How Sharp is Sharp?

J

Judge_Blackhawk

Joined
Jun 9, 2000
Messages
109
I have several knives that are sharp, they will shave hair off my arm, which my wife finds beyond funny. She says I look like I have some kind of hair disease.

Anyway, I sharpen them with a cheap Wal-Mart stone by hand and they cut everything that I have tried to cut.

It seems that most of my knife friends have this obsession with SHARP knives.

I love and baby all my knives. My wife says when I am sharpening them that I am really making love to them. My edges are all even and have a nice bevel. I am proud of my edges, but one of my buddies to my knife (benchmade Pinnacle) and said the edge was not sharp enough.

So I ask you, how sharp is SHARP? Is there different levels of hair shaving sharp?

Well thanks for the information.

Judge Blackhawk

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Government's ability to
ABUSE and EXPLOIT is directly related to it's ability to DISARM it's LAW ABIDING citizens. - Judge Blackawk
 
Judge,
I use a Spyderco Sharpmaker to sharpen with and it does a great job. A knife can shave hair at different levels. Shaving sharp means that the knife will cut the hair with slight pressure. Hair-popping sharp is a level above and means that the hair flies up as it is cut, sorta pops up. Of course an edge like this will usually dull a little quicker from that level of sharpness. Hope this helps ya.

Art Sigmon
 
Judge-
you will shortly get better replies than mine, but, to give you something to start with...absolutely there are different degrees of 'razor sharp' i recently received a BM mini-afck and an LCC from microtech and while the afck was 'hair shaving' sharp right out of the box the microtech was the sharpest knife i have ever seen. it will shave hair on my leg even if i hold the edge of the blade above skin level.

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Someday, we'll look back on this, laugh nervously and change the
subject.
 
Knew I forgot to mention something. The LCC is the sharpest out of the box knife I have ever recieved. This knife is hair-popping sharp. I mean the hair just jumps off as the edge passes over.

BTW my wife also thinks it funny that I go around with hairless parts. Even asked me if I had any unresolved sexual issues!

Art Sigmon
 
Shaving hair smoothly is one common, traditional measure of sharpness, considering how often people use razors. Another that I use is how an edge feels, where a nicely sharp edge gets 'grabby' when one runs ones fingers across the edge. Such an edge is what I call 'unforgiving' as it effortlessly and sometimes unkowningly cuts when one misuses it. For instance when I use a Mora knife on a hurried project I sometimes notice stings when I wash up, due to a small cut here or there that I hadn't noticed when it happened.
 
I consider a knife as sharp if it passes a combination of tests. One of them is the hair popping issue. Another one is if the blade stucks on your thumbnail in an angle of about 10 degrees. The third one is if the knife catches your hair as you sweep it downward your backside of the head. The third is the toilett paper test. And so on. If the blade is sharp enough, it will stand all those tests (and some more
smile.gif


[This message has been edited by dePaul (edited 09-16-2000).]
 
I while back Picked up a stag handled nickle silver Buck 110 with a Pattern welded (damascus) blade. The edge was not very sharp. However the welding was very good so I pulled out the 1000/6000 Japanese waterstone and went to work. The 1000 put a nice kitchen edge on it which would shave hair off your arm, this was OK but, I was looking to see just how sharp I cold make it.

I went to work on 6000 side after many hours the edge had the finish of a mirror. But was it sharp?

Time for the shaving test. When I ran the knife across my arm I noticed that the edge was Grabbing (snaging) the hairs and cutting them and I was not even touching my skin. If I laid the knif aginst my skin not only did it shave the hair but took some skin to.
eek.gif


Next test was to have my wife through an old silk shirt into the air and try to cut it as it floted down. It made about a 10" cut on one side and a 6" cut thruogh the other side.
biggrin.gif
The cuts were clean with no tearing.

Shortly after my wife was closing the knife very slowly when the backspring took over and cut through about 80% of one of here fingers.
eek.gif
Luckly it was only the tip but since then I keep the knife tucked away. It's actually to sharp for most people to safely handle.

I found that for every day cutting if the knife can shave hair with just a little
pressure this is good enough for most cutting chores. And this will give you good edge retention ware as the super scary sharp knifes tend to dull rather quickly.

Hope this is of some help.


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How sharp is it? It's sharper than the small end of nothing sharpend to a point.


 
Here's a test that I saw on the Himilayam Imports forum. Loop a ruber band around the blade. Stretch the band away from the edge then let go. If the knife is dull it wont cut the band. If the knife is okay it will cut the band into one piece. If the knife is really sharp the band will be cut into multiple pieces (I assume the more pieces the sharper the blade). This test may not be the most definitive but its a lot of fun to do.
 
