giant Cypress

Hatful of Hollow

One of the most well-known characteristics of Japanese chisels and plane blades is the hollow that is on the back of the tool. The reason for the hollow is to aid in sharpening. The hard steel lamination that makes up the back and cutting edge of these tools are so hard that to flatten the back without it would take a ton of effort and a very long time. By putting the hollow in the back, when flattening the tool, only the thin area behind the cutting edge needs to be flattened. It’s like the advice given for flattening the back of a western chisel to just work on flattening the 1/2” or so closest to the edge, taken to its logical extreme.

One question that often comes up with the hollow is what to do as you work on the bevel side of the tool. Eventually, the chisel gets to a point where the small flat area behind the edge will disappear and the edge winds up in the hollow, if nothing is done about it. Luckily, there is something easy to do, and that is to take your sharpening method of choice and apply it to the back of the tool. This is somewhat different than the conventional wisdom regarding western plane blades and chisels, which is to flatten and hone the back to the degree wanted, and then not to touch it again.

By working on the back of a Japanese chisel or plane blade, some of the metal surrounding the hollow is removed, which will in turn increase the flat area right behind the edge, and the tool will be back in business. Here’s a picture to help illustrate.

In this diagram, the top picture is a side view of a new Japanese chisel. You can see the hollow area at the bottom, and a nice flat area behind the cutting edge. The dark area is the softer steel, and the white area is the harder steel. The hollow still is within the hard steel layer, so that even if the hollow is ground away, you still have a layer of hard steel, albeit thin, on the back of the chisel.

The second picture is what happens after the chisel has been used and sharpened for a while. The bevel gets moved back, and comes very close to the hollow area, and will move into the hollow area unless something is done at this point.

The bottom picture shows what happens if the bottom is attended to at this point. If some metal is removed from the back side, the bottom of the chisel moves upwards to where the blue line is. This reestablishes the flat area behind the cutting edge of the chisel. A little of the depth of the hollow is lost, but that’s okay. As noted above, the hollow is fully within the hard layer of the chisel, so that you won’t wear through to the soft layer.

The other thing to notice is that I’ve drawn this diagram so that the hollow is deeper at the back end of the chisel than at the cutting edge. This is how Japanese chisels are set up, although I’ve exaggerated things in the diagram to make them more visible. The reason for this is this helps to maintain the hollow as the chisel is used up, so that the hollow won’t completely be lost as the back is ground away.

Here are the backs of some Japanese chisels that I have that shows how the hollow acts as they are used. (Please ignore the horizontal streaks on the center and right chisels. Those were a result of an experiment in powered grinding of the back gone horribly wrong.) On the left is a nearly new Japanese chisel. In the middle is a Japanese chisel that is due for some more work on the back because of how close the edge is to the hollow, although the chisel itself is still perfectly usable. The chisel on the right is well used, but the hollow is still present due to it being deeper in this area of the chisel than closer to the edge of the chisel when it was new.

The hollow on a Japanese plane blade is set up a bit differently than the hollow on a Japanese chisel, and so a different approach is needed to maintain the hollow. Again, a picture to help illustrate:

The top diagram is a side view of a new Japanese plane blade. Again, the hollow area can be seen at the bottom, as well as a nice flat area behind the cutting edge. As the plane blade is sharpened, eventually the edge will get too close to the hollow, as seen in the middle diagram. The hollow on a Japanese plane blade is much more even in depth front to back, so that over time the hollow will be lost completely if the back is just ground away in a similar manner as with a Japanese chisel.

To get around this, a procedure called “tapping out” is done. If the edge is too close to the hollow, a series of light taps are done with a pointed hammer on the bevel of the plane blade, noted by the blue arrows in the bottom diagram, which moves the hard metal downwards to where the blue line is. Once this is done, the bottom of the plane blade can be ground to increase the flat area behind the edge just like with a Japanese chisel.

You can tap out a Japanese chisel if you want, and I have done that with wider Japanese chisels on a couple of occasions. But tapping out is not as necessary on a chisel as it is with a plane blade as long as you are maintaining the back as you sharpen.