Otsuka manufactures cage mills under the license granted by Stedman Machine Company of U.S.A. In the cage mill, two disks on which many pines are arranged concentrically are set face to face and rotated in opposite directions to grind the feed by the impacts applied by the pins. The cage mill is capable of grinding lumps tens of millimeters into a product 1 mm or less in diameter. The pins are arranged in 2,4, or 6 concentric circles. The more the number of concentric circles, the finer becomes the product.