A dull rotary mower blade does not cut grass blades, it tears them, which makes for an unattractive lawn and leaves the grass vulnerable to disease. Anybody who mows grass, and especially a commercial cutter, realizes they have to regularly sharpen the blades. How often the blades should be sharpened depends on how much mowing you’re doing and the mowing conditions. Mowing in sandy or dusty conditions or in high, tough grass, such as Bahiagrass or St. Augustinegrass, will dull blades faster than mowing a bluegrass lawn in suburban Chicago, for example.
Sharpening mower blades is relatively simple, especially if you use a bench grinder or a professional blade grinder and you’ve done it a few times.
But, let’s start by discussing safety. Mowers are, by their nature, potentially dangerous even when they’re being worked on. Make sure you wear a pair of stout work gloves when removing, sharpening and replacing blades, and professional-grade goggles or safety glasses when grinding blades. Also, make sure you have a clean work area. Remove any equipment or other objects from around the bench or blade grinder.
Remove the blade: Remove the spark plug. Why chance the mower starting unexpectedly when it only takes a few seconds to remove a spark plug? Squirt some penetrating oil on the blade bolt and nut and block the blade so that it doesn’t turn when you begin wrenching the nut. Some folks use a block of wood under the deck for this purpose and others use a C-clamp.
Sharpen the blade: Hold the blade firmly so that the beveled edge on one end is easily accessible. Turn the beveled edge to the correct angle. The angle at which you grind the blade will depend on the blade manufacturer’s recommendation. Typically, the angle is 40 degrees, but check nonetheless. If you put too narrow an angle on the blade it will cut well initially, but will quickly dull and nick easily; too blunt an angle and the blade will “beat” rather than cut grass. Move the blade back and forth across the grinder with a soft touch until you get the edge you want. Grind both edges of the blade equally, attempting to remove the same amount of metal from them. Don’t force the blade against the grinder because you will likely heat the blade too much and cause the metal to loose its hardness. In other words, do not grind until the blade starts glowing orange.
Check for balance and straightness: Buying an inexpensive blade balancer is money well spent. It can tell you at a glance whether a particular blade is balanced. Observe the blade to see if it tilts to one side or the other. Remove small amounts of metal from the heavy side by moving the edge across the grinder. An out-of-balance blade will cause a mower to shake, and running a mower with an unbalanced blade will eventually damage the mower.
Remount the blade: Are you still wearing your work gloves? Good. Remember, you’re handling a sharp blade and there’s no reason to get a bad cut on your hands now.
Replacement time: If a blade is badly bent or it is cracked, discard and replace it. It’s unlikely you can straighten a bent blade without weakening it. Blades spin at high RPMs, and, as remote as it might seem, you don’t want to run the risk of a blade shattering and causing property damage or injuring someone.