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Dry machining: Machining of the future
Journal of Materials Processing Technology 101 (2000) 287±291
Dry machining: Machining of the future P.S. Sreejith*, B.K.A. Ngoi School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 27 January 1999
Abstract Machining without the use of any cutting ¯uid (dry or green machining) is becoming increasingly more popular due to concern regarding the safety of the environment. Most industries apply cutting ¯uids/coolants when their use is not necessary. The coolants and lubricants used for machining represents 16±20% of the manufacturing costs, hence the extravagant use of these ¯uids should be restricted. However, it should also be noted that some of the bene®ts of cutting ¯uids are not going to be available for dry machining and also dry machining will be acceptable only whenever the part quality and machining times achieved in wet machining are equalled or surpassed. This paper presents recent developments in the dry machining operation. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Dry machining; Temperature; Cutting tool; Coating
1. Introduction Dry machining is ecologically desirable and it will be considered as a necessity for manufacturing enterprises in the near future. Industries will be compelled to consider dry machining to enforce environmental protection laws for occupational safety and health regulations. The advantages of dry machining include: non-pollution of the atmosphere (or water); no residue on the swarf which will be re¯ected in reduced disposal and cleaning costs; no danger to health; and it is non-injurious to skin and is allergy free. Moreover, it offers cost reduction in machining. The various possible routes to achieve clean machining processes were analysed and discussed by Byrne and Scholta [1]. Elimination on the use of cutting ¯uids, if possible, can be a signi®cant incentive. The costs connected with the use of cutting ¯uids are estimated to be many more times than the labour and overhead costs [2]. Hence the implementation of dry machining will reduce manufacturing costs. In the manufacturing industry, cutting ¯uids help: to remove the heat generated due to friction during cutting; to achieve better tool life, surface ®nish and dimensional tolerances; to prevent the formation of built-up edge and to facilitate the transportation of chips. Coolants are essential in the machining of materials such as aluminium alloys and most stainless * Corresponding author. Tel.: 65-7906875; fax: 65-7911859. E-mail address: [email protected] (P.S. Sreejith).
steels, which tend to adhere to the tool and cause a built-up edge. At the same time, the coolants produce problems in the working environment and also create problems in waste disposal. This creates a large number of ecological problems, but which in turn result in more economical overheads for manufacturing industries [3,4]. If industries were to practise dry machining, then all of the above-mentioned problems should be addressed satisfactorily. The cutting ¯uid industries are reformulating new composites that are more environmental friendly and which do not contain Pb, S or Cl compounds [5]. Consumption of cutting ¯uids has been reduced considerably by using mist lubrication. However, mist in the industrial environment can have serious respiratory effects on the operator. The use of cutting ¯uids will be increasingly more expensive as stricter enforcement of new regulation and standards are imposed, leaving no alternative but to consider dry machining [6]. Many metal-cutting processes have been developed and improved based on the availability of coolants. It is well known that coolants improve the tool life and tool performance to a great extent. In dry machining, there will be more friction and adhesion between the tool and the workpiece, since they will be subjected to higher temperatures. This will result in increased tool wear and hence reduction in tool life. Higher machining temperatures will produce ribbon-like chips and this will affect the form and dimensional accuracy of the machined surface [7,8]. However, dry cutting also has some positive effects, such as reduction in thermal shock and
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 4 4 5 - 3
288
P.S. Sreejith, B.K.A. Ngoi / Journal of Materials Processing Technology 101 (2000) 287±291
