Downhole rotary cutting tool

A rotary cutting tool for enlarging an underground hole, comprising:

• a plurality of cutter assemblies distributed azimuthally around a longitudinal axis of the tool,
• wherein each cutter assembly includes a supporting structure and a plurality of cutters, the supporting structure including a leading surface, a gauge region, and a lower region, the lower region bearing a sequence of cutters which are axially below the gauge region and extends axially along the tool with each cutter of the sequence of cutters having a leading face facing in a direction of rotation of the tool, and
• wherein the cutters of the sequence of cutters are positioned at a plurality of circumferential positions such that:
• the leading faces of no more than three cutters of the sequence of cutters are aligned on any line parallel to the longitudinal axis of the tool;
• and
• the leading faces of the sequence of cutters increase in distance from the leading surface of the supporting structure as the sequence of cutters progresses axially farther from the gauge region, and
• wherein the supporting structure comprises an outward-facing surface behind the leading face of at least one cutter of the sequence of cutters and aligned with a radially outward extremity of the at least one cutter of the sequence of cutters so that the at least one cutter of the sequence of cutters does not project outwardly beyond the said outward-facing surface behind the at least one cutter of the sequence of cutters.