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Figure 211. Some typical, but extreme skin disorders – attributed to exposure to cutting uids. [Courtesy of Castrol Industrial]. Cutting Fluids Many other skin conditions can occur and their causes can emanate from a number of MWF sources – going beyond the current scope and objectives of this chapter. Although through the application of bar - rier and conditioning creams, together with clean and suitable protective clothing, coupled to good washing facilities, these factors will inevitably lessen the pos - sibility of allergic reactions and skin disorders. Tumours and Cancerous Effects However, less well known than the allergic and skin condition previously mentioned, are the other more serious debilitating health eects on the machine tool personnel exposed to MWF’s. Industrial experience suggests that continuous and long exposure to certain mineral oils can give rise to skin thickening, known as keratosis, whereby ‘warty-elevations’ (i.e. see Fig. 211b) can slowly develop over a period of some years. Hence, these warts will either: remain as they are; disappear; or in the worse case scenario, become malignant. A considerable volume of research in both the chemical and biological elds has been undertaken, in particular, into the eects of mineral oils in cut - ting uids and their aect on worker’s health. Mineral oils may contain carcinogens – chemical compounds which are active in causing cancer, with currently, a number of these compounds having been identied. ey occur in the main, as polycyclic aromatic hydro- carbons and, when present in modern rened mineral oils exist in extremely small proportions – making their ‘positive’ chemical identication exceedingly dicult to dene. Oil renement by acid treatment has now been replaced by more modern rening techniques, including solvent-rened treatment and hydrogenera - tion – greatly reducing the undesirable proportions of aromatic compounds (i.e. these latter compounds being potential carcinogens) 33 . Moreover, chemical coolants were originally based on diethanolamine and sodium nitrate, which for some time have been suspected of forming ethanolnitrosamine – another suspected carcinogen. In order to remove this pos - sible carcinogen, in 1984, cutting uid manufacturers removed the nitrates from their formulations. Finally, 33 ‘N-nitrosamines’ , and its chemical compounds are a signi- cant danger to worker’s health and, the American Environ- mental Protection Agency (EPA), stated in a report of their ndings in 1974, that: ‘As a family of carcinogens, the nitrosa- mines have no equal.’ if one considers permissible exposure levels (PEL’s) from nitrosamine sources. en, it has been stated that smoking twenty (untipped) cigarettes per day will de- liver 0.8 micrograms of various nitrosamines which al- most equates to eating a kilogram of fatty bacon per day (i.e. 6 microgrames), thus, when undertaking these seriously debilitating smoking/eating toxicity habits over a signicant period of time, they would consider - ably increase the risk of cancer. Cutting Fluid Mists Mists resulting from machining operations and their subsequent collection resulting from the application of cutting uids, are usually given a low priority by most manufacturers when compiling a list of potential capi - tal items for the workshop. To press this point still fur - ther, many companies would much sooner purchase a new machine tool, than install a special-purpose air cleaner. In the automotive industries interest in the level of air quality has some degree of importance, while elsewhere in smaller production workshops it is somewhat of a hit-or-miss aair. Given the potential worker health risks involved today, with high-speed machining (HSM) coupled to increased tooling cut - ting data and higher-pressure coolant supplies (i.e. see Fig. 195 – top), possibly the greatest threat posed to a worker is from atomised mists (i.e. sub- µm size) within the local atmosphere. Many companies that incorporate mist collection ltering, will only remove particles of >4 µm in size, leaving the critical sub-µm particles still present in the atmosphere. e earliest chemical interventions to reduce mist - ing were high-molecular-weight polymer additives, that act to stabilise MWF’s and thus suppress mist for - mation. With conventional petroleum-based uids, polyisobutylene has been the preferred anti-mist ad - ditive. While, for aqueous-based cutting uids, poly - ethylene oxide (PEO) has been utilised. Due to the susceptibility of PEO’s to shear degradation, repeti - tive additions of the PEO polymer are needed to main - tain mist reduction. Today, a newer class of shear- stable polymers has been developed to overcome the shear degradation as indicated by PEO’s. ese latest polymer products