Reamer

A reaming tool for enlarging an underground borehole, comprising:

• a plurality of cutter assemblies distributed azimuthally around a longitudinal axis of the tool;
• wherein each cutter assembly comprises a supporting structure and a plurality of hard faced cutters secured therein as a sequence of cutters which each have a hard surface facing in a direction of rotation of the tool and extending radially outwards from the tool axis to a radially outer extremity;
• the sequence of cutters including a row of cutters whose leading faces extend radially outwards for an equal distance from the tool axis and the supporting structure includes a radially outward facing gauge surface following behind the leading faces of the cutters in the row and positioned at a radial distance from the tool axis which aligns the gauge surface with the radially outer extremities of the cutters in the row at the maximum radius swept by the tool as it rotates;
• the sequence of cutters also including a further plurality of cutters between the row and an end of the assembly, where the leading faces of the further cutters extend radially outwards to respective cutter extremities at radial distances from the longitudinal axis of the tool which progressively diminish towards the end of the cutter assembly;
• the supporting structure including radially outward facing secondary gauge surfaces following behind the further cutters and positioned at radial distances from the tool axis which align the outward facing surfaces with the radially outer extremities of the cutters which they follow;
• and
• the outward face of the cutter assembly further including portions which connect adjacent gauge surfaces, and have zones which face axially towards the said end of the assembly and extend circumferentially back from the leading faces of the cutters while tapering away from the end of the assembly.