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Testing Sharpness

Sun Jul 06, 2014 8:04 am

The title is pretty self explanatory. I want to steal your sharpness tests. I'll start by listing a few I use.

-Practical Exam: The optimum method. Cut something you would normally cut.
-Thumb Test: Draw a thumb from the side of the knife over the edge. Should feel tingly and even; bumps indicate small blemishes.
-Shaving Test: Simple enough. Use the knife to shave a patch of hair. This test eventually leaves one arm looking like it had mange. A knife should be able to take off hair after a 1k grit stone, if not sooner.
-Paper Test: Holding a piece of paper in the non dominate hand, attempt to slice and then push cut part of the paper. Clean slices are very easy, clean push cuts much more difficult. I use newsprint, but I've heard other types of paper have merit and would like to hear what you all use. A knife should have some degree of success with this test off the first stone. Off the final stone it should be able to make confetti.
-Fingernail Test: Carefully and gently draw the knife down a thumb or fingernail at a low angle. It should catch. I've heard lower angles indicate better edges, but is like to know why people say that. I don't use this one often.
-Three Finger Test: I'm not telling anyone to effectively grab a freshly sharpened knife blade. I mention it for completeness but will not endorse it because blood stains everything and accidents happen.

I did a little looking and I couldn't find a thread along these lines. If there is one, feel free to merge me.

Re: Testing Sharpness

Sun Jul 06, 2014 2:08 pm

"-Fingernail Test: Carefully and gently draw the knife down a thumb or fingernail at a low angle. It should catch. I've heard lower angles indicate better edges, but is like to know why people say that. I don't use this one often."

I think this is used to ascertain whether or not you have an edge. If you have an edge, it will catch on you nail. I would not slide the knife so much as set in on the fingernail.

I would not think the angle of the nail informs the quality of the edge, rather the final apex angle of the edge. If the edge is sharpened at a high angle, you will not make contact with the edge at a more acute angle on the nail regardless of the quality of the edge.

There is another variation on this one, draw the knife edge along the nail tip very carefully. The goal here is to locate chips in the edge.

I have also seen Ken use a rolled up paper towel as a test.

Otherwise, you listed most of the tests I use.

Re: Testing Sharpness

Sun Jul 06, 2014 3:43 pm

I've never seen the rolled up paper towel test but it sounds useful.

Re: Testing Sharpness

Sun Jul 06, 2014 3:56 pm

Ask JEFFB, for his phone book video...

Re: Testing Sharpness

Mon Jul 07, 2014 12:57 am

I tend not to use my body parts for testing, only having just so many of them :)

Fingernail test. Not useful to me. All of my edges easily pass this test so why bother. Too easy.

3 finger test - while I don't use it, it tends to favor a 6k edge that has been deburred by running the edge through something. The microserrations left by removing a burr this way is the toothiness that gives it a passing grade. A more refined edge just cuts and doesn't warn. A serrated knife will pass this test quite well. A good 10k edge has trouble with this test.

Drawing the edge of the nail is useful for detecting chips that are hard to see without a loupe. Quick and useful. Not a sharpness test.

Using the edge for the task specific purpose intended - yes. Not always convenient and I would have too many piles of veggies doing this all the time :)

Shaving test(s). Just shaving arm hair is only slightly tougher than the fingernail test. Too easy. Shaving arm hair with the blade not touching the skin - hair whittling and hair popping (can't whittle because the hairs 'pop' before they can be whittled). Cutting the hairs without bending above the skin. Interesting tests and of use, but I rarely use because I don't like using my body parts for testing and want to at least appear somewhat normal to the public :) There is also a WIDE variance in hair thickness.

Mostly I use push and slice cutting paper and use copy paper. An Honest push cut - no acceleration straight down - and measuring the distance from the pinch point is quite useful and easy to perform. Must be a DRY blade. 2 inch push cuts are quite sharp in anyone's book and 1/2 inch is reasonably good. You can use zig-zag cigarette papers, phone books, etc - all different. Pick something you can get a lot of and standardize on it.

Toilet paper - for scissors, can it cut a single ply of 2 ply toilet paper?

Paper towels - this is a structurally weaker paper and favors testing for toothy edges. Slice cutting holding only one side is a useful test. A rolled up paper towel tests for a toothy edge nicely - it should cut through the roll with only the weight of the knife upon it. Not toothy enough and it glides over it.

Dumbest test I know is to test for a 'wire edge' holding a blade perpendicular to a moving leather belt on a belt grinder. Popularized by a guy who has no idea what a wire edge is, but considers himself an authority on the subject. One guess who that one is LOL.


Re: Testing Sharpness

Mon Jul 07, 2014 6:07 am

Umberto? :mrgreen:

Re: Testing Sharpness

Mon Jul 07, 2014 7:55 am

Thank you, Ken. Your illumination on paper tests is surprisingly helpful. Any further insight on shaving without being in contact with skin? I should think there would be a base minimum grit to expect to pass.

I should add that I don't use most of these myself, sticking almost strictly to drawing my thumb over the blade on other people's edges and paper and product tests on finished edges. When I started sharpening the shaving test was valid, but if you always pass it, there's not much point in it. I just wanted to list them to get them out of the way.

Umberto, no, hah. This persona non grata has a much more epic story behind it. I did that research one night when I couldn't sleep. It was surprisingly tough info to find and slightly discomforting to read.

Re: Testing Sharpness

Mon Jul 07, 2014 3:52 pm

The initials D.M. come to mind. ;)

Re: Testing Sharpness

Mon Jul 07, 2014 6:08 pm

Try thin catalog/magazine/circular pages for testing, the thinnest ones, I use like them because they have just enough stiffness to not flop around, but are weak enough that they are crushed/folded very easily by a dull spot of a chip.

I've tried setting my blades down on my finger nails, but then I just I ended up walking around with a bunch of little lines going all over my finger nails for a while.

Ron wrote:I've never seen the rolled up paper towel test but it sounds useful.

I've noticeably dulled sharp edges using this method, like testing edges between final sharpening steps... Case in point: for a multi-day hiking trip I wanted a SMALL roll of papertowels - solution? Cut a standard roll in half. Took my gyuto from being ready for a touch-up sharpening to utterly dull (it was my Kono HH, Gyuto version of this, which has rather good edge retention).

Although if you're okay with re-sharpening, I did find a really good method for testing/comparing edge retention.

Re: Testing Sharpness

Mon Jul 07, 2014 6:34 pm

This ones for Mel ;)

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