<p>A theoretical model of chip clogging in drilling is developed in the present research. It is considered that the chips are compressed plastically under pressure to become a lump at the bottom after they start clogging and that the lump is evacuated with elastic recovery. The chip lump formation process is estimated by utilizing data in literature on irregular copper powder compaction, while the lump evacuation and clogging processes are formulated by applying the Hooke’s law, mass conservation, and force equilibrium with consideration of a vicious cycle of pressure rise due to time-delay of evacuation. The derived delay-differential equation to represent the pressure rise is numerically solved together with the equation to represent the evacuation process with the elastic recovery. The developed theoretical model and simulation method are applied to drilling of copper alloy at different feed rates. Rapid increases of the simulated and measured thrust forces due to the chip clogging agree well. Furthermore, the clogging depth at which the chip clogging causes the rapid increase in the pressure/force is evaluated both theoretically and experimentally at different feed rates, and both evaluation results agree quantitatively, verifying validity of the developed theoretical model and meaning that the chip clogging mechanism in drilling is clarified. </p>