K08 - edge retention (1/8", ridged cardboard)

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Cliff Stamp
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K08 - edge retention (1/8", ridged cardboard)

#1

Post by Cliff Stamp »

This isn't going to be overly exciting unless you are interested in the real fundamentals of edge retention and especially the influence of random variation and how to deal with it.

Here is the first run which is used on really non-abrasive cardboard to give a high end benchmark :

Image

Note the obvious features :

-high scatter
-very high initial loss
-long and very stable plateau starts to set in when sharpness is about 25%
-very high total cut count, but very large uncertainty


Second run :

Image

This and all subsequent runs are done with random sampling on the cardboard. Note :

-same shape (all blunting has to have this curve)
-reduction in total cut count and uncertainty
-data starting to fit the curve much better

Third :

Image

Similar changes.

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Again, note how the changes keep decreasing as the data becomes stable.

Fifth run :

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Almost no change now, the data is decently stable.

An over view :

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Among other things this illustrates how multiple runs are needed to get a decent performance estimate and how even just a couple of extra runs will achieve this easily. Note median statistics were used in the above as they are inherently better able to handle large deviations.
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egbladesmith
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#2

Post by egbladesmith »

Being a blacksmith i definitely appreciate this data, it is very interesting. I would like to know how you gauge the sharpness in terms of percents accurately. I could look at a certain number of steels and tell you how much sharper they could be, or that i think a particular knife is only at 80%, but not with that kind of accuracy.
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#3

Post by Cliff Stamp »

There are a few ways I normally measure sharpness, in this particular experiment I am cutting light cord (Berga spinning thread) under 40 grams of tension and the length of the edge used to make a cut. Thus for example once sharpened if 0.5 cm of edge is required to make a cut, then at 5 cm it would have 10% of optimal sharpness.

This choice of thread is actually a fairly poor one for consistency however I normally do that many runs and points of collection that this is averaged out and doesn't matter, but in general a synthetic is much more consistent so there is no reason not to use one. I just use Berga because I found a huge roll of it a few years back.

You want to adjust the tension so that your optimal sharpness is ideally about 1.0 cm, as this will allow you to measure with about 5-10% precision (depending on how you deal with parallax).
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Brock O Lee
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#4

Post by Brock O Lee »

It's very interesting to see how quickly initial sharpness is lost, within 5% (at best) of the work done, if I look at the graph above.

I'm starting to wonder if its even worth the effort to sharpen a blade more than 25% of its optimal sharpness...

I normally sharpen when it doesn't shave anymore. I'm guessing at this point it hasn't even reached the 10% sharpness plateau yet!
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#5

Post by Cliff Stamp »

Brock O Lee wrote:It's very interesting to see how quickly initial sharpness is lost, within 5% (at best) of the work done, if I look at the graph above.
Yes, that is the basic mechanism, the blunting rate is an inverse proportion to the extent of blunting, so initially it is very fast.
I'm starting to wonder if its even worth the effort to sharpen a blade more than 25% of its optimal sharpness...
At first glance it seems like there is almost no reason because of the relative lifetimes, however if you start off with a blade that sharp the curve doesn't simply follow the above one with a horizontal shift, the entire nature of it changes. This is because you can't sharpen a knife to what a sharp edge - blunted to 25% looks like, if your sharpness is less than optimal it is so because the edge is left over stressed, has a significant burr, etc. all of which will greatly increase rate of degredation. As just a rough guide, in general the sharper you get the knife, the longer the edge retention, a simple proportion is a decent first estimate.

However you have to take into account the efficiency of sharpening. I normally limit sharpening to be two minutes on all such work as I don't see the sense or realism of doing an edge retention trial on a blade which took extremely long to sharpen because that result isn't going to be of practical use to anyone aside from a few people who obviously really like to sharpen. All of these edges are finished on the fine side of a benchstone (similar to an india stone) can shave, slice newsprint and will push cut it easily on a 45, and will do a true push cut very close to the point of hold. They are extremely aggressive on the slice though.

However, it does (or should) make an obvious point that if you have a small rod with you edge retention is kind of not relevant because it only takes 2-4 passes per side and you can keep the knife extremely sharp with very little work.

I normally sharpen when it doesn't shave anymore. I'm guessing at this point it hasn't even reached the 10% sharpness plateau yet!
Yes, it depends on your hair, what exactly you mean by shaving etc., but by the time it is in the 10% plateau this is fairly blunt, it is now taking ten times as much edge length to make a slice, few people who sharpen knives would let a knife go this dull. For example in your kitchen knives are they ever that dull you have to go back and forth over a tomato to cut it?
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