400 % more edge holding - convex vs flat

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I mean, I don't have enough evidence to go so far as calling it the norm, but I know a decent number of makers have expressed it as their go-to method. Like I said, the faceting both allows for faster grinding thanks to pressure against the platen and gives more consistent form to the blade, so it's a win/win process. :)
 
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This picture shows a Mora Knife edge with 5 facets I have made for some years ago.

To make facets take time becouse precision grinding are necesarry. The facets must have the same width - and it must be the same angle between them if a true convex edge shall be th result.

It would be interesting if you guys shows pictures of the facets you have made and that you describe how you made them, how you meassure the angle and what sharpener you use when you mqde the facets.

Thomas
 
You can freehand them in. I'm not sure you quite understood what I meant in the context of creating a convex. The facets don't need to be be the same amount of angular shift from one to the next, nor do they need to be the same width. You just need to do them consistently from side to side and one knife to the next. What will result is typically a compound convex curvature rather than one of a "true arc" and how you arrange those facets before blending will impact the form of the resulting curve once blended.
 
On my jig I have a known start and finish angle. I do the rest by eye, stopping often to apply Sharpie.


This is an older video, I have since removed all the voodoo and this widget goes from machine to stone, stone to stone etc.


 
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Cobalt said:
Explain to me how it is not more time consuming? You are grinding flat facets first then going back and rounding it off. Two different grinder setups. No way that is not more time consuming. You grind facets, then you go back and round off. There is no way it takes the same time to do as just grinding a convex profile. NO way.

I have met several knife makers who do full height convex grinds and love it and do it all the time. Not a big deal on a slack belt.
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Setup time is a killer
 
Mr.Wizard said:
I'm confused; which video is older, and what voodoo do you mean?
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Sorry, the first video is the oldest, I probably did not need to include the rest. That one shows the shaping of the convex within defined boundaries. Not exact shaping of the entire grind face, but makes it a lot easier than freehanding on a slack belt.

In the first one I was still fine tuning all the relationships of the guide rod to the clamp body and this created "voodoo" - workarounds that had to be used if I changed the rake path across the stone/belt. By the last two videos you can see the widget holding the guide rod to the clamp body is a lot beefier and totally redone. Angle consistency front to back and with different grind paths across the bevel (rake) is virtually 100%.

This is the setup I continue to use today if I need a guided system, works beautifully.
 
Mr.Wizard said:
HeavyHanded HeavyHanded Thanks. Is there a thread where you describe how to build your jig? If not would you consider creating one?
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There isn't an existing thread, but yes, I might go ahead and do a build tutorial.

These videos are all several years old - shared with a closed loop of folks for evaluation as I was considering building them to sell. I don't imagine I'll be going that route at this point, but they aren't that difficult to make. With two exceptions - the guide rod mount has to be cut/hand fit with a lot of precision to produce 100% repeatable edges with symmetry side to side. And the clamp belt has to be extremely tough yet not mark up the handle of whatever knife you're working on. I never did find an off the shelf solution for either of these issues.

I keep thinking I should go back and come up with a better belt at least, but it seems to be holding up well. I made two of them just in case...
 
Twindog said:
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If you look at point D as the apex of a convex blade edge, you might be able to get the gist of the discussion. The convex edge would be the green and reddish lines to northwest of point D.

Lines BD and CD are the two intersecting tangents at the apex of this convex edge. The angle of the tangents at the apex is 90 degrees. That angle is not the angle of the curved edges, it is the angle of the tangents. The tangents are straight lines. You can measure angles of intersecting straight lines. You cannot measure the angles of curved lines, although you can measure the angles of their tangents. But the tangents are not the curves lines. A convex edge has an infinite number of tangents and all have different angles. The angle of the tangents at the apex will be one of those angles.
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A curve has a unique slope at a given point, and that slope at that point defines the line tangent to the curve -- at that specific point. This has been known mathematically since Sir Isaac Newton developed differential calculus circa 1666. A first semester course in calculus is devoted to finding slopes of various curves at specific points. A key is to think in terms of infinitessimals. This brief video might give you an idea of the fundamental concept:


Using your set of overlapping circles, edge geometries can be calculated. The 2-edges dagger defined by circles centered at B and C have 90° edges characterized by bounding angles of 90° and 180° at point D and 0° and -90° point A (where 0° points to the right and 90° points up).

The edges can be calculated for the symmetrical, 4-bladed you've highlighted in yellow. The bottom edge, at point (5.00, ~1.34), has a 60° edge bounded by angles of 60° and 120°. Because the shape is symmetrical, the other three blades also have 60° edges.
 
I have few Bark River knives with full convex grinds and edges and they do not perform any better than some of my hollow or flat grinds,actually perform worse lol
 
lonestar1979 said:
I have few Bark River knives with full convex grinds and edges and they do not perform any better than some of my hollow or flat grinds,actually perform worse lol
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Sorry. I took your post to be a comment on the validity of differential calculus. I acknowledge one's right to have an opinion about blade grind performance, especially if it's rooted in empiricism.
 
