Two choils, one cut

[ian]

So, I have these two knives, a Catcheside nakiri and a Mizuno gyuto. Here are some choil shots.

Catcheside:

Mizuno:

The Catcheside is wide bevel, with a shinogi around 1/3 of the way up the blade. You can see it in the choil shot. The Mizuno also has a shinogi, but it's less pronounced and halfway up the blade. The Mizuno is asymmetrically ground (for a righty), and the Catcheside is more symmetric.

Question: The Catcheside just glides through horizontal cuts of (very fresh) onions like the onion isn't even there. The Mizuno starts wedging a bit when about 1/3 of the blade is through. Why? And what can I do to improve the performance of the latter?

I'm making the cuts via a horizontal push-cut, not a tip swipe. I think the Mizuno is already thinner than the Catcheside in the relevant part of the blade, so I don't think "just thin it" is the answer. I've been trying to convex the blade road a bit with wobble strokes, but I'm not sure how well I'm doing. Anyway, if anyone has any ideas just from looking at the choil shots (which may or may not be representative of the overall grind, but let's assume they are), I'd appreciate it.

 

[SilverSwarfer]

I wonder if the surface finish of the blade has an impact in this case.

 

[M1k3]

How wide are they at the same points?

 

[ian]

I wonder if the surface finish of the blade has an impact in this case.

Maybe, but I’d expect that would impact stiction more than wedging. Don’t know though. The Miz currently has a 1200 grit sandpaper finish. The Catcheside has a 3000 grit stone finish on the bevel and whatever the OOTB finish is above that. (Pretty smooth.) The Catcheside has an irregular surface above the bevel, but I doubt that matters.

Are wide are they at the same points?

Mizuno: .35mm at 1mm from the edge. .75mm at 5mm from the edge. 1.25mm at 10mm from the edge, 1.9mm at 20mm from the edge, 2.2mm spine in the middle of the blade.

Catcheside: .35mm at 1mm from the edge. .85mm at 5mm from the edge. 1.35mm at 10mm from the edge, 1.85mm at 20mm from the edge, 3.05mm spine in the middle of the blade.

The Catcheside may indeed be slightly thinner at 20mm from the edge, but the difference is pretty slight...

 

[Bensbites]

I am not familiar with either of these blades, but you are showing us the choil and I assume we are talking about the tip for horizontals. Is it assumed that the grinds are the same at the choil and tip?

 

[Carl Kotte]

Interesting! Sorry @ian, no relevant input. But I will follow this closely to see the mystery unravel

 

[McMan]

I'm still trying to unsee the thread title...

 

[Corradobrit1]

I'm still trying to unsee the thread title...

 

[Carl Kotte]

I'm still trying to unsee the thread title...

Haha, how come?

 

[Kippington]

From the choil shots alone you would expect the Mizuno to perform better of the two. As you've mentioned, the grind almost definately changes on the Catchside towards the tip.
I think theres an easy way to quickly test this, I'll try it at home and report back here.

 

[M1k3]

Maybe, but I’d expect that would impact stiction more than wedging. Don’t know though. The Miz currently has a 1200 grit sandpaper finish. The Catcheside has a 3000 grit stone finish on the bevel and whatever the OOTB finish is above that. (Pretty smooth.) The Catcheside has an irregular surface above the bevel, but I doubt that matters.





Mizuno: .35mm at 1mm from the edge. .75mm at 5mm from the edge. 1.25mm at 10mm from the edge, 1.9mm at 20mm from the edge, 2.2mm spine in the middle of the blade.

Catcheside: .35mm at 1mm from the edge. .85mm at 5mm from the edge. 1.35mm at 10mm from the edge, 1.85mm at 20mm from the edge, 3.05mm spine in the middle of the blade.

The Catcheside may indeed be slightly thinner at 20mm from the edge, but the difference is pretty slight...

Are the measurements at the heel or where you're trying to cut at?

 

[McMan]

I am not familiar with either of these blades, but you are showing us the choil and I assume we are talking about the tip for horizontals. Is it assumed that the grinds are the same at the choil and tip?

Good point.
Seeing what the grinds look like over the last 3" of the knife could be helpful. I use a credit card or a business card to check. This should tell you how much convex (if any) it left towards the tip. In the case of the Catchside, it might be a little more complex than just convex depending on how he forged the geometry close to the tip.