I'll ask this question again because maybe there are some new people here who can answer it:

I'm tired of people telling me qualitatively how sharp their knives are: it's "really sharp", it's "awesome sharp", it's "scarry sharp", it's "shaving sharp", it's "hair poppin' sharp". There's got to be a quantitative way to measure edge sharpness.

When the military buys a new airplane, they don't say it has to go "scarry fast", fly "really high", and care and "awesome big load". No. They use quantitative measures. They say it has to go X MPH at Y Feet MSL, carrying Z lbs. of cargo.

I can't believe that when the military buys knives the contract reads: "scarry sharp." There has got to be a quantitative measure.

It's not just the military either. Anytime a business buys something from a supplier, there has to be a specification and that spec has to have quantitative measures. Many of the knife companies we all know get some or all of their blades and even assembled knives from other companies, often Asian sources. When CRKT, for example, goes to one of their Asian sources to have one of their designs manufactured (sorry to burst your bubble, but CRKT does not manufacturer their own stuff. I drive by their "plant" frequently and it's just an office.) they have to give that manufacturer a specification, often a blue-print of what they want. That specification, those plans detail everything about the knife. The length, for example, is specified. It doesn't say, "really long". It gives an exact measure down to thousandths of an inch. That specification becomes the core of the contract between these two companies. CRKT agrees to buy the knives produced if they meet the specification. If the knives arrive and they're to short, CRKT can measure them and say, "Look, the spec clearly says that the length measured from this point to that shall be 3.250 +/- .005 inches. Here is our caliper which was just calibrated tracable to the National Institute of Science and Technology, see the little sticker right there. We can measure that and see that these knifes are only 2.905 inches. The contract says we can reject the lot of 2% are out of spec. We measured ten pieces out of this box of 500 and the longest one was 2.917". Therefore, we are rejecting this lot." The contract works the other way too. If CRKT sends a shipment back, the supplier can say, "What's wrong with these? We've rechecked everthing and they are exactly according to the spec." If CRKT's answer is "we just don't like 'em," the supplier can say, "Wait a minute. We have a contract that says that if we made these knives according to this specification, you would buy them. We made them, now you have to buy them."

As much as such a specification spells out in quantitative measures exactly how long a knife shall be, how wide it shall be, what materials it shall be made of, I've just got to belive that such a specification must also spell out how the sharp the knife must be. That's one of the most important things about a knife.

If CRKT gets a lot of knives from one of their suppliers that are way dull, then they've got a problem. They can't sell 'em, or maybe they'll have to sell 'em for a lot less or pay someone to sharpen 'em which will reduce CRKT's profit. To assure them that they're gonna get a marketable product, CRKT must be able to reject a lot based on sharpness. That means that there has to be a quantitative measure. "Scarry sharp" is not something you can measure. Even "shaving sharp" is not something you can reliably measure.

My question that nobody seems to be able to answer is this: how does the military specify the sharpness of knives they buy? How do companies like CRKT and others who out-source blades and knives specify sharpness? There's gotta be a measure.



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Chuck
Balisongs -- because it don't mean a thing if it ain't got that swing!
http://www.balisongcollector.com
 
Bors,
sorry to hear about your wife's accident and hope everything is on its way to healing back. I agree with your assesment of sharp. The hair popping edges do tend to wear quicker. The edge that requires slight pressure to shave will do for most cutting task that I face daily.
dePaul,
I had forgotten the toilet tissue test. I assume we are talking about cutting the tissue cleanly? I have three knives that will do this and two came from the factory like that.
1) Al Mar SERE 2000
2) MT LCC MA
3) MT Combat Talon II after I worked
it over with my Sharpmaker
just had to get that off my chest!

Art Sigmon
 
Hi Gollnick,
(Push)cutting means penetrating a stuff under applied force.
So take a standardized stuff, (e.g. a THIN copper wire) on a non resilient block (e.g. Plexiglass, which won't kill the edge) hold the knife on it and load progressively. At a certain force (or weight) the knife will "snapp" through the wire. This value is reproducible, as long as the geometry of the edge is "fixed" and the wire thin.
Copper is hard enough to give reasonable readings yet soft enough not to kill the edge.
If you are using "push & pull" cutting you may use a standardized rope, a well defined
applied force(best is to use a "weight"), a standardized length of pull and simply count strokes untill you are "through". See also Waine Goddards report on edge holding.
Happy sharpening
smile.gif


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D.T. UTZINGER
 
I just did the falling paper test with a kit knife I got from texas Knifemakers supply. it was a green river paring knife and after i put my personal edge on it, it could cut a piece of paper lengthwise as it is falling. pretty cool! Not bad for a $6 blade!
 
That's an interesting point, Gollnick.

