Single bevel concavity

[newtothis]

Hi all, hope you'll forgive a newbie question... Have been stone sharpening my everyday beaters for a few years and am able to get a keen (if ugly) edge on them but just bought a couple of nicer new knives that I'd like to try and maintain the finish on, maybe get into basic polishing.

A quick question on concavity of single edge and wide bevel japanese knives... have watched various tutorials suggesting single bevel blades are pretty much chisel shaped edge and should be sharpened with the bevel flat to the stone. However, I imagine that vast majority of single bevel knives are ground on circular stones and will have a degree of concavity to them? People talk about blades being 'stone ready' or not - I guess the degree of concavity in a supposedly flat bevel?

If this is the case then why the different approach to sharpening double wide bevel blades with presumably similar level of concavity? Could have misunderstood but folk seem to avoid laying the bevel flat on the stone in favour of slight secondary bevel. Is this to preserve functional benefits of slight hollow grind? To preserve blade geometry? Preserve finish? Increase longevity of the blade?

Hope these aren't completely daft questions, lots to wrap my newbie head around. Appreciate any KKF wisdom...

 

[Dan-]

Have you watched this?



Because it seems you have some misconceptions about the blade profile on a single bevel knife that that video answers.

 

[ethompson]

You've opened a massive can of worms here...

Let us start with single bevels.

You're correctly they're ground on large wheels which typically leaves areas with a hollow. That said, they're rarely fully hollow bevels as the ideal geometry for the bevel (kireha) below the shinogi is actually convex (hamaguri) rather than concave or flat. You absolutely can use a circular stone, radius platen, convex stone, etc to create a convex rather than concave surface. What they target is a slightly convex kireha. In practice it is typically flatfish to slightly convex with some concavity remaining in spaces. Stone ready would be something that is slightly convex without any of those remaining low spots from a big wheel. You will see this only on knives that someone has already sharpened well or extremely high end blades. Also not a necessity to do this right away to sharpen properly.

As you sharpen, you definitely don't want to just lay the blade flat and wail away. Most of the time you just need to apply pressure right at the edge and sharpen there. This pressure placement is enough to maintain convexity and form a fresh burr without abrading all the way up to the shinogi. Periodically you will also want to work the area at the shinogi to push the grind up maintaining the overall geometry of the knife. Some people do this each time, others only periodically, but so long as your intentional either can work.

What you don't want to do is add a sizable relief bevel because then you're dramatically changing the grind and making it harder to sharpen long-term. Anyone who has worked a secondary bevel out of a single bevel knife will tell you how much that sucks to do. Single bevels are all about maximum keenness, you don't want or need a secondary bevel (except deba but lets not get sidetracked).

For double bevels you aren't grinding to a zero edge on one side and deburring on a hollow other side. You are sharpening like any other knife at the edge most generally. Therefore the need for convexity to follow on that bevel is much less and, in fact, having some concavity will help keep the edge thin for longer, preventing the need to constant thinning. Eventually you will need to thin things out and that's when it gets complicated and nasty with hollow wide bevels - the reason I don't personally like them. But unlike a single bevel it's not always necessary.

There are also wide bevel grinds that are flat to convex with double bevels, but even those are usually sharpened with a relieve bevel.

TL;DR - double bevels and single bevels have two different applications and are sharpened two different ways. Single bevels should be slightly convex not concave or flat. Single bevels should almost never be sharpened with a relief bevel. Double bevels can be hollow, flat, or convex ground, but should almost always be sharpened with a relief bevel.

 

[ethompson]

Have you watched this?



Because it seems you have some misconceptions about the blade profile on a single bevel knife that that video answers.

I still watch Jon's videos every few months. New or experienced alike they contain critical knowledge.

 

[kujaku]

A single bevel knife has a different structure from a chisel. In a single bevel knife, the blade tapers from the heel to the tip, while the width of the kireha remains constant. This means that the kireha gradually becomes sharper from the heel to the tip, resulting in a twisted, propeller like shape.

 

[newtothis]

Thanks for taking the time, all, really helpful. Was aware of the torsion along the kireha but hadn't seen a detailed explanation of the bevel geometry from shinogi to edge. Pretty subtle differences that to my untrained hands looks like it would be difficult to preserve over time. Scary stuff.....!

 

[kdizzog6]

Thanks for taking the time, all, really helpful. Was aware of the torsion along the kireha but hadn't seen a detailed explanation of the bevel geometry from shinogi to edge. Pretty subtle differences that to my untrained hands looks like it would be difficult to preserve over time. Scary stuff.....!

My single bevels are some of the sharpest and well performing knives. When I sharpen, always flat on the back, then I lift off the shinogi line just a hair when sharpening the front side for that reason.