If you know what you are doing a belt sander, or even sand paper on a piece of glass and buffing wheel will touch up your blade to razor sharp very quickly. Lansky diamond stone will work too but you must work very carefully and slowly to ensure you keep the exact angle over the whole edge.
Never sharpen the bottom and make sure you keep the exact top angle. You can also use shims to offset changes in the angle that sometimes occurs due to sharpening or from dropping the auger on its cutting edge and bending the wings.
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