Machining of Composite Materials R. Teti University of Naples Federico l, Italy Abstract Machining of composite materials is difficult to carry out due to the anisotropic and non-homogeneous structure of composites and to the high abrasiveness of their reinforcing constituents. This typically results in damage eing introduced into the workpiece and in very rapid wear development in the cutting tool ventional achining processes such as turning, drilling or milling can be applied to composite materials, provided propel ol design and operating conditions are adopted verview of the various issues involved in the conventional achining of the main types of composite materials is presented in this paper Machining, Composite Materials, Conventional Cutting Processes ACKNOWLEDGEMENTS such as glass, graphite, boron, alumina and silicon Acknowledgements are due for papers, contributions and carbide, are highly abrasive and hard (sometimes as hard correspondence received from Messrs ("CIRP members) as or even harder than the tool material), conventiona Aspinwal, D K, University of Birmingham UK;"Balazinski machining is considered for composites because their M, Ecole Polytechnique de Montreal, Canada: *Byrne, G reinforcements are brittle and material separation is University College Dublin, Ireland; "Brinksmeier, E accomplished G deformation ahead of the tool. However, the cutting tool Naples Federico Il, Italy; *Chandrasekaran, H, Swedish materials must be attentively chosen to minimize wear due nstitute for metals Research. stockhol en: Chen to the hard abrasive constituents of the reinforcing phase J. Rotors Business Center. USA: 'Dornfeld. D in the composite representing the work material University of California, Berkeley, USA: *Geiger Machining of a composite depends on the properties and University of Erlangen-Nurnberg, Germany: Inasaki elative content of the reinforcement and Kentucky, USA; Klocke, F, Technical University of process. In addition, the choice of the spec Aachen, Germany: Komanduri, R, Oklahoma State depends upon the following factors type of machining peration, part geometry and size, finish and accul Japa-rsity, USA, *Narutaki, N, Hiroshima University, Japan:"Poll mann, W, Daimler Crysler AG, Stuttgart requirements, number of parts, diversity of parts(including Germany: Spur, G, Technical University Berlin the material of the parts), availability of appropria machine and cutting tools, availability of in-house Germany: Tomizuka, M, University of California, Berkeley technol USA: *Uhlmann, F,, Technical University Berlin, Germany ufacturing schedule, capital requirements and justification for new equipment, environmental and safety considerations, and Weig, E, HSC-Manufact. Engineering, Austria; "Weinert overall costs K, University of Dortmund, Germany; *Wertheim, R ISCAR Ltd. Israel 2 COMPOSITE MATERIALS PhD student Doriana D'Addona, University of Naples Federico I, Italy is gratefully thanked for her help and Composite materials are formed from two more support in the preparation of the text materials producing properties that could not be obtained from any one material. One of the constituent materials acts as the matrix and at least one other constituent material acts as the reinforcement in the composite Composite materials are used extensively as their higher of th specific properties(properties per unit weight) of strength to protect the reinforcement materials and stiffness, when compared to metals, offer interesting to distribute the stress to the reinforcement material(s) opportunities for new product design. However, being non to provide for the final shape of the composite part homogeneous, anisotropic and reinforced by very abrasive The role of the reinforcement material(s)is the following components, these materials are difficult to machine to provide the composite high mechanical properties Significant damage to the workpiece may be introduced to reinforce the matrix in preferential directions and high wear rates of the cutting tools are experienced The properties of a composite material depend on the Conventional machining practices, such as turning, drilling nature of the reinforcement and the matrix. the form of the and milling are widely applied to the machining of reinforcement(particles, fibres) and the relative content of composite materials in view of the availability of equipment reinforcement and matrix expressed as volume fraction and experience in conventional machining. Although some Vt=(reinforcement volume)composite volume)and vm of the materials used as reinforcement in composites (matrix volume) /(composite volume), where Vr+ Vm
Machining of Composite Materials R. Teti University of Naples Federico II, Italy Abstract Machining of composite materials is difficult to carry out due to the anisotropic and non-homogeneous structure of composites and to the high abrasiveness of their reinforcing constituents. This typically results in damage being introduced into the workpiece and in very rapid wear development in the cutting tool. Conventional machining processes such as turning, drilling or milling can be applied to composite materials, provided proper tool design and operating conditions are adopted. An overview of the various issues involved in the conventional machining of the main types of composite materials is presented in this paper. Keywords: Machining, Composite Materials, Conventional Cutting Processes ACKNOWLEDGEMENTS Acknowledgements are due for papers, contributions and correspondence received from Messrs (*CIRP members): Aspinwal, D.K., University of Birmingham UK; *Balazinski, M., Ecole Polytechnique de Montreal, Canada; *Byrne, G., University College Dublin, Ireland; *Brinksmeier, E., University of Bremen, Germany; Caprino, G., University of Naples Federico II, Italy; *Chandrasekaran, H., Swedish Institute for Metals Research, Stockholm, Sweden; Chen, L.J., Rotors Business Center, USA; *Dornfeld, D., University of California, Berkeley, USA; *Geiger, M., University of Erlangen-Nurnberg, Germany; *Inasaki, I., Keio University, Japan; *Jawahir, I.S., University of Kentucky, USA; *Klocke, F., Technical University of Aachen, Germany; *Komanduri, R., Oklahoma State University, USA; *Narutaki, N., Hiroshima University, Japan; *Pollmann, W., DaimlerCrysler AG, Stuttgart, Germany; *Spur, G., Technical University Berlin, Germany; Tomizuka, M., University of California, Berkeley, USA; *Uhlmann, F., Technical University Berlin, Germany; *Venkatesh, V.C., University of Technology Malaysia; *Weigl, E., HSC-Manufact. Engineering, Austria; *Weinert, K., University of Dortmund, Germany; *Wertheim, R., ISCAR Ltd., Israel. PhD student Doriana D'Addona, University of Naples Federico II, Italy, is gratefully thanked for her help and support in the preparation of the text. 1 INTRODUCTION Composite materials are used extensively as their higher specific properties (properties per unit weight) of strength and stiffness, when compared to metals, offer interesting opportunities for new product design. However, being nonhomogeneous, anisotropic and reinforced by very abrasive components, these materials are difficult to machine. Significant damage to the workpiece may be introduced and high wear rates of the cutting tools are experienced. Conventional machining practices, such as turning, drilling and milling, are widely applied to the machining of composite materials in view of the availability of equipment and experience in conventional machining. Although some of the materials used as reinforcement in composites, such as glass, graphite, boron, alumina and silicon carbide, are highly abrasive and hard (sometimes as hard as or even harder than the tool material), conventional machining is considered for composites because their reinforcements are brittle and material separation is accomplished by brittle fracture rather than plastic deformation ahead of the tool. However, the cutting tool materials must be attentively chosen to minimize wear due to the hard abrasive constituents of the reinforcing phase in the composite representing the work material. Machining of a composite depends on the properties and relative content of the reinforcement and the matrix materials as well as on its response to the machining process. In addition, the choice of the specific process depends upon the following factors: type of machining operation, part geometry and size, finish and accuracy requirements, number of parts, diversity of parts (including the material of the parts), availability of appropriate machine and cutting tools, availability of in-house technology, current machining practice, manufacturing schedule, capital requirements and justification for new equipment, environmental and safety considerations, and overall costs. 2 COMPOSITE MATERIALS Composite materials are formed from two or more materials producing properties that could not be obtained from any one material. One of the constituent materials acts as the matrix and at least one other constituent material acts as the reinforcement in the composite. The role of the matrix material comprises the following: - to protect the reinforcement materials; - to distribute the stress to the reinforcement material(s); - to provide for the final shape of the composite part. The role of the reinforcement material(s) is the following: - to provide the composite high mechanical properties; - to reinforce the matrix in preferential directions. The properties of a composite material depend on the nature of the reinforcement and the matrix, the form of the reinforcement (particles, fibres) and the relative content of reinforcement and matrix expressed as volume fraction: Vf = (reinforcement volume)/(composite volume) and Vm = (matrix volume)/(composite volume), where Vf+ Vm = 1
Composite materials can be classified on the basis of the Among the thermoset resins, polyester resins are lower in matrix material used for their fabrication polymer matrix composites(PMc) matrix composites are used in the fabrication of boat hulls metal matrix composites(MMC) structural panels and parts for automobiles and aircrafts ceramic matrix composites(CMc) building panels and beams, electrical appliances, water Theoretically, a multitude of materials can come under tanks, pressure vessels, etc. Epoxy resins, in addition these categories. In the following a brief description of have a lower shrinkage after cure allowing for highe some of the PMc, MMc and CMc composites most fabrication accuracy. Epoxy matrix is used commonly in ly used for industrial applications is reported CFRP and AFRP composites for aerospace applications. 