I think sharpness is something that's much easier to "test" for, rather than "measure". It's easy to measure the width of a plane's fuselage, but it's not easy to measure the width of a knife's edge. Even using an electron microscope, it's still difficult to tell how "sharp" an edge is.
 
A simple test would be something like being able to cleanly slice a sheet of specified size and type of paper. I seem to recall that Kershaw might do something similar per some pictures of knife sharpening and testing at their factory in a recent magazine article. Fine Woodworking did a test on chisels a short while back, where they sharpened the chisels, measured the roughness of the edge using a profilometer, used the chisel in a standardized test to cut oak, and then remeasured the chisel edge to determine wear. The amount of force needed to slice thru a standard test piece could also be used. I recall reading an ad of a maker of diamond knifes, and they noted that they were well suited for precision use as the force needed for cutting was so low.
 

Sharpness, as the term is commonly used here, usually seems to mean how perfect an edge is (consistent & smooth - mirrored), and how small the angle of the edge is...

What you should be more interested in is cutting ability. If you're interested in push-cutting, for things like shaving, you want a perfect edge with as small of an angle as possible. Blade design also needs to be a consideration, because a thin, chisel ground blade is probably the best design for push cutting because if its small angle (it's half that of a normal blade because one side of the blade is flat).

As other people have noted, you'll be better served by having a coarser edge if you want to use the blade for slicing rope and such. The micro-serrations bite into the rope and cut it with less necessary force...

I imagine that if you had the right equipment (an electron microscope?) you could quantitatively assess how sharp a particular blade edge is (all other things being equal - such as blade material - which would have to be compared independently for equal blade designs & edges). For push cutting, you would actually be able to measure how smooth the blade is...how perfect the angle is, how straight the edge is, etc...
For slicing, the matters would be much more complex... most likely there would be a wide range of designs for micro-serrations (just like there are designs for normal serrations). The combinations of size of the serrations, spacing of them, etc., are unlimited...and could indeed be patented. In the future, there will most likely be some sort of computer controlled sharpening machine that can make a "perfect" edge, however you design it...smooth, custom serrations, etc...it would be cool, but it would take all the fun and rewards out of sharpening by hand and doing something the "old-fashioned" way...

Well, I got off on a bit of a tangent, but I think that in response to the reply about quantitative measurement, the answer is that we don't have at our disposal the right equipment with which to analyze the edges of our blades...We can go and buy good calipers for measurement, but we can't go and pick up an electron microscope at home depot to examine our blade edges to assess "sharpness" or how perfect the micro-serrations are on a utility edge...

-ZZ
 
Edge sharpness is a subjective thing? Some prefer a smoother polished edge, which is better for push cuts and some prefer rougher edge which is better for saw cuts, ie: rope cutting. I had a CRKT Mirage that was polished and would cut hair with the lightest pressure but it didn't feel sharp when you ran your finger across it with light pressure. One of the guys I worked with commented that the edge felt dull at which point I demonstrated its profiency in cutting in bareing my arms. I agree that not only do we need to come up with a system that accurately defines sharpness, but maunfacturers would have to use the system and advertise the type of edge that came with the knife.
 
Originally posted by cerulean:
Even using an electron microscope, it's still difficult to tell how "sharp" an edge is.
Click to expand...

With something so small, like a knife's edge, we need a way to magnify it before it can be measured at all. It reminds me of the test Archimedes discovered for measuring volume. Before his bathtub "aha", how could one possible map such an irregular surface as the human body?

In the case of a knife's sharpness, the following idea came to me: What's we're trying to measure is consistent angle, and fineness of the edge (in the case of push cutting). Here is one possible way to get around the smallness issue:

Create a V in a hard substance (like a bar of steel), that is precisely the same angle as the angle you're sharpening your final bevel to. The fineness of the V is directly related to the accuracy of the test.

Now in the pit of the V, put a soft, viscous substance with a discernable color, like Crisco. If you take your kinfe, and lay it along the V so that the edge touches each point along the bottom (this will require a sweeping action for curved belly knives), you should be able to push ALL of the Crisco out. If your bevel angle is off, you will not be able to (too wide and it won't reach the bottom, too narrow and it won't have the same pushing effect). Also if your edge is blunted, you won't be able to, because it won't reach the bottom of the V (in the case of correct angle).

It's a crude test, and depends on creating a valid V. Although perhaps joining two pieces of metal -- one of which has a proper single bevel -- is the easiest way to do this.

I haven't tried this out myself; it's just a thought experiment. I'm sure there are other ways of magnifying the very small quantities we want to measure.
 
Another method that can used, and is evidently used for bevels at least, is to see how much light is scattered when a coherent light source is pointed at the edge. Commonly a laser is used. For just measuring the bevels and such a macroscopic technique works fine, while some smaller sensors would probably be needed to check for scattering in an attempt to estimate the edge condition.

 
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