hence improved tool life in an interrupted-cutting environment. 2. Developments in dry machining To pursue dry machining, one has to compensate for the bene®cial effects of cutting ¯uids. One approach towards dry machining is to have an indirect contact of coolants and thereby take away the heat generated. Research in the area of dry machining without the direct contact of coolants has lead to the following advancements: 1. An under-cooling system [9], where the coolant ¯ows through channels located under the insert, then out to the environment, without any direct contact with the cutting zone. 2. Internal cooling by a vaporisation system [10], in which a vaporisable liquid is introduced inside the shank of the tool and vaporised on the underside surface of the insert. 3. Cryogenic systems [11], where a stream of cryogenic coolant is routed internally through a conduit inside the tool. 4. Thermoelectric cooling systems [12], using a module of couples of thermoelectric material elements. When an electric current is passed through the thermoelectric elements, a cold junction and a hot junction is produced at the opposite ends of each of these elements. Another approach is to improve the properties of the tool material by making them more refractory or generate less heat during machining. There has been a continuous development in the ®eld of cutting-tool materials starting with HSS, cobalt alloys, cemented tungsten carbide, coated carbides and coated HSS, cubic boron nitride and diamond. However, the need for machining with increasingly higher cutting speeds and also to machine dif®cult-to-machine materials are imposing pressure for the development of new tool materials. As a result, newer tool materials such as ceramics and also different types of coatings on the tool materials will address the problems in dry machining to some extent. 3. Effect of temperature on machining The performance of a cutting tool is dependent on the form stability of the cutting wedge, which in turn is mostly dependent on the effective/working hardness and thermal conductivity of the tool±work materials. The working hardness of the tool material is related to the hot-hardness characteristics of the tool material. Typical hot-hardness characteristics of different tool materials are illustrated in Fig. 1. It can be seen that, barring carbon steel and highspeed steel (HSS) materials, all other tool materials exhibit a steady reduction in hardness with increase in temperature. The temperature of machining yc is dependent on the cutting
Fig. 1. Typical hot-hardness characteristics of some tool materials [13].
speed V and the type of material pair involved in machining and can be expressed as yc / V b
(1)
The hardness of the tool material is selected so as to maintain consistent replication of its nose on the work surface, to maintain the desired surface texture. A reduction in tool material hardness with increase in temperature (due to the increased velocity) is to be limited so that a high degree of ®delity of nose replication can be maintained. This calls for limiting the temperature of machining, which can be attempted by limiting the speed of machining. The situation is more aggravated during the machining of heat-insulating materials. The signi®cance of machining speed on the machining performance is illustrated in Fig. 2. It is seen that a range of cutting speed can offer a highly ef®cient machining performance. This is again re¯ected in the need for controlling the machining temperature within limits for achieving good machining performance. This aspect of a temperature limit for economical machining has to be taken into consideration for dry machining. 4. Cutting tool requirements for dry machining Dry machining in the more demanding interrupted cutting environment calls for a tougher grade of hard tool material. For this, the solution can be selection of plain cemented
P.S. Sreejith, B.K.A. Ngoi / Journal of Materials Processing Technology 101 (2000) 287±291
Fig. 2. Illustration showing the signi®cance of machining speed on machining performance [14].
carbide, K-type grade of tools. However, the application of such tools can be with only a lower order of cutting speeds, the limitation being due to cutting temperature. In order to
289
overcome this, one can make use of K-type coated cemented carbide tools. Ultra-hard tool materials such as diamond and cubic boron nitride (CBN) cutters have been found to produce better machined surface characteristics, with considerable increase in tool life due to their higher hardness and thermal conductivity. Diamond and CBN, being super-hard materials, have found numerous applications as cutting tools. Diamond and CBN are very similar in many ways. They share the same crystallographic structure and exhibit high values of thermal conductivity. However, diamond oxidises in air and is subjected to graphitisation, loosing its form stability, thus limiting its application to relatively lower temperature situations. The quality of a machined surface, especially in turning, depends largely on the form stability of the cutting tools. During turning, the cutting tool replicates its nose on the work surface, resulting in the formation of the surface texture (Fig. 3). From the surface texture point of view,
Fig. 3. Typical illustration of surface production in machining [15].
