surfaces 40 42 neck down to generally align with the reliefs 14 as illustrated in fig5 the top 22 of the bit 10 presents a planar or flat surface so that it can be easily welded as at 50 to the squared end of a prepared drill stem 12 the inner surfaces 49 of bit 10 reference fig2 adjacent the top 22 and the drill stem 12 are preferably also beveled slightly to improve heat transfer and weld penetration the horizontal cutting edge 26 of the bit 10 is particularly formed by the bevel surfaces 52 and 54 which coverage towards one another until they intersect at the edge 26 at present the cutting edge is case hardened to a rockwell hardness of 63 to a depth of 0 060 inches this has proved adequate on average to permit drilling 400 to 500 feet of bore hole between bit changes in contrast to previous bits from which only 300 to 350 feet could be obtained for the same soil conditions moreover the present bit cuts at up to a 50 faster rate which is believed to occur due to the improved tip design and hardened construction the replaceable drill bit 10 may be cast from many different metals or metal alloys for different drilling applications and could for example be cast iron it might also be hardened to different hardnesses.