Essentials of Knife-Sharpening

In my mind, the primary function of a knife is not to chop, hack, screw, pry, or split, but to cut. Don’t get me wrong; my survival knife performs all those functions as necessary, but only as necessary. The main thing I want my knife to do is cut, so I work hard to keep a good edge on it.

The primary factors of knife sharpness are blade geometry and the type of steel used to make the knife.

Sharpening Your Knife

A dull knife is like an unloaded gun, or a car with no gas — useless. And because it’s easier to keep a knife sharp than it is to get it sharp, make it a habit to give your knife a few licks on the whet stone every time you notice it getting dull. Rule of thumb — if you need to use anything coarser than a fine grit stone, you let it get too dull.

1. Use a fine grit stone for a blade that already has a good edge.
2. Use a little honing oil.
3. Sharpen at a 20 degree angle.
4. Give the knife 7 swipes on one side, then the other. Repeat with six, then five, and so on. You may need to start at 10 if it’s a little dull, or maybe 5 if it just needs a touch-up. You’ll get the hang of it with practice.
5. For a very sharp edge, repeat with a light hand on a superfine ceramic stone, no oil. Raise the angle a little to work just the edge, but not too much, or you’ll make it dull. Again, experience is the key, and you only get that with practice.

Video on how I sharpen my knives by hand:

Blade Geometry

All other things being equal, a thin blade is easier to sharpen than a thick one. Razors and surgical scalpels are very thin, and axes are very thick. Let’s take a look at various blade geometries and how they affect sharpness.

The cross section of the knife has two bevels: the primary cross section and the cutting edge. The primary cross section is the “blade grind,” and is of four general types: Flat grind; taper grind, hollow grind, and convex grind.

The cutting edge is that little narrow bit that you sharpen with your whet stone. The more acute the angle of the cutting edge, the sharper you can get the knife. But as you keep sharpening the knife, that angle tends to get more and more obtuse, making it more difficult to get it really sharp. The different grinds exhibit this tendency to different degrees.

A convex grind (like an axe) gets thicker very quickly as you move up the cross section from the edge to the blade spine:

 

A taper grind is a variety of convex grind in which the curve doesn’t reach all the way to the spine. It is characteristic of many handmade knives because it is the easiest grind to achieve by hand. It exhibits the same characteristic thickening of the cutting edge after repeated sharpening:

 

For any given maximum thickness, a hollow grind will make the sharpest edge. Moreover, as you sharpen the knife repeatedly, it will remain thin and sharp at the edge for more sharpening cycles:

 

In terms of keeping a thin edge after repeated sharpening, the next best cross section is a flat grind:

 

My personal favorite is the Scandinavian grind (aka “Scandi” grind). This is a variant of the flat grind in which the flat grind doesn’t go all the way to the spine, but only one-third of the way up the blade or so. It is the flat grind analogue of the taper grind; in other words, it is to the flat grind as the taper grind is to the convex grind.

This diagram illustrates the Scandi grind in comparison to several others:

When sharpening a Scandi grind knife like the Mora Clipper Companion, you actually sharpen the entire flat portion of the blade, then hone the cutting edge with just a few licks to make it very sharp. Here’s a fascinating discussion about the Scandi grind.

Type of Steel

The type of steel in manufacture affects many characteristics of the knife, including hardness, toughness, wear resistance, corrosion resistance, and the coarseness of the cutting edge. A discussion of all these characteristics is beyond the scope of this article, so I refer you to the Knife Steel FAQ.

As it relates to sharpness, a steel with fine carbides will in general get sharper than one with coarse carbides, because as the edge approaches sharpness, large carbides get knocked out during the honing process, leaving holes in the edge. In my experience, high-carbon steels like the very cheapest 1085 and 1095, when properly tempered, are the easiest to sharpen. This is important in the field, where your knife gets a lot of use.

Knife Maintenance Tips

• Keep your blade sharp. Otherwise it’s useless.
• Practice sharpening. Practice makes perfect, and it takes a specific skillset to sharpen knives.
• Prevent corrosion. Even “stainless” knives will corrode, but high-carbon steels are particularly susceptible. Wipe off blood, avoid salt, and apply a light coat of oil daily while in the field.
• Keep the blade cool. Don’t poke around in the fire — it will harden the blade and make it brittle. A high-carbon blade can get too hard to sharpen if heated.

~SnoMan