UKC

The scissors are blunt, so I sharpened them.  Got me thinking, what is "sharpness"?  Is it simply the radius at the edge?

Whilst I'm at it, what is "coarseness"?  I see lots of grits on sandpaper, but they seem to vary widely in what that translates to - 180 grit of one sort seems to be different to another manufacturer.

SI units for sharpness and roughness?

microns and colchester night out.

Have you taken to cutting your own hair then?

Would the sandpaper be for .......?

What about the sharpness of a point, such as a freshly sharpened pencil?

Not specifically for your question but you might be interested in Moh’s scale of hardness.

Have a look at

http://www.bessu.org/bessu.org_009.htm

I am not an expert so don't know if this is a well accepted standard.

Thanks - interesting. I think in practical terms it has to be more than radius, because a fine radius in a soft material isn't "sharp".

I knew about MOH, but wondered if there is similar for coarseness and sharpness.

I understood that Mohs scale was rather unscientific in that the gap between 9 and 10 is greater than between 1 and 9 (though how that was rationally determined I couldn't say).

I’d presume it’s simply narrowness, or for a point diameter, with the caveat that the material at that dimension needs to be strong enough not to break. 

I wouldn't say that the difference in gaps is unscientific.  It just means the scale is nonlinear which is true for the gaps between respective points throughout the scale.  It is common to have nonlinear scales in other scientific measurement quantities where there is huge variation across the full range.

Use one of these: https://www.sharpeningsupplies.com/Edge-On-Up-Professional-Edge-Tester-P156...

The sharpness of a point or a knife is different from scissors. Knives and points are sharp because they concentrate the pressure on such a tiny area that the material has to yield, whereas scissors are a shearing action that depends on the opposing surfaces being absolutely square and tightly opposed to give a clean separation. 

Then of course there's the sharpness of the average contributor to UKC, which is another thing altogether....

The sand paper one, shouldn't be an issue, the measure is the size of grain that goes through a mesh (sieve). There are gauges for roughness, as in the roughness of finish on a surface.

Good question though, well done.

Something for me do look up while I'm lock up.

Search "Brubacher Edge Sharpness Scale"

Thanks for that, I'd always been under the impression that there wasn't a measure of sharpness.

The standard you quote has problems though. It seems to depend on the geometry of an edge. I think that there is more to sharpness than that. If I make a knife out of Plasticine which has exactly the same geometry as a steel knife would you expect it to be as sharp? Also when a butcher sharpens a knife using a steel they are not reducing the radius of the edge but rather putting a burr on it, irregularities have an effect on how well something cuts.

Personally if I were coming up with a standard I think I'd base it on how "well" it cuts one or several specimen materials and specify the amount of force applied and the speed of movement of the edge across the material, something like that.

Someone made a tungsten needle that tapered down to one atom.

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[Sum of (pixel intensity squared)] divided by [sum of (pixel intensity) squared]

Doesn't apply to knives!

Does that include Viagra and Cialis?

A friend who is big into his knives tells me there are four accepted sharpness levels:

Blunt

Utility sharp: will catch your nail if you slide the blade edge sideways across the plane of your thumbnail 

Razor sharp: will shave hairs off the back of your forearm

Scary sharp: no idea how you'd test this!

shave hairs off another part of your anatomy?

Check out the thumb of the guy in the video for that instrument!

'Sharp enough for you not to realise you've cut yourself until you wonder where all the blood's suddenly gushing from'

There was a 'Rutherford & Fry' episode discussing this a couple of years ago:

https://www.bbc.co.uk/sounds/play/b09sqk04

Doesn't it also have something to do with the pressure that can be applied to the blade and speed of movement - e.g a paper cut, a concrete cutter and a face shovel

Bet it went blunt quickly .

We use these things in the semi conductor industry for measuring the properties of films of tens of nanometers thick. You'll see the sharpest one this lot sell has a radius of less than 50nm.

http://microstartech.com/index/NANOINDENTERS.pdf

Saw something in a film once of a thin silk handkerchief floating slowly down onto the static upturned edge of a Samurai sword and being effortlessly cut in half by its own momentum...

Edit: oops, pretty bad fake. Plus the embarrassment of admitting that I've actually watched that film... 

youtube.com/watch?v=r2NtAuuEYwg&

Hasn't everyone? Most of my mates have almost simultaneously just about shaved their heads.

We look like Yul Bryner's groupies.

This guy (Longstrider) is a bit of a legend in the knife sharpening world

https://www.youtube.com/watch?time_continue=4&v=uDOTXr2U0cU&feature...

Where you 'just know' that one slip shaving your forearm could result in a visit to A&E? A Guilds Of Master Craftsmen furniture maker sharpened the family axe for chopping wood, it scared me for ages until it went utility sharp.

I’ve been down a few knife sharpening YouTube rabbit holes in my time. Mirror edge sharpening is pretty impressive too. I bought some stones and am completely rubbish at it, consistently making my knives worse. Clearly a skill requiring some perseverance. 

Could probably do with spending more time cleaning his kitchen floor though!

Takes practice and time. Keeping a consistent angle is critical, I've found the Lansky system very good, with the addition of the sapphire stone you can polish the edge but still needs a finish on a strop. Once you get a consistent angle, it's all down to the wire edge. 

No they're not. What they're doing is straightening and work hardening the edge. Because they cut against bone the 'point' of the blade (looking in cross-section) gets bent over to one side or the other. Steeling unbends this very slight burr. If they press hard enough the burr will bend over the other way, then they steel the other side of the blade to bend it back to straight.  This work-hardens the edge of the blade making it more resistant to being bent in use, and keeps the edge keen without having to re-sharpen it.  It only works with a pretty sharp blade, once it gets too blunt it needs sharpening with a stone.  This 'bendy edge' principle is why axes have a steep edge angle - you want lots of material behind the edge to stop it bending - a long lasting edge that's ultimately less sharp.  A scalpel has a fine angle as you're not chopping with it - a ultra-sharp edge that's easily blunted in (mis)use.

To the OP - a sharp edge has no reflection. Hold a blade upside down in decent light and tip it over from side to side- a sharp one will go from reflecting off one ground surface to reflecting off the other.  A blunt blade will reflect off the edge itself part-way between reflecting off the ground surfaces.

I'm a joiner/carpenter and sharpening Edge tools is an integral part of my job.

There is a sharpening system out there called "scary sharp". I can sharpen my edge tools better than the scary sharp system can get them. Easily as sharp as a scalpel with a mirror finish on the honed edge.

Sharpening is all about removing the grinding grooves and micro scratches from the steel that would otherwise leave a jagged edge when you look at the edge under a microscope. 

If you're interested have a search for Japanese plane competitions where they can plane wood and get shavings as thin as 5 microns

off topic slightly

I used to work in a Histopathology lab in a hospital. We used to cut section for Electron Microscopy with  knives made from Diamonds. The sections were about 30-60nm thick (or should that be thin).

I guess the knives were sharp!

For light microscopy we cut sections of paraffin wax with knives similar to disposable razor blades. When I first started we used big heavy steel knives which were sharpened on a rotary surface with 2 grades of paste into two facets. You handled the knives with great care!

I have no idea how you measured the 'sharpness' apart from the ability to cut good sections (even thickness, no lines etc).