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TOPIC: What is "Sharp"

What is "Sharp" 6 months 3 weeks ago #16467

  • tcmeyer
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Last year, I watched a dermatologist slice a very thin sample of skin off a growth on my forehead and was amazed at the fact he didn't have to work at the slice to get the cut started. So what was different about scalpels? The sharpest knife I ever sharpened couldn't do that.

As I've mentioned in another thread, I've been studying sharp stuff, including scalpels and razor blades and have found some interesting stuff.

Most notable, and most applicable here is a box of scalpel blades I bought on Amazon. Who knew they were so cheap? 100 sterile, foil-packed blades for $13.50. A stainless scalpel handle cost $3.00. Shipped. I had cut and micro-photoed another type of scalpel but these were interesting. They had normal grind marks on both sides, but on one side, and only one side, it had been stropped. I haven't measured the grind marks yet, but they look to be 600 to 800 grit. Sort of a hybrid "toothy" edge, which, by the way, makes for a very nice precision slicing device. I was able to cleanly remove a patch of raised skin (commonly found on the hides of male humans approaching 70 years of age) from the back of my hand, with no need to work at establishing a slice line. It bit into the skin at the first application of light pressure. None of my sharpest knives would do that. Their polished edges would slide right over the skin, but then scalpels do have a more acute included bevel angle.

When I get a chance in the next couple of days, I'll post a few photos here.

I'm as fixated as any of the denizens of this forum on the achievement of a perfect edge, but have begun to become convinced that "working" sharp and mirror sharp are mutually exclusive goals.
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What is "Sharp" 6 months 3 weeks ago #16468

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Very interesting experimentation and observations. Well done!

Leo
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Leo James Mitchell
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What is "Sharp" 6 months 3 weeks ago #16469

  • LeoBarr
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We have been doing a lot of cutting tests lately and the polished edge has won every contest. We just did a test on towing straps and, I and the customer, both felt a edge with a little bite would work better but we were wrong. The polished edge went through the strap much easier. I think you will fin the same thing with the rigging knife. However the secret to any job, especially this one is a thin blade. Rigging knives have fairly thin blades but they are not usually real high quality. The next thing you want is a very narrow bevel. I just received a new knife and the bevel was much wider than I like. It was sharpened at 19 degrees and even with a polished edge it would not slide through a piece of cardboard. So I went down to 12 degrees and cut in a second bevel. I cut my primary bevel down to 20 thousandths, I think this is the absolute minimum, then I polished the bevel., without touching my 19 degree edge. Now the knife will slide through a piece of cardboard effortlessly. Often when trying to do difficult cutting jobs, people focus to much on the edge and not the shape of the blade, which is just as vital. If I was looking for the best knife for cutting this rope I would try a 4 or 5 " Japanese laminated cooking knife. These blades are stainless on the outside and have a thin layer of very had carbon steel down the middle.

This is a quote from Ben Dale of Edge Pro.

The sharpest edge I have which is untouched is that of my Rockstead Un I photographed it on a duvet and I found the blade whilst resting on the duvet had cut through both the cover and into the duvet just by been placed there I only noticed it after I had finished photographing . The Edge on the Un is finished on 2K/3K grit wet & dry sandpaper from the factory so it is slightly toothy the angles are 12Ëš at the kissaki and 15Ëšto the heel the steel is ZDP 189 so perhaps the level of polish is relative to the quality of the steel that is if the steel is strong enough to take the edge if it is not the edge may roll ; so perhaps a lower quality steel is more suited to less refinement since say a blade of lower quality steel finished with 600 grit may well chip in use rather than roll if it is more refined.

I have to say we have opened up a tin of worms perhaps other would like to add to this topic.
ScreenShot2014-03-09at10.26.52.png

The edge (slightly out of focus 400X) of the Rockstead Un
04Jun2013_1918.jpg




Leo Nav
Last Edit: 6 months 3 weeks ago by LeoBarr.
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What is "Sharp" 6 months 3 weeks ago #16479

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One has to be careful when talking about sharpness not to stray over into talking about the suitability of an edge to a certain task as I have just done.
The edge Leo Nav has just described is indeed sharp. So sharp is sharp as we see with Leo's example. There is no doubt there. I guess where I keep on erring is slipping over into talking about suitability of an edge to a task...when one does that we are then talking about edge geometry i.e. is an edge of 12 dps and an edge of 20 degrees dps that are of equal keenness suitable to make fuzz sticks and baton wood into kindling over time.
My mistake in thinking, and I suppose this is where the can of worms comes into things,is talking about durability not sharpness precisely.
Hmmm! Is that clear as mud Leo Nav or anybody. My old noodle is spinning its wheels here.

plain Leo
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Leo James Mitchell
Last Edit: 6 months 3 weeks ago by leomitch.
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Re:What is "Sharp" 6 months 3 weeks ago #16480

  • razoredgeknives
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Excellent points Eamon. People have to realize that there are two different types of cutting - push cutting and slice cutting. The former is done with a highly polished edge and the latter with a low grit edge. It all depends on what you want to use your blade for. I have found over the years that for almost all tasks the toothy low grit edge works much better for EDC tasks whereas the polished edge has a very specific limited use (i.e. Straight razor). As my diamond stones have worn in more and more, I have found myself decreasing the grit I finish with on my EDC knives - it started with 1k then went to 800, now I'm down to a 600 grit finish and am loving it! I have not noticed any edge deterioration in toothy vs polished as Leo has. To confirm this, This process/testing would need to be done in a double blind test with the same knife with two different finishes on several runs. :-)

You may be right Leo, but I'm just saying to confirm we would need to do this.