have been derived from complex: 2-acrylamido-2methlypropane sulphonic acid mono - mers, hence, providing longer-term performance in continuously recirculating aqueous-based MWF sys - tems. So, very high concentration cutting uid mists will over a short period of time cause: ‘smarting’ of the eyes; irritation of exposed skin; result in slight irrita - Chapter tion of the mouth and throat; by inhalation, will ir- ritate the lungs; by ingestion, of the stomach – it may promote nausea; and aect other internal organs. If exposed to toxic mists over a long period of time, this could cause lasting damage to both external and internal bodily-parts, with at the extreme condition, promoting the growth of malignant tumours. In order to restrict misting and minimise operator health risks, then special-purpose ltering systems have been de - veloped, which will be briey reviewed below. e conventional mist-collection technology, such as: lters; rotating drums; or cyclones; will col - lect particles of >1 µm in diameter, but cannot cope with smaller sub-µm particles. Further, it has been re - ported that brous lters once they are wet, lose ef - ciency over time – see Fig. 212. erefore, the opti - mum manner of removing sub-µm mists are by tting one of the following: High-eciency Particulate Air l- ters (HEPA); Electrostatic Precipitators (ESP’s); or Fi- bre-bed systems. Probably the two best systems for re- moval of sub-µm mist particles are the HEPA and ESP systems. Each one has its disadvantages, with HEPA l - ters being expensive and become clogged, thereby los - ing eciency. So, when disposable lter replacements are needed this hidden replacements cost, will result in both costly maintenance and disposal. While, ESP’s need frequent maintenance and cleaning, thus rep - resenting a continuous on-going cost burden. Mean - while, Fibre-bed systems oer high eciency in mist collection, but with ease of maintenance, although they are larger requiring more electrical power to op - erate them. Vegetable Oil-Based MWF’s Driven by the health and safety concerns of both workers and manufacturers alike, vegetable oil-based MWF’s have been developed, to substitute for the same machining operations as either the mineral-, or petro - leum-based uids, currently undertake. It has been reported that compared with mineral oil-based cut - ting uids, the alternative vegetable-based MWF’s, en - hance cutting performance by extending tool life while improving machined surface texture, with the addi - tional benet of being an environmentally-friendly MWF. In particular, Soybean oils have shown con- siderable promise as a practical alternative to ‘tradi - tional’ MWF’s, where they have improved component surface texture and reduced tool chatter. One of the principle reasons for these surface texture and ma - chining improvements, is that the vegetable oil-based MWF’s have enhanced lubricity, coupled with a slight ‘polar-charge’ – which acts to attract the vegetable oil molecules to the metallic surface being tenacious enough to resist any subsequent wipe-o. e oppo - site is true for a mineral-based oil, where there is no molecular charge, so oers little improvement in lu - bricity. Mineral-based MWF’s are just straight hydrocar - bon, while their vegetable oil counterparts contain oxygen, which is tenaciously-attracted to the sterile elevated temperature of the recently-machined work - piece’s metallic surface, thus it bonds more strongly – acting as a result as a better lubricant. Yet another performance benet of utilising vegetable-based oils over their mineral-based equivalents, is that they have a higher ‘ash-point’ 34 , which reduces both the ten- dency for smoke formation and re-risk. Yet another reason for selecting a vegetable-based MWF over its mineral-based counterpart, is that it has a high natural viscosity 35 . Hence, as the machining temperature increases, the viscosity of the vegetable oil drops more slowly than for that of a mineral oil. Conversely, as the temperature falls, the vegetable oil remains more uid than its counterpart mineral oil. us, facilitating more ecient and quicker drainage from both the swarf and workpiece. e high viscosity index 36 of vegetable oil ensures that it provides more lubricity-stability, across the operating temperature range being found during a range of machining operations. High viscosity allows vegetable oils to be used as a slideway lubricant and for gear lubrication in gearboxes, acting as a so-called: ‘multi-functional uid’ (i.e. see Section 8.9). Along with the above stated benets, there is also a down-side to vegetable-based uid applications, the limitations are that they lack sucient oxidative sta - 34 ‘Flash-point’ of oils, is the instantaneous ignition of the oil at a specic temperature, without the aid of a ame. So, in the case of a Soybean oil it has a ash-point of 232°C, while a typi- cal mineral oil has a ash-point of just 113°C. 35 ‘Viscosity’ , can be dened* as: ‘e resistance of a uid to shear force.’ erefore, the shear force per unit area is a constant times the velocity gradient, with the constant being the coef- cient of viscosity. SI units are: Newton-seconds per square metre (Ns m –2 ), denoted by the Greek symbol: ‘µ’. [Source: Carvill, 1999] *While another denition for a uid’s viscosity is: ‘e bulk property of a uid, semi-uid, or semi-solid substance that causes it to resist ow.’ [Kalpakjian, 1984] 36 ‘Viscosity index’ , can be dened as: ‘A measure of a uid’s change of viscosity with temperature: the higher the index, the smaller the relative change in viscosity.’ [Kalpakjian, 1984] Cutting Fluids bility for many machining applications. us, a low oxidative stability means that the oil will oxidise quite quickly during use, becoming thick as it polymerises to a plastic-like consistency. Once the oil has become too thick, or even too thin for that matter, the cutting tool’s edge(s) will quickly wear-out. Vegetable oils be - come oxidised and as a result, will chemically change, along with their viscosity and lubricating abilities. ere is some concern among users of vegetable-based cutting uids, that this oil reacts with the environment (i.e. oxygen and metals), thus breaking-down, which is not the case for petroleum-based products. Both of these uid products oxidise with heat, but vegetable oils are more susceptible to oxidation. While another Figure 212. At the lter some droplets and volatiles are re- moved from the atmosphere, but the remainder pass through and are re-entrained. Other particulates are ‘indenitely’ re- tained, but with time reduce lter eciency. Optimum lters . maximise droplet removal, while minimising evaporation and re-entrainment – at a reasonable pressure-drop. [Source: Raynor P. & Leith, D. – Univ. of North Carolina, 2003] Chapter drawback to utilising vegetable-based oils, are its lack of hydrolytic stability 37 . Typically, when making an emulsion; obviously oil and water are present; so if ox - ygen and some form of alkaline component is at hand, it may cause certain ester linkages within the vegetable oil to break down. ese broken-down components act in a dierent manner to that of the original vegeta - ble oil, thereby aecting its cutting uid performance. Conversely, mineral-based cutting oils are resistant to hydrolytic reactions. Vegetable oils can support micro - bial growth more readily than the equivalent mineral- based cutting uids. Although this vegetable oil’s bio - degradability is ideal for subsequent waste treatment, conversely, when this product is ‘festering’ in a ma - chine’s sump, it becomes both smelly and sour, via its bactericide and fungicide reactions. Finally, for many companies, probably the biggest limitation in changing over to vegetable-based products in machining opera - tions is its purchase cost. For example, canola oil, can cost up to 300% more than its equivalent petroleum- based product and to compound the nancial problem still further, costly ingredients are necessary to control oxidation and enhance its biological stability – consid - erably adding to the nished product’s cost. 8.12 Fluid Machining Strategies: Dry; Near-Dry; or Wet So far, this chapter has been principally concerned with all aspects of ood/wet coolant applications to the overall machining process. Several other complemen - tary cutting strategies can be adopted, these include: dry; near-dry; together with wet machining; thus, in the following sections a discussion of these important issues and concerns will be briey mentioned. 37 ‘Hydrolytic stability’: ester molecules consist jointly of con- densed fatty acids and alcohols; so the vegetable oils will naturally exist as esters – oen termed ‘triglycerides’ , these being a condensation of fatty acid, plus glycerine. Under the right conditions, the triglyceride can split and revert back to a fatty acid and glycerine, which acts dierently from that of the original ester. In the case of mineral-based oils, they do not contain these ester linkages and as such, will not break down, nor ‘hydrolise’. .. Wet- and Dry-Machining – the Issues and Concerns In the past twenty-ve years the cost of cutting u- ids has risen from just 3% of the overall cost of the machining process, to that of >15% of a production shop’s cost. Cutting uids and especially ones that are oil-based products have become something of a liabil - ity of late, this is due in the main, to many countries ‘Environmental Protection Agencies’ , strictly regulat - ing their ensuing disposal – at the end of their natural life. In many countries ‘spent’ cutting uids have been re-classied as either ‘toxic-’ , or ‘hazardous-waste’ , moreover, if they have been found to have machined certain alloyed and exotic material workpieces, they are under even harsher disposal regulations. Due to the increasing popularity today of high-speed machin - ing (HSM) – more will be said on this subject in the following chapter – coupled to increased cutting data and the application of coolants via high-pressure sys - tems, these factors have signicantly contributed to the creation of air-borne mists within the workshop environment (i.e. see Fig. 212). Such coolant mists now have even stricter permissible exposure levels (PEL’s) imposed in the working environment, to regu - late and control these air-borne particulates, thereby minimising workers health risks. us, the cost of: uid maintenance; record-keeping; with strict compli - ance to current and proposed regulations, have rapidly increased the overall price of cutting uids. In many manufacturing companies involved in a signicant amount of machining operations, they are consider - ing the strategy of cutting dry, to overcome the cutting uid-based costs and disposal concerns during and aer their subsequent usage. For many companies involved in signicant work - piece machining operations, they are unsure if they could cut all their components ‘dry’. Furthermore, they are under the impression that to achieve higher cutting data and ‘hard-part’ machining, then cutting uids are essential in achieving these objectives. Moreover, many companies also believe that the cost of chang - ing from a ‘wet-’ to ‘dry-machining’ operations would be prohibitively high. Neither of these impressions are true. So, by machining ‘dry’ it can be considered as a standard operational procedure for most metal-cut - ting operations, including: turning, drilling and mill - ing operations (i.e see Figs. 39, 49 and 168a, respec - tively). Moreover, it is not only possible to ‘hard-part’: turn (Fig. 15) and bore (Fig. 65b); or mill (Fig. 172); etc.; but these can now be classied as highly-prot - able ‘dry-machining’ activities. Cutting Fluids Probably the chief obstacle to dry-machining ac- ceptance, is that conventional wisdom dictates that MWF’s are vital in attaining acceptable machined n - ishes and will considerably extend the tooling’s life. In many circumstances these are valid points, but with some of the advanced cemented carbide grades and high-technology coatings, such tooling can be oper - ated at higher cutting data than was previously the case and, cope with their elevated machining tempera - tures. In fact, if interrupted cutting occurs, the hotter the cutting zone becomes, the more unsuitable will be the application of a cutting uid – as the thermal shock 38 becomes greater with a ‘wet-machining’ strategy. Present tool coating technologies are vital to dry- machining applications, as they both control the tem - perature uctuations, while restricting heat transfer from the cutting vicinity to the insert, or tool. Mul - tiple coatings act as a heat barrier because they of - fer a lower thermal conductivity to that of the tool’s substrate and the workpiece material. us, coated inserts/tooling absorb less heat and as a result, can tol - erate higher cutting temperatures, allowing more ag - gressive cutting data, whilst not debilitating the tool’s life. Coating thickness is also important, as the thin - ner the overall coatings, the better they can withstand temperature uctuations, that might otherwise arise, if thicker coatings were utilised. e main reason for this improved thermal shock performance of thinner, rather than thicker tool coatings, is that a thinner coat is less likely to incur the same stresses, hence, they are less susceptible to cracking as a result. So, by running thin coatings in ‘dry-machining’ operations, normally extends tool edge life by up to 40%, over thicker coat - ings 39 . 38 ‘ermal shock/fatigue’ , the cyclical nature of both rapid heating followed by immediate cooling – in for example face- milling (i.e see Fig. 213 – top), or when interrupted turning (e.g. when eccentric turning, or OD/ID machining with either splines and keyways present), promotes potential tool edge fracturing – resulting from the cyclic thermal stresses and in- creased temperature gradients, being exacerbated by the ap- plication of a cutting uid. 39 ‘in coats-v-thick coats’ , the former coating oers longer life than the latter coating process. Today, it is normal to utilise the coating process of: physical vapour deposition (PVD) as this type of coating is thinner and will adhere/bond more strongly, than the alternative chemical vapour deposition process. For example, a TiAlN PVD coated insert/tool can have a hardness of 3,500 Hv, withstanding cutting temperatures up to 800°C. ‘Dry-machining’ – some Factors for Consideration • Adopting a ‘dry machining’ strategy will only make sense, if all the cutting processes in the part’s manu- facture can be performed without coolant, • Only by utilising specialised cutting tool geome- tries, can ‘dry-machining’ be possible and eective, • Tooling typically having special hard multi-layered, or diamond-like coatings, etc., to isolate heat and create minimal thermal conduction across the tool/ chip interface, • Employing cutting tool materials producing sharp edge geometries – to reduce heat, • For drilling operations, utilise ‘so-glide’ coatings – for lubrication, with the necessary and appropriate ecient chip transportation geometries, • Speedy and ecient removal of both chips and as- sociated steam – by suction – are important factors in ‘dry-machining’ , • Utilise new machining concepts, plus the latest fully-enclosed machine tools – whenever possible, • Employ faster, rather than slower cutting data, to al- low the majority of heat to be conned to the evacu- ated chips. .. Near-Dry Machining e strategy of ‘near-dry’ machining is not a new con- cept, it has been in existence for more than 50 years. However, this machining and lubricating approach is still not a universal practice, which is surprising when one considers the real benets that accrue from the practice over its ‘wet-machining’ counterpart. As its name implies, in ‘near-dry’ machining little lubri - cant is used – normally vegetable-based oils, meaning that both cutting uid treatment and its disposal are eliminated. Further, instigating a ‘near-dry’ machining strategy means that there are fewer worker health risks from resultant mists, which might otherwise create: re - NB From a metallurgical/materials science viewpoint, the: TiAlN – PVD tool coating can attribute its superior mechani- cal/physical properties to an amorphous aluminium-oxide lm that forms at the tool/chip interface, as some of the alu- minium of the coating surface oxidises at these elevated ma- chining temperatures. While, even more exotic multiple-type diamond-like coatings can be applied and their like, which of- fer even greater cutting performance – in certain machining circumstances, when applied to the tool’s cutting edge(s). Chapter spiratory problems: skin irritations; etc. e ‘near-dry’ cutting approach can be exploited across a wide range of either ferrous, or non-ferrous workpiece metals. Most machine tools are equipped with the capabi- lity of supplying ood coolant to the cutting process, together with ‘through-coolant’ tooling systems, mean - ing that the cost to recongure for that of a ‘near-dry’ technology is not prohibitive. Assuming the worse- case scenario of requiring a through-coolant tool - ing system, then probably just over $5,000 at today’s prices should prove sucient capital to complete the task. Some re-tooling to complement the ‘near-dry’ machining production techniques may be necessary, allowing the precise application of lubricant to the cut - ting edge(s). Further, the user must consider a method for ecient chip removal from the cutting area. Usu - ally, with external ‘near-dry’ cutting operations, the lubricant is transported within the media of a com - pressed air application, via the correct-sized aperture nozzle – pointed toward the cutting zone. Control of the volume of lubricant delivery to the tool and work - piece area is critical, with the common misconception being that more lubricant is better! e optimum ar- rangement for ‘near-dry’ lubricant application, is when the minimum of over-spray and resultant misting does not occur. With external ‘near-dry’ operations, dispensing systems usually consists of reservoir metering pumps and valves, being mounted on the machine tool’s exte - rior – at some convenient location. While the nozzles are strategically-mounted so that they can easily be directly aimed at the tool’s cutting edge(s). Normally, the nozzles are a manufactured from either copper, or plastic and ‘snap-together’ – being much smaller in size than their ‘wet-machining’ counterparts. For internal machining operations, having tooling with ‘through-the-nose’ delivery, the lubricant is mixed with compressed air prior to delivery to the cutting zone. e admixing of compressed air and lubricant keeps the lubricant in suspension, with these oil par - ticles being broken-down into minute particles prior to being fed into the compressed air jet stream – on their way to the tooling. For ‘conventional’ ood coolant delivery the sys - tems, the coolant channels are lled with cutting uid, which inevitably nds its way to the cutting zone. If however, in a ‘near-dry’ machining conguration, a heavy mist of lubricating oil oats through the com - pressed air, attempting to negotiate all of the twists and turns on its way to the cutting zone, this may pres - ent a potential