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The drilling process is currently applied for bolt or rivet assembly of composite structures. As for metallic structures, the twist drill is the most commonly used cutter. At the hole exit, some delamination is generated with the use of a conventional twist drill. In this paper, an analytical model to calculate the critical thrust force in drilling composite materials is developed. This model relates the drilling critical thrust force to the delamination conditions (opening of cracks). It takes into account the chisel edge effect and the principal cutting edge effect of the twist drill. A close correlation between the experiment and the presented analytical model results is obtained. Finally, this study can be used to minimise drilling delamination at hole exit.