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 < 1.5 um Optimized tool Going from Fatty alcohol Short fibers-> 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 <0.8 um utting edge sharpness. As regards cutting edge geometry, care should be taken to ensure that cutting oing from dge serration and radius are as small as possible. Due to Low thickness(<3 mm)-> 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 materialNumber 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 counter￾clockwise 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