290
P.S. Sreejith, B.K.A. Ngoi / Journal of Materials Processing Technology 101 (2000) 287±291
an ideal tool is one that is able to sustain the replication of the cutting nose. Hence, the performance of the tool or the quality of the machined surface texture is largely dependent on the form stability of the cutting tool. Form stability is largely in¯uenced by the different forms of tool wear. The quality of a machined surface is important whilst evaluating the reliability and functional life of a structure. In addition, inappropriate selection of a tool material can result not only in surface deterioration but also in increase the tooling costs. Therefore, knowledge of cutting tool materials and the nature and quality of the surface that they produce is of considerable importance. For achieving the desired results in machining, one has to be well informed regarding both the work material and the cutting tools. The tools used for dry machining have to suit some speci®c requirements. The options available under these include the following: 1. The use of very high positive rake angles (308) on submicron cemented carbide tools, which will reduce the overall cutting energy signi®cantly. 2. The development of refractory-type tool materials that can withstand high temperatures. 3. The use of ultra-hard tool materials such as diamond and CBN. 4. The development of coatings on tools that can withstand high temperatures and at the same time provide a lubricating effect to reduce friction. 5. Dry machining of common metals The dry machining of cast iron is not exactly new. Cast iron can be cut dry in turning and milling operations. The dry machining of cast iron has been attempted using ceramic cutting materials and CBN at high surface speeds and feed rates by Spur and Lachmund [16]. They found that since CBN tools have the highest thermal conductivity compared with the ceramic type of tools, CBN was able to remove the heat ef®ciently from the cutting material. Hence they concluded that CBN tools were highly suited for the dry machining of cast iron at high cutting speeds. Cast iron can also be dry milled with the cermet type of tools at high speeds [5]. Here the high speed employed is not to reduce the machining time, but to reduce the tool and workpiece contact time to prevent the heat of the chip penetrating the tool. Drilling is the most widely used process in the steel industry due to the assembly of components. The main problem associated with the dry drilling of steel is the removal of chips from the drilled hole. One approach to reduce this problem is to enlarge the ¯utes and thereby giving the chips more space, helping them to come out of the hole. Another problem that can be encountered is the jamming of the drilled hole due to the expansion of the drill at high temperatures. One way of correcting this problem is to give more drill taper towards the shank.
For the continuous high speed machining of superalloys and titanium, cooling is necessary. However, in an interrupted cutting environment, the coolant if used induces thermal shocks on any cutting material. For this reason dry machining can be recommended for interrupted cutting. Aluminium and its alloys are considered to be the most critical materials with regard to dry machining. Because of their higher thermal conductivity, the workpiece absorbs considerable heat from the machining process and may cause deformation due to its higher thermal expansion capabilities. In addition aluminium alloys may cause problems related to chip formation. Hence in the machining of aluminium and its alloys, it is essential to use tools with suitable coatings. The main issues that have to be considered for the dry machining of non-ferrous metals are the achieving of higher spindle speeds, the improvement of chip ejection geometry and the design of better tooling. Diamond tools will be a major enabler of this technology because of their high thermal coef®cient, fast heat diffusion, no af®nity for aluminium and the possibility of diamond being used as a coating for other shaped tools. 6. Coated tools for dry machining Since 1970s many new developments have been made in tool coatings to improve the tool life and also to increase the cutting speed [17±19]. Coatings on cemented carbide tools were developed initially using the chemical vapour deposition (CVD) technique. Currently, for HSS tools and cemented carbide tools, coatings involving physical vapour deposition (PVD) are used. The attraction of the PVD process is that it is a much cleaner process and the formation of brittle interface between the substrate of the tool and coatings Ð which is responsible for the poor adhesion of the coatings with the tool substrate Ð is eliminated to a certain extent, since the substrate temperatures are lower compared to those for the CVD process (4508C instead of 10008C). Presently, investigations are directed towards improving the properties and performance of coating materials by reducing the spatial scale of the material system to nanometer dimensions [20]. Nano-layers of various combinations such metal and metal, metal and ceramic, ceramic and ceramic, are under investigation in view of their potential for tribological and manufacturing applications. The studies have indicated that these nano-coatings can signi®cantly increase the hardness, modulus and toughness of the tool and thereby they will be able to give a better performance in friction, wear and lubrication applications. 7. Summary Environmental laws are closing on machining coolants. Moreover, coolants give rise to environmental problems related to waste disposal. As the costs for waste disposal
P.S. Sreejith, B.K.A. Ngoi / Journal of Materials Processing Technology 101 (2000) 287±291
increase, industries will be forced to implement strategies to reduce the amount of coolants they use. Dry machining requires suitable measures to compensate for the absence of coolants. This paper gives a state-of-art and the recent advancements in this direction. Dry machining is only possible when all the operations can be done dry. Technology has to be further improved if dry cutting is to be fully employed in industries.