2.1 Polymer matrix composites(PMc) military equipments, satellite antennae, sports The most common types of reinforcement used for PMC equipments, medical prostheses, etc Thermoplastic polymers consists of flexible line A are placed before the acronym FrP to specify the strength and modulus but quite high ductility. Among the thermoplastic resins, polyamide and peek resins are used nature of the reinforcing fibres glass, carbon or aramid fibres. The fibres can be long (continuous)or short as matrix materials in FRP composites for applications in (discontinuous). Long fibres can be unidirectional(al the aerospace industry due to their superior mechanical properties and high glass transition temperature fibres parallel to each other) or woven into a fabric or Maximum service temperatures for FRP composites are cloth.Unidirectional fibres provide for the highest relatively low, as the matrix material is prone to softening. mechanical properties in a composite Glass fibre reinforced plastics(GFRP)are by far the most chemical decomposition or degradation at moderate temperatures. The same temperature limitations apply to commonly used materials in view of their high specifi mechanical properties and low cost. Carbon fibre the machining of FRP composites Table 1 reports the main mechanical properties of some einforced plastics( CFRP)and aramid fibre reinforced FRP composite materials lastics(AFRP)provide higher specific strength, higher specific stiffness and ligher weight. They are, however 2.2 Metal matrix composites(MMc) expensive and are used only for those applications where MMC are used for applications requiring higher operating performance and not cost is the major consideration emperatures than are possible with PMc materials AFRP is used instead of CFRP where strength, lightness Most of these composites are developed for the and toughness are major considerations, and stiffness and aerospace industry, but new applications are found in the high temperature performance are not automotive industry such as in automobile engine parts The common matrix materials for FRP composites ar making use of continuous fibre, discontinuous fibre or thermoset polymers(e.g. polyester, epoxy) particle reinforced MMC. Continuous fibres provide for the thermoplastic polymers(e.g polyamide, peek highest stiffness and strength properties obtainable in Thermoset polymers remain rigid when heated and consist MMc materials of a highly cross-linked three-dimensional network; they Boron-aluminium composites are one of the earliest are quite strong and stiff and have poor ductility layers of boron fibres between aluminium foils, so that the TensileElastic foils deform around the fibres and bond to each other [1] FRP odulus By reinforcing with boron, the tensile strength can be material ou(MPa)E(MPa) failure, d(g/ Increas ed by a factor of three to five while the elastic modulus can be tripled GFRP Further reinforcing materials for MMc are silicon carbide alumina and graphite in the form of particles, short fibres (whiskers) or long fibres. Aluminium, magnesium and itanium alloys are the most common matrix materials coth*10030010000200 used in MMc materials Table 2 reports the main mechanical properties of some 50-2006000-12000 13-2.1 MMC materials hort fibres Figure 1 is a plot of specific strength versus specific stiffness for various composites and conventional m materials. It can be seen that composites, in general, have compound* 1.3-19 higher specific strength and specific modulus over conventional steel, Al, Ti, and Mg alloys and mMc have properties superior to PMc compos Applications of continuous fibre reinforced use of B-Al for the fuselage of the space shuttle (V=60%) 145000 0.9 1.6 SiC-Al for the vertical tail section of advanced e orbiter Discontinuous fibre and particle reinforced MMc are low (V=60%) 0.3 1.6 cost MMC that provide higher strength and stiffness and 220000 better dimensional stability over the corresponding unreinforced alloys. Small additions of reinforcement(V 20%)moderately increase the base alloy strength and stiffness. They also increase the wear resistance and Unidir 75000 1.6 contribute toward the difficulty in machining these materials. These MMc are used for sport equipments Table 1: Mechanical properties of FRP composite automobile engine parts(pistons cylinder liners, brake "For these materials: v+= 20%-50% drums), missile guidance parts, etc
Composite materials can be classified on the basis of the matrix material used for their fabrication: - polymer matrix composites (PMC); - metal matrix composites (MMC); - ceramic matrix composites (CMC). Theoretically, a multitude of materials can come under these categories. In the following, a brief description of some of the PMC, MMC and CMC composites most commonly used for industrial applications is reported. 2.1 Polymer matrix composites (PMC) The most common types of reinforcement used for PMC are strong and brittle fibres incorporated into a soft and ductile polymeric matrix. In this case, PMC are referred to as fibre reinforced plastics (FRP). Capital letters G, C and A are placed before the acronym FRP to specify the nature of the reinforcing fibres: glass, carbon or aramid fibres. The fibres can be long (continuous) or short (discontinuous). Long fibres can be unidirectional (all fibres parallel to each other) or woven into a fabric or cloth. Unidirectional fibres provide for the highest mechanical properties in a composite. Glass fibre reinforced plastics (GFRP) are by far the most commonly used materials in view of their high specific mechanical properties and low cost. Carbon fibre reinforced plastics (CFRP) and aramid fibre reinforced plastics (AFRP) provide higher specific strength, higher specific stiffness and ligher weight. They are, however, expensive and are used only for those applications where performance and not cost is the major consideration. AFRP is used instead of CFRP where strength, lightness and toughness are major considerations, and stiffness and high temperature performance are not. The common matrix materials for FRP composites are: - thermoset polymers (e.g. polyester, epoxy) - thermoplastic polymers (e.g. polyamide, peek). Thermoset polymers remain rigid when heated and consist of a highly cross-linked three-dimensional network; they are quite strong and stiff and have poor ductility. FRP material Strain to Density failure, d (g/cmS; Tensile Elastic strength, modulus, ail (MPa) E (MPa) Eu (%) GFRP I Unidirectional (Vr = 60 %) Woven cloth* Chopped looo 45000 2.3 2.1 100-300 10000-20000 - 1.5-2.1 .. roving* (short fibres) Sheet 50-200 6000-12000 - 1.3-2.1 modulus AFRP molding compound* (short fibres) CFRP Unidirectional (Vr = 60 %) High strength Unidirectional High (Vr = 60 %) 10-20 500-2000 1.3-1.9 1200 145000 0.9 1.6 800 220000 0.3 1.6 Table 1: Mechanical properties of FRP composite *For these materials: Vf = 20% - 50%. Unidirectional (Vr = 60 %) Among the thermoset resins, polyester resins are lower in cost and are not as strong as epoxy resins. Polyester matrix composites are used in the fabrication of boat hulls, structural panels and parts for automobiles and aircrafts, building panels and beams, electrical appliances, water tanks, pressure vessels, etc. Epoxy resins, in addition, have a lower shrinkage after cure allowing for higher fabrication accuracy. Epoxy matrix is used commonly in CFRP and AFRP composites for aerospace applications, military equipments, satellite antennae, sports equipments, medical prostheses, etc. Thermoplastic polymers consists of flexible linear molecular chains that are tangled together and, as the name indicates, soften when heated; they have lower strength and modulus but quite high ductility. Among the thermoplastic resins, polyamide and peek resins are used as matrix materials in FRP composites for applications in the aerospace industry due to their superior mechanical properties and high glass transition temperature. Maximum service temperatures for FRP composites are relatively low, as the matrix material is prone to softening, chemical decomposition or degradation at moderate temperatures. The same temperature limitations apply to the machining of FRP composites. Table 1 reports the main mechanical properties of some FRP composite materials. 2.2 Metal matrix composites (MMC) MMC are used for applications requiring higher operating temperatures than are possible with PMC materials. Most of these composites are developed for the aerospace industry, but new applications are found in the automotive industry, such as in automobile engine parts, making use of continuous fibre, discontinuous fibre, or particle reinforced MMC. Continuous fibres provide for the highest stiffness and strength properties obtainable in MMC materials. Boron-aluminium composites are one of the earliest developed MMC material types. It is made by hot pressing layers of boron fibres between aluminium foils, so that the foils deform around the fibres and bond to each other [I]. By reinforcing with boron, the tensile strength can be increased by a factor of three to five while the elastic modulus can be tripled. Further reinforcing materials for MMC are silicon carbide, alumina and graphite in the form of particles, short fibres (whiskers) or long fibres. Aluminium, magnesium and titanium alloys are the most common matrix materials used in MMC materials. Table 2 reports the main mechanical properties of some MMC materials. Figure 1 is a plot of specific strength versus specific stiffness for various composites and conventional metal materials. It can be seen that composites, in general, have higher specific strength and specific modulus over conventional steel, Al, Ti, and Mg alloys, and MMC have properties superior to PMC composites. Applications of continuous fibre reinforced MMC include use of B-AI for the fuselage of the space shuttle orbiter, SIC-AI for the vertical tail section of advanced fighter planes, SIC-TiAI for hypersonic aircraft, etc. Discontinuous fibre and particle reinforced MMC are low cost MMC that provide higher strength and stiffness and better dimensional stability over the corresponding unreinforced alloys. Small additions of reinforcement (Vr = 20%) moderately increase the base alloy strength and stiffness. They also increase the wear resistance and contribute toward the difficulty in machining these materials. These MMC are used for sport equipments, automobile engine parts (pistons, cylinder liners, brake drums), missile guidance parts, etc. looo 75000 1.6 1.4
Matrix Sic whisker Flexural MMC material volume fraction, strength, toughness ou(MPa)E(MPa)Eu(%) SiaN4 4060 A2124T6(45%B) 45022000081 Al2O3 A6061-76(51%B) 000 5-65 A6061T6(45%Sc)14602000 45-55 Discontinuous-fibre mmc Table 3: Mechanical properties of Sic whisker reinforced A21246(20%sc)65012500240 CMC materials at room temperature A6061T6(20%SiC) 480 120000 5.00 3 MACHINING APPLICATIONS Particle MMC Machining of composite materials differs significantly in many aspects from machining of conventional metals and A2124T6(20%ScC) 55010500700 their alloys [3-5]. In the machining of composites, the A6061-T6(20%SC) 50010500550 material behaviour is not only non-homogeneous and anisotropic, but it also depends on diverse reinforcement No reinforcement and matrix properties, and the volume fraction of matrix A2124F 450700000900 and reinforcement. The tool encounters alternatively matrix and reinforcement materials, whose response to A6061-F machining can be entirely different. Thus, machining of composite materials imposes special demands on the Table 2: Mechanical properties of MMC materials eometry and wear resistance of the cutting tools Accordingly, tool wear mechanisms and developme must be attentively considered to establish correct cutting ool selectio In the following, applications of machining processes to 0. 34 Gr/Mg composite materials are reviewed with reference to FR 04 materials and MMC materials 0.37Gr/A As regards the machining of CMC materials, the very Conventiona small number of contributions received and the scarcity of s Steel,At,Ti,Mg information available in the open literature on this topic did not allow for the preparation of a dedicated section d02 0.60 Gr/Epox 3. 