In this article, we will answer your questions about convex grinding as best we can! We get a lot of questions about convex ground knives and how to sharpen them. Which knives are convex ground? How do I sharpen a knife with a convex edge? What tools and techniques do I need to sharpen convex knives?
What is a Convex Grind?
A kitchen knife usually has a flat edge, a Scandinavian bush knife usually has a Scandi edge and hunting knives often have a hollow edge. The grind refers to the shape in which the edge of a knife is ground. One grind that regularly raises questions is the convex grind. This is where the edge is ground in an arc from the back (top of the knife) to the cutting edge. This gives the blade its familiar convex shape, which is how you can tell it is a convex grind.
A convex edge has several important advantages. A well-convex ground knife can have a stronger edge and still cut more evenly because there is no angular edge as with flat ground. This is why this grinding method is very popular for bushcraft knives. Convex ground knives are incredibly sharp and can handle heavier work without losing their sharpness. This makes them excellent for woodworking.
Why you can do your own Convex Grinding
Do-it-yourself knife sharpening is becoming increasingly popular, whether it be flat, Scandi, or hollow sharpening. You simply hold the knife at a fixed angle and move it along (for example) a whetstone at that fixed angle. As mentioned above, a convex knife has a convex blade, but whetstones are often flat. So how do you grind a round surface on a flat stone?
You might think that a convex blade is very difficult to sharpen yourself. However, with the right knowledge, it is not a problem!
When you sharpen at a fixed angle, you are constantly concentrating on maintaining the correct fixed angle. If you choose a different angle, you can change the edge of your knife. The problem with sharpening by hand is that it is never very precise. Without a grinding angle, you will probably never be able to maintain a fixed angle. With convex grinding, the angle is not constant, so you are actually taking advantage of the lack of precision. This makes the convex grinding of a knife by hand feel very natural. Also, you are sharpening the entire blade, not just the narrow edge, so small mistakes are less noticeable. So there are a few reasons why convex grinding is easy!
Ways to grind convex
By far the easiest way is to grind on a belt grinder. The abrasive belt gives way when you hold the knife against it. This means that you always grind convex. We find the Work Sharp Ken Onion Edition with the Blade Grinding Attachment, particularly practical.
A whetstone is the most popular way to sharpen a knife. A convex edge can also be sharpened very well with a whetstone. Place the knife flat on the stone so that the edge does not rest on the stone. Now guide the knife in both directions over the whetstone and slowly turn the edge towards the whetstone until you have reached the ideal angle and the edge is completely touching the whetstone. Continue along the entire length of the blade, and then do the same with the other side of the blade. Repeat the process until the desired result is achieved.
Another option is to sand on a piece of sandpaper. This is basically the same principle as using a grindstone, except that you can put the sandpaper on a softer surface. For example, you could use a mouse pad. Move the blade across the sandpaper with light pressure, and the sandpaper will follow the convex shape of the blade. This makes it easy to sand in a convex shape.
In principle, you can also grind convex with a grinding stick. However, since you are only touching a small area of the blade with the grinding stick, you will be busy for a relatively long time.
Grinding systems or knife sharpeners are often not suitable for grinding a knife convexly, as they are designed to grind constantly at a fixed angle. On a Wicked Edge, for example, you can in principle always grind at a fixed angle and gradually shift this angle by one degree. But of course, it takes forever until you are finished with the whole blade.
What should you be aware of?
As already mentioned, a knife with a convex edge should not be ground at a constant angle. However, try to work the whole blade evenly. It can help to mark the entire blade with a felt-tip pen so that you can see which parts have been sharpened and which have not.
Sharpening a convex knife changes the surface texture of the blade. Especially if you use a rough stone or belt, you will get scratches on the blade. If you sharpen the entire convex shape, you will also see scratches all over the blade. The good thing about this is that you can see immediately if you have sharpened the whole blade. The next time you choose a higher grit, you can remove the scratches. But only when you polish or strip the blade will you be able to remove all the scratches.
Keep your wrist relaxed. If you move the knife over the stone with a loose wrist, you will never sharpen in exactly the same place or at exactly the same angle. This will automatically result in a convex edge.
Be careful not to turn the knife too far up at the end of the stone. As you sharpen, continue to turn the edge towards the stone. As soon as the edge ‘grips’ the stone, you have reached the ideal sharpening angle and do not need to turn the knife any further.
The technique is certainly important. So it’s not a bad idea to practice with a cheap or old convex-ground knife.
With a little practice, sharpening a convex edge is easier than sharpening at the same angle all the time. By sharpening a convex edge, you are sharpening the entire convex shape of the blade, not just the edge. If the convex shape extends all the way to the back of the blade, it makes the sharpening process even easier. You don’t have to stop sharpening halfway down the blade.
A belt grinder always grinds the blade convex. It is also easy to sharpen convex by hand on a whetstone because the angle is never exactly the same. You just have to find the right rocking motion.
If necessary, mark the blade with a felt pen. If there are still colored areas after sharpening, you have not sharpened it well or at all.
As with many things, the best way to learn is to start and do it yourself. So try sharpening a convex blade! To be on the safe side, test your skills on an old or inexpensive knife first. You will find it is much easier than you think!