References [1] G. Byrne, E. Scholta, Environmentally clean machining processes Ð a strategic approach, Ann. CIRP 42 (1) (1993) 471±474. [2] R. Komanduri, J. Desai, Tool Mater. Encyclopaedia Chem. Technol. 23 (1983) 273±309. [3] R.B. Aronson, Why dry machining? Manuf. Eng. January (1995) 33±36. [4] T.D. Howes, H.K. Tonshoff, W. Heuer, Environmental aspects of grinding ¯uids, Ann. CIRP 42 (2) (1991) 623±630. [5] Dry machining's double bene®t, Mach. Prod. Eng. 3 (1994) 14±20. [6] N. Narutaki, Y. Yamane, S. Tashima, H. Kuroki, A new advanced ceramic for dry machining, Ann. CIRP 46 (1) (1997) 43±48. [7] T. Kurimoto, G. Barrow, The in¯uence of aqueous ¯uids on the wear characteristics and life of carbide cutting tools, Ann. CIRP 31 (1) (1982). [8] E. Lenz, Z. Katz, A. Ber, Investigations on the ¯ank wear of cemented carbide tools, ASME Trans. J. Eng. Ind. 98 (1) (1976) 246±250.
291
[9] A. Ber, M. Goldblatt, In¯uence of the temperature gradient on the wear in turning tools, Ann. CIRP 38 (1) (1989) 69±73. [10] N.P. Jeffries, A new cooling method for metal cutting tools, Ph.D. Dissertation, University of Cincinnati, Cincinnati, USA, 1969. [11] G.M. Dudly, Machine tool having internally routed cryogenic ¯uid for cooling interface between cutting tool and work piece, US Patent 3 971 114 (1976). [12] P.G. Meyers, Tool cooling apparatus, US Patent 3 137 184 (1964). [13] E.A. Almond, Towards improved tests based on fundamental properties, Proceedings of the International Conference on Towards Improved Performance of Tool Materials, The National Physical Laboratory and The Metals Society, Teddington, Middlesex, April 28±29, 1981, pp. 161±169. [14] E.M. Trent, Metal Cutting, Butterworths-Heinemaan Ltd., Oxford, UK, 1991. [15] G. Santhanakrishnan, Tribological aspects in composite machining, Ph.D. Dissertation, Indian Institute of Technology, Madras, India, 1994. [16] G. Spur, U. Lachmund, Trockenbearbeitung von graugub mit hohen schnittgeschwindingkeiten, ZWF 90 6 S, 1995, pp. 302±305. [17] T.A. Hale, Coated cemented carbide product, 1973, US Patent 3 736 107. [18] R. Horsfall, R. Fontana, TiAlN coatings beat the heat, Cutting Tool Eng. February (1993) 37±42. [19] W. Konig, R. Fritsch, D. Kammermeier, New approaches to characterising the performance of coated carbide tools, Ann. CIRP 41 (1) (1992) 49±54. [20] G. Jayaram, L.D. Marks, M.R. Hilton, Surf. Coat. Technol. 76/77 (1995) 393±399.
Dry machining: Machining of the future P.S. Sreejith*, B.K.A. Ngoi School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 27 January 1999
Abstract Machining without the use of any cutting ¯uid (dry or green machining) is becoming increasingly more popular due to concern regarding the safety of the environment. Most industries apply cutting ¯uids/coolants when their use is not necessary. The coolants and lubricants used for machining represents 16±20% of the manufacturing costs, hence the extravagant use of these ¯uids should be restricted. However, it should also be noted that some of the bene®ts of cutting ¯uids are not going to be available for dry machining and also dry machining will be acceptable only whenever the part quality and machining times achieved in wet machining are equalled or surpassed. This paper presents recent developments in the dry machining operation. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Dry machining; Temperature; Cutting tool; Coating
1. Introduction Dry machining is ecologically desirable and it will be considered as a necessity for manufacturing enterprises in the near future. Industries will be compelled to consider dry machining to enforce environmental protection laws for occupational safety and health regulations. The advantages of dry machining include: non-pollution of the atmosphere (or water); no residue on the swarf which will be re¯ected in reduced disposal and cleaning costs; no danger to health; and it is non-injurious to skin and is allergy free. Moreover, it offers cost reduction in machining. The various possible routes to achieve clean machining processes were analysed and discussed by Byrne and Scholta [1]. Elimination on the use of cutting ¯uids, if possible, can be a signi®cant incentive. The costs connected with the use of cutting ¯uids are estimated to be many more times than the labour and overhead costs [2]. Hence the implementation of dry machining will reduce manufacturing costs. In the manufacturing industry, cutting ¯uids help: to remove the heat generated due to friction during cutting; to achieve better tool life, surface ®nish and dimensional tolerances; to prevent the formation of built-up edge and to facilitate the transportation of chips. Coolants are essential in the machining of materials such as aluminium alloys and most stainless * Corresponding author. Tel.: 65-7906875; fax: 65-7911859. E-mail address: [email protected] (P.S. Sreejith).
