The grind, of course, is what makes a knife sharp. There isn't much need to belabor the basics. The primary grind may be concave, flat, or convex. Concave, or hollow, grinds are quick to sharpen, but delicate. Flat grinds are sturdier, and simple to sharpen on basic equipment. Slightly convex grinds are sturdier still, and hold an edge very well. The primary grind may, or may not, extend all the way from the edge to the back of the blade.
Most knives, especially those from factories, combine two or three planes on each side of the blade. There is often a flat portion near the spine of the blade. The primary grind, that reduces the thickness of the blade to form the edge, may be concave, flat, or convex. There is usually a secondary bevel which is less acute and forms the edge itself. This format is easy to manufacture. The resulting edge can be reasonably sharp and holds up well. The final portion forming the edge is often at a fairly blunt angle on factory knives. If the blade is thin enough, and the final bevel is small enough, the knife can still be forced through the medium being cut. It works well enough for those who don't need, or want, a really sharp knife.
There are those however, who do want a really sharp knife, and there are other ways of shaping a blade to get a sharper edge. The most obvious is to change the final bevel at the edge to a more acute angle. Doing this freehand requires some skill to maintain a consistent angle. However, it was done just this way for many centuries, so the level of skill is obviously not beyond reach.
The process is made easier (but NOT faster) with some kind of jig to hold the knife at a constant angle. A popular version is the Lansky (TM) system. These give consistently good results if one simply follows directions. I've recently heard good things about the Spiderco version as well. A real advantage is that you can put an acceptable edge on most knives, whatever the original grind. A disadvantage is that you have to have the kit with you to resharpen, and it is slow. Another disadvantage is the marked transition from the final bevel to the rest of the blade. This increases drag through the medium being cut. The rest of the blade profile may also be unsuitable for actually cutting things. A thick blade with a deep hollow grind is particularly bad for deep cuts in semi-hard mediums like cardboard or soft wood, even though the edge may be keen. (Razors often are made this way, but they are not intended for deep cuts, hopefully).
A currently hot blade grind among custom knife makers is the Moran Edge. This is named after Bill Moran, who popularized it on his now classic knives. Bill called it an apple seed shape. This is a slightly convex profile, extending to the back of the blade, with the radius of curvature gradually decreasing toward the edge. This seemingly gives the best possible edge for general use. The slightly wider angle at the edge gives the effect of a secondary bevel. The smooth transition to the nearly parallel sides of the blade causes minimal resistance moving through the medium being cut. The edge can be made very keen, but is still durable. For many years I ground, or reground if necessary, my knives to this shape.
One disadvantage to this profile is the resharpening. It's difficult to restore an edge in the field. Most folks establish and maintain these edges with a slack belt grinder. More recently it's been found that you can do a very nice job with abrasive papers and a soft surface like a mouse pad or a few magazines. The degree of softness of the surface controls the amount of secondary bevel.
Another disadvantage is less control of the depth of cut when working in wood. This is why most woodworking chisels and knives have a flat final bevel.
The Scandinavian Grind is a wide flat bevel that runs to the edge of the blade There is little or no secondary bevel. The primary angle is engineered to match the quality of the steel and intended use. The result is a very keen edge that provides excellent control of the cut. The downside is that a full Scandinavian edge is fragile. It can be strengthened by a bit of secondary, which may be as little an aggressive stropping.
The advantage is that it can be resharpened until the blade is worn away, without changing the angle of the edge. No jigs or other gadgets are required. All that is required is to lay the bevel flat to the stone, and work the entire surface of the bevel. It forms it's own guide. This requires some patience, but minimal skill. This is the ideal form in a culture where everyone is expected to use and maintain a knife regularly. It's also ideal if you expect to sharpen your knife in the field, and don't want to carry a tool kit around. There is another advantage to the Scaninavian grind. In earlier days knives were often used for carving wood. Many household items were made by the user. The Scandinavian Grind gives excellent control in woodcarving. If you carve wood, you will probably find that the flat bevels give you better control of the cut.
When I was a child I had two Nordic knives, one from Sweden, and one from Finland. I didn't know I was supposed to work the entire bevel. That seemed like too much work. I couldn't understand why such good knives had such blunt bevels. Eventually I reshaped both knives, one to a convex profile, and the other to a shallow, almost flat, convex from the back of the blade to the edge. I still use the second one quite a lot for tasks like cutting cardboard..
Eventually it dawned on me that people who developed a pattern over several hundreds of years might know what they are doing. I started carrying a Swedish pattern with the original profile to see how it would work. It holds an edge almost as well as the convex profile, and is a whole lot easier to sharpen. I've also taken identical Swedish carving knives, reshaped one to a Moran style edge, and left the other with the Scandinavian grind. Again the Moran style edge was a little more durable, but the other seemed a little keener, and was easier to control. They were very close however, and the big difference was the foolproof resharpening of the Nordic style.
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