Rock drill

Claim 1

A rock drill for percussion drilling comprising a bit body, said bit body including a front face extending generally transversely relative to the longitudinal axis of said bit body, a substantially central button insert of hard material, said central button insert extending longitudinally outwardly of said front face substantially in alignment with said longitudinal axis, and at least a first row of circumferentially spaced outer button inserts of hard material, there being more than two outer inserts disposed on more than two different radii from said axis, said outer button inserts extending outwardly of said front face and being inclined at an angle relative to said longitudinal axis and being arranged to define an outer diameter (D) of the hole drilled by the rock drill, said central and outer inserts each including a working surface disposed on the front of a mounting portion of the insert, said mounting portion being embedded along substantially all of its length within said bit body, said working surface being characterized by an absence of an edge extending radially of said longitudinal axis when viewed along said axis, the improvement wherein the ratio of Y to D is in the range of 8% to 22%, where Y is the distance between opposed side faces on said central button insert and at least one of said outer button inserts when measured at said front face, and the ratio of X to D is no greater than 20%, where X is the outward extent of the outer end of said central button insert beyond a transverse plane through the outer end of at least one of said outer button inserts.