IADC WOB(kN/mm)(Bit dia) RPM(R/min) Applicable Fommations 111 0.3--0.75 200---80 Very soft formations with low compressive strength and high drillability .such as clay .mudstone ,chalk etc. 121 0.3--0.85 200---80 Soft formations with low compressive strength and high drillability,such as mudstone,gypsum,salt,soft shale,soft limestone,etc. 131 0.3--0.95 180---80 Soft to medium soft formations with low compressive strength and high drillability, such as medium soft shale,hard gypsum, medium soft limestone, medium soft sandstone, soft formation with harder interbeds ,etc. 211 0.35--0.95 180---80 Medium formations with high compressive strength ,such as medium foft shale, hard gypsum, medium soft limestone, medium soft sandstone, soft formation with harder interbeds,etc. 241 0.35--0.95 180---70 Medium hard formations with high compressive strength ,such as abrasive shale ,limestone, sandstone, dolomite, hard gypsum marble etc. 321 0.4--1.0 150---70 Hard and semi-abrasive formations with high compressive strength ,such as abrasive shale ,limestone, sandstone, dolomite, hard gypsum ,marble etc.