“Discovered” the Hollow Grind WOW!

lambertiana said:
This argument is only valid for shallow hollow grinds with thick blade stock. The wedging in a hollow grind blade is determined by the thickness at the shoulder and the height of the shoulder from the edge. If a deep hollow grind has the same height and thickness at the shoulder as a full flat grind has in blade thickness and height at the spine, they will behave in a very similar fashion. While it is true that many hollow grind knives have a shallow hollow grind with thick blade stock, there are many exceptions to this.
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Not from my experience. In fact not from my experience even with considerably thicker FFG blades. I'm sure there are exceptions but the argument for hollow grind seems to involve too many 'ifs' that FFG simply doesn't require.
 
Mr.Wizard said:
Very large wheels are common in knifemaking in Japan and used to form the urasuki on single-bevel knives. If a double sided tall hollow grind was thought to be superior they would already be set up to do it. However they do not, as far as I know.
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To do a hollow ground chefs knife in .080" stock, with maybe a 2.5" grind height, I think you'd be looking at a wheel that isn't practical for production manufacturing (hundreds of inches in diameter, from quick calculations). The concave grind on single bevel knives are generally in knives with thicker stock and require a much smaller wheel, from what I can tell, so I do not agree with you.

At that level of thinness, flat grinds are going to be the only reasonable way to make knives, and a flat grinder will be easier for every level of maker, from custom all the way up to giant facilities.

Edit: did some quick checking on the wheel sizes. It's not in the ballpark of a thousand inches but a wheel making a 2.5" wide grind that's .02" deep requires a wheel that's 78 inches in diameter. If you want a more slight hollow, say of .01" deep, that requires a 156" wheel.
 
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marrenmiller said:
To do a hollow ground chefs knife in .080" stock, with maybe a 2.5" grind height, I think you'd be looking at a wheel that isn't practical for production manufacturing (like hundreds or a thousand inches in diameter).
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Have you tried to draw out the dimensions you're describing? They do not make sense. You either have a piece of foil or effectively a flat grind.
 
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Mr.Wizard said:
Have you tried to draw out the dimensions you're describing? They do not make sense. You either have a piece of foil or effectively a flat grind.
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I took a fairly standard dimensioned flat ground chefs knife and considered what it would take to give it a slight hollow. You're looking at a wheel that is impractically large to do so in a way that doesn't make it a piece of foil (by limiting the depth of the grind), which is the point I'm making.

The difficulty in making a practical hollow ground knife of the same dimensions as a thin flat ground knife are what would stop someone from making a thin full-hollow chef's knife like that, not that it wouldn't cut better.

All the time in these discussions we see the common and tired point that kitchen knives are flat ground because they perform better, when the truth is that a hollow grind isn't even a realistic option at that scale, so that point is not meaningful to a comparison between the different grinds. It's possible that the differences at that scale may not be very noticeable but there's no way to tell. Meanwhile, in pocket knife land, I've still found nothing that hollow ground knives won't cut better over a similarly dimensioned flat ground knife.
 
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marrenmiller said:
The difficulty in making a practical hollow ground knife of the same dimensions as a thin flat ground knife are what would stop someone from making a thin full-hollow chef's knife like that, not that it wouldn't cut better.
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No, there is not enough material in a 0.08" stock to make a 2.5" high hollow grind of constant radius without it effectively being a flat grind.
 
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Mr.Wizard said:
No, there is not enough material in a 0.08" stock to make a 2.5" high hollow grind of constant radius without it effectively being a flat grind.
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Sure, let's accept that as true for a second. Is that an indictment of a hollow grind, or somehow making a point that flat grinds perform better?

I am aware that you can't reasonably do a hollow ground knife of the dimensions I listed. From what I can tell using arc-radius calculators, to have a reasonable cross sectional thickness and not make the edge a piece of foil, you'd need a wheel in excess of 13' in diameter or some similarly radiused platten on a grinder (this limits the depth of grind to .01", which even then is probably a little much), plus incredibly precise control to ensure that the hollow grind is properly applied and doesn't turn the edge into foil. In theory, it is very possible to do so, but the difficulty would be prohibitive and the market for it would be nonexistent. At that point, on a knife with dimensions like I mentioned, you might as well use a flat grinder, which will produce a knife with largely similar performance in an easier way.

