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Levels of sharpness? Sharpness tests?

Joined
Nov 20, 2008
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I would have to respectfully disagree with the finger test. Maybe I'm special or something but my skin is never the same on a number of levels at any given time of day, ambient temperature, mood, etc. They can be anywhere from bone dry and flaking to dripping sweat and the edge of a knife will act differently all over the range.
 

Jason B.

Knifemaker / Craftsman / Service Provider
Joined
Jun 13, 2007
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11,089
It's something you develop, its how the edge digs into the skin and yes actually cutting the first layer of skin. Most know it as the 3 finger test but for me I use my thumb. Wet hands will make the test very difficult but mood and room temp have no effect.

Truthfully, viewing edges under a microscope has given me great visuals as to what I am feeling. It has helped me understand better what my fingers are feeling and how to make better corrections in sharpening. I can feel if I still have a burr or if the apex is not fully reached or if the scratch pattern is not completely formed just by how the edge feels on my finger.
 
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Joined
Jun 14, 2013
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I actually hate finger test, although I do it. I get a little carried away at times, and cut a little deeper than I intended. Paper and hair, give me results, that are easily indicated, and predictable. Edge retention, is a tough thing to measure, and that is also a big part of "sharpness" to me. Yes steel has something to do with it, but so do angles, and edges.
 
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Nov 6, 2001
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Honestly, I have tried Murray's 3-finger test, and wanted it to work, but it just isn't there for me. I can thumb a blade and tell pretty well how sharp it is, and of course the above mentioned phone book page test.
 
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Jun 4, 2010
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I just followed Murray Carters advice from the video and started using the 3 finger test all the time and in conjunction with other tests. Its only helpful in context, you have to build up an "inventory" of memory. After a while one can get a very good feel for this. Between a close visual inspection and a three finger test, I can predict with reasonable accuracy how an edge is going to perform.

Have to put in a + for powerful microscope. Largely overkill, but much like the 3 finger test, once one gets a large enough sample of various edges it can tell you a lot about how things work at that scale. Backlighting the edge makes a big difference.
 
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Jun 7, 2005
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What do you guys use for a microscope? I have a 10x loupe but I would like to see more
 
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Jun 4, 2010
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What do you guys use for a microscope? I have a 10x loupe but I would like to see more

Most of the time I use a 12x loupe. For a microscope I'm lucky enough to have a high powered metallurgical microscope at work. I am dubious of the benefits of a microscope under 400x (aside from cosmetic study of the scratch pattern), and even then there is a world of difference between what the cutting edge looks like at 400-600 and what it looks like at 1000x.
 
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Oct 29, 2010
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Basically all I use my knives for is opening cardboard and paper packages, so slicing magazine paper is my usual test.
It is fun to get them past that level though of course!
 
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Aug 3, 2009
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I've tried the 3 finger test a number of times, but my results aren't consistent. When I sharpen to a middle grit edge (1000 JIS or Spyderco medium) the blade seems a bit sticky. I can watch my finger pads move with blade. But when I go any higher (or with a dull knife), it just seems to glide over my fingers.

This is a weird thing because psychology plays a role. I don't want to slice my fingers open, so I'm not pressing with any more than just tiny pressure. Murray says really weird things about this test like, (paraphrasing) "buffers make a blade dull like a ballpoint pen and fail the 3 finger test. They will shave and slice paper, but they aren't sharp."

Frankly that seems like complete BS to me. I think he's just going for a toothy edge that bites skin. OTOH, he's the "expert" and he should know, right? My blades that glide over my skin do shave and easily slice phonebook paper. Am I missing something that he is able to do that I can't? Or is a finer edge just not able to bite into skin and "stick" with the 3 finger test?

This has been on my mind for many, many months now, so I'm very glad someone brought it up.

If a finer edge isn't sticky, how do you (Jason, knifenut) use it to appraise finer ground edges? I'm asking Jason, but I'm also asking anyone else with experience in this.

Thanks guys,

Brian.
 
Joined
May 25, 2013
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I have never gotten great feedback from the three finger test. I use my thumb only and get a lot better feel for the edge. You just have to be a little more careful. I hold the pad of my thumb at about a 45 degree angle to the edge and then VERY lightly touch/slide my thumb on the edge about 1/16 of an inch. The idea is to see how easily the edge will "bite" into or cut through the first layer of skin. When I've got a knife right, I can feel the edge cutting through that skin layer with just the lightest feather-like touch. The tips of our fingers are quite sensitive. I have performed this test thousands of times and have never cut myself or drawn blood....and I assure you that's not because my knives are dull. As jason said, I can effectively perform this same test with a straight razor.

For what it's worth, I still like using my fingernail. That gives me a great feeling for what kind of "bite" the edge has, but the finger test gives more feedback on how refined the edge is.

Ok, take all that for what it's worth. I'm still learning. :)
 
Joined
Nov 20, 2008
Messages
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Sometimes I use my knife to open envelopes at the office it makes me think that all papers are not equal. Some envelopes will cut open like the fibers voluntarily jump out of the way and others I have to struggle with; that is with a not-so-sharp knife, when the knife is good and sharp I rarely have to struggle with any of them.
 

Jason B.

Knifemaker / Craftsman / Service Provider
Joined
Jun 13, 2007
Messages
11,089
I've tried the 3 finger test a number of times, but my results aren't consistent. When I sharpen to a middle grit edge (1000 JIS or Spyderco medium) the blade seems a bit sticky. I can watch my finger pads move with blade. But when I go any higher (or with a dull knife), it just seems to glide over my fingers.

This is a weird thing because psychology plays a role. I don't want to slice my fingers open, so I'm not pressing with any more than just tiny pressure. Murray says really weird things about this test like, (paraphrasing) "buffers make a blade dull like a ballpoint pen and fail the 3 finger test. They will shave and slice paper, but they aren't sharp."

Frankly that seems like complete BS to me. I think he's just going for a toothy edge that bites skin. OTOH, he's the "expert" and he should know, right? My blades that glide over my skin do shave and easily slice phonebook paper. Am I missing something that he is able to do that I can't? Or is a finer edge just not able to bite into skin and "stick" with the 3 finger test?

This has been on my mind for many, many months now, so I'm very glad someone brought it up.

If a finer edge isn't sticky, how do you (Jason, knifenut) use it to appraise finer ground edges? I'm asking Jason, but I'm also asking anyone else with experience in this.

Thanks guys,

Brian.

I use my thumbs mainly until they get too cut up then I switch to the 3 finger method :)

If you can move you fingers over a sharp edge regardless of grit then it is not truly sharp. I finish razors on a 15k natural stone and can still test with my fingers.

Truth is you should be using some pressure, going too light against the edge will not give you the proper feedback. It's basically resting your fingers on the edge then maybe a bit more, just enough to "feel" the edge. Once you start making the slightest of movements you should feel the edge bite the skin and you can "measure" the sharpness/refinement level. Coarser edges will bite more while finer edges give a almost burning sensation.
 
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