How sharp is sharp?

[Woodchips2]

If there was a competition to assess sharpening methods how could you measure if your tools are sharper than any other? Would it come down to shaving the hairs on your arm or cutting a chamfer on a piece of very hard wood or is there a scientific way of measuring it?

Is a new blade for a craft knife as sharp as it gets or would it get sharper if you stropped it? I often strop craft knives when they get blunt because it is quicker than changing the blade but I don’t know if they are as sharp as a new blade!

Keith

 

[MMUK]

I had an accident a few months ago trying to slice some thin plastic with a brand new Irwin bi-metal "Stanley" knife style blade in my knife. It jumped over the straight edge and took the top corner of my finger off. I didn't notice until the bench started turning red a minute or two later. Now that's sharp!

 

[Corneel]

The hanging hair test? That seems to be the ultimate test in straight razor crowds.

 

[Cheshirechappie]

There are ways. Industrially, one way is to examine the edge under high magnification, and measure any roundness of the meeting of the two surfaces, the depth of scratches on the meeting surfaces and so on. You need a toolmaker's microscope or similar, so it's not an appropriate method for the craftsman in the workshop.

For the practical woodworker, it's probably best NOT to try measuring sharpness. Just become used to the degree of sharpness you need for a given type of work, and how to get that degree of sharpness. Assess sharpness by using the tool - if it works well, it's sharp enough. That's all you need, really.

 

[Vann]

Another sharpening thread, oh goodie...

If you sharpen on an old dished oilstone it will be sharp enough and it doesn't matter how many hairs you can cut with it. Or so I've heard... :wink:

Cheers, Vann.

 

[Richard T]

For the practical woodworker, it's probably best NOT to try measuring sharpness. Just become used to the degree of sharpness you need for a given type of work, and how to get that degree of sharpness. Assess sharpness by using the tool - if it works well, it's sharp enough. That's all you need, really.



+1.

And whatever you do, don't read any Phillip Pullman. Cutting through time is not an achievable aspiration.

 

[Woodchips2]

Another sharpening thread, oh goodie...

If you sharpen on an old dished oilstone it will be sharp enough and it doesn't matter how many hairs you can cut with it. Or so I've heard... :wink:

Cheers, Vann.

Hope it doesn't get controversial
Regards Keith

 

[Fromey]

My test for sharpness is when I can simply push the edge of a piece of paper onto a blade and, with out any lateral movement, it will cut and give visibly clean edges. If I have to use any slicing action of the blade or paper to get a clean cut, it's sharp but not as sharp as it can get.

 

[RossJarvis]

How sharp is sharp? Can you cut cleanly through a whole sheet of bog-roll held between two fingers at one edge? (if you can you're a better man than me Gunga Din).

 

[marcus]

Isn't there some story about how Richard the Lionheart and Saladin met and were having a chat about weaponry, and Richard took his axe and smashed through a piece of armour. Then Saladin got his scimitar and threw a silk handkerchief into the air and cut it into two as it floated down. Something like that anyway.....

 

[Paul Chapman]

Isn't there some story about how Richard the Lionheart and Saladin met and were having a chat about weaponry, and Richard took his axe and smashed through a piece of armour. Then Saladin got his scimitar and threw a silk handkerchief into the air and cut it into two as it floated down. Something like that anyway.....

Did they then have an argument about different honing methods....... :-k

Cheers :wink:

Paul

 

[charvercarver]

A standard test is if you can slice the end grain of a piece of pine and leave a polished surface, then it's sharp enough for woodwork.

 

[marcus]

Did they then have an argument about different honing methods.......

Yes, and it got quite heated apparently.... :wink:

 

[Kalimna]

Forget Phillip Pulman - I want a blade edged in shimmering octarine

I suspect that, seriously, if you wanted to assess ultimate sharpness, then a high powered microscope is the only way to go. Or perhaps have a measuring setup that gauged the 'effort' required to pull the edge through a standard material of standard dimensions. Attach a weight to the blade and see how far it cuts through a standard material in a standard unit of time. Obviously the material would need to be homogenous, and therefore wood wouldnt be any good....

Adam

 

[Dangermouse]

Well if you go right down to what theoretically is totally sharp, you would have get two surfaces to combine on a line / edge that was infinitely thin, down to subatomic level. That of course is impossible in the real world. So if it cuts a bit of wood without sticking, jumping or buggering about, then I'm happy.

 

[Kalimna]

Ah DM, I quite agree with you, but the OP was with regard to 'is it possible to'/'how would you' measure the sharpness of one edge against another. So, not quite real world, more of a theoretical question. Is there, I wonder, a scientific unit of sharpness? In the same sort of way that a Scoville is a sort-of-scientific measure of chilli heat?

I imagine that the currently-theoretically maximum sharpness would be obtained with a sheet of graphene, or carbyne if the boffins are able to manufacture it.

Adam

 

[RossJarvis]

Another way of looking at it is how "sharp" are your "sharpening doo-dahs" If you've sharpened your tools to the best of your "doo-dahs" abilities, you aint gonna get any sharper. If you want sharper, get a sharper "sharpening doo-dah". At the moment I am considering whether to get a sharper sharpening stone at greater cost, so that I can drop it onto the quarry tiles in the larder and have more than one "Super-Wishy-Washy-Stone".

I read one article, where the "test" was the force needed to sever a sewing thread. The "tester" measured this on a fresh sharpened blade, then after planing some "unobtainium" a certain amount of time he measured again. This was actually a test of the edge strength of the material. I don't think measuring by viewing gives the real sharpness of an object, this is a theoretical measure. The thingy needs to be cutting something to give a good result. You may actually find that sharpness varies, depending on what's being cut. The sharpness of something cutting pine may be one thing, but cutting an obscure hardwood may be different. This could depend on cutting angle, clearance angle and the direction of the wind. Plus, something may be b****y sharp cutting the first shaving and be as blunt as a Chelsea Bun on the second pass.

Comment removed: as I am now sober!

(Maybe you can tell I've been at the vino and SWMBO has removed my bank card till I stop buying things I can't afford, such as Japanese Whetstones!)

 

[Cottonwood]

I know when my plane is sharp (especially the sloother) when the scratchy dull grey areas of micro tera out on the boards gradually start to turn into a burnished shiny sheen instead. Thats as sharp as I need, anything more (?) is academic and unnecessary

 

[bugbear]

If there was a competition to assess sharpening methods how could you measure if your tools are sharper than any other? Would it come down to shaving the hairs on your arm or cutting a chamfer on a piece of very hard wood or is there a scientific way of measuring it?

Is a new blade for a craft knife as sharp as it gets or would it get sharper if you stropped it? I often strop craft knives when they get blunt because it is quicker than changing the blade but I don’t know if they are as sharp as a new blade!

Keith

The kitchen knife crowd know that fineness of edge (the triangle taperingt a perfect point that we all see in our minds eye) isn't the whole story, even if it were possible. The actually cutting ability of a knife varies with shape and profile too.

Which means that "just" looking at the edge with an electron microscope (a fairly full-on thing to do to a blade) still wouldn't be a "full test" of sharpness (whatever that is).

BugBear

 

[Grahamshed]

Isn't there some story about how Richard the Lionheart and Saladin met and were having a chat about weaponry, and Richard took his axe and smashed through a piece of armour. Then Saladin got his scimitar and threw a silk handkerchief into the air and cut it into two as it floated down. Something like that anyway.....

Did they then have an argument about different honing methods....... :-k

Cheers :wink:

Paul

That meeting actually took place just before the crusades started. They were not caused by religious differences but by an argument about whether the edge should be achieved freehand or with a jig.