Not only is it frustrating to trim your hedges with dull trimmers, it's also dangerous. Dull trimmer blades tend to bounce back, away from the shrub you're cutting, rather than cleanly passing through it. Reciprocating blades pulsate at over 1,400 RPM and will cut into a leg or arm deeply and quickly. Avoid accidents by sharpening your own hedge trimmer blades. They don't require frequent sharpening--just once a season. It doesn't take long to accomplish and is well worth the effort.
Place the trimmers on a solid surface, such as a work bench, and allow the cutting bar to extend over the edge of the bench. Use a regular screw driver to pry the double blades apart slightly. Place the tip of the screw driver in the space between one of the top blade teeth and its bottom counterpart. Twist the screw driver slowly, forcing the top and bottom blades to shift position, opening the maximum gap between the top blade teeth and the bottom blade teeth.
Put on gloves and eye protection. Notice how each tooth on the blade has two cutting surfaces. Standing at the far end of the cutting bar and facing the engine, you will begin by sharpening only the surface facing you. The opposing surface will be sharpened at a later time.
Fit the rotary tool with a conical abrasive point. Angle the tool such that the abrasive point is at the same degree of slant as the teeth on the trimmer blade. Turn on the tool and move the abrasive point along the cutting surface of one tooth in short quick strokes. Do the same with each tooth surface facing you.
Turn your back to the trimmer's engine. Now the opposing cutting surface of each tooth is facing you. Sharpen each of these in the same way.
Flip the machine over, and follow the same procedure for sharpening the bottom blade. Begin sharpening the cutting surfaces of the teeth facing you. Then shift your position and sharpen the remaining teeth.
Oil the entire cutting bar when the sharpening is complete.