convex edges

[Ken S]

On a practical basis, how many knives would really benefit from having a convexed edge compared with the shallow concave standard edge of a Tormek wheel?

Ken

[tgbto]

I'd say it has to be a thick knife if the convexity is to be noticeable.

By noticeable, I mean "having an effect on the cutting action". A x° apex on a convex edge will tend to have less metal "behind the edge" than the equivalent x° apex on a concave edge. The latter could hinder cutting deeply through dense material. So I'd say everything between a hunting/bushcraft  knife and an axe.

I'm not sure why the "meat knife" they refer to in the video about the new knife jigs would be enhanced by the convex edge...

[GKC]

"On a practical basis, how many knives would really benefit from having a convexed edge compared with the shallow concave standard edge of a Tormek wheel? Ken"

Ken, as you doubtless know, there are big "convex vs concave" (and "flat vs everything") debates on the knife forums. I have a convex bushcraft knife that I feel does all of its bushcraft tasks (including staying sharp) better for me than my scandi bushcraft knives (which I also love). But I would agree with tgbto that the advantages of convex edges--if the ones suggested in the debates are real--are likely to be most pronounced on thick blades, such as bushcraft knives and cleavers.

Though one of the other threads on the new KJ-45 knife jig queried whether one could get a smooth convex bevel on a Tormek, I think it should be quite doable, especially if one begins with a sufficiently coarse wheel. I note that the Japanese surgeon with the YouTube channel "virtuovice", who is one of the big champions of convex bevels (on hunting and bushcraft knives), does it by hand on waterstones. It is painful to watch how long it takes him, but he is a master at it. Here is a link to one of his videos, in which he propounds a theory in support of the superior toughness of a convex edge.

https://www.youtube.com/watch?v=XA5AM2Lb0iY&ab_channel=virtuovice

Gord

[Ken S]

Good post, Gord.

Flat grinding was never an issue on the forum until diamond wheels. More recently, convex grinding has become an option with the new KJ knife jigs. The more I learn, the more questions pop up. Now I need to learn more about convexing bushcraft knives.................That's exciting; game on!

Ken

[Perra]

I think it will be difficult to get a nice convex edge with the new jig. It feels a bit "swaying" when you switch between the positions and difficult to move the knife in two directions at the same time and it becomes an, unwanted stop with the edge against the grinding wheel, which I find difficult to handle. As you can see on pic1-2, there is about 1.4 degrees (Slightly convex) difference between the two positions. It can work on a small edge, but on forest knives similar to F1 (https://fallkniven.se/en/home/) a medium convex or high convex is needed.
I think it is easier with a jig as in picture 31-33 which is more accurate for convex knives and where you have greater control at all angles. I will probably not grind convex with kj-45. It will be interesting to hear what you others think when you get the opportunity to test kj-45.

[Perra]

Correction:
cbwx34 helped me to understand one thing that i mist. Thanks cbwx34
I made a mistake when I wrote that it is 1,4 degrees convex.
I did not accounting for the change in the actual angle on the curved surface , just the angle change of the jig. There is probably neared 7 degrees in actual change. Sorry

[tgbto]

Here is a link to one of his videos, in which he propounds a theory in support of the superior toughness of a convex edge.

https://www.youtube.com/watch?v=XA5AM2Lb0iY&ab_channel=virtuovice

Gord

Well, that's an .... interesting theory. Mixing aerodynamics and structural physics in an innovative way, at least. Do you guys know of any research that would support the underlying assumption that for an equal apex angle, the convex edge is stronger (in terms of edge retention, I guess ?) than the straight edge ?

To be clear, and whatever the answer to the previous question, I find this theory highly suspicious. The fact that vegetable pressure crushing the bevel is the failure mode of the edge is IMHO cuckoo bananas. Unless we're talking hollow knives, but I don't think I have any...

[3D Anvil]

This article does a good job explaining what I think the Japanese gentlemen in the video was talking about:
http://site.baryonyxknife.com/blog/2014/08/11/why-convex-edges-are-awesome-its-not-why-you-think/

The bottom line, according to the article, is that the advantage a convex edge of equivalent acuteness has is that it produces less drag when cutting, and thus requires less cutting effort.  Makes sense to me, and it confirms some of my own observations.

