Foodstuff grinders utilize one or more blades and a perforated screen to comminute a feed material, that is, to reduce the size of pieces making up the feed material. The apparatus forces the feed material against the screen with sufficient force to cause portions of the material to enter the screen openings. The blades of the apparatus are driven across the screen openings periodically to cut off that portion of the material having entered the respective screen opening. This relatively small piece of material may then be displaced through the screen opening as the apparatus presses additional feed material against the perforated screen. The severed pieces of material are eventually ejected from an outlet end of the respective screen opening.
Most foodstuff grinders use either a flat plate screen with a substantially planar cutting surface, or a drum-shaped screen having a generally frustoconical inside surface making up the cutting surface. In flat-plate grinders, the blades usually extend generally transversely to an axis extending perpendicularly through a center point of the grinding plate. The transverse extending blades are connected to a drive shaft and pass over the screen openings of the flat plate to provide the desired cutting action as the drive shaft rotates about the perpendicular center axis of the plate. Grinders that have a frustoconical screen employ an auger that rotates about the longitudinal axis of the screen. The outside edge of the auger acts as a blade that passes over the screen openings periodically as the auger rotates.
In addition to grinding or further comminuting a feed material, some grinders are capable of separating out pieces of hard material that are included in the feed material. Pieces of bone, connective tissue, and sinew, are commonly included in a mass of comminuted meat that is to be further ground or sized through a grinding device. Pieces of this type of hard material that are too big to pass through the grinder screen openings are forced into a separate outlet of the grinder. However, small pieces of hard material are commonly chipped off or otherwise separated from larger pieces as the larger pieces are pressed against the grinder screen and as the blades continually move over the screen openings. Some of these chipped or otherwise separated pieces of hard material may be small enough to pass through the screen openings, and thus some pieces of hard material may be included with the meat at the grinding device output. This inclusion of small hard pieces of material in the final comminuted meat product is highly undesirable.