The DTH Casing concentric drilling system was designed to allow simultaneous drilling and holing through unstable ground conditions. The screwdriver bit with a satisfactory ring or reusable ring achieves a complete head design, which significantly improves the penetration rate of the hole and the straightness of the hole. The internal flush recess, along with the built-in casing boots, provides a good seal to the borehole buttons, thus minimizing damage to the environment around the borehole. https://www.speedmadedrill.com/d...mer-bits/dth-casing.html