Food Release: Stiction and the Grind

[Kippington]

I don't know about you guys but I'm fascinated by food release. It's surprising how little information is out there on the subject, despite it being a hugely desirable property in a knife - especially among those of us who work in the food industry.

With a craving to learn more, I recently attempted to make my first knife designed specifically to tackle this topic. The knife was a partial success, the food release is amazing but the design was too demanding on the steel. Internal stresses from the differential HT plus crazy thin cross-sections towards the tapering tip caused the blade to warp out of shape and subsequently it broke on the attempt to straighten it out again... so now I have a nice non-stick nakiri! :lol2:

I'm pretty sure I can take this experience and redo the knife with some improvements to stop this from happening again, so over the next few weeks I will re-design and create a new one while posting updates to this thread. I hope to go into some of the details of how it works, the problems this kind of grind creates, a few solutions and a bit of other fluff. Hopefully there's a bit of interest out there! :biggrin:

[video=youtube;CodmXr9MUks]https://www.youtube.com/watch?v=CodmXr9MUks[/video]

 

[Barmoley]

Sounds very interesting Kip. Thank you for sharing and please update with progress.

 

[Nemo]

Really intersted to see how it goes. Also interested in your thoughts on which grind features are best dor food release.

 

[chinacats]

Nice and I too am a big fan of good release. While I know nothing about design or knife making I'm curious why you're taking this approach vs a simple convex grind which tend to be my favorite?

 

[HRC_64]

Kip, that profile looks like a trad j-knife with the bevel on the 'wrong' side.
cool project and I'm interested to see how it plays out...


Btw, what is the cross-sectional minimum?

 

[milkbaby]

I'm assuming that you did the entire grind after HT? If not, that's worth a try although the stresses in the steel can still manifest in warp during post-HT grinding. I'm pretty sure you already knew that tho...

Is it carbon steel? I don't like to crank on the blade while cold to straighten. I always clamp with counterbend and heat to tempering temps again, check straightness after an hour or two, and repeat until straightened. Perhaps best 10-25 degrees F below the original target so as to lose the least amount of hardness as I've read you can lose a teeny bit (maybe on the order of HRc 0.5 to 1 point) repeated extended tempering.

 

[panda]

this is cool AF, keep us posted when you do food-no-sticky knife 2.0

 

[RDalman]

Nice and I too am a big fan of good release. While I know nothing about design or knife making I'm curious why you're taking this approach vs a simple convex grind which tend to be my favorite?

Can only reply for myself, but consistency. Convex involves slack belt grinding/finishing, or "wobbling" grinding and is very difficult to pull off consistently knife to knife. The "sweet spot" is very small (making wise).

 

[chinacats]

Can only reply for myself, but consistency. Convex involves slack belt grinding/finishing, or "wobbling" grinding and is very difficult to pull off consistently knife to knife. The "sweet spot" is very small (making wise).

Thanks for the explanation Robin...it's funny because this grind looks so much more difficult to someonewho doesn't know.

 

[Kippington]

this is cool AF, keep us posted when you do food-no-sticky knife 2.0

Will do!

I'm assuming that you did the entire grind after HT?

Yep, the knife was completely straight and tapered before I started on the hollow, much like this:

But while grinding out the middle, the steel got much thinner then I had anticipated. To answer HRC_64, it broke with a 0.5mm center cross-section

In a nutshell, the water quench caused the sides of the blade to expand more than the center, resulting in compression on both the left and right with a tensile stress in the middle. Grinding away much of the right side caused the left side to expand throwing everything out of whack. There's a huge amount of science involved in this, but I'll leave it there for now.. suffice it to say there's a whole lot of static pushing and pulling going on inside the knife!

 

[Kippington]

...which grind features are best for food release...

...I'm curious why you're taking this approach vs a simple convex...

Ah, now this is the science I want to get into! :lol2:

I'm sure we've all tried those laughable knives with gratons/scallops along the side after being told, "They help the air get between the food and the blade", only to find they did absolutely nothing to aid food release:

But then Glestain comes out with mega gratons which apparently work quite well (I've never had the chance to try one myself):

And later we get the el'cheapo Aero Knife, with gratons that go through the entire blade.