1 Machining of fibre reinforced plastic composites Orthogonal machining of FRP FRP composites with different fibre orientations allowed or the clarification of the cutting mechanisms taking place in FRP(Figure 2). When machining is conducted at an angle of o to the fibre orientation the laminate is Figure 1: Specific strength vs specific stiffness for varie subjected to stresses parallel to the fibres. In addition, the MMC materials. Number in front of the composite is the surface below the cutting edge is compressed. The reinforcement volume fraction [2 material failure occurring in front of the cutting edge is due to delamination. matrix fracture or fibre-matrix interface 2.3 Ceramic matrix com posites(CMC) failure, which is recognizable from the crack in the CMC materials are being developed mainly to improve the composite laminate ahead of the cutting edge. Individual fracture toughness of unreinforced ceramics which already fractures occurring in the fibres and in the matrix below the possess higher specific modulus and mechanical cutting edge are also visible and remain in the machined properties at high temperature superior to those of metals surface. As the angle between cutting direction and fibre Continuous fibres, discontinuous fibres (whiskers)or orientation increases, fibres are compressed and bent in particles can be utilised as reinforcing constituents in the direction opposite to the fibre orientation, ending up in CMC fibre breakage as a result of bending and pressure load The common reinforcement materials used in CMc are This can result in fibre-matrix interface failure which alumina and silicon carbide, a volume fraction V:= 20% of SiC whiskers added to alumina can increase the fracture directions. which are the least favourable for FRP toughness from 25 to 50 MPa. Such an increase in composites particularly at angles between 30 and 60 to toughness of a ceramic cutting tool will enable it to take the fibre direction, is reflected in a poor surface quality In heavy cuts or to perform without fracture in interrupted a composite machined at 90 to the fibre direction,the g. Conventional hot isostatic pressing techniques fibr subjected ng e used to consolidate CMC composites contrast to laminates with 0 fibres. each fibre has to be Other CMc include carbon/carbon composites in which cut separately. The compressive strain normal to the fibres high strength carbon fibres are embedded in a graphite creates problems as interfacial fractures extend into the matrix. The low density of carbon in combination with the unmachined surface, More favourable conditions develop very high strength of carbon fibres offers potential for the for fibre orientation 135. Fibres are subjected to bending development of ultra high specific strength materials and tensile stress and break in bundles problems arise Table 3 reports the main mechanical properties of some however, from the fact that individual fibres can be pulled out due to insufficient adhesion to the matrix
Tensile strength, u,, (MPa) MMC material IContinuous-fibre MMC I I Elastic Strain to modulus, failure, E (MPa) E,, (%) IAI2124-T6(45% B) I 1450 I220000 I 0.81 I Matrix material SiqNd IAl6061-T6(51%B) I 1410 I230000 I 0.74 I Sic whisker Flexural Fracture volume fraction, strength, toughness, Vf (%) uf (MPa) k (MPa) 0 400-650 30-45 IAl 6061-T6 (45% Sic) I 1460 I 200000 I 0.89 I Discontinuous-fibre MMC Al 2124-T6 (2O%SiC) Al 6061-T6 (20% Sic) 650 125000 2.40 480 120000 5.00 lparticle MMC I I No reinforcement Al 2124-F Al 6061-F IAl2124-T6(20%SiC) I 550 I 105000 I 7.00 I 450 700000 9.00 310 70000 12.00 IAl 6061-T6 (20% Sic) I 500 I 105000 I 5.50 I Table 2: Mechanical properties of MMC materials 0.6 h E z D 0.4 a 3j I- 3 5 m C u) 0 E 0 8 a v) 0.2 c 0 0.45 B/Al 0 0.34 Gr/Mg SlCfr 0. 0.37 Gr/AI Be 0 Conventional 0 Steel, Al, Ti, Mg 0 0.37 Gr/AI 1 NO.giCw/Al 0.60 IsI Gr/Epoxy 0.50 Gr/Epoxy 0 50 100 150 Specific stiffness (10 Nrn/Kg) Figure 1: Specific strength vs. specific stiffness for various MMC materials. Number in front of the composite is the reinforcement volume fraction [2]. 2.3 Ceramic matrix composites (CMC) CMC materials are being developed mainly to improve the fracture toughness of unreinforced ceramics which already possess higher specific modulus and mechanical properties at high temperature superior to those of metals. Continuous fibres, discontinuous fibres (whiskers) or particles can be utilised as reinforcing constituents in CMC. The common reinforcement materials used in CMC are alumina and silicon carbide. A volume fraction Vf = 20% of Sic whiskers added to alumina can increase the fracture toughness from 25 to 50 MPa. Such an increase in toughness of a ceramic cutting tool will enable it to take heavy cuts or to perform without fracture in interrupted cutting. Conventional hot isostatic pressing techniques can be used to consolidate CMC composites. Other CMC include carbonkarbon composites in which high strength carbon fibres are embedded in a graphite matrix. The low density of carbon in combination with the very high strength of carbon fibres offers potential for the development of ultra high specific strength materials. Table 3 reports the main mechanical properties of some CMC materials. 400-550 40-60 350-500 45-65 400-500 45 20 500-800 45-55 Table 3: Mechanical properties of Sic whisker reinforced CMC materials at room temperature. 3 MACHINING APPLICATIONS Machining of composite materials differs significantly in many aspects from machining of conventional metals and their alloys [3-51. In the machining of composites, the material behaviour is not only non-homogeneous and anisotropic, but it also depends on diverse reinforcement and matrix properties, and the volume fraction of matrix and reinforcement. The tool encounters alternatively matrix and reinforcement materials, whose response to machining can be entirely different. Thus, machining of composite materials imposes special demands on the geometry and wear resistance of the cutting tools. Accordingly, tool wear mechanisms and development must be attentively considered to establish correct cutting tool selection. In the following, applications of machining processes to composite materials are reviewed with reference to FRP materials and MMC materials. As regards the machining of CMC materials, the very small number of contributions received and the scarcity of information available in the open literature on this topic did not allow for the preparation of a dedicated section. 3.1 Orthogonal machining of FRP Investigations carried out in [6] by orthogonal cutting of FRP composites with different fibre orientations allowed for the clarification of the cutting mechanisms taking place in FRP (Figure 2). When machining is conducted at an angle of 0" to the fibre orientation, the laminate is subjected to stresses parallel to the fibres. In addition, the surface below the cutting edge is compressed. The material failure occurring in front of the cutting edge is due to delamination, matrix fracture or fibre-matrix interface failure, which is recognizable from the crack in the composite laminate ahead of the cutting edge. Individual fractures occurring in the fibres and in the matrix below the cutting edge are also visible and remain in the machined surface. As the angle between cutting direction and fibre orientation increases, fibres are compressed and bent in the direction opposite to the fibre orientation, ending up in fibre breakage as a result of bending and pressure load. This can result in fibre-matrix interface failure which extends into the unmachined surface. These load directions, which are the least favourable for FRP composites particularly at angles between 30" and 60" to the fibre direction, is reflected in a poor surface quality. In a composite machined at 90" to the fibre direction, the fibres are subjected to bending and are sheared off. In contrast to laminates with 0" fibres, each fibre has to be cut separately. The compressive strain normal to the fibres creates problems as interfacial fractures extend into the unmachined surface. More favourable conditions develop for fibre orientation 135". Fibres are subjected to bending and tensile stress and break in bundles. Problems arise, however, from the fact that individual fibres can be pulled out due to insufficient adhesion to the matrix. Machining of fibre reinforced plastic composites
-orientation fibre-/matrix fibre matrix fracture and fracture interfacial fracture ackage crack- re- matrix fracture and fracture/ interfacial fracture fibre-matrix crack-turn round lue to fibre. tension Figure 3: Examples of turned FRP parts [15] fibre matrⅸx Although the cutting of FRP composite parts is rarely Figure 2: Cutting mechanisms for FRP composites [6] desired, it can be seldom avoided for the production of the final geometry, surface quality, and form accuracy The machinability of CFRP and GFrP was deeply conventionally produced parts. Turning is applied to investigated in [7]. A model for cutting force prediction in rotation-symmetric parts such as drag links, bearings orthogonal cutting operations was presented. Three spindles, axles, rolls, or steering columns, etc. Figure 3 initially varied during orthogonal cutting shows some typical FRP parts produced by turning sing HSS tools: tool rake angle, relief angle and depth of Particular attention was given by several authors to the cut. Their effects on cutting forces were investigated and aspects of tool wear mechanisms and development in an optimal tool geometry was found The effect of tool turning of FRP composites with the aim of establishing wear on cutting forces in the machining of unidirectional correct cutting tool selection criteria. Among the possible GFRP was investigated too [8]. In [9], cutting forces were found to increase with increasing depth of cut during tribo-oxidation and surface damage, only abrasion, surface orthogonal machining of unidirectional CFRP, The effect of damage and sometimes adhesion are of significance for bre orientation on cutting forces and cutting quality in FRP machining. Wear mechanisms are primarily related to orthogonal machining of unidirectional CFRP was treated the physical and mechanical characteristics of the different in [10]. In [11], the attention was focused on the fibre- matrix systems. Glass and carbon fibres show a mechanisms of chip generation. Because of the inferior strongly abrasive behaviour because they are extremel surface quality of unidirectional CFRP after orthogonal abrasive by nature. Aramid fibres, on the other hand machining for some fibre orientations, in [12the mpair the tool due to their low heat conductivity and development of a new tool geometry to reduce work ductile behaviour. Adhesive wear occurs when carbonised material surface damage was investigated or molten matrix depositions settle on the tool surfaces In [13, 14], tool wear development was studied and In [16], an analysis of tool wear during turning of GFRP monitored using acoustic emission(AE) signal detection and cfrP with diamond coated tools was carried out. the and analysis during orthogonal cutting of different types of dominating wear mechanisms during cylindrical turning GFRP and CFRP and sheet such as cutting edge blunting elimination of the coating moulding compound(SMC). Decision making on tool wear layer, retreat of the cutting edge, and crater wear formation were characterised and their development was explained material type: tool wear discrimination was reliably glass, epoxy-glass, polyamide-carbon) with carbide achieved for gfrP but not for cfrP and SMc diamond coated, PCD and cBn tools was presented Turning of FRP The machinability of GFRP in