Rotary mower blade sharpness is the largest contributor to that golf-course look for your freshly-cut lawn. Sharp blades also help the grass-plants too, since they don't suffer the whacking they take from the rounded edges of dull blades. A sure sign of dull blades is a stringy, uneven, or ragged edge at the top of the grass blades. Sharpening and re-balancing blades can be easily performed with a power grinder, a fine finishing stone or file and an easy-to-make balancer.
Slip the spark plug wire(s) off the spark plug(s) to prevent an inadvertent mower starting while working on the blades. Consult the owner's manual to ascertain the correct direction to tip the mower for blade removal.
Tip the mower as recommended. Make sure gasoline or oil is not running out of the top of the engine. Use a box-end or socket wrench to remove the nut while holding the blade tightly from turning. An electric or air impact wrench helps too.
Remove the blade from the threaded shaft, noting the order of the nut, blade and washers.
Inspect the blade for cracks and dings on the cutting surface. Bent or cracked blades should be discarded. Clean off any grass residue from the blade with a wire wheel.
Use the power grinder to make some sharpening passes on one side and then similarly on the other side until the sharpened edges start to emerge. Keep the grinder tilted to maintain the blades original edge attack angle as you sharpen. The attack angle is the angle of the top of the edge compared with the flat plane of the spinning blade, which should be about 35 degrees which provides both a clean cut and a durable edge.
Make a balancer from a 16-penny common nail. Hold the nail horizontally and tighten in the side of a jaw vise. You can also drive the nail into the side of a stud. Put the blade on the nail like a propeller by its center hole. If it stays level, spin the blade clockwise a half-turn and if it stays level again, it is balanced. If it tilts down right or left, spin the blade clockwise a half-turn and it should tilt down the other way. The heavier side tilts down. Re-sharpen the heavy side a bit to remove a little material, re-testing the blade after a few swipes with the grinder. When the blade is balanced, gently fine-hone the edges while maintaining the angle. Blade balancers are also available at hardware outlets.
Put a little spray rust-proofer on the blade-shaft thread. Re-mount the blade assembly, retaining the correct order of friction washers, lock washers and nut. The sharp edge should be the lowest part of the blade, and the upturned lifting edge should go up towards the deck. Tighten the nut securely.
Tilt the mower back down. Wait five minutes, and then check the oil level (four cycle engines). Re-attach the spark plug wires.
Start the mower and check for unusual vibrations. If it has a vibration, check blade balance again as well as blade mounting.