Now while the Aero Knife itself is terrible, they were onto something when they advertised it as having less surface area to stick to food. In fact many of us have intuitively worked this out already, while slicing something like a tomato and seeing that food falls off much easier at the tip of the knife rather than at the heel. This is because at the tip of the knife you get:

Less blade height → Less surface area → Less food-to-blade contact → Improved food release

And that's when I realized what this bump on the blade needs to do: Emulate the 'faux top' or spine of a knife that has very little height.

And the theory seems to check out on the test knife. As you can see in the video, tomatoes get pushed off to the side, and I was surprised to see the same while dicing the onion! It's bizarre to see most of it sitting there at the end as if it hadn't been touched. It's worth noting that the first few cuts on the onion are below the faux height of the knife, and so a couple of pieces hold on at the beginning but then get pushed up and off the bump by the following cuts. After that, nothing seems to be sticking.

Armed with this knowledge, its time to design a new knife around this 'bump'!

 

[Nemo]

Makes sense. My Tristone with a double convex grind has really excellent food release for such a thin knife. I guess that the convex on the top half of the blade effectively reduces the height of the blade that the food sees.

This would also explain the S-grind's effectiveness, I guess.

 

[HRC_64]

This is interesting...but

Personally, I try to draw-cut something that presents a major stiction risk.

Theres probaly some logic why it works, empirically (or anecdotally),
but it does seem food sticks alot less this way (grind notwithstanding).

so overall this is a fascinating topic to learn about.

good thread.

 

[daveb]

Somewhat related - Pens Tiger did a evaluation of sticktion on some popular brands a couple years ago. Someone with more google-fu than I would have to link it.

 

[Kippington]

Is a draw-cut the same as a pull-cut? Because if so you would finish the cut towards the tip of your knife, aiding in food release for the reasons I've hopefully explained in my last post.

Nemo, I've never seen a Tristone before. Can you post a choil shot?

 

[Pensacola Tiger]

Somewhat related - Pens Tiger did a evaluation of sticktion on some popular brands a couple years ago. Someone with more google-fu than I would have to link it.

Totally unscientific, and I don't have half of the knives I "tested", but here's the link:

https://www.youtube.com/channel/UCc...?sort=dd&view_as=subscriber&view=0&shelf_id=0

"Many potatoes gave their all for these videos."

 

[Kippington]

Ah yes, thank you Dave and PT. Those videos will play an important roll later on because, even though I'm trying to reduce the contact between food and the blade, the area where they do touch will still need to be worked out. This happens closer to the end of making the knife.
PT, I think your tests are hiding a few secrets we can uncover! :biggrin:

 

[jessf]

sharpens like a single bevel? That bump will disappear as you sharpen, no?

 

[Nemo]

Nemo, I've never seen a Tristone before. Can you post a choil shot?

I will but it may not help. Chadd only ground double convex on the distal 2/3 because he was worried about upsetting the knife's balance if he did it the whole length of the blade.

 

[merlijny2k]

Interesting read! You make it sound like using an air-hardening steel might be more forgiving and allow for the original profile to work. Another approach I have seen a smith take is to heat the blade in the forge to quite somewhat above quenching temp, than letting it air-cool for a while. That way at the start of the quench the temp is no longer even as the outside of the steel has cooled much more to the air than the inside. That should even out the shrinkage levels to a degree.

 

[JohnnyChance]

Essentially a highly exaggerated and focused on a single side Shige or Takeda grind.

 

[Drosophil]

Now that's an S-grind! I'm going to ask the obvious: how's the wedging? Can it be made tolerable or is it a one trick pony? Not that I'm complaining, it's a hell of a trick.

 

[milkbaby]

I'm gong to make an educated guess that the deep concave grind caused warping left to right lengthwise and tacoing spine to edge to the right hand side. Maybe you could lessen the degree of warp to correct by going with a more shallow concave section and/or lower height fuller which removes less metal. Did you grind the left side full height convex or is the curvature 100% from warping only?

There are some custom Xerxes with fuller which was intended for aiding food release. Didn't know if you've seen them:

Also IIRC, Robin Dalman does his S-grind wth a slight hollow grind for the lower cutting bevel (please correct me if wrong, Robin). This would serve to drive the cut food away from the blade as the angle of approach to the shoulder increases towards the top of the hollow grind bevel.

