Thread: Convexed bevel, Good thing?

Micrographs of a convex edge.

Here is an interesting video by Dave Weaver, Dave is an SRP member but has not posted in a while. This video is part of a series on the “Unicorn edge” and buffing an edge with a buffer, posted on the Sawmill Creek forum,

While this technique will not work on a razor, it is interesting what his micrographs show of the edge taken of chisels at a side profile.

As Dave says, the technique is not new, I was first taught it by famed carver W.P. Wilcox in the early 80’s for sharpening carving chisels. W.P could not be bothered with sharpening on stones, except to remove chips and often said. “We don’t get paid to sharpen tools; we get paid to cut wood.”

And off a leather wheeled buffer they cut wood like butter.

What is new is Dave’s new look at the technique and experimentation with edge sharpness, longevity, and dramatic micrographs of the bevel side profile.

So, how does it apply to razors? Buffing and the result of buffing is exactly what we do by stropping. The strop works exactly the same way, except that it removes microns or fraction of microns, rather than thousands by buffing.

It has long been contended that stropping convexes a bevel or if done incorrectly rounds an edge. My contention has always been that convexing a bevel is a good thing and makes a stronger edge.

The best example is a Scandi grind vs a bevel grind on a knife. A Scandi grind is a convexed bevel on steroids. No question that a Scandi is a stronger edge than a bevel edge, and possibly sharper, or at least subject to less resistance. Think boat hull design.

Dave also touches on how a buffed convex edge is stronger and cuts cleaner than a Micro bevel. And the video also makes an argument that bevel angle is not as critical as once thought, that bevel shape may trump bevel angle, Dave’s convex bevels are much steeper than the honed bevel angle yet perform much better.

So, yes Dave’s cutting wood and we are cutting hair, two different kettles of fish… or are they?

What are your thoughts?

(Micrographs video Unicorn Edge)

(Dave’s first Unicorn Edge,)

One of my pet theories has been that using Slurry actually adds to the Convexity of the edge, making for a longerlasting and smoother feeling edge..

Originally Posted by gssixgun

One of my pet theories has been that using Slurry actually adds to the Convexity of the edge, making for a longerlasting and smoother feeling edge..

Interesting! Would you please share your thoughts with us?

Paul

Shigeyoshi Iwasaki recommends a convex bevel, especially for westerners. Good enough for me.

I do a little honing for a barber friend of mine.
This pic shows the convex bevel, after being stroped for about a month, without honing.

I also understand that Samurai swords were honed in this fashion.

We also know that a Arch is stronger than a triangle,especially in stone bridges, and doorways

“One of my pet theories has been that using Slurry actually adds to the Convexity of the edge, making for a longerlasting and smoother feeling edge.”

I agree, here is what I think happens. Convexing a bevel is a good thing.

No matter how hard we try to grind flat, momentum forces the pressure to the edge, in the direction of travel, edge leading. On a stone without slurry that is not as big deal.

But with slurry, picture a bunch of balls under the bevel, as the pressure moves forward towards the edge, (the direction of travel) the edge dives down into the grit and contact the stone, the more slurry the more aggression, convexing. The balls/grit assist that momentum by reducing friction. Both the grit and the stone are aggressive, so cutting is multiplied and concentrated toward the direction of travel. Which is also why incrementally thinning slurry produces better results for finishing and polishes more that it cuts. Grinding vs Polishing.

All of this is microscopic, and the weight/pressure transfer is almost imperceptible, but it happens, and we, (razor honers) are talking about removing microns of steel.

A simple example of how little force is needed to make a change, is the X stroke. That little bit of centrifugal force concentrates the pressure on the edge, add a rolling action and the pressure is magnified on a small spot at a time as the pressure shifts along the edge.

Slurry is a remarkably interesting thing, that is not studied much in honing other than how to, and probably why it is difficult for new guys to understand and master. Thicker is not always better.
Add to the pressure shift the fact that some slurry is friable and crushes to a finer consistency and fresh cutting edge, but not necessarily uniformly, (natural stone), and the process is magnified, (one of the goals of finishing on slurry). The balls or bits of grit become smaller and multiply in number and abrades the steel.

Paste on a strop works much the same with added aggression from the grit. Add to the pressure shift and rolling grit, the strop flexes at the edge to the highest point or the most pressure and it too becomes more aggressive. The paste grit is not fixed, It will convex a bevel, but if your attention wanders one can easily roll an edge.

It is also why, when lapping a precision granite plate, they use a cast iron lapping plate, that imbeds the grit to prevent it from rolling around under the plate and cutting a more uniform surface. They also routinely clean the plate to remove the swarf and get a more uniform surface.

When I visited Alex Gilmore he actually showed this to me. He did by leaving a light scrach pattern from the DN slurry. When he made a tomo slurry he did a couple of edge forward strokes and let me see it through his microscope. It was removing metal in the front of the bevel leaving the cratches in the back intact. Then he did spine leading stroke and you can see it removed the scratches in the rear of the bevel. It is small but it did prove it was doing it and you could influence it by your stroke. The fact that a strop increased it made it clear to me that it is a actual thing to take into cosideration. You can "see" it better at 400 mag and a stereo microscope than on a video screen. The 3d is lets you see the depth you can't see on a screen.

My mind is blown. This is way to advanced for me, so just want to try a simple yes/no type of question. It sounds like with a slurry and pasted strops this just sort of happens with proper technique and there is no special movements or anything to make it happen.

Yes.

Any stone will self-slurry, some softer stones more than others, that slurry is loose slurry rolling on the stone, add to that the swarf.

But technique can make a flatter bevel even on a soft stone. For example, too much pressure on a wide hollow ground razor can lift the edge off the stone, as can honing stroke, Rolling X vs straight stroke.

Compare the stria pattern of a bevel set with lapping film, Lapping film does not produce loose grit and the bevels are flatter and stria much more uniform.

A micro convex bevel is a good thing, A Scandi ground knife is a stronger edge than a secondary bevel grind.

Dagnabbit, I asked for a yes or no. I always have to read your posts two or three times and try to digest half of it. I thought I'd have a chance to get it on one go.

Seriously, though. Thank you. It's interesting how everything ties together.