Patent ID: 7958953

Claim:
A bi-center drilling tool 10 for drilling a borehole comprising a diameter 29 , the tool 10 further comprising a main reamer region located eccentrically to an axis of rotation 24 of the tool, and an out-of-balance leading blade 30 located ahead of the main reamer region, in the drilling direction, the leading blade 30 being located so as to apply a side loading to the drilling tool, the direction in which the leading blade 30 side loading acts being such as to counter a side loading applied, in use, due to the main reamer region being eccentric to the axis of rotation, wherein, a majority of first blades 18 of a mid-reamer region 16 extend substantially to a pilot gauge circle 22 of a first diameter 23 centered upon the axis of rotation 24 of the drilling tool 10 , and second blades of the main reamer region 14 extend substantially to a pass through circle 26 of a second diameter 27 eccentric to the pilot circle 22 , a part of the pilot gauge circle 22 projects outside of the pass through circle 26 , and in this part of the tool, the blades 18 of the mid-reamer region 16 extend radially outwardly only as far as the pass-through circle 26 , and not as far as the pilot gauge circle 22 , such that in operation, the second blades 18 drill the full borehole diameter 29 .