This article details experimental work performed to evaluate the effects of varying feed rate (0.08 and 0.15 mm/rev) and tool coatings (diamond-like carbon and chemical vapour–deposited diamond) on tool wear modes and hole quality when drilling 30-mm-thick Ti-6Al-4V/carbon fibre–reinforced plastic/Al-7050 stacks in a single-shot operation. At a feed rate of 0.08 mm/rev, the diametrical accuracy of holes produced by both the chemical vapour–deposited diamond and diamond-like carbon–coated drills (6.38 mm diameter) was comparable within a tolerance of ±0.04 mm even after 70 holes. However, in terms of cylindricity, holes machined with chemical vapour–deposited diamond were significantly better than those produced using the diamond-like carbon–coated drills by a factor of ∼2 (65.7 vs 140.6 µm). A similar trend was also observed for hole out of roundness. Burr height at hole entry locations (Ti layer) ranged from ∼0.03 to 0.08 mm for all trials undertaken at the lower feed rate level; however, the diamond-like carbon–coated drills generated exit burrs which were ∼4 times larger than their chemical vapour–deposited diamond-coated counterparts. Subsequent wear analysis showed that diamond-like carbon–coated drills operating at 0.08 mm/rev typically exhibited progressive abrasion, workpiece adhesion and chipping leading to fracture of the tool corner, while fracture due to fatigue was prevalent in tests carried out at the high feed rate level. In contrast, poor adhesion of the chemical vapour–deposited diamond coating to the carbide substrate led to premature flaking, severe chipping and fracture of the drill cutting and chisel edge.

The article details experimental work to evaluate tool wear, cutting forces/torque and associated hole quality/accuracy following single-shot drilling of twin layer CFRP/AA7010 stacks. A full factorial experiment was initially planned involving variation in cutting speed (60 and 120 m/min), feed rate (0.15 and 0.30 mm/rev) and drill tip geometry (double cone and flat point drills). While flank wear for the flat point drills did not exceed 40 µm even after 120 holes irrespective of operating conditions, the double cone geometry suffered catastrophic failure after only four holes at the lowest parameter combination. Therefore, the remaining tests involving the double cone drill at higher operating parameters were subsequently halted. Feed rate had a significant influence on torque in both the CFRP and Al layers, although thrust force and torque generally remained stable over the test duration. Hole diameter was typically up to 34 µm above the nominal value of 6.38 mm with corresponding out of roundness of <60 µm. Burrs were prevalent at hole exit in all tests, with an average height of ∼120 µm when drilling at the highest cutting speed–feed rate parameter combination. Similarly, the delamination factor at hole entry increased by up to 23% when operating at the higher feed rate level.

The paper details a comprehensive experimental investigation on the influence of operating parameters and cut direction (parallel and perpendicular to fibre orientation) when wire electrical discharge machining (WEDM) unidirectional CFRP composites using zinc-coated brass wire (0.25-mm diameter). A Taguchi L18 fractional factorial orthogonal array considering four variable parameters including open voltage, ignition current and pulse-on time as well as pulse-off time was carried out for each cut direction. Results showed that a ~ 16% increase in maximum material removal rate was achieved when machining parallel to fibre direction (2.41 mm³/min) compared to cutting perpendicular to the fibres (~ 2.08 mm³/min), which was attributed to the higher electrical conductivity of the workpiece along the fibre length leading to greater discharge energies. This however resulted in relatively larger average kerf widths and poorer workpiece surface roughness (Sa) caused by the Joule heating effect. Workpieces machined parallel to fibre direction were generally free of any major edge defects, in contrast to severe delamination observed on both the top and bottom surfaces of specimens cut perpendicular to fibre orientation. High-magnification scanning electron microscopy of the machined surfaces revealed the presence of adhered resin material, fibre cracking, cavities/gaps in the matrix phase and interlayer cracks on most of the samples analysed, with damage severity dependent on the operating parameters and cut direction.