Abstract: An orbital tool support system for a machine tool such as an EDM machine, and ECM machine, or a conventional milling machine, of the kind including an orbiting head comprising two head elements interconnected by an orbital drive; the head is mounted centrally between a machine mounting plate affixed in centered relation to the tool head of the machine and a tool mounting plate that supports one or more electrodes or other tool elements. A plurality of slide bearing assemblies interconnect the two mounting plates in an X-Y bridge structure, maintaining the plates in parallel fixed vertical alignment but permitting limited onmidirectional relative movement in a horizontal plane. The bridge structure absorbs the vertical reaction forces resulting from machining operations, affording a dynamically stable tool support structure, even for very large electrodes or other tools; the bridge can be integral with or retrofit to virtually any EDM machine and to a wide variety of other machine tools.