steels, which tend to adhere to the tool and cause a built-up edge. At the same time, the coolants produce problems in the working environment and also create problems in waste disposal. This creates a large number of ecological problems, but which in turn result in more economical overheads for manufacturing industries [3,4]. If industries were to practise dry machining, then all of the above-mentioned problems should be addressed satisfactorily. The cutting ¯uid industries are reformulating new composites that are more environmental friendly and which do not contain Pb, S or Cl compounds [5]. Consumption of cutting ¯uids has been reduced considerably by using mist lubrication. However, mist in the industrial environment can have serious respiratory effects on the operator. The use of cutting ¯uids will be increasingly more expensive as stricter enforcement of new regulation and standards are imposed, leaving no alternative but to consider dry machining [6]. Many metal-cutting processes have been developed and improved based on the availability of coolants. It is well known that coolants improve the tool life and tool performance to a great extent. In dry machining, there will be more friction and adhesion between the tool and the workpiece, since they will be subjected to higher temperatures. This will result in increased tool wear and hence reduction in tool life. Higher machining temperatures will produce ribbon-like chips and this will affect the form and dimensional accuracy of the machined surface [7,8]. However, dry cutting also has some positive effects, such as reduction in thermal shock and
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 4 4 5 - 3
288
P.S. Sreejith, B.K.A. Ngoi / Journal of Materials Processing Technology 101 (2000) 287±291
hence improved tool life in an interrupted-cutting environment. 2. Developments in dry machining To pursue dry machining, one has to compensate for the bene®cial effects of cutting ¯uids. One approach towards dry machining is to have an indirect contact of coolants and thereby take away the heat generated. Research in the area of dry machining without the direct contact of coolants has lead to the following advancements: 1. An under-cooling system [9], where the coolant ¯ows through channels located under the insert, then out to the environment, without any direct contact with the cutting zone. 2. Internal cooling by a vaporisation system [10], in which a vaporisable liquid is introduced inside the shank of the tool and vaporised on the underside surface of the insert. 3. Cryogenic systems [11], where a stream of cryogenic coolant is routed internally through a conduit inside the tool. 4. Thermoelectric cooling systems [12], using a module of couples of thermoelectric material elements. When an electric current is passed through the thermoelectric elements, a cold junction and a hot junction is produced at the opposite ends of each of these elements. Another approach is to improve the properties of the tool material by making them more refractory or generate less heat during machining. There has been a continuous development in the ®eld of cutting-tool materials starting with HSS, cobalt alloys, cemented tungsten carbide, coated carbides and coated HSS, cubic boron nitride and diamond. However, the need for machining with increasingly higher cutting speeds and also to machine dif®cult-to-machine materials are imposing pressure for the development of new tool materials. As a result, newer tool materials such as ceramics and also different types of coatings on the tool materials will address the problems in dry machining to some extent. 3. Effect of temperature on machining The performance of a cutting tool is dependent on the form stability of the cutting wedge, which in turn is mostly dependent on the effective/working hardness and thermal conductivity of the tool±work materials. The working hardness of the tool material is related to the hot-hardness characteristics of the tool material. Typical hot-hardness characteristics of different tool materials are illustrated in Fig. 1. It can be seen that, barring carbon steel and highspeed steel (HSS) materials, all other tool materials exhibit a steady reduction in hardness with increase in temperature. The temperature of machining yc is dependent on the cutting
Fig. 1. Typical hot-hardness characteristics of some tool materials [13].
