The corners of a sheet of glass travelling in a longitudinal direction are ground by a grinding assembly having a fixed frame; a movable frame; a grinding wheel; a supporting arm supporting the grinding wheel and fitted to the movable frame; and an actuating device interposed between the fixed frame and the movable frame, and having a first powered guide-slide assembly for moving the movable frame in a direction parallel to a longitudinal travelling direction of the sheet, and a second powered guide-slide assembly for moving the movable frame with respect to the fixed frame in a direction perpendicular to the longitudinal direction; the grinding assembly also having a locator fixed to the supporting arm and defining a stop to keep the sheet at a distance from the grinding wheel; and a compensating device interposed between the supporting arm and the movable frame, and having a third guide-slide assembly and a flexible controlled-damping device.