"What is a Convex Edge?"

This simple question seems to baffle even some of the most scientifically minded people - yet the principle is very easy to understand.

Look at the following diagram:

Simply put, the convex edge is a curve - an arc, more specifically. Seems easy enough....but now consider this:

Instead of a flat, fixed 20 degree bevel throughout, the convex edge angle changes across the entire bevel. Look at the tangent lines - it starts out more obtuse nearer the tip (shown in red) and becomes more and more acute the deeper it cuts. You get the durability of a wide bevel angle without wedging or sticking.

"It's all about material separation." - Jerry Fisk, Moran Hammer-In, 2003

It's been said that a "smaller bevel angle" can cut better, but is more fragile; while a "larger bevel angle" lasts longer, but won't slice as finely. This all becomes moot when utilizing the convex edge. Essentially, you get the best of both worlds - edge retention as well as fine cutting. Knifemakers have been putting convex edges on knives for hundreds, even thousands of years. Only since the advent of machine-made knives, has the convex edge dropped in popularity. But lately, it has been making a roaring comeback...

"What do these terms mean - full-convex, convex micro-bevel, convex grind, convex bevel, and so on?"

These terms make reference to the extant to which a certain knife has been convex ground. In my research, there are really three different kinds of convex edges:

The first (on the left) is called a Full Convex Grind. The entire face of the knife is one continuous and smooth curve. This is the most efficient use of the convex edge. However, for some, it is the most daunting to sharpen. Yet, if the knife is satin-finished, there is no worry of messing it up with scratches - it can be easily sharpened with fine grit sandpaper. There is much more room for creativity and freedom with this grind. It can be left thick as a cleaver, or tapered thinly as a slicer. It requires, however, carefully planning and grinding.

The second type (shown in middle two) is called a Convex Bevel. The bevel varies in size, but is noticeable at arms length. It is much easier to make than a full convex bevel, and has a little more flexibility in what "primary grind" it can be applied to - you'll find it primarily on flat-ground and saber-ground knives.

The third (shown in the last two) is called a Convex Micro-Bevel. Only the last 1/32-1/16" of the knife's final grind is given the convex edge. This is by far the easiest and more widely accepted application of the convex edge. You can apply it to any grind, any knife (the diagram shows a hollow-grind and flat-grind, respectively). This method takes less time, can be done by anybody with some free time, and with the right grind can be strong yet easy to sharpen and maintain.

The numbers you see below each example are the relative cross-sectional areas. For example, if a Full-Convex knife is 1.0, then a saber-ground convex beveled knife might have anywhere from 1.4 - 1.7 times as much "area". Since edge geometry does play a significant part in cutting tasks, it's important to understand how to choose the right grind to fit the specific application.

Note: The diagram above is a "real world" example. It represents actual knife cross sections - it is not merely "diagrammatic". I drew the outlines in CAD based on a knife ground out of a 1" wide bar of 1/8" thick steel. The same would apply to a 2" wide bar of 1/4" thick steel (both are common barstock sizes). The "base" for each was taken from a blade that tapers from the spine down to 1/32" near the edge. Then, I applied a typical full-convex, flat or hollow grind. The hollow grind is based on a 14" wheel to maximize cross sectional strength. What you see is what you get - except that the the diagram has been enlarged for clarity.

"Are all convex edges the same?"

Yes and No. It is a convex edge if it has an curved bevel, period. However, that's where the similarities end. It is up to the knifemaker to make a knife that fits the intended task at hand. Do not fall for the hype of the "Do-Everything" knife - it does not exist. In our day and age, we encounter specialized tasks at every corner. How many knife owners have just one knife? Would just one knife do everything demanded in the kitchen? Work as a cleaver, slicer, dicer as well as parer? Likely not. "Use the tool most appropriate to the task." That alone accounts for the hundreds of different knife designs, shapes, sizes and even grinds you can find at knife shows today.

"Is a Convex Edge sharper than other edges?"

Sharper for what? Cutting meat? what about bone? sushi? perhaps just vegetables? or maybe chopping wood? Again, the answer is both Yes and No. Let the knife perform the task for which it was intended. A highly polished edge works great for sushi, but bad for bone. A toothy edge works wonders on rope, but frustrates the wood-carver. Any edge can be made to have any of these characteristics. The convex edge, however, can be kept sharper longer if properly maintained - which is achieved by simply stropping after every other use. There is nothing "magical" about the convex edge, though - it will not cut your buddy's katana in half. The beauty of the convex edge is in the ease of acheiving and maintaining a durable, yet "scary sharp" edge. The same can be done for other edges, but it requires expensive sharpening equipment and more time "at the wheel".

"Are all Convex Edges polished? What if I want a "toothy" edge?"

To some extant:

Despite any hype you read about any of the edges, sharpeners, sharpening techniques, etc.....all edges have some degree of "toothiness" to them. Just look at an edge under a microscope - you will see scratches on even the most finely polished edges (10,000 grit).

Essentially, there are 2 steps to sharpening:

Grinding and Polishing

Grinding sets the edge bevel angle (do it once, only go back to repair dings/dents)

Polishing refines the scratches on that edge to help it cut more smoothly.

I arrived at this conclusion after much study and thought on the subject. One day, I decided to test my theory by grinding out a Convex Bevel on a knife using only a 120 grit belt - it shaved hair on my arm! The reason? Despite the coarse scratches, the convex edge that had formed was being polished by the fast-moving slack belt. The uniformity of the edge allowed it to shave hair. There is no sharpener manufacturer that will stand behind a claim that their 120 grit stone will shave hair! This is by no means news for anybody who specializes in convex edges.

The more polished the edge - the better it "push-cuts". A push cut is where you push the knife through the material without any "back-or-forth" action - like how you would chop celery.

The more "toothy" the edge - the better it "draw cuts". A draw cut is a slicing, back-or-forth action - like how you would slice bread, or a tomato. The ultimate toothy edge is a serrated blade.

Decide for yourself what your application is, and tailor your edge to meet it. With the convex edge, this is achieved simply, using only sandpaper and a strop. Just stop at the grit level that gives you the desired balance of polish and toothines, and strop.

"Can I sharpen the Convex Edge myself?"

Yes. Follow the instructions in The Troubleshooter (Coming Soon!) and you will have no problem even putting on a convex edge yourself. The sharpening and stropping is a piece of cake and not messy.

What's this I hear about "Blade Geometry"?

Some have said that blade geometry (the cross-sectional shape of the blade) plays the most important role in sharpness. I would agree - partially. I do agree that blade geometry is very important in helping the blade fit the cutting task at hand - a first for any knife. However, it is not the only nor the most important contributor to sharpness. It is merely a starting point. Understanding the separate roles of grinding and polishing will do more to achieve sharpness in the final product. For more lectures on the subject, please visit the various knife discussion forums found online.

"Ok. You've sold me. What else should I know?"

For a self-guided tour on how to make the best use of the convex edge, please visit The Troubleshooter. (Coming Soon!) I've put most all of my convex edge information there. Hopefully it will help you in your quest for the "Scary Sharp" edge.

Please feel free to email me (button at the bottom of every page) any questions, thoughts, ideas, or suggestions you might have concerning the convex edge. I am open to any discussion on this topic and as a' student of the sharp blade', I remain willing to learn.