Analysis of thrust force and characteristics of uncut fibres at non-conventional oriented drilling of unidirectional carbon fibre-reinforced plastic (UD-CFRP) composite laminates

Carbon fibre-reinforced plastic (CFRP) is an often-used structural material in the high-tech industries, like aerospace, wind turbine, sport, automobile, robotics and military. Due to both the growing application area of composites, and the advanced construction requirements, the used thickness of the CFRP plates increases, and the necessity of drilling holes on the sides of the plates (normal II direction) becomes even more important. Many researchers studied the machinability of UD-CFRP using numerous drilling experiments at the normal I direction. However, drilling experiments at normal II and axial directions were not published yet. The main objective of the present study is to analyse and discuss the influence of a non-conventional drilling direction on hole-quality parameters and on the thrust force. Drilling experiments were carried out in unidirectional CFRP at non-conventional drilling direction, based on central composite inscribed design. Influences of feed rate and cutting speed were analysed using response surface methodology (RSM) and analysis of variance (ANOVA) techniques. Characteristics of uncut fibres were analysed using digital image processing (DIP). The results have proved that the effect of the cutting speed is more significant when drilling UD-CFRP at the non-conventional drilling direction than at the conventional one. Furthermore, the specific feed force (k(f)) in the case of the non-conventional drilling direction was more than three times higher than the k(f) in the case of the conventional one.