Preliminary study on the effect of drill geometry on the surface quality of CFRP holes

Composite materials are of major interest to the aerospace industry. In order to facilitate the composite assembly operations, the drilling process becomes imperative. When using carbon fiber reinforces plastics (CFRP) composite, its anisotropic and non-homogeneous structure makes it difficult to machine high quality holes. The fiber pull-outs that occur in the holes is of major interest due to often being the origin of failure.  An experimental study on drilling holes in CFRP was conducted using different drill geometry along with varying speeds and feeds. The drills used were all tungsten carbine/cobalt with varying point, helix and rake angles. Confocal and SEM images were used to determine the size of fiber pull out. Fiber pull out was observed most often in a fiber cutting angle of 120-160 degrees.  Lower rake angle tools created deeper fiber pull outs with a smaller effected area.  Although, higher rake angle tools created shallower fiber pull outs over a greater area. Knowing the locations and size of fiber up out for varying drills allows us to better predict fiber pull out and point us toward tooling that will create higher quality holes.