Objective Sharpness Testing?

O

Oakengroves

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Sep 4, 2012
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I'm out of arm hair and did not have much luck digging through the forums for information regarding a fundamental question:

What is the best, objective, test for sharpness to determine the relative quality of an edge?

We've all tried any number of sharpness tests: (1) shaving arm hair; (2) cutting printer paper; (3) cutting single ply toilet paper; (4) cutting paper towels; (5) cutting newspaper; (6) cutting magazine or add paper.

Which of these common tests is the best benchmark for testing sharpness? Many of the sharpness tests above appear incredibly relative and it is difficult to distinguish between "this knife is sharper than that knife" on many of these tests (i.e.-even some dull knives will slice printer paper).

Are there better (simple) tests for objectively testing sharpness?
 
Microscope maybe?

knife60.jpg
 
Phone book paper is the best IMO. Push cutting with no snags is what your looking for. If its a freshly sharpened knife on a system like the WEPS, then hair off your head is the best sharpness tester. Cleanly cutting a hair in half under its own weight is what your looking for.
Tissue & toilet paper wont tell you much because no 2 brands are the same.
 
I use push cutting phone book paper as my test. I can tell how sharp it is by the sound of it and how much drag, I guess, I get when push cutting. I only free hand sharpen and have never attempted to whittle hair or tree top hair. I've never had a edge sharp enough to cut free hanging paper towel. I would imagine if you could hold the paper towel and cut it like shaving slices of phone book paper your edge is light saber sharp. I'm still fairly new to sharpening and learn more with every knife.
 
Ditto, for phonebook paper. Every blade that I've been able to make push-cut and/or slice individual sheets cleanly and repeatedly, has also performed to my expectations in other 'real world' cutting tasks. Sometimes not quite shaving, but most are. An edge that can do this, slicing from heel to tip cleanly and repeatedly without snagging or slipping, is in good shape. Even very small, very fine imperfections like burrs/chips/dents or just incomplete apexes will be revealed by watching how the edge propagates through the paper during a SLOW slice, from heel to tip. I've never been disappointed in 'testing' my edges this way (save for blades of cheap steel that take a good edge, but won't be durable; but that's not any fault of the testing method). Knives that don't pass this test won't be going into my pocket for EDC until they do.

I don't like relying on shaving/hair-whittling alone, because that'll only tell you about the apex at a very small portion of an edge (in direct contact with a hair), and also won't distinguish between a fully-sharp edge and a fine burr or wire that's doing all the shaving, but folds under anything heavier. The phonebook paper always finds those flaws; they can't hide if you're paying attention.

Shaving with a new edge is always fun and rewarding, but I consider it to be secondary to proving the edge is 'real world' sharp and durable first. If it still shaves after checking for that, it's gravy. :)


David
 
You can tell quite a bit by using newspaper or similar. I usually start out at a diagonal and then make my cuts more and more straight across the grain. You can tell even more by noting the sound its making and amount of hitching etc. You can even use this for very coarse edges that are well done. While they probably won't be crosscutting the paper with a push, you can still tell plenty by how little pressure it takes to draw cut across the grain.

Another test is to do a shallow swat at hanging paper towel or TP using just the belly, no tip. If it parts easily that's good 'bite'. Turn the paper towel sideways and see how it cuts with a push or shearing cut.

None of these will tell you a thing about longevity, but with some experience you'll have a pretty good idea of how you're liable to do in real use.
 
Newspaper or phone book pages will give a very good indicator if you can slice clean with no drag. But I like to use the old fashioned hairy twine. The yellow stuff that has lots of fibers sticking out all over. If I can slice slowly into the twine, and feel the edge biting into it, and going clean through with no sliding, then I'm happy with it. I don't like any edge sliding on fibrous materials.

Once you get an edge sharp enough to push cut phone book pages, and do clean cuts through the old hairy sisal rope/twine, then that is good enough to do whatever you need to do with a blade. You can take it farther, but to what end? After a certain point, it gets kind of pointless.

Carl.
 
I just use copy paper and news paper. I think copy paper works fine, I can tell if my knife is sharp by the way it feels going thru the paper, how clean and how fast.
 
I'm proud to push cut phone book paper. Sharpened my new back pocket this evening. Push cuts yellow pages along the whole edge. Took one of the wife's hairs ( not on her head ) and could cut it off hanging with a swipe about four places up the blade, it still wouldn't whittle hair freely but if I helped it very carefully it would a little. This was following the factory bevel about 22 deg per side. Now how will it wear?
 
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