Testing Sharpness with the Thumb

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well...i just feel that i dont need to cut things if it's not really necessary. i'll test my edges once in awhile on paper or whatever, but not every time.
 
About the only thing I use thumbing for is lateral thumbing to feel for a burr, and even that is unreliable.

For example, the sharpest edge I've seen was a 'stropped burr' edge on a soft Buck blade. The burr was thick enough to strop without bending. It failed the thumb test, since it was not bent to either side. Beautiful edge that would push-cut a ripe tomato, but collapse immediately with any serious pressure on the edge.

I use newsprint-slicing for checking progress during honing. It's a beautiful media for testing sharpness - very uniform media, visual feedback, audible feedback and pressure feedback. But it only tests the single parameter of sharpness. Nothing is established about how serviceable the blade is for the full range of cutting tasks normally encountered.

For example, if your blade is used for slicing everything from envelopes to hardwood, then perhaps hardwood-slicing should be enrolled in your testing:
- Hone, newsprint-slice-test and repeat until it's 'sharp'.
- Take a few full-edge full-pressure slices of something like a hardwood broom handle.
- Re-test for sharpness.

That's pretty much the method I use when determining the best bevel/microbevel combination for a new blade.
 
Cliff: Try a "zero angle" cut on the side of your thumb. You can touch the side of your thumb tip to the blade at a zero angle. The knife is held in the right hand, and the right thumb is used. As soon as the thumb is moved (I mean instantly), an extremely thin bit of skin should be removed from your thumb. Depending on which direction the ridges in your thumb print run, you may be able to produce 3 or 4 curls, which are the individual ridges being shaved off. You only need to move your thumb perhaps .03" to perform this test.

If you don't have callouses, I'd find another test.
 
Greg said:
Tell them that knives don't cut laterally--only longitudinally--and have them check the sharpness in that direction (carry band-aids).:D
Greg
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I like this idea.:thumbup:
 
rj martin said:
You can touch the side of your thumb tip to the blade at a zero angle. The knife is held in the right hand, and the right thumb is used. As soon as the thumb is moved (I mean instantly), an extremely thin bit of skin should be removed from your thumb. Depending on which direction the ridges in your thumb print run, you may be able to produce 3 or 4 curls, which are the individual ridges being shaved off. You only need to move your thumb perhaps .03" to perform this test.
Click to expand...

Can all of your blades so that R.J. irregardless of the steel or the sharpening angle, the Pathfinder for example?

-Cliff
 
Cliff: The Pathfinder is a slightly different animal, because it's made for heavy, heavy use. But, if I were to sharpen it, even with the thicker edge and steeper bevels, I believe it would be able to shave skin off my thumb.

If you own one, send it back to me with $30 to cover FEDEX and I'll sharpen it for you the "RJ Martin" way.
 
Not yet, but is on a list of knives I have to buy. I severely doubt you will be able to pry it out of my hands though once obtained.

-Cliff
 
Cliff: I retract my original statement. I KNOW I can shave the fingerprints off my thumb with a big, heavy duty knife like the Pathfinder. I'm making Black Birds now, they have a 10", S30V blade at about .040 thickness, and I have shaved skin with every one as described.
 
Nice to know. Nice looking knife by the way, not of personal interest, but looks like a clean execution of that design.

-Cliff
 
I have used the thumb method before, although I do not know if it is the same as is being discussed here. I do not try to 'feel' if the edge is sharp; I run the knife on my thumb pad almost parallel and see/feel if the edge slices a thin layer of skin off of my thumb. Does anyone else here do a test similar to this? I find that a highly polished edge slices almost effortlessly, while a course edge is more grabby (but still slices well if sharp).
 
I like to use the filter of a cigarette to test the sharpeness of my blades.
I hold the cigarette with one hand exposing the filter.. the. I try slicing a 1/8 of the filter off the end.. if you slice it all the way off and it's smooth.. It's extremely sharp. if the fibers fray, then your not finished just yet.
When I sharpen my big blades I like to use a wire wheel to knock off the left behind medal that hangs to you blades edge..I do it back and forth on both edges until I don't see the rolled edge anymore. And then it's been a lot more sharp than it was. A wire wheel on my bench grinder..ha go figure.
 
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