precision turning by means of tools made of various materials and geometries was A significant amount of research work has been carried investigated experimentally in [17]. It was found that, by out in applying turning processes to the various FRP proper selection of the tool material and geometry composites with different cutting tools excellent machining of the workpiece is achieved and the Turning, together with drilling, milling and sawing, belongs surface quality relates closely to the feed rate and the tool to the most important cutting technologies for the Flank wear as well as retreat and rounding of the cutting machining of FRP [15]. Turning differs from milling and edge are the most frequently observed wear effects during sawing mainly because an almost constant engagement of cutting of FRP [18-20]. Hereby, the wear speed is mainly the tool exists. Apart from fluctuations in stress caused by elated to the fibre content [18]. Furthermore, crater we the different cutting behaviour of the fibres and the matrix occurs only to a minor extent [19]. The cause for this wear a quasi-continuous cut exists during turning of FRP behaviour results from the discontinuous chip formation The machinability of FRP is primarily determined by the during cutting of FRP. Hence, fracture on the face occurs physical properties of the fibres and the matrix as well as only to a minor extent, whereas fracture on the flank is the tation and volume fraction whi main reason for the examined wear types [19, 22, 23].The and carbon fibres break in a brittle manner under bending selection of clearance angle is therefore stresses, aramid fibres undergo shearing fracture under ecommended to improve the tool life. However, it must be high deformation bending and tear under tensile loading noted that this causes a weakening of the tool that may composites is much easier than that of unidirectional FRP. clearance angle has to be determined for every to stimum Moreover, the machining of short fibre reinforced omote cutting edge chipping. Thus, an opt
Figure 3: Examples of turned FRP parts [15]. Figure 2: Cutting mechanisms for FRP composites [6] The machinability of CFRP and GFRP was deeply investigated in [7]. A model for cutting force prediction in orthogonal cutting operations was presented. Three parameters were initially varied during orthogonal cutting using HSS tools: tool rake angle, relief angle and depth of cut. Their effects on cutting forces were investigated and an optimal tool geometry was found. The effect of tool wear on cutting forces in the machining of unidirectional GFRP was investigated too [8]. In [9], cutting forces were found to increase with increasing depth of cut during orthogonal machining of unidirectional CFRP. The effect of fibre orientation on cutting forces and cutting quality in orthogonal machining of unidirectional CFRP was treated in [lo]. In [Ill, the attention was focused on the mechanisms of chip generation. Because of the inferior surface quality of unidirectional CFRP after orthogonal machining for some fibre orientations, in [I21 the development of a new tool geometry to reduce work material surface damage was investigated. In [13, 141, tool wear development was studied and monitored using acoustic emission (AE) signal detection and analysis during orthogonal cutting of different types of composites: unidirectional GFRP and CFRP, and sheet moulding compound (SMC). Decision making on tool wear state was performed through graphical examination and neural network computation of AE spectrum features. Different results were obtained according to the composite material type: tool wear discrimination was reliably achieved for GFRP but not for CFRP and SMC. Turning of FRP A significant amount of research work has been carried out in applying turning processes to the various FRP composites with different cutting tools. Turning, together with drilling, milling and sawing, belongs to the most important cutting technologies for the machining of FRP [15]. Turning differs from milling and sawing mainly because an almost constant engagement of the tool exists. Apart from fluctuations in stress caused by the different cutting behaviour of the fibres and the matrix, a quasi-continuous cut exists during turning of FRP. The machinability of FRP is primarily determined by the physical properties of the fibres and the matrix as well as by the fibre orientation and volume fraction. While glass and carbon fibres break in a brittle manner under bending stresses, aramid fibres undergo shearing fracture under high deformation bending and tear under tensile loading. Moreover, the machining of short fibre reinforced composites is much easier than that of unidirectional FRP. Although the cutting of FRP composite parts is rarely desired, it can be seldom avoided for the production of the final geometry, surface quality, and form accuracy of conventionally produced parts. Turning is applied to rotation-symmetric parts such as drag links, bearings, spindles, axles, rolls, or steering columns, etc. Figure 3 shows some typical FRP parts produced by turning. Particular attention was given by several authors to the aspects of tool wear mechanisms and development in turning of FRP composites with the aim of establishing correct cutting tool selection criteria. Among the possible wear mechanisms, which include abrasion, adhesion, tribo-oxidation and surface damage, only abrasion, surface damage and sometimes adhesion are of significance for FRP machining. Wear mechanisms are primarily related to the physical and mechanical characteristics of the different fibre-matrix systems. Glass and carbon fibres show a strongly abrasive behaviour because they are extremely abrasive by nature. Aramid fibres, on the other hand, impair the tool due to their low heat conductivity and ductile behaviour. Adhesive wear occurs when carbonised or molten matrix depositions settle on the tool surfaces. In [16], an analysis of tool wear during turning of GFRP and CFRP with diamond coated tools was carried out. The dominating wear mechanisms during cylindrical turning such as cutting edge blunting, elimination of the coating layer, retreat of the cutting edge, and crater wear formation were characterised and their development was explained. In [15], a survey on the possibilities and variants of application in turning of different types of FRP (polyesterglass, epoxy-glass, polyamide-carbon) with carbide, diamond coated, PCD and CBN tools was presented. The machinability of GFRP in precision turning by means of tools made of various materials and geometries was investigated experimentally in [17]. It was found that, by proper selection of the tool material and geometry, excellent machining of the workpiece is achieved and the surface quality relates closely to the feed rate and the tool. Flank wear as well as retreat and rounding of the cutting edge are the most frequently observed wear effects during cutting of FRP [18-201. Hereby, the wear speed is mainly related to the fibre content [18]. Furthermore, crater wear occurs only to a minor extent [19]. The cause for this wear behaviour results from the discontinuous chip formation during cutting of FRP. Hence, fracture on the face occurs only to a minor extent, whereas fracture on the flank is the main reason for the examined wear types [19, 22, 231. The selection of a large clearance angle is therefore recommended to improve the tool life. However, it must be noted that this causes a weakening of the tool that may promote cutting edge chipping. Thus, an optimum clearance angle has to be determined for every tool
In [20, 21], investigations on cutting of CFRP showed that the hardness and microstructure of the cutting edge for 4 various PCD tools exert a significant influence on the effectiveness of FRP machining. Coarse-grained PCD PCD tools, in particular, reveal higher resistance to wear than E103 medium- and fine- grained Pcd types. The wear appears in the form of cutting edge rounding, chipping and crack formation on the different PCD types. Carbide tools also display a flank wear yet more irregularly and with a tool life significantly shorter in comparison. The wear is haracterised by scratches and chippings. A longer tool life is achieved by PCd and Tic or Tac free carbide types 10 due to their higher thermal conductivity pressure strength and wear resistance. Interrupted cutting during turning of CFRP causes a higher wear than continuous cutting with carbide tools under equal cutting conditions In [15], tool wear was studied during turning of GFRP obtaining good results with carbide and PCD tools. Yet, the wear for these two tool materials is considerably different. Carbide tools exhibit mostly flank wear and unding of the cutting edge. Crater wear does not occur deposited on the tooray but carbonised chip material is Figure 5: Tool life for carbide, diamond-coated and PCD also show flank wear. However, the rate of wea development is clearly slower in comparison with that for tools vs cutting speed in turning of GFRP (Vi= 35%)[15] carbide tools. Furthermore the cutting speeds attainable th PCd tools are much higher than those possible with A comparison in tool life of uncoated and diamond-coated carbide too carbide tools shows the protective effect of the diamond Figure 4 shows the influence of cutting speed on tool life layer on the carbide substrate, granting protection against luring turning of different GFRP composites: unidirectional the diamond-coated tool life is surpassed by PCD tools lass cloth/epoxy with fibre volume fraction V, =55% The degradation of adhesion of the diamond layer to the EPRU 5), bidirectional glass roving fabric/epoxy with Ve carbide substrate is the main reason for this behaviour 45%(EPR 8), glass mat/polyester with Vt= 35%(UPM Tool wear in CFRP machining is significantly different from tool wear in GFRP machining. As Figures 6 and 7 performance was verified with increasing cutting speed illustrate, a minor relation of tool life to cutting speed exists he EPRu 5 composite displays the worst tool life, the EPR 8 composite an intermediate tool life, and the UPM for CFRP machining in comparison with GFRP machining 72 composite the best tool life behaviour. This can be igure 4 and 5). The flatter trend of tool life versus cutting peed for CFRP is related to the lower temperature explained by the fact that lower glass fibre volume development during machining due to the much higher fractions result in lower thermomechanical stresses of the heat conductivity of carbon fibres. Thus, higher cutting cutting edge and, consequently, higher tool life values speeds can be utilised during CFRP turning shown in Figure 5 during turning of the UPM 72 thermoset matrix CFRP(Figure 6)indicates that much composite. The superiority of diamond-based cutting materials, PCd and diamond-coated carbide, over higher cutting speeds can be used with PCD tools. It must monolithic carbide tools is clearly seen. While the tool life trend is decreasing for carbide tools, a linear development the two tool materials. While during machining with carbide tools a vb 0.2 mm was used as tool life is evident for diamond-coated tools and pcd tools criterion the vB for Pcd tools was reduced to 0. 1 mm. If a standard VB =0.2 mm were taken as a basis, the total volume of material removed for PCD would surpass the total volume of material removed for carbide by 250 times Moreover, the tool life for both cutting tools shows that with increasing cutting speed the temperature influence on the UPM 72 tool life increases. The steeper rise in tool life for PCD tools demonstrates the extended range in cutting speed g [EPR Figure 7 shows a comparison between uncoated and diamond-coated carbide tools in turning of thermoplastic 10 matrix CFRP. The tool wear during turning of polyamides matrix CFRP with diamond-coated tools is characterized by small chippings of the coating. The degree of chipping increases with the engagement time of the cutting edge up to little beyond the contact area between chip and rake face. The base carbide becomes smooth and the cutting edge rounded. The sharp-edged transition between carbide and coating layer or tool face is also subject m/min 10 to the abrasive action of the carbon fibres and is removed he direction of the chip flow. Independently of the cutting parameters, thermoplastic matrix deposits form on Figure 4: Tool life of diamond-coated carbide tools vs the tool face and flank but are periodically removed during cutting speed in turning of different GFRP composites [15] turnIng
