5 Things I Was Wrong About Video

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Larrin
You can see that the initial cut length with a smaller angle is considerably higher and that the difference holds basically to the end of the test. This finding is significant because some have speculated that lower angle edges start out sharper but a more obtuse edge lasts longer [2]. And with the high wear that occurs in the CATRA test it isn’t likely that the situation would reverse with even further cutting. The initial blunting rate is relatively rapid regardless of angle and it then begins to level out. The highest TCC measured was over 1000 mm with an angle of 20°, and this decreased all the way to under 100 mm with 56°. In a CATRA study by Bohler-Uddeholm [3] with a range of steels, but unspecified edge geometry or sharpening, 154CM was measured at 547 mm, and M390 was measured at 959 mm. The 547 mm value would be with an edge angle around 30° in this study if other parameters are similar. So if the edge angle of a 154CM knife is reduced from 30° to 20° then it can match or exceed a steel with 75% greater wear resistance.

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Millscale
I agree that edge retention can be influenced by the wear resistance of the alloy, regardless of the edge thickness, but shouldn't the higher cutting ability and the lower forces applied lower the amount of stress on the very apex, decreasing damage anyway?

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jasonstone20
cKc,
How does BTE thickness effect edge retention? That is what we are trying to figure out.

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Ryan Nafe
The last point about holding the blade at the cryogenic temperature for longer not doing anything was kinda odd because he referenced the HRC number as evidence that it doesn’t do anything meaningful. Well the HRC number isn’t the point anyway, the point of cryogenic quenching is to lower the percentage of RA, not to gain Rockwell points. Now offhand I’m not sure what difference would actually occur from different hold times, but either way he’s not understanding the point of the quenching.

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https://www.bladeforums.com/threads/a-model-for-cutting-ability.421313/
Recently I proposed how to model edge retention and showed a few examples of it in use :

[www.bladeforums.com]

[www.bladeforums.com]

Now for a harder question, can you model cutting ability? Under a specific amount of force can you predict how deep a blade cuts and how this changes with repeated cuts? Can you model this :

chart2.gif



These are CATRA tests done by Buck knives to promote the Edge 2000 sharpening method. Buck reduced the angles from 35-50 degrees included to 26-32 degrees included and switched to a harder sharpening medium and thus removed the convexing of the cloth wheels. This work (2001) showed blades which cut better initially but cut better for longer. The effect of geometry was so significant that a 420HC blade with the flatter and more acute edge outcut both a ATS-34 and BG-42 blade with the older thicker convex edges. Now if you also re-profiled the BG-42/ATS-34 blades they would of course outperform the 420HC blade as shown here :

chart1.gif


Now this had been known for years to those active on rec.knives and I had discussed it in detail in many reviews showing how changing the angle and as well the grit had the same effect. This was based on work by Swaim and Talmadge. Swaim had been doing it for years when I entered the discussions (lurking) in 1998. Now as to the model - cutting ability is basically inversely proportional to the force on the blade. Now ignoring friction (which is usually valid for several reasons) the force on a blade due to wedging is just a constant determined by the cross section which doesn't change during the cutting plus the force against the very edge which is essentially the bluntness which I have modeled before. Thus the cutting ability is then :

Code:
CA(x)=CA_0/(1+b*x^c)

Where CA(x) is the cutting ability after a given amount of cuts in the test medium (x). CA_0 is the initial cutting ability with no blunting, which is a function of the stiffness of the media and the shape of the blade and initial sharpness. The constants b and c relate how and at what rate the blade blunts. These are the same coefficients as noted in the above linked posts as that part of the equation is of course the equation which I previously used to model sharpness. Ok, now does it work :

buck_catra_bg_420HC.png


Those points are data digitized from the CATRA graphs which is why the axis are scaled (yes I could scale it back, there just isn't any point). The model replicates both early and late stage cutting ability. As noted eariler, Swaim predicted this behavior back in 1998 where he first noted his opposition of greater angles=better edge retention, one of many myths which still persist. I'll be using this model shortly to discuss the hemp cutting and other work I have done. I wanted to use a CATRA model first as it is completely independent data. I will also be using examples of other similar tools, like dental scrapers and such just to show the more general implications. Note I didn't quote the numbers that come out of the model in the above because I didn't have the raw data. I will have all of this for the data I have collected so I'll discuss some of the issues in more detail when I use that.

