| 摘要 |
<p>Cutter head with tips below is followed up the string by a wedge head and retainer head arranged so that the retainer does not rotate where the cutter and wedge head turn together during drilling. Mud is flushed down through the string from the surface, through the heads and back up. The griff stem weighting the heads from above is in two halves, pinned together and capable of riding up and down the strig so that the cutter, wedge and retainer heads can be surfaced with the griff stem for tip tool changing without stopping the mud flow.</p> |
