What's new

Different kinds of 'sharp'

This is an amalgamation of a few posts I've made in various threads to explain a bit about sharpness in relation to razors and knives, because I think it's quite an important thing to understand when using, and especially honing different kinds of blade.

Sharpness is not an absolute characteristic of an edge itself in which a single test can tell you whether one thing is 'sharper' than another, because it also depends on what you're cutting. If you take a razor and a kitchen knife and try to shave with them your razor will cut the hair very easily and closely, but not your skin. Whereas the knife may cut your skin, but probably not your hair very well. And neither are going fell a tree.

---

Here's a rubbish drawing of a knife cutting a tomato. Where 'b' is the angle of your edge (sharpening angle), 'a' is 90 degrees minus 'b', and 'c' is 90 degrees minus the angle you're cutting the tomato at:

IMG_E2994 (1) (1).JPG



In order to make the knife cut the tomato well we need to increase the frictional force between the two. ***Everything about how anything cuts anything is about how friction works.***

F = uN.

The Frictional Force (F) equals the Coefficient of Friction (u) multiplied by the Normal Force (N). So to make the knife cut the tomato we want to try to increase either u or N, or both. Let's first look at N...

The main reason that ripe tomatoes are used often when testing the edge of a kitchen knife isn't because they're unusually hard or tough - the skin is fairly comparable to many other fruit and veg, it's because the flesh is unusually soft. And that massively decreases the Normal Force which is exerted upwards by the tomato against your knife. Here are another two rubbish drawings:

IMG-3006 (1) (1).jpg



In the first the knife cuts straight down and the Normal Force acts straight up against it. But unfortunately you don't always want to neatly bisect a tomato; you cut it more like the second drawing, in which 'd' is the angle 'a' minus 'c' from the first pic. Meaning we only get a small proportion of the Normal Force, which I'm calling N1. We can see if we decrease the sharpening angle 'b', then we increase 'a', hence increase 'd', and subsequently increase the value of N1. So a more acute edge will cut the tomato better.

But there's a slight problem with that, because after you cut through your tomato you hit your chopping board, which is hard. And if your edge is too acute then it will be frail, and crumple when it hits the board, and now your knife is blunt. So we can't just take the edge angle down as low as possible to increase N to the max, we need to try to increase the Coefficient of Friction (u) in some way too...

The other reason tomatoes are used in this test is because the skin is quite smooth, and that decreases u. Try cutting a slippery wet tomato vs a dry one and you'll see the importance of the friction coefficient. But apart from making sure your tomato is dry there is another way to increase u - by finishing the knife on a lower grit stone to leave more 'teeth' on the edge, making it like a microscopic saw. And using a push or a pull cut (literally a sawing motion) rather than chopping straight down to amplify the effect.

So in fact: While we might think that finishing an edge at a higher grit will make it sharper, for many applications it doesn't.

---

Now let's go back to looking at our razor...

A general purpose kitchen knife is designed to cut most foods well, whereas a razor is a highly specialised instrument designed to cut some things very well and other things very badly. You don't actually want a razor to be particularly good at cutting a tomato, because your face likewise has a relatively tough outer skin and soft flesh, and if you had a high coefficient of friction you'd cut yourself. It should cut a tomato better at bevel set than it does at finish.

So you refine and polish the edge with a series of progressively higher grit stones and you cover face in lather, which reduces 'u'. And you shave with light pressure at a low angle, which reduces 'N'. And all this means that your razor (hopefully) glides smoothly over your skin, while effortlessly cutting your hair.

---

If you take two different types of edged instrument that have apexes that meet - you can't really say that one is 'sharper' than the other. Knowing what you're trying to achieve is essential, because there are different kinds of sharp.
 
I will say that I can appreciate the thought that went into this and I will also say that you may be right, and I may be wrong but... I completely disagree. We can still be frens though.


Haha!

(Anything in particular, or just the whole lot...?)
 

Rosseforp

I think this fits, Gents
If you take two different types of edged instrument that have apexes that meet - you can't really say that one is 'sharper' than the other. Knowing what you're trying to achieve is essential, because there are different kinds of sharp.
That is what my finger said as I was setting the bevel on my boys axe with a file. A coarse file at that. A nice clean 1/2" slice just deep enough to sting and not draw blood. Will the axe cut hair? Sure, with a nice swing behind it, it will, but shaving with an axe? I think not.

There is a different kind of sharp.

~doug~
 
That is what my finger said as I was setting the bevel on my boys axe with a file. A coarse file at that. A nice clean 1/2" slice just deep enough to sting and not draw blood. Will the axe cut hair? Sure, with a nice swing behind it, it will, but shaving with an axe? I think not.

There is a different kind of sharp.

~doug~

Indeed!

I almost included a bit about axe sharpness in the op, but didn't in the end because I don't use axes very often and frankly I'm not very good at sharpening them, so it would have just been theory. Whereas I am pretty good at sharpening (kitchen) knives and alright at razor honing, so understand them better on a practical level.
 

Legion

Staff member
I agree with all of the OP, and I think that was my "ah ha!" moment with knife sharpening was when I stopped trying to turn every knife into a razor. I could make an edge very sharp, but the edge had no durability. My main metric of knife sharpness was, and still is to an extent, shaving arm and leg hair. And that is a good indicator of sharpness in a razor sense, but doesn't show how well the teeth or micro serrations might work when cutting food.

