Subsequent drilling on pilot holes in woven carbon fibre reinforced plastic epoxy laminates: the effect of drill chisel edge on delamination

Experimental results on thrust force and quantitative delamination are compared after drilling selected woven epoxy carbon fibre reinforced plastic (CFRP) laminates using two different drill bits. The results were acquired from two separate drilling modes: the normal drilling mode and the subsequent drilling conducted on some predrilled pilot holes. The use of pilot holes in drilling fibre reinforced plastics (FRPs) has been recommended by various researchers, following the basic concepts of drilling of metals and theoretical modelling. According to researchers, the drilling thrust force is directly correlated to the hole exit delamination; therefore, a pilot hole should minimize the hole exit delamination by nullifying the so-called chisel edge effect of a drill bit. A chisel edge, which is always regarded as the most inefficient part of a drill bit, is the biggest contributor to the thrust force. In this paper, despite the observed tremendous drops (as high as 92 per cent) recorded in the experimental thrust force magnitudes while conducting subsequent drilling on predrilled pilot holes, no improvement could be witnessed in the quantitative results on the induced hole entry and exit delamination. The main inference of this research is that the main cutting edges of a drill (along with their corners) do play a decisive role with regard to the hole exit delamination, instead of the role of the chisel edge alone.