Namely, I've noticed that knives I've free-hand sharpened (which have convex edges) sometimes seem to slice paper more cleanly than slab-sided edges from fixed-angle sharpening systems, even when the fixed-angle edge scores 20-30 points lower on a BESS tester.  When I say more cleanly, I guess what I mean is that it's a quieter cut.  I think that's because the convex edge doesn't have the sharp shoulder of the flat edge.

[RichColvin]

Ken,

Some chip carvers and axe users prefer this as they say it works better..

Rich

[tgbto]

I totally understand what you say about the edge producing less drag and requiring less effort during the cut. That's not quite being "stronger" though, and the bevel is thinner...

As the article referenced by 3D Anvil says :

little comparative testing has been done between two otherwise identical knives of equal edge angle with "V" and convex edges to provide substantive evidence for any sort of improvement in edge retention. If there is one it's simply that for equal amounts of force applied to each blade the thinner convex one will be able to cut deeper, and thus cut more material for equal energy expenditure, but the actual sharpness of the edge (or the thickness of the apex on the terminal "edge" bevel) will be affected about the same since it has the same edge angle and degree of wear resistance.

[Perra]

My experience is that there is a big difference between having a convex knife from the start compared to converting a straight knife to a convex one. A convex edge is stronger and more material on than on a straight edge as picture A shows. Picture B shows what happens when you sharpen a straight knife to convex. If you turn a convex edge into a straight edge, you also lose material just like turning a straight edge into a convex edge. It depends on what you start with.
On a convex original knife, the radius of the convex sphere is often smaller than if you turn a straight edge into a convex one because you cannot add material. The radius is drawn green in the image. The only way to reduce the radius is to move the tip so that the blade becomes less high.

[tgbto]

Hi Perra,

It seems to me you're making the confusion that is referred to in 3DAnvil's posted link. And often made to support the fact that a convex edge is stronger. Both knives in your picture don't have the same apex angle. So yes, the knife with the higher angle will be "tougher" in that hitting it against a solid piece of wood will see it failing later.

... BUT :
- The point here is to compare two knives with the same apex angle, one being later convexed by thinning its shoulders (using the technique demonstrated with the KJ-45 or whichever other one)
- As a side note, Wootz's studies have also shown that apex angles higher than the optimal angle (ie that at which the edge stops failing from being too brittle/weak) actually result in lower edge retention.

Cheers,

Nick.

[jvh]

Hello,

I fully agree with tgbto:

...compare two knives with the same apex angle...

I remember a similar discussion - What happens when sharpening a convex blade on a Tormek (if the angle is followed exactly) is shown here: https://forum.tormek.com/index.php?topic=4507

I recommend to draw the theory in CAD, then it is possible to simulate exactly what happens, and how the grinding method / grinding angle affects the result.
I'm not saying that there is no difference between convex and hollow grinds, but you need to compare comparables (same apex angle) and look at the variations that exist between these grinds. If you draw it, it doesn't look that bad, personally I would focus on the apex angle and blade thickness parameters. It doesn't make much sense to me to do a convex grind on thin blades or full flat / high flat blades, but let each person grind to their own...

jvh

[Ken S]

Perra,

I understand and agree with your logic as you state here:

"On a convex original knife, the radius of the convex sphere is often smaller than if you turn a straight edge into a convex one because you cannot add material. The radius is drawn green in the image. The only way to reduce the radius is to move the tip so that the blade becomes less high."

My problem with it is where does one find convexed (or unsharpened) knives from the factory?

The practical question for me is, Do the practical benefits of convexing outweigh the extra steel needed to be ground off of a knife to convert a concave grind to a convex grind? The extra first sharpening would both slightly shorten the useful life of the knife and require more time. I suspect the answer to this question would start with, "It depends".
"Depends" means considerations like the intended use, future sharpening conditions, and the steel of the knife.

I can see possible benefits from convexing some knives.

Ken

[Perra]

Hi Ken
Here can you find convex knifes
https://fallkniven.se/en/home/