downhole drill

downhole drill

[′dau̇n‚hōl ′dril]
(mining engineering)
A hammer or percussive drill in which a reciprocating pneumatic piston is located immediately behind the drill bit and can follow and enter the bit down the hole, for minimizing energy losses.
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A successful operation to retrieve downhole drill string, tools, or pipe product can save a contractor from costly failure of a directional drilling project.