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?
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.
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 :-)
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.
Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.
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.
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
kindest regards, Nick.
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.
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