You will need something like the following set of tools:
1. A large adjustable (“Crescent”) wrench.
2. A large and sturdy flat blade screwdriver
3. A small jar or tube of lapping compound. You can usually find this at automotive supply stores, where it is sold for engine valve lapping. Lapping compound is nothing more than fine grit dispersed in a paste compound; there are two basic types, water-based and oil-based. You can use either, though I have a slight preference for oil-based.
4. A couple of sheets of copy paper, cut in one-inch strips.
Measuring the health of your mower
To measure the health of your mower you will cut the strips of copy paper as though they were blades of grass. Stick one of the strips up through the bottom of the mower till it is visible above the cutting bar, and advance the cutting blade by hand carefully with your hand (the blade should come up from the bottom as it crosses the cutting bar). Note whether the paper gets cut cleanly by the blade, or whether it binds, or whether the blade slides over the paper without cutting it. Check each of the five or six blades at several spots on the cutting bar (at each edge and in the middle). You want to get a feel for the situation: does the paper slip through without being cut in a lot of places? You’ll need to move the cutting bar quite a bit, relatively speaking, to bring it into alignment. Or does it mostly cut the paper, with only a tight spot or two? Not much movement of the cutting bar will be needed.
Reverse the motion of the wheels
The next step is set mower to run the blades backward when you push the mower backwards. Normally when you flip the handle over and push the mower backwards the wheels turn and make a tick-tick noise, but the blades don’t turn. The reason we want the wheels to turn backwards needs a little explanation. Look at the following diagram:
The diagram is a side view of the cutting bar (lower left) and a simplified blade (upper right). The diagram shows the blade in two positions: at the start of contact during sharpening (1), and at the end of contact during sharpening. The small circles represent the sharpening grit. Notice how in position 1, the leading edges of the cutting bar and the blade are softened by the grit as the two pieces engage. However, as the objects leave contact the grit has been spread evenly between the blade and the cutting bar in a film that is one grit diameter in depth, and the trailing edges are actually made sharper.
When you start the sharpening process some of the blades might be at various distances from the cutting bar. If you’ve hit a stick and jammed the blades, that blade gets thrown out of whack. What we’re going to do is bring the blades and grit and cutting bar ever so slightly into interference while keeping the blades moving, and the grit will slightly wear away the cutting bar and blades to produce a consistent clearance gap that is the same for all the blades. This gap is tiny, and if done right the mower will cut paper or grass when we’re done sharpening.
To reverse the motion of the blades, begin by removing the hubcaps using a thin-bladed flat screwdriver. On a Craftsman mower this will expose the outer head of the axle bolt; your mowever may be different, but the basic idea is to use a wrench on the inside and a large screwdriver on the outside to loosen and remove a wheel. Take care when you remove the wheel, because there is a driven gear on the blade shaft that you don’t want to drop. Remove the other wheel as well.
The driven gear on the blade shaft has a ratchet mechanism inside it. The ratchet action is achieved with a specially shaped interior on the gear that mates with a cross-wise key on the shaft. The key has alternating sharp edges and soft edges; the soft edges slide over corresponding soft edges on the inside of the gear when the mower is moving backwards, which allows the blade to remain motionless. When motion is reversed, and the mower is going forward, the sharp edges of the keys catch on sharp edges inside the gear, and the blade is driven. What we’re going to do is reverse the blade motion by swapping the two internal gears side to side - take the left side gear and move it over to the right side, and vice versa.
Reassemble the wheels with the swapped driven gears. Verify that the ratcheting action has been reversed: when pushed backwards, the blades should rotate across the top and down to the cutting bar.