In the past I have Free hand sharpened jointer knives, which works fine with 6" long knives, but for 12" long knives, straightness is more critical. I have used my dad's planer sharpener, but was later dismayed to realized that I ended with a slightly curved edge, so it's really no better than doing it freehand, as long as I check the straightness of the blades a few times while I do it and adjust. But that still leaves an edge that isn't entirely straight.
Sending the knives away to be sharpened is no guarantee they will be straight either. If they work too fast, the knives will heat up near the cutting edge, which causes them to curve slightly while sharpened. Once they cool down, they curve back and the edge won't be straight.
Here removing the knives from my 12" homemade jointer. In 2012, I wrote about how the glue in plywood can damage the knives. And those very were still on the knives, so I hadn't actually changed the knives in 6 years. The way I use the jointer, I know the right side has nicks in it, and the left side is clean, so if I plane some questionable wood, I always do that on the right, and if I need a nice finished surface, on the left side.
I found this granite slab, which I figured would be perfect for sharpening the knives using a fine sharpening sandpaper.job.
I have seen videos on doing this, and people making jigs and such, but an important aspect they usually ignore or aren't aware of is that supporting the knife so it's straight is just as important as the flatness of the sharpening surface.
For example, this pretend wooden jointer knife touches the granite along it's entire length when I press it down, but looking along it, it's actually quite crooked.
Basically, as long as the knife is curved to the side a bit, given that it hits the granite at an angle, that results in a curved edge. I'm illustrating this in the extreme case with this piece of paper.
So I needed something to support the knife straight. I had some ground steel bars that looked quite straight, but pulling the dial indicator along them as they were resting on the granite, it was curved.
Even the piece of aluminium extrusion that I used to check the straightness of the knives in this article also measured curved. Some cold rolled steel I checked turned out not to touch the granite in the middle, and if I flipped it over, it still didn't touch in the middle.
I have had great experience with the straightness of these 2" (5 cm) wide aluminium rulers (I use these rulers to check jointers). So I checked it with one of those. It didn't touch in the middle either. I measured a 0.45 mm gap under the middle of the 60 cm long ruler. Further checking, the granite has a very consistent curvature everywhere, so it was actually ground slightly spherical. I worked it out that the curvature has a 100m radius.
But I had another chunk of polished granite kicking around the property, and checking this one, it was adequately straight and flat. But it's 10 cm thick, quite heavy. I loaded it onto the homemade lawn tractor trailer just by tipping it onto the trailer, which was itself tipped. I decided to leave that one on the trailer for while I'm doing the work.
If you don't have a flat piece of granite but have a glass topped cooking stove in your kitchen, check that stove. All the ones I ever checked were amazingly flat.
I used three C-clamps to hold the knife to the aluminium and started sharpening. I positioned the clamps such that the clamp would rub against the sandpaper when the knife was at the right angle.
But with the clamps made of softer steel than the knives, it felt like I was sanding more clamp than knife, so I made an outrigger arm which I clamped to the straight piece of aluminium to hold the angle. A few pieces of wood stacked behind allow me to do fine adjustments of the actual angle.
Checking that I have the angle right by pressing a small piece of wood against the knive's bevel and comparing it to the outrigger piece I had clamped on.
I was using wet sandpaper, but then realized it wasn't sanding along the whole edge. I confirmed this by putting marks along the knives with an indelible sharpie marker, then grinding a little and checking which marks had disappeared.
The problem was that the knife wasn't perfectly straight to begin with, so I needed to take material off. I switched to some coarser dry sandpaper (didn't have anything coarse in the wet variety). Once I could tell that I was grinding along the whole edge, I switched back to the fine wet honing sandpaper.
Checking with the sharpie again with that, I was satisfied that I had a good straight edge.
I set jointer knives typically about 0.1 mm or 0.004" higher than the outfeed table. The knives need to be slightly higher because the wood always yields slightly as the knives cut, Also, as the knives get nicked, the resulting ridges in the wood still needs to be as high as the outfeed table. If the knives are too low, straight stock ends up riding up on the edge of the outfeed table without actually cutting much off.
This piece of paper was just the right thickness. And I'm using two aluminium extrusions as a straight edge to get the level on the cutter head, with the edge of the knife directly above the shaft. In the past, I would have used some weights to weigh down these pieces, but I had the idea of putting some magnets into the extrusion to hold them down.
The springs in the cutter head push the knives up until they hit the aluminium, and then it's a matter of tightening the jack screws on the wedge parts to lock the knives in place.
With the knives installed, I manually turn the cutter head and watch how far it drags the aluminium extrusion resting on the outfeed table (more on that). I checked it several spots along the head for each knife.
It's very satisfying using a planer or jointer after the knives have been sharpened. The wood just comes out super smooth.