Sharpening a rotary mower blade involves more than simply running a file over the edge. Special equipment is required, and blades must be balanced as well as correctly ground. With practice the job goes swiftly, but beginners may be better off buying new blades. Improper sharpening could ruin the temper of the blade or put extra stress on the mower and shorten the lifetime of important machine parts.
Check the blade for obvious damage including cracks, bends and missing pieces. Don't bother trying to sharpen or straighten a badly damaged blade--buy a new blade to replace it.
Place the blade on the balancing cone and note the heavier side. Sharpen the heavier side first.
Adjust the tool rest table of the bench grinder so that the grinding wheel touches the middle of the blade's beveled edge when lying flat on the tool rest. Always adjust the settings with the grinder at a dead stop and unplugged.
Stand to one side of the grinder and turn the machine on. Wait a moment to be sure there are no dangerous vibrations or obvious safety problems.
Hold the the mower blade in the left hand and rest the back or un-beveled side of the heavier side on the tool rest table of the bench grinder. Apply controlling pressure to the blade with the right hand.
Advance the cutting edge of the blade to the wheel, beginning at the hub of the blade and drawing the blade steadily across the grinding wheel and off the table. Immediately dip the blade in the bucket of water for cooling.
Repeat the procedure and keep count of the number of strokes made across the wheel. Stop when new metal shows along the full length of the cutting edge and--ideally--the full width of the bevel.
Revolve the blade in the left hand, gripping the sharpened side, and grind the other side in the same way. Count the passes and try to finish the blade in the same number of strokes or less.
Finish balancing by grinding steel from the back of the blade near the end. Check the balance frequently until the blade rests on the balancing cone without tipping to either side.