Sharpness levels defined

zyhano

zyhano

Gold Member
Joined
Dec 3, 2009
Messages
1,593
So, i'm whittling hair, shaving it off my arm against the grain, have it pop off from my arm, slicing through paper, push cutting through newspaper, and I' m wondering where I am with my sharpening...
Can it get sharper and how can I tell..
Also I find the names confusing. Juranitch defines shaving sharp and razorsharp but here on the forum you can also find scary sharp and tree topping sharp and loads of other terms. It confuses me and it's hard to compare things when you do not have the same words for same things.

I hope to have it all cleared up for everybody so we can have an inventory here for common sharpness levels from dull to sharp, how to get there and how you can tell it is so.

Can someone please define sharpness levels in the following way:
name
How to achieve it normally
How you can tell

I'll start
0. Abused. Use your knife in ways it was not made for - Your edge is chipped and nicked, rolled over, planes of metal from either side of the knife do not come together.
1. Dull. Use your knife a lot and don't sharpen it - Knife won't cut very well, force is needed to make the knife cut or slice, tendency to crush soft material when trying to slice.
2. shaving sharp. Sharpen the knife on an appropiate abrasive and get the planes of metal from both sides together to form an edge. Shaving sharp can be reached up from XC dmt stone or 220 grit japanes waterstones and up - Knife shaves hair from arm, rough cutting feel to it when slicing tomatoes.
3. <your turn guys>

Please feel free to add or insert levels and to alter or expand on how to get ther and how you can tell.
When/if there is a consensus and the info is valuable, i can edit the first post here to just get that info in this post



Enjoy and I hope we can get somewhere here..
 
Whether you have one or not, your input should be better than that knifenut ;)
 
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I can't label it, it for me is a continuous progression and even within one grit you can have many levels of sharpness. I don't see it like a stack of larger blocks but as a smooth road that keeps going up.

Words like hairpoping and tree topping relate to exactly what the edge does, but again as a description of sharpness they really don't mean much because a edge can be sharpened to many different grits at a number of angles and do these things.

This type of labeling would only work if everyone used the same abrasive and technique, otherwise the limits are endless to your question.
 
Yes, but we need a common ground. The alphabet allows us to create words, and from that poetry. Having some definitions will allow for better understanding. Arguing if you need 26 letters or less or more in the alphabet would never have produced script.

So, again, please chip in, your experience is highly valued and you will be able to provide some useful insights
 
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I've been thinking about this problem myself actually. So far I'm only able to test in a couple of ways but only up to a point. Push cutting paper and getting very fine curly slivers. Then of course there's shaving hair but this a method of dimishing returns because I sharpen more often than I can regrow hair! The last one is seeing if the edge will catch on my finger nails.

After that though I can't tell if I'm making progress of not. I've never managed to get to a place where I can whittle hair although this is likely to do with the blade geometry than my skill.
 
The best I have achieved so far is free hair whittling. It has been quite a learning experience for me because when I first got my sharpmaker, I could get hair scraping sharp with my first 2 knives on it but could hardly get a hair popping edge. Then I got my DMT aligner and got it easily hair popping but struggled to get a hair splitting edge. Now all my knives are easily hair splitting sharp but I still can't get free hair whittling easily on all my knives. I can only get it consistently on some.

Now I'm thinking of lowering bevel angles just to try to get free hair whittling edges. I know that I will constantly strive to get better edges and I'm having fun discovering a lot of little things that can even be difficult to describe that make a difference in the final result.

It has been said that insanity is repeating the same things and expecting a different result. Well then, I am now officially insane. :D
 
zyhano said:
Yes, but we need a common ground. The alphabet allows us to create words, and from that poetry. Having some definitions will allow for better understanding. Arguing if you need 26 letters or less or more in the alphabet would never have produced script.

So, again, please chip in, you're experience is highly valued and you will be able to provide some useful insights
Click to expand...

This is much akin to the debate between people and laws that govern them.

Laws are black and white or "digital" if you like (like a scale of sharpness definitions would be)

People (and nature, and reality for that matter) is "analog"....infinite shades of gray...

I appreciate what you are trying to accomplish, but you will likely get a lot of words and little agreement on concepts. Best of luck to you on this though...I would love to see some standardized rules to play with, but SO many of the sharpness tests are at least partially based in cutting technique and are therefore more of a "trick" or "technique" than a test.

Here are some examples:

Free hanging hair whittling--Variables: hair thickness, moisture, and relative health. Take a couple hairs from different sources and you will quickly understand what I mean. These variables also apply to nearly every other test involving hair (shaving, tree-topping, popping, etc).

Paper cutting--Variables: the paper, thickness, rag content, contaminant content, etc. (same can be said for string, rope, and cardboard...do not get me started on variability of cardboard)

On another portion of the spectrum. There are different types of sharpness when you start considering slicing vs push cutting. There are different types of performance with different angles of bevels.

I did a series of demonstrations recently that showed a polished super low bevel angle knife cutting Kevlar rope with far less force than a 20 degree beveled knife with similar polish and a 30 degree knife with factory edge...then I dulled the thin edge to the point where it would barely shave a few hairs and certainly would not shave cleanly (it would be considered much duller than the 20 or 30 degree knives by any form of hair testing), but it still cut kevlar with significantly lower force than these other "sharper" knives.

This will not be a simple spectrum to define...
 
To me, a blade is sharp when it will "push-cut" newsprint both along and across the bias, and the edge is mirror polished.

If it "feels" sharp, IMHO, it isn't and you have more work to do. :p

It should shave dry arm hair smoothly without any feeling of resistance. :thumbup:
 
My method (to impress customers :)). dropping a large tomato from about
18" onto the blade and watching it split in half.
 
