Even in something as simple and mechanical as knife sharpening, the difficulty comes down to not knowing what we're aiming for, and as a result aiming for the wrong thing. Sharpening becomes easy when you aim at sharp.
Everything the normal consensus on sharpening says is important is completely backwards, because it's lost in the proxies that proxy for the wrong thing.
- "Angle consistency is the most important thing!" — consistent angles are bad, actually.
- "You have to form a burr!" — forming a burr is bad too.
- "Deburring is haaaaard!" — deburring is trivial. The difficulty isn't what you think it is, and there are better ways.
Angle consistency
Inconsistency in angle just means convexity. That's it. And convexity is good.
"But if you start at 20 dps and then wobble more obtuse, you'll ruin the apex!"
Yeah, so don't do that. Start as low as you need to in order to end up at your desired apex angle.
There are many reasons convexity is good, assuming sanely chosen convexity profile. The obvious one is that the laws of nature aren't piecewise linear. The needed strength, and therefore needed thickness of a knife, is smoothly varying. Under a toy model of "side load at apex," the optimal is parabolic, for example. For any given durability, the angle needed drops off rapidly as you get away from the apex.
Knives with even somewhat optimized geometry cut much better, because they're not such thick wedges.
The only real downside of unintentional inconsistency is that it eats into the margin for intentional inconsistency. However, with optimized geometry, there's quite a bit of room to play with. The primary grind of a knife is in the ballpark of 3 dps, while the apex is in the ballpark of 15 dps. Sharpening angle should vary across this whole range (and more, as we'll see), so if you wobble in a ±5 degree window, who cares.
Another reason convexity is good is that it allows you to do less work. If you want a 20k grit apex, you only have to polish the apex to 20k grit. Increasing the angle every time you switch grits means you have less and less work to do. On a regular pocket knife, a reasonable start is to use 120 grit to blend the shoulder of the secondary bevel into the primary to keep it small, 1k to prepare the secondary bevel for an apex, and apex with whatever the hell you want. 5 dps, 10 dps, 15 dps. Wobble to your heart's content. Or at least, ±5 deg or so.
Burr management
The central problem of sharpening — the only thing that makes it harder than "throw it on an angle grinder until the two bevels meet at an infinitely thin line" — is the problem of the burr.
The burr is a consequence of the fact that abrasion takes force. The abrasive particles need to dig into the steel in order to cut some steel out, and this force pushes on the steel that doesn't get removed too. Burrs form when this force pushes steel out of the way of getting cut off.
So what determines whether a burr will form? Well, when there's little steel to support it. Acute angles, thin pieces of steel. In other words, burrs form when sharp. The problem gets a bit clearer when said that way, huh?
The reason people think "deburring is hard!" is because they think sharpening is "form burr, then remove burr." If you smuggle in the presupposition that deburring needs to leave the edge sharp, no wonder you're experiencing deburring as hard. You're trying to deburr in the exact context that promotes burr formation!!!
Okay, so how about we don't do that.
The reason people use "form burr" as the first step of their sharpening process is because burrs don't form until the steel gets thin. They don't form until you get sharp and then keep going, so the burr becomes a sign of "okay, we've done enough grinding" — because it's a sign of having done too much grinding. The saner way to approach sharpening is to not try to do too much. Grind until it's sharp, and then stop. This has other benefits, like not stressing or wasting steel, but "don't try to do what you already know to be too much" is enough.
Of course, burrs happen; if you aim at sharp you will err on both sides. So we need a way to deal with burrs when they form. The most effective ways to remove a burr will result in a less sharp edge. Like "jointing," which is basically just "dulling on the stone" by cutting straight into the stone. Or like 45 dps edge-leading passes, which fold the burr backwards and immediately cut it off, also resulting in a very obtuse microbevel. This is another reason for pretty wild angle inconsistency; we're trying to accomplish pretty wildly different tasks at different parts of the process, and the ideal angle for each is not the same when you're trying to do something different.
Jointing and high angle microbevels are taking steps backwards in terms of "thin, acute", but — you can take steps forward again. Think not "form burr, remove burr." Think "stop just before burr. If burr, back up and try again."
It's like golf
In golf, sometimes we hit the ball too far and have to turn around and hit it back. It would be foolish to try to hit it too far, though. Or to take 20 strokes to make absolutely sure we never hit it too far, just because it's hard to get the ball into the hole in exactly one swing once we turn around.