speed V and the type of material pair involved in machining and can be expressed as yc / V b
(1)
The hardness of the tool material is selected so as to maintain consistent replication of its nose on the work surface, to maintain the desired surface texture. A reduction in tool material hardness with increase in temperature (due to the increased velocity) is to be limited so that a high degree of ®delity of nose replication can be maintained. This calls for limiting the temperature of machining, which can be attempted by limiting the speed of machining. The situation is more aggravated during the machining of heat-insulating materials. The signi®cance of machining speed on the machining performance is illustrated in Fig. 2. It is seen that a range of cutting speed can offer a highly ef®cient machining performance. This is again re¯ected in the need for controlling the machining temperature within limits for achieving good machining performance. This aspect of a temperature limit for economical machining has to be taken into consideration for dry machining. 4. Cutting tool requirements for dry machining Dry machining in the more demanding interrupted cutting environment calls for a tougher grade of hard tool material. For this, the solution can be selection of plain cemented
P.S. Sreejith, B.K.A. Ngoi / Journal of Materials Processing Technology 101 (2000) 287±291
Fig. 2. Illustration showing the signi®cance of machining speed on machining performance [14].
carbide, K-type grade of tools. However, the application of such tools can be with only a lower order of cutting speeds, the limitation being due to cutting temperature. In order to
289
overcome this, one can make use of K-type coated cemented carbide tools. Ultra-hard tool materials such as diamond and cubic boron nitride (CBN) cutters have been found to produce better machined surface characteristics, with considerable increase in tool life due to their higher hardness and thermal conductivity. Diamond and CBN, being super-hard materials, have found numerous applications as cutting tools. Diamond and CBN are very similar in many ways. They share the same crystallographic structure and exhibit high values of thermal conductivity. However, diamond oxidises in air and is subjected to graphitisation, loosing its form stability, thus limiting its application to relatively lower temperature situations. The quality of a machined surface, especially in turning, depends largely on the form stability of the cutting tools. During turning, the cutting tool replicates its nose on the work surface, resulting in the formation of the surface texture (Fig. 3). From the surface texture point of view,
Fig. 3. Typical illustration of surface production in machining [15].
290
P.S. Sreejith, B.K.A. Ngoi / Journal of Materials Processing Technology 101 (2000) 287±291
an ideal tool is one that is able to sustain the replication of the cutting nose. Hence, the performance of the tool or the quality of the machined surface texture is largely dependent on the form stability of the cutting tool. Form stability is largely in¯uenced by the different forms of tool wear. The quality of a machined surface is important whilst evaluating the reliability and functional life of a structure. In addition, inappropriate selection of a tool material can result not only in surface deterioration but also in increase the tooling costs. Therefore, knowledge of cutting tool materials and the nature and quality of the surface that they produce is of considerable importance. For achieving the desired results in machining, one has to be well informed regarding both the work material and the cutting tools. The tools used for dry machining have to suit some speci®c requirements. The options available under these include the following: 1. The use of very high positive rake angles (308) on submicron cemented carbide tools, which will reduce the overall cutting energy signi®cantly. 2. The development of refractory-type tool materials that can withstand high temperatures. 3. The use of ultra-hard tool materials such as diamond and CBN. 4. The development of coatings on tools that can withstand high temperatures and at the same time provide a lubricating effect to reduce friction. 5. Dry machining of common metals The dry machining of cast iron is not exactly new. Cast iron can be cut dry in turning and milling operations. The dry machining of cast iron has been attempted using ceramic cutting materials and CBN at high surface speeds and feed rates by Spur and Lachmund [16]. They found that since CBN tools have the highest thermal conductivity compared with the ceramic type of tools, CBN was able to remove the heat ef®ciently from the cutting material. Hence they concluded that CBN tools were highly suited for the dry machining of cast iron at high cutting speeds. Cast iron can also be dry milled with the cermet type of tools at high speeds [5]. Here the high speed employed is not to reduce the machining time, but to reduce the tool and workpiece contact time to prevent the heat of the chip penetrating the tool. Drilling is the most widely used process in the steel industry due to the assembly of components. The main problem associated with the dry drilling of steel is the removal of chips from the drilled hole. One approach to reduce this problem is to enlarge the ¯utes and thereby giving the chips more space, helping them to come out of the hole. Another problem that can be encountered is the jamming of the drilled hole due to the expansion of the drill at high temperatures. One way of correcting this problem is to give more drill taper towards the shank.
