Abstract: An extendable, retractable, erectable arm assembly for housing and supporting a water conduit extending outward from a tool located in a borehole, to a nozzle to produce a high pressure water cutting jet is provided. The arm assembly includes a plurality of interlocking arm segments for housing, supporting and moving the conduit through an angle of about 90.degree. from a stowed position to an operating position. The arm segments include integral, separable hinges capable of being interlocked such that pivotable movement is possible along any side thereof. These segments have mating apertures therethrough to accommodate the conduit. The assembly also includes an erecting device which provides compression on the segments and which gives the arm rigidity during movements. An alignment device is also included which keeps the segments aligned when the arm is deflected.