 

[TB_London]

I'm making the cuts via a horizontal push-cut, not a tip swipe.

people seem to be missing this bit....

sounds like stiction is causing the difference to me

 

[RDalman]

Haha, how come?

Guess noone is answering because you're better off not knowing really.
Old gross video that circulated the web long ago, called 2 girls 1 cup. Don't look it up, it's one of those you really would have preferred not to have seen.

 

[Carl Kotte]

Guess noone is answering because you're better off not knowing really.
Old gross video that circulated the web long ago, called 2 girls 1 cup. Don't look it up, it's one of those you really would have preferred not to have seen.

Thanks Robin! You took one for the team here. You relieved me from my ignorance, and saved me from relieving it in a way that would have caused me a lot of grief. Now I can continue to think of vardagspolerade ollon instead.

 

[ian]

Thanks, all. And apologies for the title. (Not really.)

The measurements are taken over the middle third of the blade, which is where I was doing the horizontal push cut. As @TB_London says, the tip geometry isn’t relevant here. (I’d love to give you a middle of the blade cross section as in Science of Sharp, but I worry about the impact that would have on the performance of the knife.) I get wedging when splitting an onion in half too. It’s true, though, that maybe what I’m feeling should be called stiction. What’s the difference between stiction and wedging while cutting through a dense object anyway...? Is stiction the disease and wedging the symptom? Hmm, maybe @SilverSwarfer and TB_London are right about this. I did thin the Miz for a while, so maybe I just didn’t blend the bevels enough?

Btw, how do y’all blend bevels when going for convexity? Do you actually do wobble strokes? I find them a bit awkward to control, and it’s too easy to hit the edge when doing them. Or do you ‘blend’ by just making a third intermediate bevel? Or do you actually just use something with a softer backing for this instead of a stone? I’m thinking I may try some in-hand sandpaper for a while perpendicular to the edge. (Maybe rubber backed?) I’d like to create a continuous convex halfway up the knife.

Interested to hear what the test is you’re thinking of, though, @Kippington.

 

[Bensbites]

Guess noone is answering because you're better off not knowing really.
Old gross video that circulated the web long ago, called 2 girls 1 cup. Don't look it up, it's one of those you really would have preferred not to have seen.

I had to google it. I read the synopsis, not watch the video. I am done for the day!

 

[suntravel]

I think the Mizunos grind ist flatter than the catchside wich is more convex, and maybe the edge on the catchside is thinner, both good to easy cut an onion.

A surface to smooth on the sides can also be a cause to get stuck because of more adhesion on the Mizuno.

Regards

Uwe

 

[Nikabrik]

Or do you actually just use something with a softer backing for this instead of a stone? I’m thinking I may try some in-hand sandpaper for a while perpendicular to the edge. (Maybe rubber backed?) I’d like to create a continuous convex halfway up the knife.

I've used sandpaper over a washcloth, and I was pleased with the results. The washcloth happened to be on top of a stone, but it could be any workable hard surface.

Previously, I've used sandpaper on a mousepad for pocket knives; that does a lot of the work for you, but you'd have to be really careful to not damage the edge of a j-knife.

As to stiction vs wedging - have you read Kippington's thread "Food Release: Stiction and the Grind"? It's pretty insightful on both issues.

 

[ian]

I've used sandpaper over a washcloth, and I was pleased with the results. The washcloth happened to be on top of a stone, but it could be any workable hard surface.

Previously, I've used sandpaper on a mousepad for pocket knives; that does a lot of the work for you, but you'd have to be really careful to not damage the edge of a j-knife.

As to stiction vs wedging - have you read Kippington's thread "Food Release: Stiction and the Grind"? It's pretty insightful on both issues.

Nice, hadn’t heard of the washcloth idea. Will have to try.

I’ve read Kip’s thread with much interest. The fishhook grind is super cool. However, I think a simple convex is all I’m aiming for here. Anyway, just curious what people’s methods are for that.

Found a link in an earlier thread to this nice Carter vid about sharpening convex edges on bench stones:



He does it better than I do, but I still worry about hitting the edge during it. Maybe this is unavoidable, but I’d like to do the convexing without shortening the blade at all if possible. Probably I should just chill out, though.

 

[Kippington]

You can try this test. Honestly, It's a bit silly and inconclusive, but I like it.

Lay the knife on it's side on a flat surface, but with the handle hanging off the edge out into empty space.
Push down on the tip at the edge. The spine should react by lifting, increased movement representing more thickness on the grind in the area.

