Abstract: Apparatus for machining an extremely large and very heavy workpiece, such as a runner of a hydro-turbine machine, in situs. The hub of the hydro-turbine runner is utilized as a support for a rotatable beam which is arranged to carry a depending grinding or cutting tool in position to machine the wear ring surface of the turbine. Positioning of the grinding tool both in a vertical direction and a radial direction is effected to control the grinding operation. The power drive grinding tool is bodily rotated around the wear right by rotating the entire supporting beam around an axis which is concentric with the axis of the runner.