Guys, tnx so far. But the question is if you can suggest things for the list. A sharpness level with a name and what it does.

Can someone define razorsharp and scary sharp to start with? And how to get there?
 
zyhano said:
Guys, tnx so far. But the question is if you can suggest things for the list. A sharpness level with a name and what it does.

Can someone define razorsharp and scary sharp to start with? And how to get there?
Click to expand...

Zyhano, 'scary sharp' is a technique for sharpening, not a degree of sharpness. It was started by the wood carvers who use sandpaper on glass to sharpen their gauges and knives. "Scary Sharp" is the name of this system, not the results. The term became corrupted here in BladeForums to mean really ridiculously sharp results! :eek:

Razor sharp means just that; an edge sharp enough to shave hair. Of course, an 16 year old Swede with light blond hair will not need the degree of sharp that a 65 year old Italian with tough black hair need to hack through 5:00 shadow. Cutting the hair off our arms may well be a far cry from trying to shave the stubble off Grandpa's neck.

I think that if we want to codify 'sharp,' we need a measuring device that we all have access to. I'd recommend the 'RazorEdge' Tester
http://tinyurl.com/34rwp38 Costs 10 bucks and works like a charm. I've been using one for years when I come across and edge that I'm just not sure of. I've had the same one for years, so I guess it's a long lasting product too. Very handy, takes up no space, cheap, and effective. It would be a good universal benchmark for discussions. (Yes, a Bic pen can be used, but then we're getting back into the problem of people using different products to compare edges. There are lots of different Bic pens...)

Stitchawl
 
I always thought scary sharp was when the knife is so sharp the hairs jump out of the way before the blade even touches them.
 
I like thread cutting on a postal scale to measure sharpness. For me, it is an easy way to gauge sharpness across different knives without huge variation due to grind or weight. If I sharpened a kitchen knife, and it came out really well, how did it actually compare to my SAK? It is easy to check with thread. How do both of those knives compare to a razor blade? It is easy to check.

While I don't do it, it seems like nozh's 21? cut method could weed out a lot of variables, though at the expense of a considerable amount of labor. I typically make 3 or 4 cuts, and if they aren't all very similiar, make a few more, and then use the average of several cuts.

If someone put together a standard, then I would imagine that the variables could be reduced to a manageable amount for the purposes of discussing sharpness.
 
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Any Cal. said:
I like thread cutting on a postal scale to measure sharpness. Like was mentioned, there are always a lot of variables. Using a standardized thread though, and taking a few measurements, to my mind, appears to be a somewhat consistent way of determining sharpness.
Click to expand...

What sort of 'standardized' thread? A particular brand? A particular color in that brand? Embroidery thread vs linen thread vs cotton thread? Keep in mind we have a very International group here. What is available in Chicago or Kuala Lumpur might not be available in Johannesburg or Berlin.

How long should the piece be? Stretched tight or slack? Push cut or slice?

Standardizing isn't an easy process... :o But... if we could figure it out, it really would give us a great reference point! :thumbup:


Stitchawl
 
As I've recently learned from 2 very experienced people here the only really sharp knife edge is one that does what you need it or want it to do. In sharpness tests, like slicing and cutting media, technique goes a long way.

Ex. If you can split a 1" length of hair with your knife, I may not be able to repeat it even if I have a more refined edge than you. This may be because my hands are not steady enough. So that is technique related.

OTOH, the variables involved. Take that same hair splitting test. I may be splitting a coarse strait hair easily but you might not be able to split a fine, blond, wavy hair with that same knife.
 
The edge angle and type will be the biggest factor followed by the grit it was finished at, the type of steel, the main grind, thickness behind the bevel, the HT, cutting technique, media being cut, atmospheric conditions, and HUNDREDS of other things will effect your test results.

Through experience and looking at edges under high magnification I've been able to tune my finger tips to the different levels of sharpness and what microscopic imperfections feel like. I CANNOT describe this feel in words nor can I lable it with a name like scary sharp, its simply a sense I've developed over time.

My first post though is the only true way to measure how sharp your edge is. You must find out the micron or sub micron width of the apex between edge angle A and B.
 
stitchawl said:
Standardizing isn't an easy process... :o But... if we could figure it out, it really would give us a great reference point! :thumbup:

Stitchawl
Click to expand...

I don't know if this is available in other countries, but it is pretty common in the US. http://sewingsupplywarehouse.com/coats230.html

Let the thread lay slack, as it is most repeatable. Pushcut, as it is most repeatable. Go slowly, so it is easy to read the scale, and for me, the readings tend to be higher when moving slowly.

There are a million ways of taking averages, but I would probably do something like measure 5 times, throw out the high and low, and average the other 3.

If you have a system, it is easy to build a jig to put on a scale that will allow you to make the cuts in rapid succession without retying thread.

There will still be a lot of variables, but I would hope that something like that would weed out enough of them that a 1oz edge could not appear to be a 2oz edge to someone else.

Who else has some Derby DE blades for a standard?:D
 
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Looking at knifenut's post, it would be true that there are a lot of variables. The main thing that one could learn from any given test is that one particular edge performed better on a test than another did, it wouldn't guarantee suitability for any other task.

Just because one edge cut cardstock longer or easier doesn't mean that it will cut cardboard, thread, or hair longer or easier.
 
Exactly, each knife would need its own test per the type of abrasive used. Sharpness itself can be altered by a different abrasive of the same grit too, edge retention and steel structure can also be effected by this.

Splitting hair and the hanging hair test plus feel is all I use, I've found it to be the most consistent in gauging sharpness. As of now its the only thing I can find that can be cut to very small pieces and show similar results. You must also watch for and know the variables involved with this test method.
 
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