2 different types of sharpness?

[voltimando]

I've been sharpening various plane and chisel blades for a few years now and i'd like to ask if anyone else has noticed a certain phenomena. There's you ordinary 10k-20k grit glasspaper or Japanese waterstone sharpness, but many times on the way up through the grits, i've definitely noticed an inexplicable zone in which the blade cuts better than the best scary sharp edge. I would call this zone 'rough sharp' and it occurs just after the first or second coarse-mid grit wire removal. Try it with a loosely held sheet of A4. Scary sharp need to be at their best to cut the top edge of a slack piece of paper. The rough sharp has no trouble and seems to be better at cutting for longer. Has anyone else noticed this low level, but very effective 'rough sharpness' zone? cheers, Nick.

[RicodJour]

That's probably due to the rough edge acting like a serrated knife.

R

[Puckdropper]

RicodJour wrote in news: [e-mail]:

I have found that a slightly dull knife tends to cut through potatoes really well. I think sometimes you can get things TOO sharp, and that will cause difficulty.

Puckdropper

[DJ Delorie]

I used to sharpen my wife's hairdresser's scissors (she moved, the new hairdresser hasn't asked yet). She'd get them all shiny and polished-sharp from the factory, and I'd re-sharpen them to only 1000 grit so the hair wouldn't slip out of them when she cut.

I'm guessing the same thing is happening with your paper - the sharp edge is sliding over the paper without grabbing, and the serrations help it grab and tear (not slice!) through the paper.

But paper is not wood, and this serrated half-sharp is not ideal for plane blades, chisels, etc. Unless, of course, you use your chisels for sawing through paper.

[DJ Delorie]

Another related story: Long time ago, a travelling salesperson tried to sell us a set of knives (she was a friend of a friend, and doing it for school credit). She asked for our sharpest knife, and demonstrated that it couldn't cut through a rope as well as their knives. I noted that if I knew she was going to cut a rope and not meat, I'd have given her a different knife - then proceeded to easily slice through the rope with our serrated bread knife :-)

[Leon]

I doubt that duller is sharper. I would be willing to bet that the rougher grit provides a surface that has less friction. Mirror smooth will create a lot of drag and more of the surface comes in contact with the wood. My old Craftsman cast iron TS with its visible and easy to feel ground swirl marks provided much less friction than my Jet cabinet saw. The rough surfaces make less contact with the wood and therefore have less friction.

[alexy]

I believe it was on the rec that I read that the best medium for sharpening a knife to be used to cut rope is concrete. The resulting edge has enough "bite" to effectively saw the rope.

To the OP: A better comparison than cutting paper with a cut that includes motion along the edge is to cut end grain of wood, keeping the blade's motion strictly perpendicular to the edge, like the action of an un slewed plane.

[voltimando]

thanks for the many replies. I think you might be right, friction and drag seem to also explain the rough sharp's similar effectiveness on end grain. When i trim a moulding plane wedge mortice, with a very scarey sharp, you can feel the 'stop go' grabbing of the blade back on the very smooth and frictional wood. Combine that with the sawing effect and you've explained the phenomena. Many thanks for your suggestions. kindest regards, Nick.

[George]

Each of the leftover pieces of metal from your medium grit has an edge or edges as sharp or sharper than the final continuous edge from stropping. Sandpaper is sharp because the particles are, right? Not going to cut a lot with the paper.

So if you skew your cut to allow the blade to saw the fiber, and it's fiber, which is why it works like the serrated knife on the rope, you take advantage of this reality. Sadly, these kind of edges aren't really very durable, because the pieces break off like the grit from the glue on sandpaper. A continuous stropped edge will support itself much longer.

Woodturners mostly go from the wheel to the wood with a toothed edge, but they have the rotation to bring the wood to it, and just have to put the edge at a skew angle. A little bit of "bite" makes a more stable entry, according to some.

[Maxwell Lol]

Think of those serrated Ginzu knifes...

[RayV]

Yeah but I love my Cutco 'fishermen's friend'... Had to by one for my MIL once she tried it.