In [20, 211, investigations on cutting of CFRP showed that the hardness and microstructure of the cutting edge for various PCD tools exert a significant influence on the effectiveness of FRP machining. Coarse-grained PCD tools, in particular, reveal higher resistance to wear than medium- and fine-grained PCD types. The wear appears in the form of cutting edge rounding, chipping and crack formation on the different PCD types. Carbide tools also display a flank wear, yet more irregularly and with a tool life significantly shorter in comparison. The wear is characterised by scratches and chippings. A longer tool life is achieved by PCD and TIC or TaC free carbide types due to their higher thermal conductivity, pressure strength and wear resistance. Interrupted cutting during turning of CFRP causes a higher wear than continuous cutting with carbide tools under equal cutting conditions. In [15], tool wear was studied during turning of GFRP obtaining good results with carbide and PCD tools. Yet, the wear for these two tool materials is considerably different. Carbide tools exhibit mostly flank wear and rounding of the cutting edge. Crater wear does not occur in any significant way but carbonised chip material is deposited on the tool rake face during cutting. PCD tools also show flank wear. However, the rate of wear development is clearly slower in comparison with that for carbide tools. Furthermore, the cutting speeds attainable with PCD tools are much higher than those possible with carbide tools. Figure 4 shows the influence of cutting speed on tool life during turning of different GFRP composites: unidirectional glass cloth/epoxy with fibre volume fraction Vf = 55% (EPRU 5), bidirectional glass roving fabridepoxy with Vf = 45% (EPR 8), glass matlpolyester with Vf = 35% (UPM 72). For all GFRP materials, a decrease in tool performance was verified with increasing cutting speed. The EPRU 5 composite displays the worst tool life, the EPR 8 composite an intermediate tool life, and the UPM 72 composite the best tool life behaviour. This can be explained by the fact that lower glass fibre volume fractions result in lower thermomechanical stresses of the cutting edge and, consequently, higher tool life values. The influence of various cutting tool materials on tool wear is shown in Figure 5 during turning of the UPM 72 composite. The superiority of diamond-based cutting materials, PCD and diamond-coated carbide, over monolithic carbide tools is clearly seen. While the tool life trend is decreasing for carbide tools, a linear development is evident for diamond-coated tools and PCD tools. Figure 4: Tool life of diamond-coated carbide tools vs. cutting speed in turning of different GFRP composites [15]. Figure 5: Tool life for carbide, diamond-coated and PCD tools vs. cutting speed in turning of GFRP (Vf = 35%) [15]. A comparison in tool life of uncoated and diamond-coated carbide tools shows the protective effect of the diamond layer on the carbide substrate, granting protection against abrasive wear and thermal wear. At high cutting speed, the diamond-coated tool life is surpassed by PCD tools. The degradation of adhesion of the diamond layer to the carbide substrate is the main reason for this behaviour. Tool wear in CFRP machining is significantly different from tool wear in GFRP machining. As Figures 6 and 7 illustrate, a minor relation of tool life to cutting speed exists for CFRP machining in comparison with GFRP machining (Figure 4 and 5). The flatter trend of tool life versus cutting speed for CFRP is related to the lower temperature development during machining due to the much higher heat conductivity of carbon fibres. Thus, higher cutting speeds can be utilised during CFRP turning. A comparison between carbide and PCD tools in turning of thermoset matrix CFRP (Figure 6) indicates that much higher cutting speeds can be used with PCD tools. It must be noted that different criteria of tool life were adopted for the two tool materials. While during machining with carbide tools a VB = 0.2 mm was used as tool life criterion, the VB for PCD tools was reduced to 0.1 mm. If a standard VB = 0.2 mm were taken as a basis, the total volume of material removed for PCD would surpass the total volume of material removed for carbide by 250 times. Moreover, the tool life for both cutting tools shows that with increasing cutting speed the temperature influence on the tool life increases. The steeper rise in tool life for PCD tools demonstrates the extended range in cutting speed compared with that for carbide tools. Figure 7 shows a comparison between uncoated and diamond-coated carbide tools in turning of thermoplastic matrix CFRP. The tool wear during turning of polyamides matrix CFRP with diamond-coated tools is characterized by small chippings of the coating. The degree of chipping increases with the engagement time of the cutting edge up to little beyond the contact area between chip and rake face. The base carbide becomes smooth and the cutting edge rounded. The sharp-edged transition between carbide and coating layer on the tool face is also subject to the abrasive action of the carbon fibres and is removed in the direction of the chip flow. Independently of the cutting parameters, thermoplastic matrix deposits form on the tool face and flank, but are periodically removed during turning
Direction of vibration Eo Tool Figure 8: Mechanism of ultrasonic vibration cutting PCD ve= cutting speed f vibration frequency: IT cutting distance during one period of tool vibration [24] This cutting speed is called"critical cutting speed" in the vibration cutting and is calculated by ve 2raf, where a is the amplitude of the tool vibration and f is its frequency ( ir 100 this study Vc=110 m/min) Cutting speed'Vc(m/min) The mechanism of ultrasonic vibration cutting is shown in Figure 8. The performance of ultrasonic vibration cutting Figure 6: Tool life for carbide and pcd tools vs. cutting strongly depends on the cutting distance during one speed in turning of epoxy matrix CFRP(Vt= 40 %)[15] period of the tool vibration IT Vo/f. It was experimentally confirmed that lr must be smaller than the fibre diameter (7 um in this study) to take advantage of ultrasonic 100 vibration cutting. By making l smaller than the fibre diameter the matrix and the fibre, which have different O DCC mechanical properties, can be sheared separately Hereby, the fibres do not prevent the shearing of the plastic matrix and, consequently, the surface quality is 50 improved even if the angle between fibre orientation and cutting direction is 90 A comparison of surface roughness between conventiona and ultrasonic vibration cutting is shown in Figure 9. Wher Ir is larger than the fibre diameter, the surface roughness 8 in ultrasonic vibration cutting is similar to that of conventional cutting(Figure 9a). On the contrary, when IT is smaller than the fibre diameter, the roughness in ultrasonic vibration cutting becomes smaller than that of conventional cutting(Fi 103mmin3.103 Conventional cutting vibration cutting Cutting Speed Vc 8 Figure 7: Tool life of diamond-coated and uncoated 6 54 carbide tools vs. cutting speed in turning of polyamide matrix CFRP (V+=40%)[15] As regards cutting parameters, speed and feed primarily nfluence the life of the cutting edge(Figure 7 4590 0 45 The tool life of both uncoated and diamond-coated carbide Fibre orientation(deg) Fibre orientation(deg)D tools reveals that wear is reduced by the diamond layer (a)Ir =18 um>7 um An increase in thermal stress of the cutting edge is (b)hr=36μm<7pm connected with the increase in cutting speed. Due to the high thermal conductivity of the diamond layer, an Figure 9: Surface roughness for conventional and ultrasonic vibration cutting [24] increase in thermal load capacity is available and ccordingly, higher cutting speeds are allowed for because of the difficulty in machining CFRP composites 目 Conventional cutting■ Vibration cutting h high efficiency, in [24] it was proposed to apply ultrasonic vibrations in turning of CFRP pipes using a iamond-coated tool The performance of the ultrasonic vibration cutting was 810 evaluated in terms of cutting force, burr formation and 12 surface roughness Ultrasonic vibration cutting allows to obtain good surface quality when machining difficult-to-cut materials. This is due to the fact that the ultrasonic vibration avoids the 04590 continuous contact between the tool rake face and the Fibre orientation(deg) Fibre orientation(deg) hip. As reported in [25], when the cutting speed beco faster than the speed of the tool vibration, the tool rake Figure 10: Cutting forces for conventional and vibration face is not separated from the chip and consequently cutting for a cutting speed 4 m/m ultrasonic vibration cutting loses its effectiveness corresponding to IT =3.6 um [24
