Investigation of drilling performance and mechanical properties of fiber reinforced glass-epoxy composite with cryogenically treated special design drill bits

In this study, the cutting forces, delamination factors, surface roughness, chip shapes and processing times of fiber reinforced glass-epoxy composite drilled with specially designed 8 mm diameter HSS drills with different point angles (118°, 128° and 138°), helix angles (26°, 33° and 40°) and clearance angles (8°, 10° and 12°), with and without deep cryogenic treatment (-196°C) application were investigated. Drilling operations were performed at three different spindle speeds (800, 1200 and 1800 rev/min) and feeds (0.01, 0.02 and 0.03 mm/rev). In addition, the changes in the microstructure of the cutting tools were examined by XRD analysis. The tool wear, chip temperatures and cutting tool temperatures were measured by performing consecutive drilling operations on the composite workpiece. Moreover, the tensile forces of composite workpiece samples prepared according to ASTM standards were investigated. The smaller and homogeneous distribution of the carbide grains of the deep cryogenically treated HSS drill and the increase of the carbides with higher hardness values due to the phase change increased the hardness of the cutting tool. During drilling operations, it was observed that the cutting forces decrease depending on the increase in the number of revolutions and feed, and the most suitable cutting tool according to the cutting forces (Fx) was the deep cryogenically treated HSS drill with 128° point angle, 33° helix angle and 12° clearance angle. Also, it was found that the average surface roughness (Ra) values increased depending on the increase in the number of revolutions and feed, and accordingly, the most suitable cutting tool was the deep cryogenically treated HSS drill with 128° point angle, 26° helix angle and 8° clearance angle. The delamination factors were negatively correlated to the number of revolutions, but positively correlated to the feed, and the most suitable cutting tool according to the delamination factors was the deep cryogenically treated HSS drill with 118° point angle, 26° helix angle and 8° clearance angle. The volume of chips formed during drilling operations increased as the feed rate increased and decreased as the number of revolutions increased. Processing times of deep cryogenically treated HSS drills were generally shorter than untreated HSS drills. Chisel wear, outer corner wear and flank wear occurred in drills during the cutting tool life experiments. The tool life of HSS drills with deep cryogenic treatment increased owing to lower wear values. The temperatures of the chip produced by the untreated HSS drills were higher than that of the deep cryogenically treated HSS drills. Drill temperatures were higher in untreated HSS drills. Tensile forces were higher due to less deformations in composites drilled at low feeds and with deep cryogenically treated drills.