The sane way to golf is to aim for the hole. Every time.
- If you don't hit it far enough, keep going.
- If you go too far, turn around. Come back.
- Aim at the hole, every time, switching clubs and oomph as appropriate.
Aim at an acute, narrow, burr-free apex. Aim at sharp. If you haven't gotten there yet, keep going. If you're almost there, switch to a finer stone or at least lighter pressure. If you form a burr, aim to just barely cut it off — then reapproach from the "under-ground" side, because that generally leaves cleaner apexes and less stressed steel than approaching from the "over-ground" side. I guess it's not quite like golf.
Knowing how far you are from the hole
The question now is how to know how far we are from the hole. There are multiple ways to do this, but basically: by looking at reflected light, or by feel.
The forward pass of this is basically "plateau sharpening," or "three-step sharpening" the way Cliff Stamp talked about. He would cut into the stone to remove any damaged steel, which also gives a flat to reflect light shined into the edge. The brightness of this reflection goes with the remaining width, and when it is no longer visible it must be quite close to apexing. We're going by quantity of light reflected, so we can infer thickness far below what we have the optical resolution to resolve.
If you go too far and form a burr, the burr will show up under light too. The burr is where the steel folds over, so it will reflect light back at a different angle than the rest of the bevel. You just have to think a bit about geometry to figure out what the angles at which it reflects say about what's going on at the edge.
It's also possible to go "by feel," which brings up my favorite test.
The high-angle test: burr-agnostic sharpening
As mentioned earlier, one of the benefits of convexity is that you have less steel to remove at each step the smaller the section of bevel you're working on. Microbevels — and especially high-angle microbevels — take this to the extreme. Instead of matching angle and grinding the whole bevel down, you deliberately increase the angle above the rest of the bevel and grind only the microscopic amount of metal needed to get the edge down to an (obtuse) apex.
When the angle is very high, like 45 degrees per side, this is both extremely effective at removing any burr that might exist and very fast at bringing the edge width down to zero-ish. The test, therefore, is to see what happens when you do 1–2 light passes per side at 45 dps. Is it decently sharp? Like, starting to shave arm hair / cut paper towel?
- If so, you know you've reached the apex, and just need to remove 1–2 pps of obtuse microbevel before you have the keen, burr-free, acute apex you're looking for.
- If not, you know you have to keep grinding.
There's no such thing as "do I have a burr? Ground too much or not enough?" anymore, because any burr that isn't obvious without looking is removed, putting you back on the "under-ground" side, even if just slightly.
Most people do burr-based sharpening. Cliff Stamp advocated for burrless sharpening. What I'm talking about here is burr-agnostic sharpening — I don't know nor care if I get a burr when sharpening.
When you sharpen this way, burrs just do not matter at all. Nor does angle consistency.
You just grind away with a coarse stone at a low range of angles until you think you might be close. Then you test, with a couple of high-angle passes. If it fails, you go back to grinding. When it passes, you're done "shaping" — and it's time to actually sharpen.
Setting the apex
This is the part where we set the actual edge apex that determines how sharp our edge is. We therefore want to be very careful to remove material with minimal force toward burring.
- Use absolute minimum force. Literally. As little force as you can use while remaining in contact with the stone. 5–10 g, on the scale.
- Use sharp abrasives, since they take less force to cut. Diamonds are great.
- Use a clean surface. Waterstones can release abrasive which will fuck up your edge when you grind in the edge-leading direction — which you want to do, because when you push the steel into itself it's harder for it to bend out of the way. So clean it off, use a hard waterstone if you're going to use one, or just buy a diamond plate for $2 on AliExpress and break it in.
It takes ~4–10 passes to remove the obtuse microbevel, depending on things. You can test sharpness against arm hair or whatever, but open-loop it's usually close enough in 10 to cut arm hair above the skin and easily cut your tomatoes or whatever, kinda regardless of grit. For most practical purposes, this is plenty sharp.
For the absolute highest levels of sharpness, we need to get the forces even lower. Science of Sharp has the best guide to straight razor sharpening, by a large margin. The trick? Use microscopic abrasives embedded in cloth to remove the burr by rounding over the edge, then more microscopic abrasives on a somewhat less compressible backing (e.g. 1/4 micron diamond on leather) to reapproach without forming a burr. Same principles, performed with a designated putter.