It was previously reported that good hole quality with no delamination or deformation can be achieved when choosing the proper speed/feed ratios and proper drill bits [1]. Hard metal tools are recommended when drilling GLARE on CNC machines. GLARE structures are usually produced in large panels (more than 2 metres), and machining is often required to bring those panels into the desired dimensional requirements and prepare them for assem- bly [1]. Machining GLARE is carried out by conventional and non-conventional material removal methods. The conventional methods most frequently used are edge milling and drilling. For example, holes are drilled into GLARE panels to join them together using mechanical fasteners while edge milling is used to give the panels the desired contour shapes for mating purposes [191]. Most machining operations on GLARE such as edge milling and drilling are performed after the components are manufactured [191]. Among the noncon- ventional machining processes are abrasive waterjet and laser cutting [1]. The challenges in machining GLARE arise from its hybrid structure which differs in many aspects from machining metals or composites individually. Machining GLARE can be seen as machining a material that continuously undergoes two distinctly different cutting phases. The homoge- nous ductile aluminium metal sheets with high thermal conductivity undergo shearing and plastic deformation followed by strain hardening and the formation of a continuous chip. The abrasive yet brittle glass fibre-epoxy prepregs and poor thermal conductivity undergo brittle fibre fracture. Also, the mismatch in the thermal expansion coefficients of metal and
composites constituents of GLARE could lead to thermal stresses which could adversely affect the machined part quality.
Despite that a signficant amount of heat is taken away by metallic chips or through the direct contact between the workpiece and the cutting tool when machining GLARE, using high cutting speeds is limited by the poor ability of glass fibre prepregs to dissipate the heat away from the cutting zone. The low thermal conductivity of the prepregs limits the tolerance of glass epoxy matrix to withstand high temperatures in high-speed machining. Adding to that the relatively low service temperature of the FM94 film adhesive (104 ◦C) used for bonding GLARE components together. Therefore, machining at high speeds would expose GLARE to excessive heat for a prolonged period of time which could adversely impact the quality of the machined part. Another issue to consider when drilling GLARE is the cutting tool material. The cutting tool should be capable of withstanding the abrasiveness of glass fibres and have a low tendency for chip adhesions and build up edge to improve the borehole surface quality. It was previously reported that when machining GLARE three important phenomenas should be carefully monitored and studied [1] these are:
1. Tool wear due to the abrasive nature of the glass fibres layers of the laminate: previous tests on different cutting tools materials showed that polycrystalline diamond PCD and solid cemented carbide drills with coatings are most suitable for machining GLARE. Whereas coated and uncoated high-speed steel HSS tools proved to be undesirable due to the high hardness of S2 glass fibres in comparison with most common cutting tool materials used in machining operations. The hardness of S2 glass fibres is 6.5 on mineral hardness scale Moh [192] which equals 84 HRa on a Rockwell A-hardness scale. The high hardness and abrasive nature of glass fibres in GLARE can cause rapid tool wear. Other types of fibre metal laminates such as those containing carbon fibres will also exhibit similar tool wear problems to GLARE [1]. For FMLs that contain aramid fibres, the sharpness of the cutting tool will deteriorate causing an incomplete cut and lose fibres protruding from the edges of the laminate.
2. Delamination: the delamination in GLARE was found to be related to the helix angle of the cutting tool for both drilling and milling operations. Studies on machining GLARE showed that helix angle in milling operations and feed force for drilling op- erations should be kept as low as possible to reduce peel and shear cutting forces and minimise delamination. In drilling, the delamination in GLARE takes place when the feed force is too high as the cutting tool is exiting the laminate, the high feed force pushes the last layers away causing them to separate from the laminate. For milling operations, a large helix angle causes the top layer to peel off the laminate leading to
delamination.
3. The heat affected zones at the edges of the laminate: the tool-workpiece interaction in milling and drilling operation raise the temperatures around the edges of the machined part. The rise in workpiece temperatures was found to increase with the increase of cutting speeds and thickness of the workpiece. The rise in workpiece temperature be- comes more critical when machining thick laminates using laser jet cutting operations. The use of coolants is suggested when machining GLARE to overcome the heat issues in some machining operations such as drilling and milling.
A very limited number of studies were previously reported on the machinability of fibre metal laminates. Early literature during the development of GLARE reported that the feed rate, lubrication and laminate thickness in milling operations had a major influence on the quality of the milled edge. It was found that the tool wear and fracture increased with feed rate increase, the rise in feed rate can reduce the laminate edge quality and cause delamination [1]. The previous investigation on the cooling/lubricant influence on milling GLARE showed that using coolant is preferable over semis dry cooling especially when machining thick laminates over 6 mm or when machining at high cutting speeds and feed rates. This is due to the limited ability of the semi-dry cooling to effectively remove the heat generated during the milling process. It was concluded that semi-dry cooling is recommended for thin GLARE laminates less than 3 mm [1].
The machining of thick laminates increase the cutting forces and generated heat due to larger contact area with the cutting tool which raises the temperatures. The rise of temperatures can cause the chips and fibres to adhere on the cutting tool or protrude the laminate edges causing delamination and deformations to the material. Delamination at the upper and lower parts of the laminate is likely to occur when improper cutting parameters are used. Drilling holes in GLARE can be performed manually using a hand-held drill or automatically of up to 700 holes using stable CNC machines with good quality and no delamination or deformation [1].
