Analysis of thrust-induced drilling in composite materials using a hemispherical drill

Although composite parts are produced to a near-net shape, drilling is frequently required to make precise holes for assembly. A hemispherical drill has a quadratic distributed load over a central circular area of the radius to produce a concave, a hemicave, or a hole in structural parts. However, a hemispherical drill is characterized by a relatively high thrust force in drilling composites due to the negative rake and null cutting speed at the near-tool center, which is similar to cutting phenomenon in the chisel edge of a twist drill. An analysis of thrust-induced delamination when hemispherical drilling composite materials is presented in this study. The theoretical analysis shows that the thrust force for a hemispherical drill that produces delamination increases as the thrust ratio (alpha) decreases. This induces delamination in composite materials. The maximum theoretical thrust force for a hemispherical drill occurs at alpha = 2, which is about 30 % of the value for a twist drill. In other words, a hemispherical drill produces lower bearing stresses than a twist drill, which can cause separation of the plies at the exit, as the interlaminar bonding yields. The results agree with industrial experience, which shows that a hemispherical drill causes more delamination during drilling. The proposed model shows that alpha must be selected when a hemispherical drilling is used for composites.