Norton Crystolon Bench Stone
Anthony Carrico
Bench Stone. I've attached a photo for reference. You can see a
fingerprint sized low spot just back from the tip. The diagonal
scratches are from the bench stone, the parallel lines are from the
manufacture's grind. I've also used this stone to grind plane iron
bevels as Brent recommends.
The grinding isn't quick, and I wondered if the stone is fouled with the
swarf or glazed or whatever. Not having a brick or sidewalk that I would
consider flat, I stuck some 80 grit sandpaper on glass (the green
background in the photo) and rubbed the stone on that. Indeed rubbing
for 30 seconds or so seems to do some good. I see more metal in the oil,
but this doesn't last long.
Coarse Crystolon is supposed to "the fastest cutting oil stones", but
how fast is fast?
--
Anthony Carrico
Brent Beach
Shavings, especially chisel work, are on the order of 0.01". If there is a hollow in the back it is on the order of 0.001", probably less. It cannot affect your use of the tool.
Before you started flattening, the metal on the back at the edge was probably ok. Now, the metal has been shattered by the abrasive. It may be slightly flatter but it is weaker. Coarse grits - and coarse is anything above a couple of microns - affect the substructure of metal. Abrasives under 2 microns remove metal without affecting the substructure. Limit your use of coarse abrasives at the edge - on either side. You can restore the edge by honing both sides on very fine abrasives to remove the shattered metal. Most people worry about back bevels on chisels, so unless you can work with a back bevel on your chisel that option is not open to you.
Having said that, wear on chisels is not like wear on plane irons. You can easily dull a plane in an hour of steady use. It would be very hard to dull even a mortise chisel in an hour. My chisels rarely need sharpening. Less durable steel at the edge of a chisel resulting from working it with a coarse abrasive may reduce the time between sharpening but you still won't be sharpening very often.
Flattening chisel backs must be one of the lowest return on investment things you can do in woodworking.
On the speed of various abrasives - Silicon Carbide is pretty fast. Not as fast as diamond, but pretty fast. A coarse stone is around 220 grit, so not really coarse when you can buy 48 grit sanding belts. When you are working an areas as large as the back of a chisel, there are thousands of points of contact. The pressure per grit particle ends up being very low. Even when grinding a full primary bevel, where the area is much smaller than a chisel back, progress can be quite slow.
The key to quick work is working very small areas - micro bevels and very fine abrasives work out well together.
A dull chisel has a wear bevel on both front and back at the edge. This wear bevel will be much larger - factor of 10 - than the slight hollow and will make your chisel seem dull even when the front side has a well formed edge bevel. Dealing with that requires either extensive grinding from the front until the wear bevel is gone, or back bevels.
Anthony Carrico
the shattered edge isn't the end of the world, as long as I learn my
lessons.
Your explanations about wear bevels and back bevels on plane blades are
very convincing, but they leave me scratching my head when it comes to
chisels.
As I'm sure you've read, Leonard Lee says, "if you did not first lap the
chisel but devoted your attention only to the bevel, regardless of how
well you honed that bevel, it would still be intersecting with a grooved
face and the resulting edge would be ragged." This makes sense, but you
say not to flatten the back ("face" in Lee's terminology), so what do
you actually do to the back of your chisels?
I can understand that trying to get rid of a hallow that is back from
the edge is foolish, but I can't imagine you do nothing to the back and
leave the factory grind as one half of your edge. Do you hone (with 15u
paper to start, for example) a small area near the edge, essentially a
very slight, almost zero degree, back bevel?
--
Anthony Carrico