<p>It is commonly observed that for down-the-hole percussive drilling the rate of penetration increases monotonically with the applied weight-on-bit up to maximum (the sweet spot) beyond which the rate of penetration plateaus or decreases with further increase of weight-on-bit. The purpose of this paper is to comprehend the “sweet spot” phenomenon. To do so we build a simplified model of a down-the-hole air hammer drilling system that couples a realistic model of the hammer, experimentally derived bit–rock interaction, and simple boundary condition at the surface. The model predicts that the weight-on-bit significantly affects the hammer frequency and, as a result, the bit penetration rate. We further show that the variation in the impact frequency leads to a variation in indexing angle that, in turn, leads to non-monotonic relation between the rate of penetration and the weight-on-bit with a pronounced ‘sweet spot’. The results of this paper can be used for real time optimisation of percussive drilling.</p>