... : 1, Very soft with low compressive strength, such as soft marl, clay, unconsolidated sandstorm, salt, etc. ... r/m.2, Soft with low comprssive strength, such as shale, clay, sadstne, soft limestone, etc. Recommended ... .3, Medium soft with hard abrasive treak, such as hard shale, soft limestone, sandstone, etc. Recommended Bit ... , Rotary speed, 200~40 r/m. F/FC series tricone rock bits for oil well drilling ...