Robotic approach to improve turbine surface finish

This paper describes the approach taken by Alstom and Hydro-Quebec (HQ) in the development of robotic polishing to improve turbine efficiency by reducing surface roughness. Modern, large hydraulic turbines are profiled by a 5-axis milling machine and are polished manually. By robotizing the polishing, it becomes possible to obtain a better surface finish at a reasonable cost, and to reduce hydrodynamic friction loss. HQ's portable robot Scompi was used to perform the polishing. Recent developments made by the supplier of the abrasives have resulted in their increased durability and improved productivity. A technique was developed to select the polishing process parameters best suited to a given surface waviness and roughness. A polishing test was carried out on a full-scale Francis turbine blade. The surface finish was lowered from Ra=15µm to Ra=0.1µm and the waviness (scallop 0.2mm tall and 30mm wide) was grinded away at a rate of 5 hour/m2.