how sharp is sharp?

Since just about everybody, if not everyone, here EDCs a knife or blade of some sort or another... I'd like to ask, how do you test your knife for sharpness?

How sharp is sharp enough?

Thanks in advance,

I use my RSK every day...I like to keep it pretty sharp. Havent gotten it razor sharp yet, but, its still plenty sharp to cut through anything from packing tape, plastic, string, to cutting fruit in half on a daily basis. I usually sharpen it once a month or so-nothing fancy, just use a standard diamond sharpener. The sharper the knife, the less apt you are to hurt yourself (as long as you keep clear of the business end).

I think you need to ask about what you are slicing. A good edge for meat is not a good edge for shaving or wood carving.

The first check for me is if it looks sharp.
If you look at the edge straight on with the light behind you a full edge will reflect a line of light while a sharp edge reflects no light.
Then I see how it cuts. For woodworking any dull spots on the blade show up on the cut surface of the wood.
They kind of go together. When the blade loses its bite it needs sharpening.

Other than that I don't really check edges except by using them.

Edit: I have been known to shave a bit of hair off my arm as a check while it am putting a fine edge on, but that is just the carving chisels and my razor.
I don't think a kitchen knife needs to be stropped and having a bit of tooth to the edge lets it slice meat and tomatoes better.

Personally, I like them sharp enough to pop hairs off my arm. That's not sharp enough to shave my beard with but sharp enough for the basic tasks I ask of a knife.

Same here. I think any knife that slices paper cleanly is sharp enough. You need to strop the edge, buffing wheel with a polishing compound or a superfine waterstone (3000+ grit) to shave properly but for most utility tasks it's just not necessary.

I keep the crappy blade on my Leatherman Wave sharp enough that it "digs" into my thumbnail. That's sharp enough for most tasks I do each day with it.

If I can comfortably touch the edge and not draw blood, it's dull as heck.

I use the paper test as well... If it slices without really "pulling" on the paper, it is good for me.

As long as it goes through butter on a hot day -

Arm-hair shaving. I look like a cancer patient after I get a new knife because I'm showing everyone how sharp it is.

Are we talking a thin edge or a sharp edge? Mine are sharp enough to slice paper without grabbing, it's what I consider a good utility sharp -- don't need to shave with it. I do not thin the edge or otherwise change the blade profile.

hanging hair test

It depends on what I use it for.

When I used to cut a lot of rope, I had a blade that wouldn't cleanly slice through a sheet of paper but would put the hurting on some braided rope. I considered it more then adequately sharp for the job.

The knives I use for splitting bones on wild game are generally the same way. Don't know a way to test them other then rip a few rib cages with em and see what edges work best.

The knives I use for skinning are kept as sharp as possible without stropping.

My general utility/EDC knives are kept just sharp enough to do all my daily tasks without extra effort required by dull blades. As soon as I notice they are getting to the point that they -need- sharpened (usually a few nicks in the blade, and generally just dull) I'll sharpen them up enough to slice paper and let them be for another 4 to 6 months. Rinse repeat.

The only knives I obsess over are my kitchen knives. I don't make them razor sharp, but they receive a touch up on a -very- regular interval.

There is no upper end on sharpness. Sharper is always better. A knapped edge on glass or obsidian is many times sharper than any conventional metal blade and they aren't perfect. But there are practical limits for how much time and effort I'm willing to dedicate to getting a sharper edge.

If a knife edge won't skate on my thumbnail, it digs in, it is sharp enough for most uses.

Materials used for knife blades are a compromise between, toughness, flexibility and shock resistance and the ability to be sharpened to an edge and hold that edge. Most common alloys used for knives won't hold a very fine edge in practical use. Those alloys, or materials, like the ceramics, that are hard enough to hold a very fine edge are often so brittle that the blades can fail catastrophically in use. Some are so brittle they can snap if dropped. Extreme cold can induce similar brittleness in some otherwise practical alloys.

I hold the blade out, cutting edge toward the sky and then toss a silk scarf up in the air and let it slice in two as it falls in slow motion to the floor... oh wait... that was a dumb a$$ movie my girlfriend made me watch.

It's one thing to cut WITH the grain of wood fibers (the old whittled stick approach), but the old woodcarver's used to test a knife's sharpness by trying to cut cleanly ACROSS the end grain on a stick of soft pine. If a knife can do that without digging in, chattering, crushing or popping off little chunks of wood, and leave a clean polished surface in the blade's wake, then that knife is SHARP. It is very satisfying to get a knife this sharp because the blade makes a pleasant "whisking" sound as it slices directly across the wood fibers. It also puts a pleasant piney scent in the air.

