Chisel Edge and Pilot Hole Effects in Drilling Composite Laminates

Previous studies have shown the severe limitations that have to be placed on machining forces when drilling composite laminates due to their propensity for delamination. Delamination, which consists of separation between the plys in a laminate, is due to the relatively poor strength of these materials in the thickness direction. In drilling, delamination is initiated when the drilling force exceeds a threshold value, particularly at the critical entry and exit locations of the drill bit. While abrasive machining results in damage-free holes in most composites, such processes are slow and expensive when compared to drilling with conventional twist drills. Here it is shown that the chisel edge in such drills is a major contributor to the thrust force that is the primary cause of delamination when drilling composite laminates. In this study, a series of drilling experiments were conducted on carbon fiber-reinforced composite laminates to determine quantitatively the effect of the chisel edge on the thrust force. In addition, tests were conducted to determine the effect of pre-drilling the laminate with a pilot hole. The results show a large reduction in the thrust force when a pilot hole is present which, in effect, removes the chisel edge contribution. An analytical model that incorporates the presence of a pilot hole is also described. The results from the thrust force-feed relationships show good agreement with experimentally determined values for the thrust force for a wide range of feeds for drilling tests conducted on laminates with and without pilot holes.