How sharp is sharp?

[MountainManJim]

My tests are:

Time to Sharpen - Fingernail Test: If the edge slides off my thumbnail, it's time to sharpen the knife.

Done Sharpening - Flashlight Test: Using a cheap, multi-LED flashlight, I try to get a reflection off of the very edge of the edge. If I can't get a reflection, I'm done. But, I'm not really done. I'm checking this while sharpening with the medium grit stones. So, this test determines when I move on to the fine and extra fine stones.

Jim

[Nifty_Nives]

My test is pretty simple, but really effective.

I pull a hair from my head thats a decent length. Then i hold the hair tight, and gently run the blade of the knife along the hair, and if it scrapes away a SECOND strand (cutting the strand of hair in two) without cutting all the way through a hair, then i usually say thats sharp. I will also shave sections of my forearm with my knife to see if it is sharp. I just run it along my arm and apply pressure. You may also try peeling layers from paper without tearing all the way through it to test the sharpness of a blade.

[xavierdoc]

Something like this. :)

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Looks pretty sharp, although I'd call that a drag cut (slicing) not a push cut? Try the same thing with the blade perpendicular to the tissue paper and with out sawing or drawing motion=push cut, IMHO.

My video of cutting newspaper is also not a true push cut.

[Ankerson]

Looks pretty sharp, although I'd call that a drag cut (slicing) not a push cut? Try the same thing with the blade perpendicular to the tissue paper and with out sawing or drawing motion=push cut, IMHO.

The blades never moved.

[xavierdoc]

The blades never moved.

I know what you mean- you weren't moving the blade back or forwards; I was just summarising what I think of as a push cut (which may not be the same as other people.)

Try the same test but with the edge of the knife perpendicular/right angles to the tissue paper. It is a much stiffer test of sharpness, though I'm not 100% sure why. I have lots of knives which can pass the test with the blade at a steep angle, but only a few can do it at 90 degrees to the tissue paper.

I'm not having a go, or anything, both tests are valid yet differ.

[unit]

Try the same test but with the edge of the knife perpendicular/right angles to the tissue paper. It is a much stiffer test of sharpness, though I'm not 100% sure why.

It is because at an angle the effective separation angle is lower than the edge bevel angle. In other words, by holding the knife at an angle you can make a 30 degree inclusive edge separate like a 20 or 25 degree edge.

Either way, the knife is pretty sharp and will not retail that level of sharpness more than a few cuts...but that is a few cuts more you can do before needing to return to the stones.

[Ankerson]

I know what you mean- you weren't moving the blade back or forwards; I was just summarising what I think of as a push cut (which may not be the same as other people.)

Try the same test but with the edge of the knife perpendicular/right angles to the tissue paper. It is a much stiffer test of sharpness, though I'm not 100% sure why. I have lots of knives which can pass the test with the blade at a steep angle, but only a few can do it at 90 degrees to the tissue paper.

I'm not having a go, or anything, both tests are valid yet differ.

I would have to profile the edge to about 10 degrees inclusive to do that.

That's not gonna happen on a One of a kind Custom Knife (Read Very Expensive) like the ones in the video are. Both are at 32 Degrees Inclusive.

[Frank Castle]

If it cuts paper cleanly and catches my nail then it's sharp enough.

[SnowCrash]

So I have a question for you guys- when you have a knife that has a razor edge, the finest of the finest, and you do not use it for like....a month. Does it lose the edge at all? Or does it retain the edge until you actually cut something?

[unit]

Hard to say...my knives never seem to go very long between uses.

I would suspect that over a month there would be significant oxidation/contamination to degrade the edge slightly...but that is just a hunch. By "significant", I am really not talking about very much. It takes VERY little to make a noticeable difference in sharpness (considering the silly tests we play around with).

Think about this...if you polish a mirror, then put a drop of water on it and let it dry. That spot left behind is scale (salts left behind by the water). That would be more than enough to foul any edge to the point of failing any of these tests (TP, hair whittling, etc.) I imagine this same process occurs all the time on my blades as they go from warm pockets to cold weather and back (water condenses, or fogs up the edge...then slowly dries).

[SnowCrash]

Ah, that's what I was afraid of. I was hoping science wouldn't exist for once and let me keep an eternal edge if I didn't use it :p

[The Mentaculous]

My standard is just to push cut through typing paper with little resistance. I know I could have a more scientific way of testing, and I'm sure people who spend the time and effort re-profiling and polishing bevels can get theirs way sharper, but as long as I can zip through typing paper with ease, I'm happy.

[dbcad]

Good thread, I'll try some of the techniques found here :)

Sharpened the 3-1/8" D VG-10 Mule again yesterday. In a few minutes I was able to get it so it would cleanly get very thin curlies from a USPS postal reciept with no effort. I have to note that the original edge angle I put on the Mule was a few degrees smaller than what came from the factory. The SM just creates a small bevel on the edge at 30 deg.

It does need to be touched up a little more frequently than other vg-10 blades I have, but i love the performance :D

[Crocodilo]

My test is pretty simple, but really effective.

I pull a hair from my head thats a decent length. (...)

I might have a problem with your method. I recon the longest hair I have on my head are either eyebrows or nose&ears hair... pretty much all the rest gets shaved every morning!

[Evil D]

I might have a problem with your method. I recon the longest hair I have on my head are either eyebrows or nose&ears hair... pretty much all the rest gets shaved every morning!

LOL same here...i'm bald as a baby's butt. Still, there's a different when you shave a few hairs off your arm...sometimes they'll just come off on the blade, other times they'll JUMP off your arm...they actually seem like they're popping off. That's what they call hair popping sharp. That's what i go for.

[Hector Castro]

I test the sharpness of my blades, by cutting those cheap grocery store bags or
Saran wrap. Hold a piece with one hand and push cut through the material. Your knife has to be razor sharp to do this!

[FLYBYU44]

Ah, that's what I was afraid of. I was hoping science wouldn't exist for once and let me keep an eternal edge if I didn't use it :p

Straight razors go dull from lack of use, it's micro corrosion on the edge that breaks it down and makes them dull, I guess a sharp knife would react in a similar manner. My test for sharpness is if it will easily shave arm hair, that's as sharp as I need it. If it can shave facial hair with ease (A few knives I have can) then it is what I consider super sharp.

[dcmartin2001]

Wow! Thanks everyone for contributing to this thread! Like I said in the original post, tuning a blade is as much fun as owning one. Lots of great ideas posted here and I plan to use everyone of them.

[Piet.S]

For my users, I use the hair-popping test. For those knives I have sharpened for the sake of sharpness, hair whittling or free-standing cigarette paper slice is the benchmark.

This video (not mine- Longstrider on BritishBlades) illustrates the latter test.)
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Thanks for this vid.
I have met Ian a couple of times at the Gembloux knife show.
Great guy with a lot of wonderfull stories.
And yes, he does know his bit about sharpening a knife properly.
Quite well, I must say.
He can do this cigarette paper test anytime.