... with low comprssive strength, such as shale, clay, sadstne, soft limestone, etc. Recommended Bit type: GHJ517, Drilling weight:0.35~1 ... soft with hard abrasive treak, such as hard shale, soft limestone, sandstone, etc. Recommended Bit type: GHJ537, Drilling weight 0.5 ... KN/mm, Rotary speed, 200~40 r/m. GY series milling tooth tricone rock bits for oil well drilling