At this point I'm finding my most successful kitchen edges are a hybrid or compound edge. Where I use a coarser stone like a Shapeton Glass 500 or a soft Arkansas to sharpen, then refine the edge a little with something harder and finer, but without removing all the scratches from the first stone. Right now that seems to be giving me the best of both worlds.
 

Rosseforp

I think this fits, Gents
Indeed!

I almost included a bit about axe sharpness in the op, but didn't in the end because I don't use axes very often and frankly I'm not very good at sharpening them, so it would have just been theory. Whereas I am pretty good at sharpening (kitchen) knives and alright at razor honing, so understand them better on a practical level.
The decal said this was a boys axe, I presume that refers to the size of the blade as there was another boys axe with the same size blade and a shorter handle.
20220909_162503[1].jpg
This is actually the first axe that I have sharpened, after using it to assist in the removal of some elm tree roots in the back yard.
After the file I smoothed it out a bit with a 1000 grit stone. Sharp enough to cut flesh and wood, the angle measures 19.5 degrees.
20220909_162611[1].jpg

I'm not going to shave with it.

~doug~
 
The decal said this was a boys axe, I presume that refers to the size of the blade as there was another boys axe with the same size blade and a shorter handle.
View attachment 1520041
This is actually the first axe that I have sharpened, after using it to assist in the removal of some elm tree roots in the back yard.
After the file I smoothed it out a bit with a 1000 grit stone. Sharp enough to cut flesh and wood, the angle measures 19.5 degrees.
View attachment 1520042
I'm not going to shave with it.

~doug~


Interesting now that I think about it a bit more, because wood is both a lot harder and tougher than most other things we cut by hand, so requires very particular instruments, which demolnstrate all this in quite exaggerated ways. The edge of an axe can be extremely polished because it relies on a massive amount of kinetic energy - the Normal force is huge an the coefficient of friction tiny.

A saw is completely the opposite; with a sky-high friction coefficient and tiny Normal Force. If you tried cutting wood with a tool/method where they were both in the middle you wouldn't get very far.
 

Legion

Staff member
Interesting now that I think about it a bit more, because wood is both a lot harder and tougher than most other things we cut by hand, so requires very particular instruments, which demolnstrate all this in quite exaggerated ways. The edge of an axe can be extremely polished because it relies on a massive amount of kinetic energy - the Normal force is huge an the coefficient of friction tiny.

A saw is completely the opposite; with a sky-high friction coefficient and tiny Normal Force. If you tried cutting wood with a tool/method where they were both in the middle you wouldn't get very far.
I know the competition wood cutters hone their axes to a near razor edge. In fact, they use razor hones to do so. I’m sure they hone between events, so maybe longevity isn’t a primary concern.
 
[Obviously the first post is somewhat simplified, as there are lots of other real world factors involved. But in essence it’s all about how they impact on friction.

In fact much of day-to-day physics is about friction. If you didn’t have the Normal force we’d all sink through the floor cos of gravity!]
 

rbscebu

Girls call me Makaluod
I know the competition wood cutters hone their axes to a near razor edge. In fact, they use razor hones to do so. I’m sure they hone between events, so maybe longevity isn’t a primary concern.
That has more to do with the bevel angle being suitable for the steel used.
 

David

B&B’s Champion Corn Shucker
I know the competition wood cutters hone their axes to a near razor edge. In fact, they use razor hones to do so. I’m sure they hone between events, so maybe longevity isn’t a primary concern.
I’ve seen a video on YouTube just guessing from the mid 80’s of two guys in Australia restoring an old building traditionally. The opening scene shows them sitting on a porch sharpening their axes. Guess what stone both of them are using..
 

Legion

Staff member
I’ve seen a video on YouTube just guessing from the mid 80’s of two guys in Australia restoring an old building traditionally. The opening scene shows them sitting on a porch sharpening their axes. Guess what stone both of them are using..
Stupidly expensive Norton barber hone?
 

Legion

Staff member
Found the video. It was filmed in 1984 so the popularity of the Norton Razor Hone goes pretty far back.

These guys knew their way around an axe. Bill Boyd passed away last year at 85 years old.

Honestly, sell that thing to superstitious competitive choppers. There are probably dozens of old barbers hones that would put the same edge on an axe.
 

David

B&B’s Champion Corn Shucker
Honestly, sell that thing to superstitious competitive choppers. There are probably dozens of old barbers hones that would put the same edge on an axe.
I should, but I keep this one around for the collection just in case the kids are interested in that sort of thing. They will probably just sell everything on eBay. 50 years from now all the hipsters will be raving about the “Fairview Tennessee basement cache”.
 
Sorry for the hijack, Oli.


Not at all - I find it all very interesting!

Q. for you: Here's another old antipodean axe sharpening vid. About the 15 min mark her mentions what he's using, it's slightly difficult to hear but after a bit of sleuthing it seems to be the 'Punjab Stone', another two sided old American BH. Ever had one?

You ought to be buying them up, cornering the market, building some hype, and then flogging for a small fortune... ;)

 

David

B&B’s Champion Corn Shucker
Not at all - I find it all very interesting!

Q. for you: Here's another old antipodean axe sharpening vid. About the 15 min mark her mentions what he's using, it's slightly difficult to hear but after a bit of sleuthing it seems to be the 'Punjab Stone', another two sided old American BH. Ever had one?

You ought to be buying them up, cornering the market, building some hype, and then flogging for a small fortune... ;)

Can’t make it out. I think that’s just something he made up to keep all the other blokes from buying up all the Nortons.
 
Top Bottom