Josh
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What is "Sharp" 6 months 3 weeks ago #16481

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The tin is fully open now we will be able to discuss this add infinitum I do think that geometry leads over edge refinement for apparent sharpness if we think of the pure physics of this perhaps one way to prove this would be to take two identical edges of reasonable thickness and sharpen one with a single bevel as far as strops and the other with appropriate thinning and then see how they cut.
One fairly good paper test is to put the knife into an open palm and then draw the paper across the blade without gripping the knife I think this helps to show a truly sharp edge or back to Clay's edge tester I think the length of material to cut through is relevant since this shows how easily the blade slips through the medium it is cutting where as a thin medium to cut is not so useful for showing the cutting ability of the blade.
Leo Nav
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Re:What is "Sharp" 6 months 3 weeks ago #16482

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That would be a very interesting test. To define such a test might be difficult but it sure would be something to watch and quantify the results yes?! :woohoo:

Leo
Never go anywhere without your knife!
Gibbs rule number 9

Leo James Mitchell
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What is "Sharp" 6 months 3 weeks ago #16483

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Quoddy99 wrote:
Thanks for the replies. It still confuses me a bit as I sharpen away. Since the whole process is fairly new to me, I'm probably getting better at it without realizing it. I do believe that many of my early sharpening processes were hurried, i.e., not done sufficiently or correctly which may be showing more now that I have some experience, albeit minor, with the WE.

A friend from Portland came to visit last weekend and brought four of her kitchen knives. Two were 8" Chicago Cutlery chefs. Two were Henckels: one 8" and one 3" paring.

~~~~
For me, it seems in my current experience, that if I'm not 100% sure I got a burr on both sides at the 400 grit level, then my blades just won't be what I want them to be.

So, if I'm not sure, I keep stroking/caressing until I am sure. Then, I progress. I'm working to develop the sensitivity to sense the burr at 600. But, as long as I'm sure at 400, then the rest of the progression simply requires me to pay attention and give each side equal care.

For example: the 3" Henckel paring. The other three blades I got the burr, progressed through the grits and Wow. The paring? There was one little 3/4" segment that I could never feel a distinct burr. It was kinda, sorta there but not reassuringly definite.

I had left it for last; was hurried a bit so left it that way.

Trying to weave two points together -- what my friend hadn't see was my prior experimentation on her blades.

Fortunately, I had noticed that while the two Chicago Cutlery initially looked identical, they had different "serial" numbers on the blade near the handle. By observation I saw that one had a wider, thinner bevel and one had a narrower, thicker bevel. Didn't find a lot about it online but it appears the narrower, thicker bevel is for chopping. The wider, thinner bevel for slicing.

My friend also didn't see that I worked the blade up through the Fine Ceramics and stropped. I am still startled when a blade simply falls through paper -- brings a smile to my lips...

But, then I tried to slice a tomato, cucumber and carrot and simply wasn't happy.

So, I went down to the 800/1000 and "roughed" the edge back up. Then, gave it few strokes each with the Course/Fine ceramics. Good on paper; not my favorite (for these particular blades) on veggies.

Did it again with just the 800/1000 and was happy. Actually did quite well on paper but on those veggies!!!???!!! Cutting tomato & cucumber was like cutting air. Cutting carrot like cutting butter. It still startles me when a blade just falls through a veggie.

And that darn paring knife. Draw the paring knife through paper and it was butter -- except that little 3/4" section I wasn't sure of.

Now, on the veggies? The paring knife was near perfect. Couldn't even feel the sticky spot..... (even though I know it is still there.)

~~~~

Get the burr.....

One of the real tricks the experts here teach is that you are really tuning the sharpening process to the person, the blade and the task. Putting, at least, those things together and people will be stunned with the edge you give.

(I believe there is, at least, as much Art as there is Science in this sharpening gig.)
~~~~

For Now,

Gib

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"Things work out best for those who make the best out of the way things work out."

"My goal is to be a good, practical knife sharpener. My dream is to polish molecules."
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Re:What is "Sharp" 6 months 3 weeks ago #16484

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My gut feeling is along with you on this a toothy edge is more appropriate for EDC this is I am sure true . I do however feel that geometry is also very important I believe Josh would support this .
I am also unsure as to the need for this with superior steels which I am currently unsure of the real answer for this and hope that eventually we come up with a real irrevocable answer to this.

Leo Nav
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What is "Sharp" 6 months 3 weeks ago #16487

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Yes Gib I agree...there is a lot of art along with the physics in tuning a blade, as you have discovered. I guess one could write a book titled, Zen and the Art of Sharpening. Using your senses is important.
I keep on using the word 'caressing' as you have noted, because to me it says clearly what I should be doing to the edge. I will stop there in my description of caressing so as not to stray into the wrong territory and further embarrass myself! :lol: WHoops! There I go again!

Leo ;)
Never go anywhere without your knife!
Gibbs rule number 9

Leo James Mitchell
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