A combination drill bit for continuously drilling an annular, downwardly deepening hole coaxially surrounding an upwardly projecting cylindrical core having an upper end, and progressively crushing axially successive increments of said core from the upper end of said core,
- said drill bit comprising:
- a generally cylindrical drill bit body having an upper end provided with means for fastening the drill bit body to means for rotating the drill bit;
- said drill bit body having a radially outer sidewall surface, and, coaxially therewith, means defining a downwardly opening internal cavity, thereby defining an annular cylindrical portion of said drill bit body, having an annular, axially downwardly facing lower end;
- said cavity increasing in diameter at a level which is above said lower end, whereby said internal cavity is undercut and has an axially short band of reduced internal diameter adjacent said lower end;
- a plurality of downwardly acting cutting elements mounted on said lower end of said drill bit body and distributed across the radial extent of said lower end, so that as said drill bit body is rotated in a rock formation said cutting elements cut a downwardly deepening annular hole into the rock formation, leaving a coaxial, upwardly projecting core of rock having an upper end, said core progressively entering said cavity from below as said annular hole is deepened;
- said cutting elements being made of at least one of polycrystalline diamond compact and ceramic material;
- means defining internal drilling fluid delivery channels extending downwards in said drill bit body and opening into said cavity and at sites arranged for supplying drilling fluid to said cutting elements mounted on said lower end of said drill bit body;
- means defining channels opening generally radially through said drill bit body between said cavity and said radially outer sidewall surface of said drill bit body;
- means defining a plurality of angularly spaced external longitudinal channels on said radially outer sidewall surface of said drill bit body for circulating drilling fluid and cuttings upwards in said hole in said rock formation;
- a core crushing tool mounted to said drill bit body and disposed in said cavity above said band of reduced internal diameter;
- said core crushing tool including downwardly acting rotary crushing means having an effective diameter which is greater than that of said band of reduced internal diameter;
- said crushing means being mounted for rotation relative to downwardly acting cutting elements mounted on said lower end of said drill bit body;
- said channels having inlet ends opening into said cavity at respective sites located axially between said core crushing tool and said band of reduced internal diameter of said drill bit body;
- the distance axially of said drill bit body between said core crushing tool and said downwardly acting cutting elements on said lower end of said drill bit body being such as to provide, in use, that said core has an axial length which is between 0.5 and 2 times the outer diameter of said core;
- and
- the inner diameter of said band of reduced internal diameter and the outer diameter of said radially outer sidewall surface of said drill bit body being such as to provide, in use, that said core has an outer diameter which is at least 0.4 times the outer diameter of said hole in said rock formation.