Knife sharpness - rope cutting - edge testing machine

Singularity :

... some are talking about doing some \"fundamental\" research on knives, edge profile, blade profile, blade outline, steel, grinds (...), and some others are talking about testing \"a\" knife.

Essentially yes. There is a large difference between posts of the type "I fully endorse this knife" and posts which comment on aspects of knife performance. If you are just exploring knife performance you can be very narrow in focus and still give useful information. You can use a knife for one task and then make informative comments in that regard, but this usually isn't usually going to make for a very strong "review" of the knife as it could ignore critical aspects.

Though I agree that edge-holding as being part of the parameters for a \"high-end knife\", I still think it must fit in an \"acceptable\" range rather than as an absolute value.

Well yes, but isn't this true of everything. All performance aspects are connected to negative attributes, make an edge more obtuse and it gets more durable, make it more acute and it cuts better. So you just state where the performance falls, give some references to other blades for comparison and let the reader decide what is good enough for them.

The rope cutting is OK, though correlation to me is not an on off switch, but something that goes from hightly correlated to not correlated.

Yes, for example knife performance is generally very weakly correlated to price. However wear resistance is highly correlated to carbide content and more specifically, if Vanadium is present it pretty much swamps out everything else to the extent that wear resistance is proportional to its percentage. Slicing ability is highly correlated to push cutting ability as they both share a common dependence on blade geometry. You change a blades ability to push cut by altering the geometry and the slicing ability goes up (or down) in the same manner simply because all slicing contains push cutting (the material gets wedged apart in both cases). The change in load will be slightly lower for slicing though because there is less side wedging with slicing becuase it isn't a direct compression because of the shearing effect, the percentage reduction in load is probably more directly correlated on a 1:1 basis.

What is the point of getting factual data if it is to empirically decide of corellations afterwards ?

I am not quite sure what you mean here. The understanding of correlation comes from doing the specific testing and then normal use. If you do some test and find that it doesn't correlate to any regular blade use then you stop doing it as it isn't useful except for curiosity sake.

[rope cutting machine]

... this automated process would test \"conservation of cutting ability\" in function of the amount of material cut, and not \"edge holding\", which is just one parameter of this conservation.

I don't see the distinction there. Yes there are other properties that influence cutting ability, (blade and handle geometry for example), but nothing except the edge condition is changing during the cutting, so how isn't it simply a matter of edge retention?

how do you easily measure sharpness ?

Cut something in a way that can be quantified and which doesn't strongly depend on the blade geometry. I currently push cut baisting thread and measure the amount of force required, and slice 1/4" poly under a 1000 g load and measure the edge length required. Hemp is better to use than the poly, I continue to use it just because I bought a lot of it awhile ago before I figured this out. There are lots of other ways. Don't totally discount the ability to go by feel though, with enough experience you can tell apart very small differences in sharpness.

-Cliff
 
I don't see the distinction there. Yes there are other properties that influence cutting ability, (blade and handle geometry for example), but nothing except the edge condition is changing during the cutting, so how isn't it simply a matter of edge retention?

There is one, and you pointed to it in another thread.
As for things changing durin the cutting, I agree, thoug when you change the knife, lots of things change.


A++
 
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