as an example, if we had a flow of neat oil of about 10 ml/h, with a continuous use of 8 h/day (considering just one work shift per day), we would have, at the end of the day, 80 ml of consume. At the end of the month (with 22 days of work) 1760 ml of fluid would be consumed. In three months more than 5 liters of fluid would be pulverized. Some synthetic fluids at a concentration of 5% may have lower consumption than that. Considering a machine tank of 60 liters it would demand 3,15 liters of fluid to have this concentration. These products may have a continuous use with a life longer than six months. Even considering possible losses, the consumption of this product in this period could be lower than when pulverization is used.

Noise – A line of compressed air must be used for MQF, which works intermittently during the whole process. These air lines make a lot of noise, usually higher than what human ear can support (< 80 dB). Besides this damage to human health it makes the communication more difficult, what is also bad for the environment.

Actually, the attempt of using MQF for machining may be considered an intermediary situation between the conventional use of fluid and the dry cutting. But the results published up to now does not allow to conclude that this procedure can be widely used in the industry.

To avoid all this problems, the ideal situation is the dry cut, called by some researchers ecological machining. A lot have been done lately on this subject. Some experts (Batzer e Sutherland, 1998 e Graham, 2000) states that either when cutting fluid application is not clearly cheaper or when technical reasons demands its application, the ecological issues should be priority. Moreover, as already stated, there are some cases where dry cutting is economically advantageous.

 

References

BATZER, S.; SUTHERLAND, J., 1998, "The Dry Cure for Coolants Ills" Cutting Tool Engineering, June, pp 34 – 44.        [ Links ]

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Manuscript received: December 2000, Technical Editor: Átila P. S. Freire.

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