If your filament is getting eaten up by the gear before it enters the extruder channel to be heated up (AKA "grinding"), the most likely cause is an entirely or partially clogged extruder. If this is the case you will need to consult the tips on "How to fix a clogged extruder". If the extruder is not proper clogged, there could be some other things ailing your filament passing. One such indicative sound to either a clogged extruder or grinding can be a clicking sound as the gear moving the filament jumps. Check and see if the gear is first of all tight on the shaft (the set screw entering the side of the gear should be flush against the flat side of the shaft, and tight). The grinding may be the result of the filament rate feeding too fast, or at not a high enough temperature for the material; the filament does not melt in time and cannot move through the extruder fast enough, so the gear starts spinning and grinding on the jammed up filament -in this case you need to adjust accordingly. The grinding or "slipping" could also be caused my a tight and excessive buildup of filament residue and dust caught in between the teeth of the gear; thus the gear is inadequite to grip the filament and just makes friction which doesn't adequately feed the filament. To clean the gear you can use a needle, toothbrush or compressed air.