In general, tiller blades are not sharpened once the tiller leaves the factory. By the time they wear away enough to dull the edge, they are usually also scored, bent or rusted to the point that sharpening is not a possibility. However, if your blades are straight and haven't yet begun to rust, it is possible to sharpen them without destroying them.
Remove the spark plug wire before doing anything else with the tiller. This prevents accidental starts, which can cause severe or fatal injuries. To determine where your spark plug wire is located, consult your owner's manual. If you do not have one or have misplaced it, you may be able to find a free copy at ManualsOnline.com.
Raise the tiller to a comfortable work height by placing it on ramps, blocks or a workbench.
Grind the edge of each tine square and sharp using a 24-grit wheel on a right angle grinder. Measure each tine's angles and thickness with a ruler, calipers and protractor or other gauge to ensure that they are even with all the other tines.
Grind the tips of each tine to a sharp triangular edge on both sides. Check the angles on each tine tip to ensure that they are as equal as possible.
Lower the tiller to the ground. Replace the spark plug wire and test the freshly-sharpened blades to ensure that they are turning smoothly and digging into the ground effectively.