How sharp is sharp?

G

geohans

Joined
Dec 24, 2013
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How do you test your blade for sharpness?

I'm fairly new to good quality knives, and just got a good blade from a reputable mfr. My usual amateur "scrape your thumb over the edge" test of the factory edge was not terribly impressive; my Emerson does better. But the geometry of the blade is entirely different, so I don't know if that's meaningful.

I tried slicing single pieces of paper, hand held; the results are very dependent on the exact angle of attack into the edge of the paper.

So-- other than a microscope, or "does it cut what you need it to cut?", how should I test the edge for sharpness objectively?

Thanks.

BTW 6in S35vn hollow grind blade.
 
I'm no expert but... I use my arm hair and magazine paper. If it can cleanly remove hair without pulling and push (push not slice) magazine paper, then it's good to go. Many people have a much higher definition of sharp but that's how I go about it.
 
A knife is really sharp when I can push cut phone book paper or I can wave as I push cut magazine paper. Check out some Rocksteads.
 
A blade is at the correct level of sharpness when it does the job its designed for, without failing and can maintain that level of sharpness over an extended period of time. In other words; don't use a scalpel to chop wood and don't use an ax to remove someones kidney. :)
 
Push cut and wave cut phonebook paper with no snags is good enough for me. If I'm feeling ambitious I have a giant industrial supply company catalog with paper noticeably thinner and flimsier than phonebook paper.
 
How long is a piece of string?

Sharp can best be defined as a perception of your past experiences. If your past experiences involve a lot of plastic knives then a steel edge even remotely sharp would be impressive. If you are used to using straight razors then even very sharp knives might seem dull to you. As a solid baseline I recommend purchasing a quality utility razor, its a very thin and highly refined cutting edge that will give you a good standard for sharpness.
 
If I can slice brawny paper towels I'm good. Or phone book paper silently (any knife cuts phone book paper, quietly takes a sharp sharp edge).
 
Sharp is subjective, I've talked to some guys who think they have sharp knives because they can cut printer paper without tearing it.

Some guys think catching a few hairs on the arm is sharp, some cut toilet paper or use the hanging hair test.

I get my straight razor to the point where laying a hair on the edge will cut it in half with a very satisfying pinging sound, for me, that is sharp.

For pocket knives, I run the blade across the top of my hair and I can feel hairs popping off, a quick look on the side of the blade shows how it cuts. If I feel the blade dragging during the test I know I'm whittling hairs and if I feel no pulling then it's almost sharp to shave with.

A sharp blade down the arm won't catch on hair and you won't feel it tugging, this is true for pretty much every grit especially 400 and up.

One thing to keep in mind is the angle of the edge and geometry of the blade, not everything will excel at the same cutting task and personal preferences need to be taken into account.

The generic answer for "what is sharp?" Is that a blade is sharp when the edge bevels cleanly apex.

Some things are counter intuitive, I've found that a 1k finish on a knife can cut a loaf of bread with no squishing and no or very few crumbs compared to my bread knife. I've also found that polished edges go right through tomatoes, contrary to what I've heard so many say about wanting a toothy edge.

Find what works and strive to improve.
 
If you can shave with an ax, its not ground correctly to be used as an ax. If a tomato slicer is ground so it can be used as an ax, its not ground correctly either.
As a knife maker I don't see sharp as the issue as much as I do having the correct edge grind. Once the grind is correct; take it to its maximum potential with your preferred sharpening technique. If you approach sharpening with this mind set, your blades will perform better and for a longer period of time.
 
Fred.Rowe said:
If you can shave with an ax, its not ground correctly to be used as an ax. If a tomato slicer is ground so it can be used as an ax, its not ground correctly either.
As a knife maker I don't see sharp as the issue as much as I do having the correct edge grind. Once the grind is correct; take it to its maximum potential with your preferred sharpening technique. If you approach sharpening with this mind set, your blades will perform better and for a longer period of time.
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I think he is asking for a good test for a KNIFE that will be carried and used for a variety of standard tasks. I certainly don't think I've ever seen a hollow ground S35VN axe.
 
I think the urban legend of being able to cut a silk scarf with just the weight of it with any blade is sharp.
 
If it'll cleanly slice phone book paper that's plenty sharp for me. I can't imagine needing my knife any sharper than that.
 
bpeezer said:
I think he is asking for a good test for a KNIFE that will be carried and used for a variety of standard tasks. I certainly don't think I've ever seen a hollow ground S35VN axe.
Click to expand...
I understand the question but this point is valid; whats it being used for. I grind a lot of blades for many purposes and that is the question I always ask; "what are you going to use this knife for, from that I decide what primary bevel, what material thickness and what secondary bevel is needed. I understand the fascination with sharp, but its not the place to start, its where you end up. Many on this forum know more than I do about edge perfection, but I've made a thousand knives and I understand cutting things.
 
Much has been said that I agree with, however in the interest of adding something new I'll just say that i'm satisfied when the edge feels suitably "sticky" with extremely light contact from the fingertips.
 
Yeah, you pretty much nailed it, there's a reason no one uses tactical kitchen knives and a reason so many people compliment Opinels on their slicing. I do have to add for clarity that an axe can be sharp enough to remove hair cleanly and still have the right geometry.
 
Fred.Rowe said:
I understand the question but this point is valid; whats it being used for. I grind a lot of blades for many purposes and that is the question I always ask; "what are you going to use this knife for, from that I decide what primary bevel, what material thickness and what secondary bevel is needed. I understand the fascination with sharp, but its not the place to start, its where you end up. Many on this forum know more than I do about edge perfection, but I've made a thousand knives and I understand cutting things.
Click to expand...

I have no doubt that you understand cutting things, the only reason I said anything is because the OP specifically said besides "does it cut what you need it to cut?" That being said, there is nothing wrong with taking a blade to a higher level of sharpness than necessary for light use tasks. I take every one of my EDC blades to hair whittling sharp. They definitely don't need to be that sharp, but their performance doesn't suffer from being taken there.

Like most here, I think that a simple paper test will tell you a lot about an edge. The same paper can be used for different levels of sharpness, based on sound (as others have mentioned). And if the knife struggles to cut paper at all, I believe it's not sharp enough regardless of intended use.
 
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