Just my 2 cents, no science to back me up.

My work knives were either convex or flat grind on the edge.
After many years of using both, I find that a convex edge will last much longer than the flat edge.
My only true flat edge would be my yanagi.

Even my CRK are all convexed by me, and I found that having a slight curve (removing as much shoulder as possible) was the best result in performance and durability. It will look flat but has a slight curve when you see the reflection.

I remember reading somewhere about an apple seed shape was studied for blade angles. It was strong and sharp.
 
HwangJino said:
After many years of using both, I find that a convex edge will last much longer than the flat edge.
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In my limited experience (I have one BRKT knife, a Mini Fox River with a CPM-3V blade), my observations are the same.

I've found much of the discussion in this thread confusing, so I watched the video in the original post again. Stuart is talking about convex grinds, not just convex edges. A convex grind maintained by stropping should inevitably result in a convex edge. Much of the discussion here seems to be focused on the merit of convexing the edge of a blade having a non-convex grind. From what I've gathered, unguided hand sharpening will result in an edge being convexed because of the human inability to maintain a constant angle on the blade relative to the stone. Some argue this inevitability is beneficial because of the inherent superior stability and strength of the resulting convex edge.

My Mini Fox River holds its edge seemingly longer than my flat grind blades. Much of this difference is undoubtedly due to its 3V blade. I have also noted that it seems much easier to restore its edge to sharpness. The only negative aspect of this knife's performance is it is not a good slicer. It will do the job, but not with any elegance if the object being sliced has any depth. For example, I like to slice an apple before eating it, but the Mini Fox River cleaves it more than slices it. The healthy width of the blade spine, at 0.15 inch, surely plays a role in this, but I suspect the convex grind contributes. Interestingly, my recently acquired Kershaw Cryo, with its hollow saber grind, also cleaves my apples. I was surprised to see this, as I expected a hollow grind to be a better slicer. However, as the blade penetrated to the depth of the unground blade, which is 0.12-inch wide, the apple splits. My best slicers have flat saber or full flat grinds.

It's my understanding that the Japanese call convex grinds apple-seed grinds. I'll have to examine a seed next time I dissect an apple.
 
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There's a bunch to unpack in those statements. Basically there's sound geometrical reasons for why you're experiencing what you're experiencing...but most of it has more to do with deflection and displacement than anything else. Hollow grinds essentially "put off" deflection until the end of their grind height, which is the reason it sunk in to the depth of the plunge line and then split. It was wedging at that point. The Mini Fox River was probably due to sheer thickness. An apple can only deflect so much before it breaks. As such, thin grinds are needed to cut through them cleanly so that they deflect the flesh of the apple as little as possible. The same goes for things like carrots. In meat or in shallow cuts in a semi-flexible medium like a bell pepper, your hollow grind would have had less trouble.
 
I've found apples to be a great teaching aid about the attributes or drawbacks of any given blade geometry. I completely agree, the overall thinness of the grind, all the way to the spine, makes the most positive difference. And with some blades, the cross-sectional shape and/or crispness of the spine itself can be either a plus or a minus, once the blade sinks into the apple to the full depth of the spine. Apples have a tendency to pinch a blade like a vise, sometimes very tightly. More so with thicker blades. And the edges of the spine can sometimes give it some extra grab or traction, in halting the blade's penetration into the apple. Using a tall blade, like a chef's knife or a santoku or a relatively thin-ground cleaver, can eliminate the issues with the spine. Then, what's left is the overall thinness of the blade again, which will account for most other issues in slicing an apple.

For narrow blades though, my favorite apple knife is still a simple Victorinox parer. Very thin & flexible flat grind all the way to a thin, unobtrusively ground spine, which gives the apple very little to grab onto in cross-section. So the blade goes through it like it was a stick of butter.
 
SharpPancake said:
Please, please, please. I dont want to turn this thread to personal attacks and stuff like that (I think it will happened). I want a normal discussion.
Long time ago, I watched this video.
I know, Bark River and Mike Stuart again. But please, I dont want to read the same sh.t over again, how shady he is and so on. I want to discuss some thing that he said.

So, tl;dr version: he says, that a bit more obtuse convex edge has more edge holding as normal flat grind ,,scandi“. I think that is just nonsence. Even convex has a little V on the edge, so if you have 20° per side flat or 20° convex (made by slack belt) I think the edge holding will be the same.

What do you think?
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When splitting e. g. dry oak wood with a knife, it is subjected to very high lateral forces. If the fibers are twisted the knife can get stuck in the wood. A blade made of too hard steel will break, a blade made of tough steel will deform near the edge. This is even more pronounced the thinner the blade (flat grind). A convex or saber blade will probably resist better to lateral deformations. If you don't baton big logs, the interest of a convex blade is reduced.
 
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