The late Donald Crabtree was the dean of american knapping with obsidian. his french counterpart who worked flint couldn't even work with obsidian without considerable practice. The stuff will literally flake to one molecule thick ( I know, I saw one under an electron microscope.)
Crabtree in fact made up a surgical kit for his heart surgeon. the doctor had to practise using the tools, retraining his hand muscles to the greater cutting ability. Crabtree underwent heart surgery, and the finer incisions healed so rapidly glass instruments are now common.
That is sharp. But the advantages of metal gives us different standards. I don't make many fuzz sticks in the wild places out of paper. Testing the knife on what you actually do, like our member butchering game is the best guide.
In the real world, there is sharp,and there is sharp.
An apocryphal story is worth remembering. During the Crusades, King Richard met Saladin ( in reality, they never met.)
Richard drew his great sword and with a thunderous shout cut an anvil in half.Saladin threw a silk sash into the air, turned his scimitar blade upwards and watched the falling silk fall in two pieces.
Meanwhile, I believe several ancestors of contemporary knifemakers were stabbing castle walls.

That story is not just apocryphal, there are two other major fallacies. First, no sword could cut an anvil in half (especially not a medieval European one - plenty of research has been done in that department recently). And second, Saladin couldn't have had a scimitar because it simply did not exist as such at the time. In Egypt and Syria straight swords were still predominant at the time. The saber became popular only a couple of centuries later and the scimitar was a more recent development still.

If the story is that inaccurate I wonder if the silk scarf cutting test makes any sense at all?!

I think it depends on the use. An old style shaving razor might cut through thick rope no problem, the first time and be useless after that.

I have blades with VG-10 and S30V steel and while VG-10 can get noticeably sharper it also gets dull quicker. That must be why Doug prefers S30V in the field.

My sharpest kitchen knives might have a problem with the skin of a tomato but my knives with a a bit rougher edge manage to cut through no problem.

I lightly drag the blade across my thumb - maybe at a 45 degree angle - and if it catches pretty good on the friction ridges then I feel it sharp enough for what I need.

I sharpen my knives using a Spyderco Sharpmaker which does a great job pretty easily. If you're using a Sharpmaker on one of Doug's Mkx knives, don't forget to use the 30 degree holes to maintain the original angle.

KenK, I do hope you mean your thumb NAIL ! Anyway your reply is the closest to what i use but i use a plastic pen and hold the knife at app. 45 degree angle to test the edge then reverse the pen & knife position so as to test "both" sides of the edge. Like the thumb nail it should not slip or its not sharp. Thing is I just recently learned this method from the internet and thus I'm not sure just how good it works so I thought i'd compare the methods here with this one.

No, I mean the fleshy part of my thumb. That's just how I've always done it. I don't push hard at all. Being that its my thumb, I can feel the feeling of the blade catching on the friction ridges of my fingerprint. I can really distinguish between slightly different sharpnesses.

Think of it like you hold the blade while shaving the arm, but its just on my thumb, which provides good feedback.

Thats the way I was taught from my father. You can really tell the difference in sharpness that way. You know its damn sharp when you go to do it like you always have and cut yourself. I now do different methods, but the thumb across the edge is my quick check on my EDC knives if they come into question...

Using a thumbnail to test an edge is pretty common. The actual thumb, less so.

One traditional Japanese test was to slice a slave or prisoner. A top quality blade was reported to be able to cut a man completely in half diagonally with a single stroke. I'm not sure if that is stronger than cleaving an anvil or sharper than slicing a dropped piece of silk but it is certainly a testament of toughness and sharpness.

Given that the police generally frown upon slicing live people in half I think such testing will recede into history and legend.

I use a few methods, one being the "does it shave" method - and different parts of the body take different sharpness

The other is slice paper, but the trick here is a hold a 8.5x11 sheet by one corner, and slice, pulling the blade so that every part of the blade gets tested. If there is a dull spot, the paper rips instead of cutting. BTW if you want to test for REALLY sharp, get thin "tissue" paper. Not the stuff you blow your nose on, but the stuff they use to wrap clothes when they are putting them in a gift box. The thinner paper makes it harder. Old phone book paper is also good, nice and thin

I saw an interesting treatease (sp?) on the difference between Japaneese and "western" swords, and the person pointed out that western sword makers COULD make swords that could be made as sharp, but they had a serious problem agains plate mail and even chain mail. The HUGE problem (and the whole reason for the diferential hardening of a traditional "Eastern" sword) was that they were VERY high carbon, and the edge had a real tendency to chip. In fact, he pointed out that if you look at REAL "Samurai" swords, you will almost always find chipping on the edge. This hardness issue lead to a different style of actual use, instead of "hacking" at your opponent, you tended to make a slicing cut, and the whole technique taught to use the sword is to make the sword do a pulling/slicing cut, and to lessen the initial "impact" of the blade on the target

Not to hijack the thread but the above is not entirely correct. It's true that the fighting style in Europe was somewhat different. There were many different styles of swords used in Europe, some highly specialized and some "all-purpose". Up until about the mid-13th c. most European swords were relatively thin, excellent cutters. Later on as armor improves, swords become longer and stiffer and there is more emphasis on thrusting.