The first reported study on drilling GLARE can be traced back to 1994 during the devel- opment of fibre metal laminates by Coesel [193]. The thesis was a part of the IOP-project: Manufacturing of fibre metal laminates, which investigated the factors that influence the drilling performance of GLARE. The study investigated the effect of drill type, tool material, cutting speed, feed rate and drilling machine type on hole quality in terms of its accuracy, burr height, roundness and delamination. The final quality of the machined GLARE panels was evaluated using crack initiation tests on open hole specimens and fatigue behaviour of riveted countersunk lap joints. Results from drilling GLARE indicated that solid carbide
drills had better wear resistance than HSS drills using stable drilling machines, good hole quality was achievable up to 3000 holes using the solid carbide drills. Whereas poor hole quality and delamination were observed prior drilling 150 holes.
The drilling tests were conducted under dry conditions as it was reported that cutting fluids are not required in drilling GLARE if stable drilling machines are used along with carbide and solid carbide drills. However, this could be due to the use of thin laminates (¡ 2 mm) in the study. Moreover, cutting speeds and feed rates did not show to have an influence on the hole accuracy. From the four different types used, solid carbide and HSS-TiN coated drills produced most accurate holes. The HSS-TiN drill produced holes with high roundness error. It was also found that increasing the thickness of the laminate increased thrust force and torque. The inspection for delamination from visual inspecting and C-scan techniques showed that it occurred faster when using the HSS drills, whereas for the carbide coated and solid carbide drills no delamination was observed even after drilling 3000 holes. It was also concluded that the maximum thrust force was a critical factor on delamination. Besides, the thrust peak corresponding to the bottom layer, the heat generated during the drilling process, the drill geometry and the distribution of the critical thrust force along the cutting edge all had an influence on delamination.
It was also reported that burr heights were noticeable when using HSS drills due to the rapid wear of the cutting tool and depending on the feed rate used in carbide coated drills. Additionally, solid carbide drills did not produce burrs in all holes, while thrust force and torque increased with a number of holes drilled when using HSS drills due to the increase in tool wear. It was concluded that cutting forces increased with feed rate increase and that the drill size and chisel edge had an influence on the cutting forces while drilling different layups of GLARE showed that cutting forces increased with the increase of the laminates thickness and that the increase in lay-up thickness had no influence on the hole quality. In his thesis, Coesel did not study the delamination factor due to difficulties in measuring it visually or using the C-scan method. The surface roughness of the holes was not measured due to the small thickness of the tested laminates while burr height was measured using a micrometre and it was concluded that the actual shapes of the burrs might have changed during the measurement process. Therefore, it was assumed that all burrs were crushed which means that the accuracy of the actual burr height was affected by the measurement technique used. Also, the selection of cutting speeds and feed rates were restricted because of the limitation within the CNC machine and the pre-requisites of the experiment.
A more recent study on drilling GLARE like fibre metal laminates was conducted by Ty- czynski et al. [194]. Their drilling trials were carried out on fibre metal laminates samples made of Al2024 sheets with a thickness of 0.3 and 0.5 mm and the glass fibres were R-
type and prepregs which had a nominal thickness of 0.25 mm. The type of glass fibres used in the study is not the one which is used in standard GLARE grades (S2 glass fibre). Additionally, the stacking sequence of the prepreg laminates was a quasi-isotropic system made of (0/45/45/90/90/45/45/0). Also, the thickness of the prepreg layer was 0.5 mm and consisted of more than 4 prepreg layers. Their findings indicated that the cutting forces increased with the increase of feed rate and laminate thickness. Additionally, it was found that the cutting forces required for drilling GLARE were higher than those required for drilling GFRP and less than those required for drilling Al2024 alloy. The observed that poor hole quality at in the form of deteriorated edges at the exit of the hole and fibre pull out through the thickness of the workpiece when drilling at the highest feed rate. The damage in the glass fibre layers was more severe in the thicker laminates, while the hole size accuracy was not affected by feed rate. However, it was observed that the hole size in glass fibre layers was closer to the nominal size of the cutting tool than that in aluminium sheets. In addition, the internal surface quality of the drilled holes was poor and deteriorated, and burrs were formed around the hole edges and tended to increase with feed rate increase. However, no delamination was identified on the internal surfaces of drilled holes.
A very recent study on drilling GLARE was conducted by Pawar et al. [64] to understand the cutting mechanism of GLARE by analysing the cutting forces and the acoustic emission energy (AE). The study investigated the influence of cutting parameters and tool geometry on delamination, hole size and burr formation. The study compared four different types of solid carbide cutting tools including two flutes, three flute, four facet and eight facet drills as shown in Table 2.14. Their findings showed that the feed rate had the major influence on burr formation. Results also showed that the two facet drill outperformed the other cutting tools in terms of eliminating delamination and producing acceptable burr formation. The four and eight facet drills produced poor hole quality and uncut fibres around the edges of drilled holes, while delamination increased with feed rate. They also found that undersized holes were produced with exception to those drilled using the three flute drill. Burr heights were comparable and did not exceed 250 microns when drilling using the two flute, three flute and four facet drills, the eight facet drill produced large burrs which exceeded 1000 microns. The entrance burr height increased with feed rate increase while the exit burr height decreased with spindle speed increase. Similar results were found for the burr width.
T able 2.14: Summary of the p revious studies on drilling of GLARE laminates.[64, 193, 194]