Figure 8: Mechanism of ultrasonic vibration cutting. vc = cutting speed; f = vibration frequency; IT = cutting distance during one period of tool vibration [24]. This cutting speed is called "critical cutting speed" in the vibration cutting and is calculated by vc = 2naf, where a is the amplitude of the tool vibration and f is its frequency (in this study vc = 110 m/min). The mechanism of ultrasonic vibration cutting is shown in Figure 8. The performance of ultrasonic vibration cutting strongly depends on the cutting distance during one period of the tool vibration IT = vJf. It was experimentally confirmed that IT must be smaller than the fibre diameter (7 pm in this study) to take advantage of ultrasonic vibration cutting. By making IT smaller than the fibre diameter, the matrix and the fibre, which have different mechanical properties, can be sheared separately. Hereby, the fibres do not prevent the shearing of the plastic matrix and, consequently, the surface quality is improved even if the angle between fibre orientation and cutting direction is 90". A comparison of surface roughness between conventional and ultrasonic vibration cutting is shown in Figure 9. When IT is larger than the fibre diameter, the surface roughness in ultrasonic vibration cutting is similar to that of conventional cutting (Figure 9a). On the contrary, when IT is smaller than the fibre diameter, the roughness in ultrasonic vibration cutting becomes smaller than that of conventional cutting (Figure 9b). Figure 6: Tool life for carbide and PCD tools vs. cutting speed in turning of epoxy matrix CFRP (Vf= 40 %) [I51 Figure 7: Tool life of diamond-coated and uncoated carbide tools vs. cutting speed in turning of polyamide matrix CFRP (Vf = 40%) [I 51. As regards cutting parameters, speed and feed primarily influence the life of the cutting edge (Figure 7). The tool life of both uncoated and diamond-coated carbide tools reveals that wear is reduced by the diamond layer. An increase in thermal stress of the cutting edge is connected with the increase in cutting speed. Due to the high thermal conductivity of the diamond layer, an increase in thermal load capacity is available and, accordingly, higher cutting speeds are allowed for. Because of the difficulty in machining CFRP composites with high efficiency, in [24] it was proposed to apply ultrasonic vibrations in turning of CFRP pipes using a diamond-coated tool. The performance of the ultrasonic vibration cutting was evaluated in terms of cutting force, burr formation and surface roughness. Ultrasonic vibration cutting allows to obtain good surface quality when machining difficult-to-cut materials. This is due to the fact that the ultrasonic vibration avoids the continuous contact between the tool rake face and the chip. As reported in [25], when the cutting speed becomes faster than the speed of the tool vibration, the tool rake face is not separated from the chip and consequently ultrasonic vibration cutting loses its effectiveness. Figure 9: Surface roughness for conventional and ultrasonic vibration cutting [24]. Fibre orientation (deg) Fibre orientation (deg) Figure 10: Cutting forces for conventional and vibration cutting for a cutting speed = 4 m/min corresponding to IT = 3.6 pm [24]
Feed direction Feed direction Cutting direction 88 c2 0.1mm 0.1mm (a) conventional cutting (b)Vibration cutting Figure 11: Microscopic observation of the cut surface when fibre orientation is 0"[24] 7.5 Typical thrust force vs. time plot for a single drilling operation on CFRP [26] (a) Conventional cutting b)Vibration cutting Figure 12: Microscopic observation of the edge when : fibre orientation is 90[24] Cutting forces were also investigated to confirm that the critical limit of hr in CFRP cutting was correct. The cutting forces for different fibre orientations in conventional and ultrasonic vibration cutting are shown in Figure 10. In cutting of CFRP the thrust force is higher than the principal force. The average thrust force in ultrasonic vibration cutting becomes less than half of the thrust force in conventional cutting if IT <7 um The improvement of the surface quality was confirmed by microscopic photographs(Figures 11 and 12). Arrows in 7.5 the figure show the cutting and the feed direction For fibre Time t(s) orientation 0, in conventional cutting lots of fibres are Figure 14: Typical torque vs. time plot for a single drilling ulled out Figure 11a). In ultrasonic vibration cutting operation on CFRP [26] those fibres are absent (Figure 11b). In addition, for fibre orientation 90, in conventional cutting fibres are not cut a This is followed by a sharp reduction of the force and a he edge of the surface( Figure 12a). In ultrasonic vibration slight drop of the torque due to the fact that the tip of the cutting, however, those fibres are not visible(Figure 12b) tool has broken through the back face of the workpiece Drilling of FRP A number of research workers have investigated the the laminate, the reduction in force becomes more gradual drilling of different FRP composite materials using various and the torque is seen to slightly increase cutting tool materials Finally, the force and the torque drop to zero as reaming In [26], the use of high performance carbide drills in drilling takes place CFRP epoxy matrix composites was studied. To reduce As the number of holes drilled increases so does the high wear rate of the carbide drills, speciality coatings magnitude of both the maximum torque and the the including titanium nitride(TiN)and diamond-like-carbor force values (DLC)can be used. The performance of the coatings was Similar profiles were noted for both the uncoated and the analysed in terms of damage to the composite and thrust coated tools force and torque produced during drilling Figure 15 shows a combination of the maximum thrust Figures 13 and 14 show typical thrust force and torque force and torque for uncoated and coated drills profiles, in the case of an uncoated tool. The general form Also included are the flank wear results which show wear of the thrust force and torque profile, comprises six main in the order of 0.07 mm after 32 drilling operations stages. Initially there is a sharp increase in thrust force The maximum thrust force, maximum torque and flank nd torque due to the initial entry of the drill into the ear curves for the three drill types exhibit similar trends composite. This is followed by a further increase in the Both the thrust force and torque curves rise sharply in the force and torque as the second cutting edge enters the nitial stages after which the subsequent rate of increase is workpiece. The maximum force and torque occur as the seen to reduce tip of the tool breaks through the bottom ply of the A change in form of both these curves is apparent in the aminate region 5<n<10, drilled holes
Figure 13: Typical thrust force vs. time plot for a single drilling operation on CFRP [26]. Figure 12: Microscopic observation of the edge when fibre orientation is 90" [24]. Cutting forces were also investigated to confirm that the critical limit of IT in CFRP cutting was correct. The cutting forces for different fibre orientations in conventional and ultrasonic vibration cutting are shown in Figure 10. In cutting of CFRP the thrust force is higher than the principal force. The average thrust force in ultrasonic vibration cutting becomes less than half of the thrust force in conventional cutting if IT < 7 pn. The improvement of the surface quality was confirmed by microscopic photographs (Figures 11 and 12). Arrows in the figure show the cutting and the feed direction. For fibre orientation O", in conventional cutting lots of fibres are pulled out (Figure Ila). In ultrasonic vibration cutting, those fibres are absent (Figure 11 b). In addition, for fibre orientation go", in conventional cutting fibres are not cut at the edge of the surface (Figure 12a). In ultrasonic vibration cutting, however, those fibres are not visible (Figure 12b). Drilling of FRP A number of research workers have investigated the drilling of different FRP composite materials using various cutting tool materials. In [26], the use of high performance carbide drills in drilling CFRP epoxy matrix composites was studied. To reduce the high wear rate of the carbide drills, speciality coatings including titanium nitride (TIN) and diamond-like-carbon (DLC) can be used. The performance of the coatings was analysed in terms of damage to the composite and thrust force and torque produced during drilling. Figures 13 and 14 show typical thrust force and torque profiles, in the case of an uncoated tool. The general form of the thrust force and torque profile, comprises six main stages. Initially, there is a sharp increase in thrust force and torque due to the initial entry of the drill into the composite. This is followed by a further increase in the force and torque as the second cutting edge enters the workpiece. The maximum force and torque occur as the tip of the tool breaks through the bottom ply of the laminate. Figure 14: Typical torque vs. time plot for a single drilling operation on CFRP [26]. This is followed by a sharp reduction of the force and a slight drop of the torque due to the fact that the tip of the tool has broken through the back face of the workpiece. When the first chisel edge breaks through the back face of the laminate, the reduction in force becomes more gradual and the torque is seen to slightly increase. Finally, the force and the torque drop to zero as reaming takes place. As the number of holes drilled increases, so does the magnitude of both the maximum torque and the thrust force values. Similar profiles were noted for both the uncoated and the coated tools. Figure 15 shows a combination of the maximum thrust force and torque for uncoated and coated drills. Also included are the flank wear results, which show wear in the order of 0.07 mm after 32 drilling operations. The maximum thrust force, maximum torque and flank wear curves for the three drill types exhibit similar trends. Both the thrust force and torque curves rise sharply in the initial stages after which the subsequent rate of increase is seen to reduce. A change in form of both these curves is apparent in the region 5 < n < 10, drilled holes
Thrust Force %0一 88见 Torque (a)n=1 (b)n=1000 5101520253035 Figure 17: Hole exit in drilled GFRP(Ve Work thickness: 10 mm: drill; fish tail carbide drill dial Number of holes drilled n 10 mm; feed: 0. 1 mm; cutting speed: 163 m/min (301 Figure 15: Variation of maximum thrust force torque and In [30], the problem of burr generation in drilling of GFRP flank wear with number of drilled holes [26] composites with different cutting tools is studied. The fish x Uncoated tool, o dlc tool tin coated tool tail drill is found to be very effective in suppressing the generation of burr. Several grades of carbide materials In [27 the tool life of uncoated and diamond-coated were tested as fish tail drills. Among the tested carbides carbide tools in drilling of GFRP composites was studied K01 and K10 showed the highest cutting performance and (Figure 16). The comparison of the tool life of the different drill wear depended only on the fibre type and volume types of tools illustrates the protective effect of the lese drills, tool wear causes diamond layer. In addition to the protection against generation of burr after a certain length of drilling. Figure abrasive wear, the diamond layer also protects against 17 shows an example of burr after drilling 1000 holes on a thermal wear. a shift of the tool life line towards highe values is obtained for higher cutting speeds. Nevertheless mainly caused by the outer corner wear of the drill the tool life curve of the diamond-coated carbide bends at To get a longer tool life, it is necessary to use higher wear high cutting speeds. This indicates the thermal failure of resistant tool materials and diamond is the most suitable the substrate material. An increase in cutting speed is a trial diamond endmill with sintered diamond blades connected with an increase in cutting temperature On the Compared with carbide drills, the wear of diamond development and, on the other, by the decrease in tool life endmills was very small and after drilling 1000 holes the for uncoated carbide tools due to the insufficient heat burr was scarcely generated. In addition, the torque and esistance of the substrate [28] thrust for diamond endmills was less than a half of that for [29] an overview of the potential uses of PCD in FRP carbide drills. The roughness of the hole wall drilled with composite drilling was shown and PCD tools were carbide drills and diamond endmills was compared. Holes compared to carbide tools in terms of both economics and drilled with fish tail carbide drills have a high roughness uality. It was found that drilling processes performed on with Rmax 30 um after drilling 1000 holes. Holes drilled with diamond endmills have a low roughness with rmax <5 implemented. PCD is an economical alternative to carbide um even ater drilling 1000 holes despite the higher cost because tool life is longer and As