A tip for thinning asymmetrically

[cotedupy]

Here's a little tip for thinning knives with slight asymmetry to the grind, or if you want to get some slight asymmetry into a 50:50 grind...

Apply more pressure on trailing strokes when thinning the body side, and more pressure on leading strokes when thinning the food release side. The bevels will naturally end up with a slight bias.

 

[captaincaed]

That makes perfect sense - nice insight.

Purposeful or accidental discovery?

 

[jaydee]

Hmm, in a perfect world edge leading and trailing strokes remove the same amount of metal at the same angle so help me, i cannot see how you would endup with a bias?

 

[captaincaed]

I think it's the small hand motion that differs each direction as you drag the wide bevel surface across the stone.

To make an analogy, I personally find these small hand motions create a dome on any stone I flatten by hand. Getting to machine flatness requires a surprising amount of care. Doesn't matter a bit for knife sharpening, but it was enough I noticed it when razor honing, where the angles are more fixed and precise.

 

[Delat]

I use digital angle gauge while sharpening and for me anyway it aligns with what @cotedupy and @captaincaed are saying. On the push stroke away from the body (edge trailing for me) my hands hold a slightly higher angle. On the transition to a pull (edge leading) my hands dip just a touch to a lower angle.

There’s some sort of body mechanics at play that I’ve been able to reduce but not overcome over the past couple years, but I suppose it adds a bit of convexity just behind the apex.

 

[cotedupy]

Hmm, in a perfect world edge leading and trailing strokes remove the same amount of metal at the same angle so help me, i cannot see how you would endup with a bias?

This is absolutely a valid question, and in theory it would hopefully work like that.

In practice though; a combination of what @captaincaed and @Delat said above, but also importantly - how cladding / softer steel* behaves trailing vs leading, means that that you won't abrade in exactly the same way.

Here's a hyper-extreme example. This knife has basically had the bevel re-ground, with the same pressure at the same places, but exclusively trailing on the blue side, and exclusively leading on the red side:

---

* And acttually - this effect may well be less marked with monosteel knives. I do far more thinning of san mai blades than monos.

 

[ian]

To make an analogy, I personally find these small hand motions create a dome on any stone I flatten by hand. Getting to machine flatness requires a surprising amount of care. Doesn't matter a bit for knife sharpening, but it was enough I noticed it when razor honing, where the angles are more fixed and precise.

Feel like maybe this is more about the pressure getting distributed over a smaller area when you’re flattening near the edges/corners, so you abrade more there. Or maybe, yea, there’s pressure differences just coming from body mechanics, which I guess is the point you’re making.

how cladding / softer steel* behaves trailing vs leading, means that that you won't abrade in exactly the same way.

Confused by this. How does cladding know if it’s trailing or leading? Is it the abrasion that happens right at the lamination line that you’re talking about? That’s the only place I see where the the situation isn’t entirely symmetric.

Def makes sense that angles tend to be lower on trailing strokes.

 

[big D]

I seem to recall science of sharp showing photos of edge trailing and leading. The angles are not the same. Could be wrong, but steel moves and edge trailing moves towards the edge and produces a smaller angle than a knife held at X angle. Edge leading the steel moves upwards and the angle becomes thicker/larger, than a knife held at X angle. That was how I interpreted it anyway.

 

[M1k3]

Probably has more to do with rotational force, and accentuating or neutralizing the effect. I think. Makes sense to me at least.

 

[ian]

I seem to recall science of sharp showing photos of edge trailing and leading. The angles are not the same. Could be wrong, but steel moves and edge trailing moves towards the edge and produces a smaller angle than a knife held at X angle. Edge leading the steel moves upwards and the angle becomes thicker/larger, than a knife held at X angle. That was how I interpreted it anyway.

Interesting!

Is “steel movement” much of a thing when sharpening? I’m assuming this is some sort of plastic deformation or something. I mean, sure it happens, but does it move enough to measurably affect the geometry of the knife?

I bet the science of sharp stuff is about edges, not thinning. When you’re sharpening, it makes sense to me that edge trailing and thinning are different, even apart from the difference in usual angles you get from body mechanics. I feel like the way we press on a knife when sharpening, the edge is more likely to bend during edge trailing motions, which results in a thinner apex. And probably there’s other stuff going on where if your stone is muddy then edge leading dulls the apex with the slurry more than edge trailing does. With edge trailing, the little abrasive particles will first contact the face of the knife, then sorta be rolled under the knife past the edge, lifting and bending the edge up a bit as they roll under it. With edge leading, the particles may just whack into the edge.

However, if you’re talking about cladding, I don’t think the above is relevant.

 

[cotedupy]

Confused by this. How does cladding know if it’s trailing or leading?

Err, I believe you are the science and maths sharpening guy round here, so actually you’ve gotta tell me what the feck is going on...

Just to be clear, our respective remits are:

Ian
- Synths
- Knowledgable advice
- Difficult sums

Oli
- Washitas
- Wales
- Whimsy

 

[big D]

@ian
--
Is “steel movement” much of a thing when sharpening? I’m assuming this is some sort of plastic deformation or something. I mean, sure it happens, but does it move enough to measurably affect the geometry of the knife?=-----

Probably irrelevant, and I never should have mentioned steel movement. Was simply a throwback of my interpretation from long ago which I disproved to myself, also back then. Don't know why I seem unable to shake that stupid thought. Dead wrong, and never should have been written..

---I bet the science of sharp stuff is about edges, not thinning. ----

Yes, on edges. But again, disregarding metal moving it still stands that two blades sharpened at _stupid numbers 14Deg, ET ends up a 12.5, and EL ends up at 15.3. This exactly, while on the edge, mimics cotedupys observations.

---When you’re sharpening, it makes sense to me that edge trailing and thinning are different, even apart from the difference in usual angles you get from body mechanics I feel like the way we press on a knife when sharpening, the edge is more likely to bend during edge trailing motions, which results in a thinner apex.-----

All sounds reasonable. But an edge with an arc should only touch one specific spot, on a stone. Being flexible allows a portion of the edge to touch the stone. Therefore a edge under pressure will both flatten, and the angle will change. But it should not change because of direction. So why do the results change between ET and EL? Perhaps even though the edge mimics this thread, it my be a completely separate thing with a different answer.
Good to see you Ian.
@cotedupy - nice little tip.

 

[simona]

Curious. Are you swapping hands for ET and EL or turning the knife over on its axis?

 

[big D]

Personally I am comfortable doing both. Most recently I have been holding the knife in one hand for the most part. I still occasionally go back to swapping hands because I do like it, just dislike taking the time to securely cover the handle. Upon occasion I revert back to swapping hands when I have to sharpen in a tighter location.