Mechanical excavating machines including rotary cutter and suction tube

A mechanical excavating machine comprising a rotary cutting tool which is capable of cutting through compacted soil, hard core and like materials, a suction tube having an inlet opening adjacent to the cutting tool, a suction device for applying suction to the tube remote from the inlet opening to cause material cut away by the cutting tool to be sucked inwards through the inlet opening, and a driving mechanism for rotating the tool, the cutting tool and part at least of the suction tube being carried on a mounting which is adapted for movement in horizontal and vertical directions, the inlet opening of the suction tube being nonrotatable, the rotary cutter being mounted for rotation about said inlet opening, and the axes of the cutter and suction tube being coincident and vertical, said cutter being provided with a sensor means for providing a signal for signifying the approach of the cutter to a buried metallic object, said sensor means comprising an electro-magnetic metal detector which includes at least three coils mounted on said cutter, at least one of which said coils is an exciter coil and at least one of which said coils is a receiver coil, means for supplying alternating current to said exciter coil and a detector for detecting any change in the net signal induced in the receiver coil.