regards the geometry of fish tail drills, clearance angle higher processing speeds can be used point angle and helix angle were examined. As the elastic deformation of composites is rel machining, the contact area between the flank of the drill and the finished surface may become quite large when the 100 cle To find out the suitable clearance angle, drilling tests were conducted with fish tai carbide drills and variable clearance, point and helix angles. The most suitable angles for drilling gFrP were clearance angle 15, point angle 75, helix angle 35 DCC Figure 18 shows the photos of the exit side of holes aft drilling 1000 holes on 10 mm thick GFRP with each drill The effect of machining parameters on the cut quality and the mechanical behaviour of GFRP composites was verified in [31] during drilling tests. A correlation between width of the damage zone and drilling speed and feed ratio was found: the higher the ratio, the better the cut quality Carbide Dynamic modelling and adaptive predictive control of thrust force in the drilling of CFRP composite laminates and control of CFRP composite laminates were presented n[33] the thrust force defined by the discrete Hocheng Dharan equations was compared with the experimental Cutting speed vc(m/min) thrust force at the pre-exit drilling phase. A theoretical study and a series of experiments were conducted to Figure 16: Tool life of carbide and diamond-coated carbide develop a dynamic model of the process which was used tools vs cutting speed in drilling of GFRP(V+=55%) to design a supervisory adaptive predictive controller that Drill diameter: 10 mm: work thickness: 18 mm. estimates model parameters and applies predictive control feed: 0.08 mm[271 for force regulation
(a) n = 1 (b) n=1000 Figure 17: Hole exit in drilled GFRP (Vf = 60%). Work thickness: 10 mm; drill: fish tail carbide; drill diam.: 10 mm; feed: 0.1 mm; cutting speed: 163 mhin [30]. Figure 15: Variation of maximum thrust force, torque and flank wear with number of drilled holes [26]. x Uncoated tool, DLC tool, + TIN coated tool. In [27], the tool life of uncoated and diamond-coated carbide tools in drilling of GFRP composites was studied (Figure 16). The comparison of the tool life of the different types of tools illustrates the protective effect of the diamond layer. In addition to the protection against abrasive wear, the diamond layer also protects against thermal wear. A shift of the tool life line towards higher values is obtained for higher cutting speeds. Nevertheless, the tool life curve of the diamond-coated carbide bends at high cutting speeds. This indicates the thermal failure of the substrate material. An increase in cutting speed is connected with an increase in cutting temperature. On the one hand, this can be explained by a crater wear development and, on the other, by the decrease in tool life for uncoated carbide tools due to the insufficient heat resistance of the substrate [28]. In [29] an overview of the potential uses of PCD in FRP composite drilling was shown and PCD tools were compared to carbide tools in terms of both economics and quality. It was found that drilling processes performed on FRP composites are strongly dependent on the tools implemented. PCD is an economical alternative to carbide despite the higher cost because tool life is longer and higher processing speeds can be used. Cutting speed vc (rnhin) Figure 16: Tool life of carbide and diamond-coated carbide tools vs. cutting speed in drilling of GFRP (Vf = 55%). Drill diameter: 10 mm; workthickness: 18 mm; feed: 0.08 mm [27]. In [30], the problem of burr generation in drilling of GFRP composites with different cutting tools is studied. The fish tail drill is found to be very effective in suppressing the generation of burr. Several grades of carbide materials were tested as fish tail drills. Among the tested carbides, KO1 and K10 showed the highest cutting performance and drill wear depended only on the fibre type and volume fraction. Even with these drills, tool wear causes the generation of burr after a certain length of drilling. Figure 17 shows an example of burr after drilling 1000 holes on a 10 mm thick GFRP laminate with Vf = 60 %. The burr is mainly caused by the outer corner wear of the drill. To get a longer tool life, it is necessary to use higher wear resistant tool materials and diamond is the most suitable. A trial diamond endmill with sintered diamond blades brazed on a carbide substrate was used for drilling GFRP. Compared with carbide drills, the wear of diamond endmills was very small and after drilling 1000 holes the burr was scarcely generated. In addition, the torque and thrust for diamond endmills was less than a half of that for carbide drills. The roughness of the hole wall drilled with carbide drills and diamond endmills was compared. Holes drilled with fish tail carbide drills have a high roughness with R, = 30 pm after drilling 1000 holes. Holes drilled with diamond endmills have a low roughness with R, < 5 pm even after drilling 1000 holes. As regards the geometry of fish tail drills, clearance angle, point angle and helix angle were examined. As the elastic deformation of composites is relatively large during machining, the contact area between the flank of the drill and the finished surface may become quite large when the clearance angle is small. To find out the suitable clearance angle, drilling tests were conducted with fish tail carbide drills and variable clearance, point and helix angles. The most suitable angles for drilling GFRP were: clearance angle 15", point angle 75", helix angle 35". Figure 18 shows the photos of the exit side of holes after drilling 1000 holes on 10 mm thick GFRP with each drill. The effect of machining parameters on the cut quality and the mechanical behaviour of GFRP composites was verified in [31] during drilling tests. A correlation between width of the damage zone and drilling speed and feed ratio was found: the higher the ratio, the better the cut quality. Dynamic modelling and adaptive predictive control of thrust force in the drilling of CFRP composite laminates were developed in [32]. The characterisation, modelling and control of CFRP composite laminates were presented in [33]; the thrust force defined by the discrete HochengDharan equations was compared with the experimental thrust force at the pre-exit drilling phase. A theoretical study and a series of experiments were conducted to develop a dynamic model of the process which was used to design a supervisory adaptive predictive controller that estimates model parameters and applies predictive control for force regulation
aluminum /CFRP/titanium Diameter tolerances caused by dissimilar lastic moduli of the materials Major CFRP flank Shim-lay C=10° Intense tool wear Delaminations, erosion Figure 19: Aspects concerning drilling of composite-metal MateriaL: Al/ CFRP/ TiAI6V4 hickness 10/10/10mm Figure 18: Influence of clearance angle on burr generation after drilling 1000 holes on GFRP(V+=60% d=6.0 mm Work thickness: 10 mm: drill fish tail carbide drill diam Tools 12 mm; cutting speed: 181 m/min; feed: 0.05 mm [30] Carbide K10 Carbide K30F In [34] the authors emphasize that in many aerospace Diameter D 91mm85/91mm pplications dissimilar material stack-ups of composites ind aluminium and/or titanium are used for wing No, of flutes 3 tailplane structures. Such structures contain holes for various purposes. The machining characteristics of these materials, when sandwiched together, introduce a unique Point angle o set of drilling problems(Figure 19). Apart from the intense Rake angle y 0° tool wear caused by the high loads during the process, the erosion and abrasion in the cfrp layer are also critical for uncoated aerospace fabrication In addition, industrial needs such as Boring parameters the avoidance of cooling lubrication also increase the efforts for successful machining and require highly efficient Cutti d machining technology. Therefore, extensi ge set-up of Feed reverse after 5/10/15/20/25/30 mm e experiments Feed f 0.15mm ere undertaken to find solutions conomic drilling processes on multi-layer materials(Table 4). Different drill configurations were selected and MQL(fattyalcohol/ester) investigated by characterising cutting forces, tool wear hole quality and chip formation. The investigations showed Table 4: Experimental conditions in the drilling of an that drilling of multi-layer composites requires adapted aluminium/CFRP/titanium sandwich structure [34] utting parameters tools and cooling lul brication Figure 20 shows the feed forces for a drilling process on an AlCuMg2/CFRP/Al6V4 sandwich structure using a N Fatty alcohol onventional drilling tool with a diameter d =9. 1 mm and stigations have shown that dry machining of titanium involves increased ip formation problems [35 onsequently, the drilling experiments were carried out with minimum quantity lubrication. The increase of feed 3 forces clarifies that the forces vary considerably between le material layers. The feed force when drilling the titanium layer is clearly higher and increases with growing 公 N+(additivated umber of holes caused by extensive tool wear 200 igure 21 shows the course of tool wear for a conventional tool and a tool with optimised geometry. The latter was designed with a diameter step of 8.5 to the carbide specification was changed to a micro-grain carbide and the tool surface was treated by micro-blasting 2 For conventional drilling tools, the number of holes is Number of holes limited to 15 no matter what cutting fluid is used. The optimised tool shows a significantly better wear behaviou Figure 20: Feed force when drilling the aluminium/CFRP/titanium sandwich structure [34]
Figure 19: Aspects concerning drilling of composite-metal sandwich materials [34] Process Workpiece a = 20" Figure 18: Influence of clearance angle on burr generation after drilling 1000 holes on GFRP (Vf = 60%). Work thickness: 10 mm; drill: fish tail carbide; drill diam.: 12 mm; cutting speed: 181 m/min; feed: 0.05 mm [30]. Drilling Material: Al / CFRP / TiA16V4 In [34] the authors emphasize that in many aerospace applications dissimilar material stack-ups of composites and aluminium and/or titanium are used for wing or tailplane structures. Such structures contain holes for various purposes. The machining characteristics of these materials, when sandwiched together, introduce a unique set of drilling problems (Figure 19). Apart from the intense tool wear caused by the high loads during the process, the erosion and abrasion in the CFRP layer are also critical for aerospace fabrication. In addition, industrial needs such as the avoidance of cooling lubrication also increase the efforts for successful machining and require highly efficient machining technology. Therefore, extensive experiments were undertaken to find solutions for the set-up of economic drilling processes on multi-layer materials (Table 4). Different drill configurations were selected and investigated by characterising cutting forces, tool wear, hole quality and chip formation. The investigations showed that drilling of multi-layer composites requires adapted cutting parameters, tools and cooling lubrication. Figure 20 shows the feed forces for a drilling process on an AICuMg2/CFRP/TiAIGV4 sandwich structure using a conventional drilling tool with a diameter D = 9.1 mm and two different cutting fluids. Several investigations have shown that dry machining of titanium involves increased tool wear and chip formation problems [35, 361. Consequently, the drilling experiments were carried out with minimum quantity lubrication. The increase of feed forces clarifies that the forces vary considerably between the material layers. The feed force when drilling the titanium layer is clearly higher and increases with growing number of holes caused by extensive tool wear. Figure 21 shows the course of tool wear for a conventional tool and a tool with optimised geometry. The latter was designed with a diameter step of 8.5 to 9.1 mm. Moreover, the carbide specification was changed to a micro-grain carbide and the tool surface was treated by micro-blasting. For conventional drilling tools, the number of holes is limited to 15 no matter what cutting fluid is used. The optimised tool shows a significantly better wear behaviour. Tools d = 6.0 mm Conventional I Optimized Cutting speed vc Feed f 1 Carbide K10 1 Carbide K30F 20 m/min 0.15 mm Diameter D I 9.1 mm I 8.5/9.1 mm No. of flutes I 3 I 3 Helix angle 6 Point angle o Rake angle y Coating uncoated uncoated Boring parameters I Feed reverse after I 5/10/15/20/25/30 mm Cooling I MQL (fatty alcohol / ester) Table 4: Experimental conditions in the drilling of an aluminium/CFRP/titanium sandwich structure [34]. Number of holes Figure 20: Feed force when drilling the aluminium/CFRP/titanium sandwich structure [34]