Interested in hearing more of your thoughts and seeing the next iterations!

 

[HRC_64]

Intersting point above.
curious how this works out.

Question about that xerxes....

What are peoples thoughts about
taking mass out of the distal spine
like that?

pros/cons?

 

[zetieum]

I don't know about you guys but I'm fascinated by food release. It's surprising how little information is out there on the subject, despite it being a hugely desirable property in a knife - especially among those of us who work in the food industry.



With a craving to learn more, I recently attempted to make my first knife designed specifically to tackle this topic. The knife was a partial success, the food release is amazing but the design was too demanding on the steel. Internal stresses from the differential HT plus crazy thin cross-sections towards the tapering tip caused the blade to warp out of shape and subsequently it broke on the attempt to straighten it out again... so now I have a nice non-stick nakiri! :lol2:

I'm pretty sure I can take this experience and redo the knife with some improvements to stop this from happening again, so over the next few weeks I will re-design and create a new one while posting updates to this thread. I hope to go into some of the details of how it works, the problems this kind of grind creates, a few solutions and a bit of other fluff. Hopefully there's a bit of interest out there! :biggrin:

I would indeed expect good FR from this geometry, however this geometry may be poor in hard food. Carrots etc. Especially if one wants to cut somthing else than fine slices: I would expect it not to cut straight.
Have you tried a big carott/potatoe?

 

[LifeByA1000Cuts]

I guess no thickness profile trick will ever keep brunoised garlic or chiffonaded hard leaves from having to be scraped off the blade with a dough scraper, then off the dough scraper with the knife, repeat 2-4 times?

 

[Kippington]

To answer a few questions above:

Yes, it does wedge a bit and steers to the left a little. However, now that I've learned how the bump work, I reckon I can shrink the size of it without compromising its ability to release food.
Basically the plan is to create a knife with better food release than a convex/flat grind knife of the same thickness. If I can't do that, all the theory I've gone through would be incorrect and all that effort would be for naught.

Milkbaby you are correct about the 'tacoing'. The edge on the left side was convexed on the grinder though.
It's hard to see it, but I have done the slight hollow grind for the lower cutting bevel. Works a treat for food release and has the added benefit of making the knife thinner behind the edge.

Changing the depth of the concave section and/or lowering the height of the fuller makes a huge difference to food release.
I'll go into more detail later, I g2g to work.

 

[jessf]

My first knife had a granton edge and after 10 years of use i had sharpened it down to the scallops. Mind you i didn’t thin anything as i went as i didn’t know thinning was a thing. So, as single and double beveled knives can be thinned, what would you suggest as a long term strategy for maintaining a knife with this profile?

 

[Kippington]

The easiest thing to do would be to sharpen it as a normal double bevel. When it needs thinning you'd have to remove some of the bump a bit at a time, which would send it further up the face of the blade until eventually it would disappear completely. It would become a very thin knife at this point...

As I mentioned above, I'm working on designing a knife with a smaller bump that hopefully retains its non-stick properties. I believe a bump designed like this would work better the original, without having to grind out as much of the blade middle:

And if it's efficiency increases, I can change the green area to a thinner cross-section to help reduce wedging.

The problem is... I don't think I can make that sharp indent look nice in reality. I'm currently waiting on a much smaller contact wheel to arrive in the mail - half the size of the original one. I will try to use it to get a similar effect.

Before I start, what do you guys think? Is there something I've missed?

 

[Kippington]

Welp, I've started on the knife. I've gone through the standard steps of forging, heat-treating and the grinding of the taper. Now for grinding the 'bump'... or in this case, a 'step'.

This style of knife is a first for me, so I jumped straight in with a willingness to fail. As of yet it's turning out really well and surprisingly easy - no problems have come up either. Last time the knife had already warped out of shape by this stage.

The step gets smaller as it heads towards the tip of the knife. This is because of the taper. If the step had been ground the same depth all the way, it would've gone though the thickness of the tip.

In these pictures it weighs 192 grams and the edge length is roughly 230mm. I'll start grinding the bevels and see what happens. Looking forward to testing this baby out on some food!