Abstract: The invention relates to improvements in polishing machines incorporating a rotating plate. The upper face of the rotating plate is provided with a groove of circular section, in which is removably retained the base of a collar or screen, which defines on said plate two separate zones for rough-machining and finishing. The invention also relates to the mode of heat-conditioning the plate with the aid of nozzles disposed in the bottom of the tank for spraying a liquid against the lower face of said plate.