Place the grinded workpiece in the star wheel, and there are grinding discs at the top and bottom. The outer gear ring, central gear, planetary gear and lower grinding plate rotate in the same direction. The upper and lower discs rotate in opposite directions. When working, the workpiece will have both rotation and revolution (planetary motion), so that the workpiece can get a uniform and non repeated grinding track. The upper and lower grinding discs can be loaded and have a fixed floating, so that the parallelism and flatness of both sides of the workpiece can be improved. Since the upper and lower sides of the workpiece are processed at the same time, the workpiece is not easy to bend and deform.