Contrary to the popular belief, medieval European swords were for the most part quite light, no heavier than a katana per comparable blade length. Also, they were not meant to actually cut through armor. If you had to fight an armored opponent you'd ideally only use the point and target the weak spots in armor. But the sword was not in fact the primary weapon against armor, there were better, specialized tools for the job (mace, war hammer, pollaxe etc.).

The trick is, most European swords were either slack quenched or quenched and tempered to a relatively low hardness. In effect, the edge wouldn't last that long but the blade was very springy and tough.

The Japanese on the other hand never really figured out the tempering process. Hence they used clay and used a complex method of patternwelding, taking soft iron for the back of the blade and steel for the edge. A very similar process was known in Europe but was no longer used after the early middle ages because it wasn't cost-effective and you could get better results with more homogenous steel.

As a result, the edge on Japanese swords was fully quenched and very brittle while the back was very soft and tough - but not actually flexible because in effect it wasn't heat treated at all. It would bend under heavy stress rather than spring back. So metallurgically speaking, it was far from an ideal solution. The only reason why it worked is that the Japanese used lighter armor, largely lamellar made of hardened rawhide. A 15th or 16th c. Japanese sword would've been fairly useless against a European plate harness. Of course, it could still be used with great effect against an unarmored opponent. But so could a European sword from the same period.

Metallurgically, the very best medieval swords may have come from Iran and India. In those parts of the world relatively homogenous steel was already known in the antiquity, hence the famous Damascus blades. Billets of crucible steel were also exported to Europe. They were very expensive because of high production cost but could be forged into excellent blades.

For my really sharp blades, I like to see them cleanly cut a styrofoam cup without a lot of sawing motion. But those are generally only the smaller secondary blades on multi-blade pocket knives. Most knives just need to cut paper cleanly before I consider them sharp enough.

Sharp enough to roll cut completelythrough a piece of 12 oz leather I have. Another good test is to split shave the cover of a typical magazine so that I can pull a layer off without penetrating it completely, like splitting the thickness in half.

If I'm going for ultimate sharpness, I'll get a piece of newspaper or phone book, and hold it vertically in my left hand, from the top. With the knife in my right hand, I'll try to push cut straight down, measuring how far from the pinch grip my left hand has. Can't cut away from my left hand, has to be straight down, no slicing. My best is 4 inches.

If modern technology can detect the finest point of the edge, it is not sharp enough.
But seriously.
for an EDC knife i try shaving my arm. If, as is often the case, I've exhausted that method, i just use my thumb like KenK. For larger blades like a machete or sword i fell a small sapling. If it cuts through in one stroke leaving the cut face smooth, I'm satisfied.

One thing I don't care for is constantly working the edge of my carry knives after nominal use. Even with my culinary stuff, I don't like the notion of touching up the edge with anything more than an occasional swipe on a steel to straighten it back out. Unless I intend to cut rocks all the time (in which case I have a good chisel set for that sort of stuff), I shouldn't need to keep working the edge on a stone or a piece of carbide or my diamond wheel.

Once I set the edge with my diamond wheel and rouge polish it (in most cases), that edge should last me at least a couple months of regular use without having to sharpen it again. Right now my Leatherman is about as sharp as it was when I put the wheels to it last August, and that means it is still sharp enough to scare me a little. I admit I've only cut with it maybe a dozen times or so since then. However, the little Gerber Shortcut in my pocket gets used at least once a week, and I haven't sharpened it since I got it last September.

I've found that for crosscutting tree branches, heft is at least as important as edge. I wouldn't expect my Cold Steel SRKs to go through branches anywhere near as effectively as my Busse Battle Mistress, and I know the SRK has a sharper edge to it.

There have been more than a few people I've met who are bit OC about keeping an edge on their knives. Grandad was one of them. In his case I think knife sharpening was his "Captain Queeg" process, probably from sitting around the fire with nothing better to do on most nights.