Knives should be replaced with blanking inserts when not in use for any length of time, says Jim Hunter, and it's better to remove the knives when baling hay or straw so that the exposed tips of the retracted knives are not subjected to wear. Sharpen dull knives, typically every 500-1000 bales depending on material baled a disappearing scissor emblem on the E-Link controller is another sign of dull knives. Dealers may have a special grinding machine for sharpening the knives at the correct angle, otherwise work on the flat side using an emery disc rather than a stone to avoid over-heating and loss of wear-resistant tempering. Knives sharpened to the point where there's little or no serration left should be replaced.