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Drilling performance of carbon fiber reinforced epoxy composite when machined dry, with conventional cutting fluid and with a cryogenically cooled tool

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Although cutting fluids and cryogenic cooling have been shown to improve the performance of metal machining processes, there has been little research on their application to conventional drilling of carbon fiber reinforced plastics (CFRPs) where the tool wear and damage are known to be a problem. This work investigated the drilling of a CFRP when machined dry, with conventional cutting fluid and with a tool cooled to liquid nitrogen (LN2) temperature. A Guhring DIN 6537 K, TiAlN (titanium, aluminum, nitride) coated, solid carbide twist drill was used to produce 6 mm diameter through-holes in an 18 mm thick plaque of woven carbon fiber/epoxy composite using a constant cutting speed and feed rate of 94 m/min and 0.065 mm/rev respectively. Performance was evaluated based on thrust force, tool wear, entry and exit delamination, damage of the drilled surface and temperature generated during cutting. It was found that the highest forces and tool wear were produced when drilling with cutting fluid, followed by drilling with a LN2 cooled tool and the lowest by drilling dry. A Scanning Electron Microscopy (SEM) investigation of the drilled surface indicated a more brittle appearance of the machined surface produced with cutting fluid and LN2 cooled drills compared to that produced by dry machining which showed evidence of more ductile deformation of the epoxy. Based on these observations, it is suggested that the reduction of the drilling temperature due to the application of cutting fluid and LN2 cooling resulted in a higher strength and abrasiveness of the workpiece during machining. This, combined with the associated faster rate of tool wear, resulted in the higher forces observed when drilling with cutting fluid and the LN2 cooled drill. However, it was also shown that drilling with cutting fluid and the LN2 cooled drill produced less exit delamination and a reduction in the damage of the drilled surface compared to dry machining. The lowest level of damage was observed with the LN2 cooled drill. It is proposed that this reduction in damage (whilst cutting forces increased) was due to the increased strength of the workpiece during drilling as a consequence of the lower cutting temperature. This indicated that the application of cutting fluid or LN2 cooling to the drilling process can improve the drilling performance with respect to the quality of drilled hole compared to dry machining, even though there was no improvement with respect to thrust force and tool wear.

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