IADC WOB(kN/mm)(bit dia)   RPM(r/min)   Applicable Formations 435 445    0.35--0.95  150--60  Very soft formations with low compressive strength and high drillability,such as clay,mudstone,chalk ,etc. 515
525    0.3--0.8  200--80  Soft formations with low compressive strength and  
high drillability,such as mudstone,gypsum ,salt,soft shale,soft limestone,etc. 535
545    0.35--0.9  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. 615
625    0.35--1.0  180--80  Medium hard formations with high compressive strength,such as hard shale,limestone,sandstone,
dolomite,etc 635
     0.4--1.1  150--60 Hard formations with high compressive strength ,such as sandstone,limestone,dolomite,hard gypsum,marble,etc