The theory of a razor sharp cutting edge

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What's the video about?

This video covers the basic theory of edge tool sharpness: What is the definition of a sharp edge? what is a dull one? What has to be done to make a dull edge sharp and durable?

The video also briefly outlines my daily sharpening routine.

Video transcript: 

To help you understand sharpening, I'd like to go through a bit of basic theory about what a sharp edge is, and how you take a dull edge and make it sharp.

So what *is* a sharp edge? The best definition that I've come across is that it's "a zero-radius intersection between two planar surfaces". But what on Earth does that mean?

Basically, it means that it's the edge where two flat surfaces meet at a radius that's as close to zero as possible.

Of course, in reality it can never be zero. The molecules in the steel have a certain size and the particles in the abrasive we use for sharpening have a certain size, too.

But a zero-radius intersection is what we aim for. The smaller the radius, the less effort it will require to cut through the wood. And because less force is required, the wood fibers will have less of a tendency to tear out, which will result in a cleaner cut.

Once you start using your sharp tool, the metal at the edge will start to wear away, and the intersection will grow bigger. In the beginning, this will not be a problem. But as the radius grows larger, the tool will become increasingly difficult to push through the wood, and the surface quality will get worse.

That's when you need to sharpen your tool. Or, in technical terms - you will need to re-create the zero radius intersection at the edge.

With boatbuilding tools such as hand planes and chisels, this is usually done by removing material from the *bevel* side of the edge. When enough material is removed, the large radius will be gone and the zero radius re-established.

This work is usually done using a coarse abrasive, such as a bench grinder, sandpaper or a coarse sharpening stone.

So when you're back at zero radius, your tool is essentially sharp. But! If you were to take a look at the edge in a microscope, you would see that it is quite jagged. This jagged edge is fragile, because the tips, or peaks, can break off very easily when you start using the tool. This will happen fast, and the tool will feel dull again.

That's why we want to *polish* the edge. By polishing the edge, using successively finer grits of abrasive, the little "mountain peaks" at the edge will become smaller. These small peaks will be fragile, too, but once they break off, the tool will still feel sharp and cut well.

So polishing your edge is not just about making it sharp - it's also about making it durable. With a well-polished edge you won't need to sharpen as often.

Now I'd like to briefly illustrate to you how I usually sharpen my tools.

First, we have the dull tool. You can see that the edge is rounded over. This drawing is of course exaggerated. In reality the only way you can see if an edge is sharp or dull is by checking whether it reflects light or not. A sharp edge will look "black" and not reflect any light.

Perhaps you can also see that the bevel side of the edge is slightly concave. This is because I use a grinding wheel when I do my rough grinding.

So the first thing I do is remove steel from the entire bevel side of the edge. This will get me close to a zero-radius intersection again.

By the way, I always grind at a 25 degree angle. We'll get back to angles for grinding and honing later.

If you don't have a bench grinder, it's also possible to do the grinding manually on sharpening stones or sandpaper. I like to use the bench grinder because it takes care of this rough part of the sharpening process quickly and easily.

Once I've removed almost all of the large-radius edge, I stop.

The abrasive on the grinding wheels is very coarse. This makes it cut fast, but it also leaves a very rough edge, with a lot of the brittle "peaks" I mentioned earlier. So now I want to hone and polish the edge to make it sharper and stronger.

For this, I'll be using my stones. These are actually not stones, but steel plates with diamond dust on them - but they work just like other sharpening stones. The abrasive here is much finer than the abrasive on the bench grinder wheels. They leave a finer surface, but they also remove much less steel.

So to make the honing and polishing job easier and faster, I'll just be removing material all the way out towards the edge. The primary bevel was ground at 25 degrees. But now I'll hone the tool at *30* degrees. This creates a so-called "secondary bevel", or "micro bevel" at the very edge of the tool. Because I've increased the angle, only very little steel needs to be removed. This is good, because the sharpening stones are quite fine, so they only remove very little material.

Why 25 and 30 degrees? Well, because these angles are a good compromise between sharpness, edge durability and ease of sharpening - for most woodworking tasks. But you can get great results with other angles, too. The most important thing is to settle on a primary and a secondary angle that you use - and stick with these angles. When you do that, re-sharpening your tools will be quick and easy.

I first use my coarse sharpening stone to create this micro bevel. When I've reached the zero-radius intersection - a thin sliver of steel will curl back and form a hook burr on the back of the edge. This hook burr can be hard to see, but it's easy to feel with your fingers. So I hone until I can feel a hook burr all the way along the edge. Once the burr is there all the way, I know that the entire edge has been renewed.

Then I move on to my fine sharpening stone. This stone will remove the scratch marks from the coarse stone. I'm still honing at 30 degrees.

Finally, to make the edge really sharp and strong, I want to polish it. I'm using a leather strop for this, charged with a very fine honing compound.

This leather strop is simply a flat piece of wood with some leather glued onto it. A fine 6000 or 8000 grit sharpening stone could do the same job. But a strop with honing compound is a cheaper alternative that I think works just as well for most boatbuilding tools.

First, I strop the back of the tool, and then the bevel side. I switch back and forth until the hook burr falls off.

Perhaps you have noticed that this is the first time I've touched the back of the tool during this entire sharpening procedure. This may seem odd, given that the back of the tool is just as important for creating the polished zero-radius cutting edge we're looking for.

The reason is that the back of the tool is already polished. There are no big scratch marks from the grinder that needs to be removed. So all I need to do with my strop is to turn the hook burr around and perhaps do a few more strokes to maintain this nice, shiny polish.

Now, I'm done sharpening my tool. I've created a zero-radius intersection, and I've honed and polished it to make it sharp and strong.