Verifying edge sharpness?

N

Naphtali

Joined
May 20, 2002
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148
Please describe an effective process to verify sharpness of knife edge without a physical "cutting" test.
 
An even Apex width of less than 100 nm without a burr verified on an electron microscope with the right macroscopic geometry of the blade - maybe ?!
 
Three finger sticky test and close visual inspection. Rest edge on thumbnail and see how easy it catches.
 
The 'Razor Edge' company used to sell an edge tester. It was just a plastic rod, similar to a cheap ball-point pen (which works just as well.) You place the edge on the rod at various angles to see where it catches. Quite like doing the same on a thumbnail, but with more constancy.


Stitchawl
 
It's this simple? For years I've been using my thumbnail as a test for some cheaply made kitchen knives, but I never trusted something this crude as being useful for hunting knives and other fine cutlery.
 
Naphtali said:
It's this simple? For years I've been using my thumbnail as a test for some cheaply made kitchen knives, but I never trusted something this crude as being useful for hunting knives and other fine cutlery.
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I actually rely more on how an edge feels to me when I touch it and how it looks to me under strong light than on any other test.
 
I'll have to practice that. But I think if I moved my fingers that much on my mnandi right now, it would be the three bandaid test lol!

Thanks y'all
 
Naphtali said:
Please describe an effective process to verify sharpness of knife edge without a physical "cutting" test.
Click to expand...

I'm of the belief that a cutting test is the definitive test of sharpness for an instrument made for cutting. ;)

Sometimes a visual or 'feel' test of an edge can be deceptive in showing how sharp an edge is. And it can deceive either way, good or bad: a highly-polished edge that doesn't 'feel' sharp on the fingertips, with a lack of the toothy 'bite' usually expected in such a test, can sometimes slice you wide open, and you won't even feel it. Other edges that 'feel' wicked-toothy on the fingertips can often be much blunter when doing real-world cutting (ragged burrs on a thickly ground edge can deceive this way). Some edges will shave hair, but fail immediately after cutting into anything more substantial (paper, wood, cardboard); that's a clue the 'shaving sharpness' was likely just a sharp burr which folded over.

My own mantra is: 'The cutting never lies'. Slicing into the edge of a piece of phonebook paper can show a lot about how sharp an edge is. Slicing SLOWLY into the paper from heel-to-tip will tell you which edge portions are sharp, which aren't sharp (edge slides over the paper), which areas have burrs needing cleaning up (snags), and where possible edge damage (nicks, chips, dents, etc; they'll also slide over or snag) might be. If the edge just falls through the paper almost under it's own weight along the full length of the edge (heel to tip), that's a strong indicator the edge is in good shape. Try the same after cutting into the edge of a piece of wood 2 or 3 times; if it's still slicing effortlessly through the paper then, the edge is in GREAT shape, as it proves the edge is both sharp and reasonably durable (no sharp burrs folding over, in other words).

With more experience, it does become possible to SEE when an edge is as sharp as you want it, as inspecting the edge directly under a bright light will reveal a lot. Any reflected light coming from where the apex should be is indication that more refinement is needed. Over time, after correlating how an edge CUTS with how it LOOKS, you might occasionally know you can skip a cutting test after working an edge on the stones. When you see effectively NO LIGHT being reflected back from the apex, a trained eye can recognize "OK, that's where I want it to be; it's good to go." But even then, it's always fun and rewarding to immediately zip a sheet of phonebook paper in half, or sweep a swath of hair from your arm after already SEEING your edge is ready. :)


David
 
By touch, works for me. But you have to be careful:(. Otherwise, if it shaves cleanly. The ultimate test is whether you can create isotopes by shaving off electrons;).
 
TravisH said:
I certainly don't move my fingers as much as Murray demonstrates. ;) I'm sure he's exaggerating the motion for demonstration purposes.

If it's a sharp edge, you'll know it/feel it with little motion. Indeed, sometimes just pressure is enough.
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I usually just lightly rest them on it and slight wiggle is plenty to tell what's going on. More movement should show the tops of the fingerprints being lightly cut, depending on the edge finish and geometry.