You can actually use this test to help find a correct finger placement while sharpening a convex edge on a flat stone.
Using the same test above, watch the movement of the spine as you press down close to the cutting edge, then push down closer to the spine until the spine only reacts about half as much. The location you were pushing on is where you want to apply pressure during sharpening, if you want to make the grind more convex. Following this helps address the problem:

I still worry about hitting the edge during it.

 

[ian]

watch the movement of the spine as you press down close to the cutting edge, then push down closer to the spine until the spine only reacts about half as much

That’s helpful. I was indeed monitoring the amount the spine would rise, but I think I’ve been too concerned with getting the convexity to come close to the edge, so didn’t think of stopping at *half* the possible rise. It’s true that having a bit of a flat bevel near the edge isn’t a bad thing, after all. I’ll try that.

 

[Kippington]

Yeah it works well for understanding pressure placement.
I forgot to add, you can use this test to roughly gauge whether the heel of the knife has a different grind to the tip, but of course you'll need to factor in the distance of the spine from the edge when observing how much the spine moves... coz trigonometry.

Getting the knife to 'wobble' on the stone is really easy. You just need to reverse what you've learned after all this time trying to sharpen straight and flat.
If you use finger pressure to press the blade into stone, as you push the blade away from you the friction will naturally make the knife want to roll and the contact spot will be slightly further away from the pressure location, relative to you. On a pull stroke the pressure will come towards you. Using this, you can easily get the knife to 'wobble' on it's own, on either side of wherever you apply sharpening pressure.

It’s true that having a bit of a flat bevel near the edge isn’t a bad thing, after all. I’ll try that.

Concave grinds can be difficult to define accurately, but they always have a shinogi/spine side and an edge side. You can define one of these sides as a reference to start on, moving the grind over to the other side. Or you can define both sides and work on the middle where they meet.
For me personally, the edge side is important enough that I need to start there, moving back towards the spine. If you have a crisp shinogi line on the blade, perhaps it's a better idea define the spine area first and - from there - move the grind down towards the edge. Here's an example of the latter with ABS Mastersmith Kyle Royer (grinding starts at 3:15). He references a "top edge" in the video, that's just the top of the double-edged blade.
.
Note that you need extra thickness along the edge to do it as shown in this video. On a knife which has already been sharpened, this method means you'll always end up with a thinner cross-section above edge than before - which is not always a good thing.

 

[ian]

Note that you need extra thickness along the edge to do it as shown in this video

I take it you mean "if you want your blade road to be more convex than it already is (e.g. if it starts off as a flat bevel), then you need the edge to be fat unless you want to lose a lot of height"? If so, I agree. For some reason, my Mizuno came fat behind the edge and extremely convex, so this wasn't a problem. It's much better now: little less convexity, much smoother cut. That said, I suppose all this is more about when I was initially putting on the multiple bevels on the blade road, rather than when I was trying to even it out with wobble strokes.

----

FYI, I tried the method you suggested for convexing the blade road with wobble strokes, with success I think. Stiction seems improved a bit. (Although upon reconsideration, it wasn't even that bad before... partly, it's my fault for comparing it to the Catcheside, which basically no stiction whatsoever.) I was able to avoid the edge pretty well; a very subtle burr appeared, but nothing that I had to drop below 6000 grit to clean up. I wasn't able to keep from scratching up the shinogi and a little above it, although that was easy to clean up. I guess I could have avoided hitting above the shinogi by making sure to stop my stroke and reset every time exactly on the part of the bevel just below the shinogi, but what a pain... I feel like if I'm going to be doing this for 20 minutes, I need to at least be continuously moving. Also, it's a bit harder to keep everything uniform when you're going essentially perpendicular to the edge. I usually have the motion of the blade nearly parallel to the edge when thinning, for distal uniformity, but this was harder.

I'll try @Nikabrik's suggestion for sandpaper over a washcloth next time. I didn't this time since it seemed like I'd run into problems avoiding the edge when doing the convexing with a soft backing. I'll try it out next time when I'm not using a knife that's already pretty thin behind the edge.

Incidentally, the shinogi on this knife came pretty wavy OOTB, although well defined. It's funny: they polish the knife in a way to make it seem like there's a perfectly straight shinogi, but it's not near where you'd expect from the polish. It's about the same now as before, just uniformly a bit higher up after all the thinning.