Can CATRA Predict Rope Cutting Performance?

Discussion in 'The Kitchen Knife' started by Larrin, Feb 11, 2019.

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  1. Feb 11, 2019 #1

    Larrin

    Larrin

    Larrin

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    There has been some question about whether CATRA is a useful test for actually predicting the edge retention of a knife. After all, it is a test performed by a robot, not a human, and the knife cuts cardstock with sand in it, which doesn't seem like a very realistic material. So I took three big datasets where people reported their own rope cutting experiments, and then compared those with what we would expect a CATRA test to tell us. https://knifesteelnerds.com/2019/02/11/can-catra-predict-rope-cutting-performance/
     
  2. Feb 11, 2019 #2

    Barmoley

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    Really cool article. It is amazing how well CATRA results correlate with "real world" results, regardless of what the haters say. You guys should check it out.
     
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  3. Feb 11, 2019 #3

    Larrin

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    Thanks!
     
  4. Feb 11, 2019 #4

    HRC_64

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    There should be enough similarities between cutting different organic/fiber substrates
    (pulp->paper, hemp->rope, etc) that we should be paing attention (re: food).

    This has been a good series to follow along with, so thanks for keeping up the work.
     
  5. Feb 11, 2019 #5

    Larrin

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    You bring up a good point, There is a tension when it comes to generalizing experiments to larger conclusions. With CATRA I wrote this article because people claimed the test was not close enough to “real world.” After this article there are one or two people saying that rope is not a realistic material because we are more likely to cut meat, vegetables, etc. After most articles I write there are typically a couple comments where someone says, “That’s interesting, but when you say that (this property) is controlled by (this variable) you’re wrong because that wouldn’t be true when (performing this action with a knife).” So am I overgeneralizing or are they moving the goalposts? I honestly can’t say but it does become tedious when the comment on every article is “That’s great, but get back to me when you do the analysis on something else because then it will actually mean something.”
     
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  6. Feb 11, 2019 #6

    Michi

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    Thank you for that article, that made for a really interesting read!

    One question: the first diagram shows a graph of edge angle vs number of cuts. Unless I'm mis-reading that, the most acute angle (20º inclusive) had the highest edge retention. I'm surprised by this. I was always under the impression that a more acute angle provides better sharpness, but blunts more quickly than a less acute one.

    As to how realistic a rope cutting test is, my gut feeling says "probably quite realistic". The one thing that might make a difference is that, when cutting food, I'm typically exerting less force than I would for a sisal rope. I have no idea how much (if any) difference that might make.

    But, your results are supported anecdotally, I think. There doesn't seem to be any disagreement that PM steel will retain its edge longer than, say white #2.
     
  7. Feb 11, 2019 #7

    bahamaroot

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    There are always the naysayers that were born to argue against anything you present them...

    Great article by the way!
     
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  8. Feb 11, 2019 #8

    Larrin

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    I wrote about that a lot more in the CATRA articles that are linked near the top of this one. When it comes to pure cutting, more acute edges cut much longer. Part of the reason is that they cut more in the beginning when their cutting ability is higher. Also the knife has to be more dull to have worse cutting ability than an edge with a more obtuse edge. The situation is different when chipping is the primary edge-loss mode.
     
  9. Feb 12, 2019 #9

    Michi

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    Thanks for that. I just read the first linked article. Great information!

    OK, so if I'm reading you right, in kitchen use, it is board contact that dulls the blade more than actual cutting? (Intuitively, that would make sense, because none of the food I cut is anywhere near as hard as a cutting board.)
     
    Last edited: Feb 12, 2019
  10. Feb 12, 2019 #10

    Larrin

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    The cutting board is likely to be the main thing that dulls the knife, yes. Thinner and more acute still cuts better and longer, until you reach the point where the edge is deforming or chipping.
     
  11. Feb 12, 2019 #11

    podzap

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    Over the past few months we have changed all of our cutting boards to composite and end-grain, which are said by many to be the most edge-friendly options.

    Interesting that people keep doing rope cutting tests today with metal blades because back in the day when rope needed to be cut in a real hurry, dudes were packing obsidian (black volcanic glass) blades. There was a reason why they didn't use metal for the task even back then :)
     
  12. Feb 14, 2019 #12

    never mind

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    Thank you so much Larrin, that’s why I like science. For this issue, I thought about one thing.


    So, first, from the paper, 20° knives cut about 1,000 times (5x), while 50° cut about 200 times (1x). Thank you. Before, many heretics believed that bigger edge angles increase edge retention for sure because it is bigger, let’s make knife edges less steep to increase edge-retention, which is false. Thus, if the knives won’t chip, the benefit is that the smaller edge angle knives will out cut bigger edge angle knives.


    Then, how about micro-bevel? If one knife is 20° and another also “20°” but with 40° to 50° Japanese-style big micro-bevel, which knife will out cut other knife (better edge-retention)??


    I am aware that many believers say micro-bevel (bigger edge angles?) increases edge-retention for sure while it reduces sharpness (smaller edge angles?) as a trade-off. Are their beliefs valid or heuristic, regarding this paper?


