Convex sharpening: not complex at all!
We get a lot of questions about knives with a convex edge and how to sharpen them. Which knives have a convex edge? How do I sharpen a convex edge? Which tools and techniques do you need to sharpen a convex edge? In this article we will try to answer all your questions as best we can!
What is a convex grind?
A kitchen knife is usually enhanced with a flat grind, a Scandinavian bushcraft knife with a scandi grind and hunting knives with a hollow grind. The grind stands for the way the edge of your knife is sharpened. A grind which has led to a lot of questions is the convex grind. Here the grind runs from the spine (the top of the blade) towards the edge without interruption. As such the blade gets that famous convex shape that characterizes the convex grind. Do you want more information about the different grinds? If so check out the different grinds.
A convex grind has a couple of important advantages. A good convex knife can have a stronger edge, and still have a smoother cut. You namely miss the angular shoulder a flat grind does have. As such this type of grind is very popular for bushcraft knives. Knives with a convex edge are incredibly sharp and can handle heavy-duty tasks without losing their sharpness. They are therefore also perfect for wood work.
Why sharpening convex knives is easy
Sharpening your own knives is getting more and more popular. Sharpening a flat grind, scandi grind or hollow grind is done the same way. You hold the knife at a fixed angle and move the knife, in the same angle, alongside, for instance, a sharpening stone. As mentioned before, a convex edge has a curved blade. However, sharpening stones are flat, so how do you sharpen a round surface with a flat stone?
Many think that a convex grind is difficult to sharpen. However, with the right knowledge sharpening a convex edge is actually really easy!
When you sharpen at a fixed angle you are constantly focused on sharpening your knife at the right angle. As soon as you change the angle you can change the edge of your knife. The problem is that sharpening by hand is not very accurate. You will never be able to sharpen with a fixed angle without using sharpening aids. When you sharpen a convex edge you don't use a fixed angle. As such you benefit from the lack of precision when you sharpen by hand. As such sharpening a knife with a convex edge by hand feels very natural. In addition, with a convex edge you don't just sharpen the very narrow edge but the entire blade. As such you have more room for small errors. As you can see there are many things to be mentioned that emphasize that sharpening a convex edge is easy!
Ways to sharpen a convex edge
The easiest way is using a sharpening machine. The grinding belt moves with you as you hold the knife against it, which causes the belt to bend. As such you will always sharpen with a 'convex' angle. We believe the Work Sharp Ken Onion Edition with the Blade Grinding Attachment is perfect
Sharpening on sharpening stones is the most popular way to sharpen knives yourself. You can easily sharpen a convex edge with a sharpening stone. Place the blade on the sharpening stone to make sure the edge is not resting on the stone. Sharpen back and forth over the entire length of the sharpening stone, and slowly turn the edge towards the sharpening stone until you are using the right angle and the edge completely touches the sharpening stone. Do this until you have sharpened the entire blade and sharpen the other side of the knife afterwards. Repeat this as often as possible until you have reached the desired result.
Another option is sharpening the knife with a piece of sandpaper. Sharpening on a piece of sandpaper on a hard surface is the same as when sharpening with sharpening stones. However, when you use sandpaper you can create your own soft surface. Think, for instance, of a mouse pad. By lightly applying pressure you press the blade and the sandpaper into the mouse pad to make sure the sandpaper follows the convex edge of the blade. As such you can easily sharpen a knife with a convex edge.
It is, technically speaking, not impossible to sharpen a convex edge with a sharpening steel. However, because of the small surface with which the knife hits the sharpening steel, it will take forever to sharpen your knife.
Sharpening systems or sharpeners are often not the best option to sharpen convex knives. Sharpening systems are designed to constantly use the same fixed angle. On the Wicked Edge, for instance, you could sharpen your knife with a fixed angle and afterwards constantly move one degree until you have sharpened the entire blade. This, however, could take quite a while.
What do you need to pay attention to?
As mentioned before you shouldn't sharpen a convex edge with a fixed angle. Try to sharpen the entire blade at the same time. It could help to use a marker and colour the entire blade to see which spots you did and did not sharpen.
Sharpening a convex edge, however, does change the finish of the blade. Especially when you sharpen on a coarse stone or belt you will see scratches on the blade. Because you sharpen the entire convex edge you will see these scratches on the entire blade. This does mean, however, that you can see if you have sharpened the entire blade. By increasing the grain size you can remove these scratches. However, only when you polish or strop the blade you will completely remove the scratches.
By loosely moving your wrist over the sharpening stone, you will never sharpen at the same place or use the same angle. As such you automatically create a convex edge.
Make sure that at the end of the stone you don't turn the knife too far upwards. Gradually turn the edge towards the stone as you sharpen the knife. As soon as the edge 'cuts' into the stone you have reached the ideal cutting angle. Don't turn any further!
Technique is very important so it won't hurt to first try it out with a cheap or old knife with a convex edge.
It might take some getting used to but sharpening a convex edge is easier than sharpening using the same angle. With a convex grind you sharpen the entire curve of the blade and not just the edge / the cutting surface. If the curve runs over the entire blade (up to the spine), it is even easier to sharpen. You namely don't have to stop sharpening halfway through the blade.
Sharpening on a sharpening machine always leaves you with a convex edge. Sharpening by hand leaves you with a slightly convex edge because you never really use the same angle. You could, however, with the right rocking movements, easily sharpen a convex edge on a flat sharpening stone.
You could always use a marker to cover the blade. If you still see some colour when you are done sharpening, there are parts of the blade that still need to be sharpened. Constantly check the blade as you sharpen the knife.
As with most things you learn most by doing it. That is why you should definitely try to sharpen a convex edge! Use, just in case, an old or cheap knife with a convex edge when you get started. You will see that it is not as difficult as you thought! Are you still left with some questions? Please don't hesitate to contact us.