lubrication clogging/starvation prob - lem. Hence, for a successful ‘near-dry’ delivery system, the lubricant channels need to be smooth and even, with direct ows from the coolant pump to the cutting zone. A basic misapprehension by some machine tool designers and manufacturers, is that copious volumes of ood coolant are necessary to remove large quanti - ties of swarf. In fact, just the opposite can occur, as wet chips will not only pack tightly together but have a surface tension property to them, tending to make them adhere to machine tool surfaces (i.e. see Foot - note 29, ‘Lang’s chip-packing ratio’ in Chapter 2). is is not generally the case for ‘near-dry’ lubrication, as the chips here, have a thin layer of non-oxidising lu - bricant surrounding them and with their evacuation velocity – aer being machined, coupled to gravita - tional eects, means that they will fall to the bottom of the swarf tray, or into the chip conveyor. It is good working practice to use the external air-only supply’s blow-o nozzles to clear away chips form the cutting area 40 , however, it is not recommended to use the oil/ mist to achieve chip clearance, as it will simply blow the lubricant straight past the cutting edge(s), while probably creating an unwanted oil-misting problem. It is possible to incorporate both ‘wet-’ and ‘near-dry’ lubrication systems onto the same machine tool. It has been reported that for external/internal work the change-over from one system say, from ‘wet-’ , to the other – ‘near-dry’ , takes about 3 minutes to complete. For ‘near-dry’ machining to be successful, it de - pends upon various factors, including: workpiece ma - terial to be machined; tool geometry and its coating(s); speeds and feeds selected; plus other important fac - tors. If applied correctly, ‘near-dry’ machining has sig - nicant direct and indirect benets to the machining process as a whole. Economics of: ‘Dry-’; ‘Near-Dry’; and ‘ Wet-Machining’. For any tool and workpiece lubrication strategy to operate eectively, a range of cost factors need to be considered, regardless of the method of machining 40 ‘Chilled compressed air’ , has been successfully utilised in the past for not only removing chips from the cutting vicinity, but on certain materials, the continuous application of chilled compressed air acts simply as a form of ‘basic lubricant’ for the cutting process in hand. Cutting Fluids undertaken. In Fig. 213, a table has been constructed to show the relative merits of the three machining strategies previously discussed, namely: ‘dry-’; ‘near- dry’; or ‘wet-machining’. e cost component for each of these lubrication strategies has been broken down into its relevant parts, with some of them not being ap - plicable to every lubrication application. If one ignores the individual cumulative factor in the overall cost and simply looks at the ‘bottom-line’ , namely, the total relative costs for each process, then a clear message is being given here! Explicitly, that ‘wet-machining’- in certain cases, when compared to ‘dry-machining’ is Figure 213. Indicates the comparative costs for utilising either: ‘dry-’, ‘near-dry-’ or ‘wet-machining’ strategies. Chapter >330% more expensive overall, this being a good rea- son to look carefully at employing ‘dry-machining’ techniques – when applicable! In Appendix 14, a MWF ‘trouble-shooting guide’ has been included, to help establish the relative causes and remedies for certain uid-related problems – as they arise. References Journals and Conference Papers Antoun, G.S. e Pressure’s On to Improve Drilling [High- Pressure and Volume Coolant Supply]. Cutting Tool Eng’g., 59–68, Feb., 1999. Batzer, S. and Sutherland, J. e Dry Cure for Coolant Ills. Cutting Tool Eng’g., 34–44, June, 1998. Benes, J. 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Types of Cutting Fluids used on Turning/Ma- chining Centres. Int. Conf. on Industrial Tooling (South- ampton, UK), Shirley Press, 140–150, Sept. 1995. King, N. Ah, the Smell of It [Microbiological Colnat Ef- fects]. Manufact. Engr., 7–9, Dec., 1991/Jan. 1992. Krahenbuhl, U. Lightening [Coolant Lubricity]. Cutting Tool Eng’g., 28–34, Sept., 2002. Leep, H.R. Investigation of Synthetic Cutting Fluids in Drill- ing, Turning and Milling Processes. Lubric. Eng’g., Vol. 37 (12), 715–721, 1981. Manfreda, J. and Elenteny, D. Cool Savings, Cutting Tool Eng’g., 42–51, Feb., 2006. Phillips, D. Under Pressure [High-Pressure Coolant Deliv- ery]. Cutting Tool Eng’g., 48–53, Jan., 2000. Phillips, D. Dry Run [Dry Drilling]. Cutting Tool Eng’g., 34–40, Feb., 2000. MCCabe, J. Dry Holes – Dry Drilling Study. Cutting Tool Eng’g., 40–50, Feb., 2002. Protch, O. Fluid Cutting. Cutting Tool Eng’g., 40–43, Dec., 2001. Rahman, M., Senthil Kumar, A. and Salam, M.U. 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