Conventional core bits in widespread use typically have a ring of cutting teeth at their forward end which extend inwardly beyond the bit wall and outwardly beyond its exterior thereby to cut an annular ring having a desirable degree of clearance with the main body of the bit. Such bits present a problem, particularly in smaller diameter sizes thereof, as respects the removal of the core at the end of a drilling operation. This is because of the tendency of the core to hang up on the inwardly projecting teeth and inner shoulder of the cutting ring. This renders removal of the core time consuming and frustrating. Not infrequently, outwardly protruding portions of the core break off and this necessitates restarting the removal operation with a lighter weight residue core. A common experience is that the removal operation consumes more time and energy than that associated with drilling the hole.
The only prior teaching known to me for assisting the removal of a core from a tubular drill is that contained in U.S. Pat. No. 2,188,631 to Kraus wherein the drill stem is equipped with an internal compression spring which applies pressure to the core as drilling proceeds. This spring would not overcome the problem of hang up of the core in a drill bit equipped with a cutting ring having teeth projecting inwardly of the ID of the drill stem. Moreover the presence of the compression spring seriously limits the depth of the hole which can be cut with such a bit. No other prior teaching known to me provides a satisfactory solution to the problem.
USPTO Backgrounds
USPTO Backgrounds