by David Bassing (adapted from The Wooden Post, vol. 1, December 2015)
My presentation [at the 2015 Kezurou-kai Event – Oakland, CA] concentrated on flattening and re-establishing the ura (backside) of Japanese chisels and planes. Use of a kanaban (flat steel plate, about 3.5”x12”x3/8”) and carborundum grit along with tapping out was demonstrated.
When a blade’s cutting edge is badly cracked or chipped a grinder can be used to speed the process of removing metal and getting the edge angle back to what the user feels is right for the type of wood being worked. I have always favored a hand-cranked grinder. It is slower and takes a bit of hand control to achieve good results but there is very little risk of the hard steel getting too hot and possibly drawing the temper out. Grinding causes a hollow grind on the bevel, which should be removed by sharpening on water stones, before attempting to tap out. This insures that the hard steel is well supported by the softer iron above it.
I use the end grain of an 8×8 Douglas fir block with a bevel of about 45° cut along one corner. This bevel is the surface that the blade rests on when tapping out. The blade is then hit on the bevel (I use the corner of my hammer) starting in the middle and then working outwards. It does not take much to have the desired results. The amount and force needed to push the hard steel down is really a matter of experience. When first trying, I would suggest tapping a bit and then use the kanaban. With just a bit of water, no grit, and rubbing back and forth with pressure applied with a backing stick, the areas that are touching the steel of the kanaban will show up as a different shine than the low areas. What you are looking for is the spot in the middle of the ura along the cutting edge to begin to show a polish. If not, return to the block and tap out a bit more.
When using the kanaban and grit (120-180 grit) the key is to have the steel plate well supported and at a level that allows one to push down and have control. A small amount of grit and a couple of drops of water is placed on the steel plate and holding the blade with a backing stick begin to rub back and forth. Initially there will be a loud scratching sound but this will quickly diminish as the grit begins to break down. Continue rubbing, using pressure, making sure the surface does not dry out. I will dampen the surface with a spray of spit so that I don’t stop the rubbing motion. Eventually the grit is worn down and by continuing the rubbing the plate will begin to dry. At this point continue to rub, concentrating on flatness. At some point it will be difficult to rub the two together. If done well the back side will be pretty well polished and the flat will be re-established behind the cutting edge.
Finish with a polishing stone to achieve the desired finish. Sharpen the bevel. Get back to work.