Conventional tool Conventional macroscopic geometry of tool cutting edge End of tool life Low cutting edge radius, r =10-15 um Ester High surface quality of rake face and flank, Ra Long fibers Carbide tools ->Pcd tools Figure 22: Tool requirements for milling of GFRP and CFRP Figure 21: Tool wear when drilling the uminium/CFRP/itanium sandwich structure for different tool specifications [34] The standard tool geometries cannot be applied to machining of AFRP composites because the individual M∥ ing of FRP aramid fibres can be separated in a clean cut only under Milling operations conducted on FRP parts, as opposed to simultaneous prestress. Accordingly, the tool geometry metal parts, are characterized by a low ratio of material must allow for prestressing of the aramid fibres before the removed to total part volume. Milling is used, as a rule, as cutting process begins [37-39] corrective end-machining opera oduce a high cutting edge sharpness and a small cutting edge The fibre type radius are further requirements reinforcement architecture and matrix volume fraction are To minimize the friction at the tool rake face and the most important factors governing tool selection and therefore, the tendency to build-up. tool must satisfy machining parameter setting high requirements in terms of face and flank surface In the case of glass and carbon fibre reinforcement, it is quality due to the high friction coefficient of the tough aramid fibres In the case of aramid fibre reinforcement, it is the cutting Tools made of fine grain carbide prove successful in tool geometry that dictates the choice of the cutting tool of AFRP composites [6]. The tool requir The behaviour in machining operations is determined AFRP milling reported in Figure 23 illustrate the mainly by the characteristics of the fibres reinforcing the differences in tool characteristics compared with tools composite. This exerts a major influence on process used for GFRP or CFRP milling(Figure 22) parameter selection or on the suitability of tool concepts Delamination and top layer fraying can only be avoided in The fibres are characterized by their high tensile strength milling of AFRP composites by using tools with a counter modulus of elasticity(higher than those of the plastic clockwise spiral, The tool used depends on the thickness matrix)and low strain at failure(lower than that of the of the part to be milled For thin laminates, opposed helical plastic matrix). Additionally, there is a vanety of thermal tools prove effective. The forces thus released point from characteristics, depending on the fibre type, which differ the top and bottom layers towards the middle considerably from those of the plastic matrix workpiece. This variant requires accurate axial alig ool selection of the workpiece. When thicker parts are milled The hardness of the glass and, more becomes clogged up in the middle with fibre material. split of the helix milling cutters should be used for such parts. The carbon fibres results in a high level constantly alternating stress prevents the fibres from machining. Since this wear manifests e all in avoiding the cutting edge. The high dynamic stress, which tool cutting edge rounding, the cutti can result in strong vibration and chatter, is possess a high degree of resistance to ab asion an disadvantage for these tools. PCD tipped tools can only be chipping Fine grain carbide from the K 10 group or, better, PCD complex geometry of such tools. Only tools having PCD cutting edges set at opposed helix angles soldered onto materials are unsuitable because their low strength and high brittleness make them very sensitive to shocks solid carbide shanks have found general acceptance for esulting in tool cutting edge spalling and their low heat certain applicat conductivity does not allow for the dissipation of the heat generated during FRP composite machining. Due to its Special macroscopic geometry of tool cutting edge low wear resistance, CBN, which is as expensive as PcD esents no advantage over the latte Very high resistance to wear In order to ensure that the glass and carbon fibres are Low cutting edge radius, r=10-15 severed in a clean cut, it is very important to ensure a high Very high surface quality of rake and flank, Ra High thickness he pronounced susceptibility of these fibres to brittle fracture, tool geometries correspond approximately to Carbide tools -> Pcd tools those of the tools used in metal working. These requirements are summarised in Figure 22. Distinctions in tool selection can be drawn between areas of application on the basis of the fibre type fibre length and fibre volume Figure 23: Tool requirements for milling of AFRP composite materials fraction in the FrP composite material
Number of holes Figure 21: Tool wear when drilling the aluminium/CFRP/titanium sandwich structure for different tool specifications [34]. Milling of FRP Milling operations conducted on FRP parts, as opposed to metal parts, are characterized by a low ratio of material removed to total part volume. Milling is used, as a rule, as a corrective end-machining operation or to produce defined, high quality surfaces. The fibre type, reinforcement architecture and matrix volume fraction are the most important factors governing tool selection and machining parameter setting. In the case of glass and carbon fibre reinforcement, it is the cutting tool material, that dominates the tool selection. In the case of aramid fibre reinforcement, it is the cutting tool geometry that dictates the choice of the cutting tool. The behaviour in machining operations is determined mainly by the characteristics of the fibres reinforcing the composite. This exerts a major influence on process parameter selection or on the suitability of tool concepts. The fibres are characterized by their high tensile strength, modulus of elasticity (higher than those of the plastic matrix) and low strain at failure (lower than that of the plastic matrix). Additionally, there is a variety of thermal characteristics, depending on the fibre type, which differ considerably from those of the plastic matrix. Tool selection The hardness of the glass and, more especially, of the carbon fibres results in a high level of wear during machining. Since this wear manifests itself above all in tool cutting edge rounding, the cutting edge should possess a high degree of resistance to abrasion and chipping. Fine grain carbide from the K 10 group or, better, PCD are, therefore, suitable as tool materials. Ceramic materials are unsuitable because their low strength and high brittleness make them very sensitive to shocks, resulting in tool cutting edge spalling, and their low heat conductivity does not allow for the dissipation of the heat generated during FRP composite machining. Due to its low wear resistance, CBN, which is as expensive as PCD, presents no advantage over the latter. In order to ensure that the glass and carbon fibres are severed in a clean cut, it is very important to ensure a high cutting edge sharpness. As regards cutting edge geometry, care should be taken to ensure that cutting edge serration and radius are as small as possible. Due to the pronounced susceptibility of these fibres to brittle fracture, tool geometries correspond approximately to those of the tools used in metal working. These requirements are summarised in Figure 22. Distinctions in tool selection can be drawn between areas of application on the basis of the fibre type, fibre length and fibre volume fraction in the FRP composite material. 0 Conventional macroscopic geometry of tool cutting edge 0 Very high resistance to wear 0 Low cutting edge radius, r = 10-15 pm 0 High surface quality of rake face and flank, Ra c 1.5 pm 0 Going from: Glass fibers -> Carbon fibers Short fibers -> Long fibers Carbide tools -> PCD tools then: Figure 22: Tool requirements for milling of GFRP and CFRP composite materials. The standard tool geometries cannot be applied to machining of AFRP composites because the individual aramid fibres can be separated in a clean cut only under simultaneous prestress. Accordingly, the tool geometry must allow for prestressing of the aramid fibres before the cutting process begins [37-391. A high cutting edge sharpness and a small cutting edge radius are further requirements. To minimize the friction at the tool rake face and, therefore, the tendency to build-up, the tool must satisfy high requirements in terms of face and flank surface quality due to the high friction coefficient of the tough aramid fibres. Tools made of fine grain carbide prove successful in milling of AFRP composites [6]. The tool requirements for AFRP milling reported in Figure 23 illustrate the differences in tool characteristics compared with tools used for GFRP or CFRP milling (Figure 22). Delamination and top layer fraying can only be avoided in milling of AFRP composites by using tools with a counterclockwise spiral. The tool used depends on the thickness of the part to be milled. For thin laminates, opposed helical tools prove effective. The forces thus released point from the top and bottom layers towards the middle of the workpiece. This variant requires accurate axial alignment of the workpiece. When thicker parts are milled, the tool becomes clogged up in the middle with fibre material. Split helix milling cutters should be used for such parts. The constantly alternating stress prevents the fibres from avoiding the cutting edge. The high dynamic stress, which can result in strong vibration and chatter, is a disadvantage for these tools. PCD tipped tools can only be used in special cases as it is extremely difficult to grind the complex geometry of such tools. Only tools having PCD cutting edges set at opposed helix angles soldered onto solid carbide shanks have found general acceptance for certain applications. 0 Special macroscopic geometry of tool cutting edge 0 Very high resistance to wear 0 Low cutting edge radius, r = 10-15 pm 0 Very high surface quality of rake and flank, Ra ~0.8 pm 0 Going from: Low thickness (c 3 mm) -> High thickness then: Carbide tools -> PCD tools Figure 23: Tool requirements for milling of AFRP composite materials