Yes, a cut test is ideal, but then you have to know what type of edge you're shooting for if trying to do so rapidly with a single test, say if sharpening three or four tools in a row. With enough practice, a visual inspection should be enough as long as its an edge finish/preparation that one is familiar with.

Here's a good test, but one I haven't gotten around to doing yet....

[video]https://youtu.be/XSvrIZKI8mk[/video]
 
Naphtali said:
Please describe an effective process to verify sharpness of knife edge without a physical "cutting" test.
Click to expand...

Two processes: As a baseline, examine the apex under a strong directed light source. If light reflects from any part of the apex, it is not sharp. Second is Muarry Carter's 3-finger test, which, with experience, can give a fairly good read on how sharp a blade is without a cutting test.

Of course, slicing and push cutting newsprint are essentially entirely non-destructive cutting tests to an edge, so I don't know why you wouldn't be willing to use them.
 
HeavyHanded said:
Three finger sticky test and close visual inspection. Rest edge on thumbnail and see how easy it catches.
Click to expand...

1. I still can't really get the hang of the three finger test. Only every now and then do I seem to make a "sticky" edge. Even edges that cleanly slice phonebook paper, right off of the gray rods on the sharpmaker. I keep trying, but I sort of feel like I'm missing something.

2. Thumbnail: For the past year or so, I've been using the "lightly cut into your thumbnail" test, as suggested by several Japanese sharpening sources and videos. I found that on MANY edges, blades bite very strongly into my thumbnail when trying to cut into it. Note: When I say "cut into it" I mean sliding it with very, very little pressure across my thumbnail. In no way am I trying to cut.

But I've now found that with highly polished edges, there's no bite whatsoever. It just smoothly glides over the thumbnail like it's on glass. Which is disconcerting, making me think the edge isn't sharp. But these polished edges do all of the polished edge cuts: Phonebook paper, hair, etc.

Finally, I think Martin (Dr. Heavy Handed) is describing the technique of holding the blade at an angle to the thumbnail and seeing if it will stick to it, or slide off, perpendicular to the blade. The closer to zero degrees between the blade and the nail you can get, the sharper it is. Am I describing your test correctly Martin?

Brian.
 
I try slicing the thin paper from the ads in the sunday newspaper. It takes a pretty sharp edge to bite into the edge of the paper. I think shaving tests might be better but I have limited amounts of hair and even more limited amount of skill and nerve.
 
bgentry said:
1. I still can't really get the hang of the three finger test. Only every now and then do I seem to make a "sticky" edge. Even edges that cleanly slice phonebook paper, right off of the gray rods on the sharpmaker. I keep trying, but I sort of feel like I'm missing something.

2. Thumbnail: For the past year or so, I've been using the "lightly cut into your thumbnail" test, as suggested by several Japanese sharpening sources and videos. I found that on MANY edges, blades bite very strongly into my thumbnail when trying to cut into it. Note: When I say "cut into it" I mean sliding it with very, very little pressure across my thumbnail. In no way am I trying to cut.

But I've now found that with highly polished edges, there's no bite whatsoever. It just smoothly glides over the thumbnail like it's on glass. Which is disconcerting, making me think the edge isn't sharp. But these polished edges do all of the polished edge cuts: Phonebook paper, hair, etc.

Finally, I think Martin (Dr. Heavy Handed) is describing the technique of holding the blade at an angle to the thumbnail and seeing if it will stick to it, or slide off, perpendicular to the blade. The closer to zero degrees between the blade and the nail you can get, the sharper it is. Am I describing your test correctly Martin?

Brian.
Click to expand...

Brian, that's it on the thumbnail test. Start at roughly the angle you were grinding at and see if it will catch and at what angle using very light touch. This method has been shown to me by several people and is sort of old fashioned. The mother in law used it back when dental hygienists sharpened their scalers on Arkansas stones (maybe they still do), and the father in law still uses it on his knives and chisels.
 
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