As a side note, the CATRA curves also show the self-sharpening effect which I first noted in late 1999 during work with Boye blades. The sharpness can decrease then rebound as the edge chips, wears smooth, and this pattern repeats. I had been meaning to look at these curves in more detail for awhile but got distracted with other work. Note in the above I modeled the force in a simple manner :

F_w+F_e

Or force due to wedging (F_w) plus force on the edge (F_e), you can actually break this up and model the force due to wedging in components and thus separate the effect of edge angle and thickness. You can also use this model to do something like compare the edge retention of two blades at different geometries because it is a multi-variable model which includes both geometrical and steel properties. Thus for example you could model the effect of different steels and geometries and thus predict how they would act at different angles or finishes. With a decent spread of steels over different properties (hardness, wear resistance, carbide size) you could also predict the exact properties needed in a steel to give the desired cutting abilities.

Once you have these two coefficients you can also calculate the ratio F_e/F_w which then tells you the fractional dependence of the edge retention on the cutting lifetime. This for example is very high for hemp but very low for soft woods. This shows that you can cut soft wood very well with a blunt blade because most of the force is due to wedging, but trying to cut hemp rope with a dull blade isn't productive. This is why for example makers trying hype blades will do something like cut up a coffee can and then slice wood showing the "superior" edge retention of the knife. In fact because the F_e/F_w ratio is so low for that media the test is nothing but hype. So as a side note, this also lets you quantify hype.

-Cliff

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cKc (Kyley Harris)

this depends on the steel to a degree, but in general its correct that cryo only needs to reach the temperature, and not hold the temperature.
the cryo is converting retained austentite into martensite as an act of dropping to that temp. once it is done, holding it wont convert more.

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Ryan Nafe


this depends on the steel to a degree, but in general its correct that cryo only needs to reach the temperature, and not hold the temperature.
the cryo is converting retained austentite into martensite as an act of dropping to that temp. once it is done, holding it wont convert more.

I missed the distinction between holding it there and just getting it there. Getting it there is what I was thinking of. I haven’t read anything on extended times but I’ve seen quite a lot on the effects of the cryogenic quenching itself.

On a similar note, I found out that I once again have access to a Rockwell C scale hardness testing machine. Some of you may remember the thread I made 3 or 4 years ago on the subject. I’ll make a new thread on that subject when I start testing knives again and annotate it with the numbers found on previous knives in the old thread. First one up for testing is the SPY27 steel Native 5. Second knife for the testing will be the Benchmade S90V hunting knife that should be arriving in the mail today.

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cKc (Kyley Harris)
in regards to s90v.. note that if it is a coated blade, you need to remove the coating to perform an accurate test.

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Cliff Stamp
es, I watched a program on discovery awhile back in which it showed how the basic axe progressed as the metals got harder and stronger and allowed a refinement of the taper from the crude heavy convex bevels of the softer metals to the very strong steel alloys and the very flat and thin bevels. So the basic idea to minimize cross section is a very old one.

Unfortunately on the forums the discussions were often lead by the people selling the knives and thus steel was the focus and the idea that knives cut due to steels was a major point and you often saw argued things like forging improving cutting ability. Some of this still exists now where you will hear forged blades have "better balance" and similar.

Swaim was one of the first guys to stand up and say that was all wrong. Look at the shape of the knife to see how it will perform. How is it ground, what is the weight, where is it balanced. He also discussed both the center of mass and the impact node, noting for example a Cold Steel khukuri was inferior to a Ontario machete because the khukuri had too shallow an impact zone which had too much vibration, especially towards the tip.

Lee's book on sharpening discusses many similar issues, noting the use of very thin bevels, coarser grits for slicing and it predates the discussions on rec.knives by several years. I doubt Lee would claim he invented much of the practices either.

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Ryan Nafe
The slight recurve might be a problem though, it might make it hard to apex it cleanly on a diamond plate.

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Millscale
I agree that edge retention can be influenced by the wear resistance of the alloy, regardless of the edge thickness, but shouldn't the higher cutting ability and the lower forces applied lower the amount of stress on the very apex, decreasing damage anyway? I don't know, maybe someone will enlighten us.
Onestly I've never been a fan of super-high carbide steel, I believe ease of grinding for a knife is a much better feature than higher carbide content, especially since this generally implies lower toughness, tendency to chip,, coarser grain structure, stubborn burring, thicker edges at higher angles to reach stability and so on.
Seems odd.

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Cliff Stamp
From first inspection this seems reasonable because a more acute edge angle, again we are talking about the part which does the cutting here, not the part which is behind it, should be easier to wear, fracture and deform. Swaim however countered this arguement on rec.knives with the proposal with two contentions. First off all, if the angle is lower it actually takes more steel to be removed to get to a given thickness and second the forces on an edge are lower due to the higher cutting ability as edge angles are reduced. Thus he proposed it isn't necessarily true that if you grind an edge at 15 degrees that the cutting ability will decrease quickly below the knife sharpened at 20 degrees. Again, there is no mention of relief grinds, this is an issue of the angle at the edge itself. Now I had much data collected which supports Swaims hypothesis but there are always people who as Cashen is fond of noting :