    I am also aware that maybe big 40°–50° micro-bevel increases edge-retention due to burr & wire edge reduction, while if you can have real 20° with minimal wire edge, this knife will out cut the “20°” with 40° to 50° micro-bevel?


    What’re your thoughts, ideas and experiences regarding this?


    Oh, I also LOVE the 52100 article so much! Oh my goodness! Thank you about that too!
     
  13. Feb 14, 2019 #13

    Larrin

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    I don't know too much about the actual performance of micro-bevels. The normal proposal is that the larger angle micro-bevel gives the edge some more strength while maintaining most of the cutting ability of the low angle primary bevel. I don't have any experiments to confirm anything one way or the other. I suppose it acts sort of like an obtuse edge with a very tiny behind the edge thickness.
     
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  14. Feb 14, 2019 #14

    Michi

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    I have no idea whether a micro-bevel provides advantages or not. With my engineering hat on, some observations (or hypotheses, if you like):
    • The actual cutting is done by the apex; that absolutely tiny line where the two planes of the bevel (micro or otherwise) intersect. If that apex is narrower, it'll go through stuff more easily.
    • Once the apex has penetrated some distance into the food, the food has to go some place. The only place it really can go is sideways because it needs to make way for the blade that keeps moving down.
    • How easily the food can go sideways depends on how thin the knife is behind the edge. If the blade is really thin, it doesn't have to push the food sideways as far as if the blade were really thick.
    • Given two blades with the same apex, but different thickness behind the edge, the thinner one will go through the food with less down-force than the thick one. (That's like the difference between an axe and a log splitter.)
    • With a micro-bevel, we have a nominally very thin edge (like an axe) that, for the last fraction of a millimeter, is actually a much thicker edge (like a log splitter).
    Picture a super-thin blade going through food. Now picture a super-thin blade with a micro-bevel going through food. What has actually changed? Only one thing I can see, off-hand:
    • The angle of attack of the apex. (The actual apex is no different; it's just that, for a tiny fraction of a millimeter, with a micro-bevel, food needs to be pushed sideways further than without a micro-bevel.)
    In theory, a micro-bevel should cause a (minute) increase in cutting force for the same performance. I have absolutely no idea whether that actually means better edge retention, and I don't know whether a less acute sharpening angle or a micro-bevel actually "supports" anything. The actual cutting is done by the apex. The point of the bevels isn't to cut, but to push food out of the way.
     
    Last edited: Feb 14, 2019
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  15. Feb 14, 2019 #15

    stringer

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    I have recent anecdotal experimental evidence on this. I have pretty much kept the original zero edge geometry of a Watanabe Pro 270 I purchased about 6 months ago. It's had 2 full progression shapton glass sharpenings and gets touched up once or twice per week on a naniwa super stone 2000. I have tried it with and without micro bevel. Without the micro bevel it gets microchipping very quickly but has more aggressive cutting edge. Retention is great but the micro chips get annoying because they catch on stuff like eggplant skin. With the micro bevel it's not as scary sharp but is more resistant to micro chipping so it goes longer between touch ups. I have noticed similar behavior on other knives but I've never been able to keep as acute an edge on anything else I've owned so the affects were not as pronounced. This is with blue #2 hardened to I think like 63. I don't have experience with more exotic high alloy stuff.
     
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  16. Feb 14, 2019 #16

    Michi

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    Thanks for that! Intuitively, it makes sense that a micro-bevel reduces the fragility of the edge a bit.
     
  17. Mar 1, 2019 #17

    Luftmensch

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    That PhD training is shining through. Nice article @Larrin - novel idea to correlate the results in the way that you did!

    Mostly, people don't understand how to construct experiments and interpret the results. Maybe they don't even know where the goal posts are!! It doesn't really matter if silica impregnated paper or rope are not close analogues to meat or vegetables - that is not the point. They are simply a consistent/repeatable material designed to blunt a blade. Other parameters (e.g. edge angle or steel composition) can be controlled and tested around those relatively consistent sources of blunting.

    Anyway.... nice work on correlating the CATRA and rope tests!
     
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  18. Mar 1, 2019 #18

    Luftmensch

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    Do you think edge retention is often confused with impact resistance? The main reasons I can see for an obtuse edge are either the steel is too soft to productively support it (the edge will just deform during the first cut) or the edge will chip.

    The difference between butchers cleavers and chinese cleavers is a good comparison. Butchers cleavers are designed for high impact chopping (potentially through hard material). The steel is softer and the cutting edge is generally obtuse. Chinese cleavers are a general purpose kitchen knife designed for slicing, dicing and mincing. High impacts will cause the edge to chip. Quite different applications with different steel hardness and edge angles to suit - despite a similar general shape.

    Garasuki/Honesuki/Deba can be quite hard but are an example of having more obtuse cutting angles to put more material behind the cutting edge. Again, this is primarily for impact resistance rather than edge retention. Even so, it is still possible to chip these knives if too much force/torque is used when cutting through bone. Cutting bone is not their raison d'etre, they are really designed to withstand chipping when glancing across bone and the softer material between joints. A side effect of this hardiness is that they can generally cope with cutting through small chicken and fish bones.
     
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