For the continuous high speed machining of superalloys and titanium, cooling is necessary. However, in an interrupted cutting environment, the coolant if used induces thermal shocks on any cutting material. For this reason dry machining can be recommended for interrupted cutting. Aluminium and its alloys are considered to be the most critical materials with regard to dry machining. Because of their higher thermal conductivity, the workpiece absorbs considerable heat from the machining process and may cause deformation due to its higher thermal expansion capabilities. In addition aluminium alloys may cause problems related to chip formation. Hence in the machining of aluminium and its alloys, it is essential to use tools with suitable coatings. The main issues that have to be considered for the dry machining of non-ferrous metals are the achieving of higher spindle speeds, the improvement of chip ejection geometry and the design of better tooling. Diamond tools will be a major enabler of this technology because of their high thermal coef®cient, fast heat diffusion, no af®nity for aluminium and the possibility of diamond being used as a coating for other shaped tools. 6. Coated tools for dry machining Since 1970s many new developments have been made in tool coatings to improve the tool life and also to increase the cutting speed [17±19]. Coatings on cemented carbide tools were developed initially using the chemical vapour deposition (CVD) technique. Currently, for HSS tools and cemented carbide tools, coatings involving physical vapour deposition (PVD) are used. The attraction of the PVD process is that it is a much cleaner process and the formation of brittle interface between the substrate of the tool and coatings Ð which is responsible for the poor adhesion of the coatings with the tool substrate Ð is eliminated to a certain extent, since the substrate temperatures are lower compared to those for the CVD process (4508C instead of 10008C). Presently, investigations are directed towards improving the properties and performance of coating materials by reducing the spatial scale of the material system to nanometer dimensions [20]. Nano-layers of various combinations such metal and metal, metal and ceramic, ceramic and ceramic, are under investigation in view of their potential for tribological and manufacturing applications. The studies have indicated that these nano-coatings can signi®cantly increase the hardness, modulus and toughness of the tool and thereby they will be able to give a better performance in friction, wear and lubrication applications. 7. Summary Environmental laws are closing on machining coolants. Moreover, coolants give rise to environmental problems related to waste disposal. As the costs for waste disposal
P.S. Sreejith, B.K.A. Ngoi / Journal of Materials Processing Technology 101 (2000) 287±291
increase, industries will be forced to implement strategies to reduce the amount of coolants they use. Dry machining requires suitable measures to compensate for the absence of coolants. This paper gives a state-of-art and the recent advancements in this direction. Dry machining is only possible when all the operations can be done dry. Technology has to be further improved if dry cutting is to be fully employed in industries.
References [1] G. Byrne, E. Scholta, Environmentally clean machining processes Ð a strategic approach, Ann. CIRP 42 (1) (1993) 471±474. [2] R. Komanduri, J. Desai, Tool Mater. Encyclopaedia Chem. Technol. 23 (1983) 273±309. [3] R.B. Aronson, Why dry machining? Manuf. Eng. January (1995) 33±36. [4] T.D. Howes, H.K. Tonshoff, W. Heuer, Environmental aspects of grinding ¯uids, Ann. CIRP 42 (2) (1991) 623±630. [5] Dry machining's double bene®t, Mach. Prod. Eng. 3 (1994) 14±20. [6] N. Narutaki, Y. Yamane, S. Tashima, H. Kuroki, A new advanced ceramic for dry machining, Ann. CIRP 46 (1) (1997) 43±48. [7] T. Kurimoto, G. Barrow, The in¯uence of aqueous ¯uids on the wear characteristics and life of carbide cutting tools, Ann. CIRP 31 (1) (1982). [8] E. Lenz, Z. Katz, A. Ber, Investigations on the ¯ank wear of cemented carbide tools, ASME Trans. J. Eng. Ind. 98 (1) (1976) 246±250.
291
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