What I am driving at is that the commonly bandied point that "kitchen knives are flat ground, so that's evidence they slice better than hollow grinds" isn't valid. It comes up in every one of these discussions, but you can't really compare a flat and a hollow ground thin chef's knife to determine which is better, because the latter knife doesn't really exist. There are hollow ground asymmetric specialty cutting tools, as you mentioned, but those are usually much thicker, are made with specific cutting tasks in mind and aren't intended to compete in ease of slicing most materials with a thin chef's knife. When comparing blades on folding knives, which are often of dimensions where you can hollow or flat grind equally effectively, I find there is little compelling evidence that hollow grinds slice worse than similarly dimensioned flat ground knives.
 
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marrenmiller said:
Edit: did some quick checking on the wheel sizes. It's not in the ballpark of a thousand inches but a wheel making a 2.5" wide grind that's .02" deep requires a wheel that's 78 inches in diameter. If you want a more slight hollow, say of .01" deep, that requires a 156" wheel.
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Well now I had to try it myself. If I assume that the hollow grind finishes with the sides above the edge parallel I get this formula:

r = d/4 + h^2/d

Where d is the combined depth of the grind on both sides, and h is the height of the grind. For example for 0.080" stock taken to 0.010" behind the edge, d = 0.070, so for a 2.5" high grind the radius is 89.3".
 
I like math, but I don’t like it that much. the talk of hollow ground kitchen knives had me scratching my head though. I’ve never seen one.
 
Mr.Wizard said:
Well now I had to try it myself. If I assume that the hollow grind finishes with the sides above the edge parallel I get this formula:

r = d/4 + h^2/d

Where d is the combined depth of the grind on both sides, and h is the height of the grind. For example for 0.080" stock taken to 0.010" behind the edge, d = 0.070, so for a 2.5" high grind the radius is 89.3".
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My quick determination of the wheel diameter was only based on the rough assumption that I wanted to replace the straight line making up primary bevel grind with an arc (with a width of 2.5" and a depth of .01", for a slight hollow), and so I used this resource. That wasn't accounting for the grind being parallel at the edge, for sure. One way or the other, it seems you'd need a huge radius.
 
Looking back at all the knives I have owned and used, I can honestly say that no full flat ground blade has ever out performed a hollow ground blade, all other things being equal.
But often, the hollow ground blade performs better than the full flat ground blade.

Just what my expirence has taught me.
 
marrenmiller said:
To do a hollow ground chefs knife in .080" stock, with maybe a 2.5" grind height, I think you'd be looking at a wheel that isn't practical for production manufacturing (hundreds of inches in diameter, from quick calculations). The concave grind on single bevel knives are generally in knives with thicker stock and require a much smaller wheel, from what I can tell, so I do not agree with you.

At that level of thinness, flat grinds are going to be the only reasonable way to make knives, and a flat grinder will be easier for every level of maker, from custom all the way up to giant facilities.

Edit: did some quick checking on the wheel sizes. It's not in the ballpark of a thousand inches but a wheel making a 2.5" wide grind that's .02" deep requires a wheel that's 78 inches in diameter. If you want a more slight hollow, say of .01" deep, that requires a 156" wheel.
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It would be possible to do that particular grind using a radiused platen, although the hollow would so shallow that you might as well just flat grind it. I have a 36" radiused platen, but I don't see any reason why you couldn't make one any size you desired.

I highly doubt that any factory would do such a grind though.

The link below shows how radiused platens work.

 
eveled said:
Radius platen. Something I never knew existed never knew I needed and now I want one. Thanks a lot.
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Ha! That's how knife making goes! Always one "last" piece of equipment to buy.
 
That’s the worst part I don’t make knives. I’m just a tool junkie.

I’m glad you posted it. I knew there had to be a way than the 100” wheel. Lol.
 
Pomsbz said:
Not from my experience. In fact not from my experience even with considerably thicker FFG blades. I'm sure there are exceptions but the argument for hollow grind seems to involve too many 'ifs' that FFG simply doesn't require.
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I have done side by side comparison cuts in heavy cardboard with full flat and high hollow grinds and the hollow grinds performed admirably, just as well as the full flat blades with similar overall height to thickness ratios.

FortyTwoBlades said:
Hollow grinds done right are wonderful performers. Most hollow grinds are not done right.
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This.
 
Mitchell Knives said:
It would be possible to do that particular grind using a radiused platen, although the hollow would so shallow that you might as well just flat grind it. I have a 36" radiused platen, but I don't see any reason why you couldn't make one any size you desired.

I highly doubt that any factory would do such a grind though.

The link below shows how radiused platens work.

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Yup, I mentioned a radiused platen later on. I still don't think it's something a production company would use to hollow grind a thin chefs knife, though I'd love to see one.
 
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