Assessment of drillability of rocks is vital in the selection, operation, and performance evaluation of cutting tools used in various excavation machinery deployed in mining and tunneling. The commonly used rock drillability prediction methods, namely, drilling rate index (DRI) and Cerchar hardness index (CHI) have limitations in predicting the penetration rate due to differential wear of the cutting tool in rocks with varied hardness and abrasivity. Since cutting tools get blunt differently in different rocks, the stress beneath the tip of the bit decreases until it reaches a threshold value beyond which the penetration rate becomes constant. In this research, a new composite penetration rate index (CPRI) is suggested based on the investigations on four metamorphic rocks viz. quartzite, gneiss, schist and phyllite with varied hardness-abrasivity values. The penetration-time behavior was classified into active, moderate, passive, and dormant phases based on the reduction in penetration rate at different stages of drilling. A comparison of predicted penetration rate values using DRI and CPRI with actual penetration rate values clearly establishes the supremacy of CPRI. Micro-structure and hardness-based index was also developed and correlated with CPRI. The new indices can help predict cutting tool penetration and its consumption more accurately.

岩石可钻性评估对于采矿和隧道掘进中部署的各种挖掘机械中使用的切削工具的选择、操作和性能评估至关重要。常用的岩石可钻性预测方法，即钻速指数（DRI）和塞查尔硬度指数（CHI），由于刀具在不同硬度和磨蚀性的岩石中的不同磨损，在预测穿透率方面存在局限性。由于切削工具在不同岩石中的钝化程度不同，因此钻头尖端下方的应力会降低，直到达到阈值，超过该阈值时，穿透速度就会变得恒定。在本研究中，基于对四种变质岩的调查，提出了一种新的复合穿透率指数（CPRI）。石英岩、片麻岩、片岩和千枚岩具有不同的硬度 - 磨蚀性值。根据钻进不同阶段钻速的降低，将钻进时间行为分为主动、中等、被动和休眠阶段。使用 DRI 和 CPRI 的预测渗透率值与实际渗透率值的比较清楚地确立了 CPRI 的优势。还开发了基于微观结构和硬度的指标，并与 CPRI 相关联。新指标有助于更准确地预测刀具渗透率及其消耗量。使用 DRI 和 CPRI 的预测渗透率值与实际渗透率值的比较清楚地确立了 CPRI 的优势。还开发了基于微观结构和硬度的指标，并与 CPRI 相关联。新指标有助于更准确地预测刀具渗透率及其消耗量。使用 DRI 和 CPRI 的预测渗透率值与实际渗透率值的比较清楚地确立了 CPRI 的优势。还开发了基于微观结构和硬度的指标，并与 CPRI 相关联。新指标有助于更准确地预测刀具渗透率及其消耗量。