A tool centers an elongated drill bit within an elongated canal surrounded by bone tissue. A sleeve having an expandable diameter is internally threaded. The sleeve threadedly engages an externally-threaded cylindrical guide. A key includes a handle at one end of an elongated rod. The rod includes transversely-protruding buttons for engaging the guide so that rotation of the handle produces rotation of the guide to cause radial expansion of the sleeve. This results in symmetrical contacting of the interior surfaces of the bone tissue, thereby creating a vertical, centered channel with respect to the bone, that extends through the tool for guiding the drill bit.
