Two on Test (Part 4)

In the previous post I spent some time fitting the hoops, kashira, to the chisel handles of these two Japanese bench chisels, o-ire-nomi. The next step is to prepare the cutting edge.

Japanese chisels, marking knives, and plane blades are put together with a traditional technique involving forge welding a hard cutting steel to a soft iron backing. The forge welding and subsequent cooling often can cause some amount of warping/cupping, and it is up to the skill of the smith's technical expertise to minimize the effect. Essentially, the cutting steel, as it cools and shrinks, tends to exert a disproportionate deforming effect on the soft iron backing, and it is not uncommon to find brand new chisels and planes in which the underside of the blade, or ura, is cupped hollow to some degree. Sometimes the ura has been flattened well after forging but if a few months have gone by residual stresses can continue to work themselves out and cause the tool to be a little less flat. Hence the reason the tool comes to you new in the box in a roughly sharpened state only.

One could place the ura of the tool on a surface plate and see what thickness of feeler gauge can be inserted here and there, however having numbers to work with is not necessarily all that useful. It is more to the point to determine what sort of shape the ura has before embarking upon a sharpening process. For instance, with many brand new plane blades, it is often the case they you need to tap out right away, before doing any sharpening at all. It depends upon the condition of the ura.

With chisels, since they do not fit into a wedged opening in a plane block, the procedure is a bit different. Rarely does one have to tap out a chisel, unless an obsession with a beautiful ura has taken over one's mind. Watch out for that.

The first thing I do is flatten a finishing stone and then rub the chisel, ura side down, on the stone a few times to see what sort of marks are left on the stone. Those marks tell me about the condition of the ura.

Starting with the Yamahiro then:

The Yamahiro leaves two streaks on the stone, one at the cutting edge area, and one at the far end of the hollow along the edge of the stone.  A chisel with this shape of hollow will be somewhat prone to digging in when paring and if you were to simply work on flattening the entire back you would produce an ura with an unattractive appearance, one in which the landing at the blade edge is wide, and in which the landing at the far end of the hollow is also thick/wide. It will also be impossible to produce an even finish on the hollow as one slides the back on and off the stone in flattening. This shape of hollow means I need to give some extra effort in flattening the steel near the cutting edge and avoid placing the entire hollow on the stone for the time being.

Let's see how the Kunitoshi does:

Here we have the exact opposite situation to the Yamahiro - the mid section of the hollow is riding on the stone, and the cutting edge and rear end of the hollow are riding clear. A blade left in this condition would not pare well when slid along its ura, and the temptation would be there to angle the chisel up at the handle end to get the blade to engage, which it would, but not always especially predictably.

Of the two chisels, I find the condition of the Kunitoshi to be somewhat easier to deal with. I only need to keep attention on the cutting edge area while sharpening and feather in the ura length up and back until I have put about half the chisel on the stone, more or less. There will be some tendency to make the sides of the hollow, the legs (ashi) as they are termed, a bit fat as the flattening proceeds, however I will do what I can to minimize that effect while creating a ura that is flat from the cutting edge back about 2/3rds of the way along. The landing on the Kunitoshi at the cutting edge is generous so there isn't much to be gained by tapping out. That process of flattening will make for a decently functional chisel, and as with any striking chisel I don't tend to fuss the back as much with setting the chisel up for paring work. It is a chopping tool mostly. For paring, you want the back as flat as you can get it for as long a distance as is practical, without making a mess of the ura. I have a separate set of paring chisels dedicated especially to that use.

So, first I work the two chisels on the coarse stone, which is a 1000 grit ceramic:

I do use two hands and unfortunately lack a third hand for taking photos, hence the one-handed staged photo.

The Yamahiro bevel was a little out of whack, leaving the edge non-90˚ in  relation to the side, so I concentrate my efforts on the high side to bring it down. Here's the appearance after the initial round of work:

The coarse stone does the most to affect the shape of the tool so it is important to keep it flat and to use the stone to get the tool the shape you want it to be. I re-flatten my stone several times in the course of working the bevel.

A little further along and you can see that the bevel is 98% flat now, with a slight untouched spot left in the lower left corner of the softer iron portion:

Once the bevel was completely flat, I re-flatten the stone and work the ura, which removed the wire-edge from the tool.

The Kunitoshi next. It didn't require much adjustment for shape and was done relatively quickly:

Then on to the next stone. I proceed on a pattern of grit doubling, more or less, so my next stone is 2000-grit, also ceramic (Kita-Yama brand).

The Yamahiro is starting to get flatter and cleaner:

The Kunitoshi after a round on the 2000:

Again, after the bevels, I work the ura on the same stone. When I'm done, I re-flatten the stone and put it away on the drying rack.

The next round is on the 5000 grit ceramic. Here's the ura on the Yamahiro after that go-round:

The bevel after the 5000:

And the Kunitoshi after the 5000 grit round, ura side:

Bevel:

Normally, I would stop at 5000 grit for a striking chisel, however this is the initial set up of the tool so I like to take the back out to the next level, which is 10,000 grit.

The Yamahiro ura after 10,000:

A check to see if the chisel will shave some hair off the back of my hand:

And same with the Kunitoshi:

Both chisels are white steel so my expectation is that they both would take a really keen edge. It will be up to the smith's skill though to see how durable those edges are. So far, I found the sharpening on the ceramics to be pleasant, especially on the 5000, which is a Shapton Pro stone. The Japanese would say that the grinding 'taste' was good (Try this bit of Japanese on your friends: togi aji ga ii desu ne). The 1000 I have (New Kent brand) was the least tasty of the lot, but it still cut adequately. Probably there is a more suitable 1000 grit stone for white steel. I'll do some digging around and see what I can find. Where's a knowledgeable sharpening stone guy when you need one?

Right, well that pretty much does it for setting up Japanese bench chisels. Over the next month I'll put these chisels to good use in a variety of woods and then come back with a 5th post in this thread and give my general impressions of the two brands. Thanks for tuning in and hope to see you next time. Comments always welcome.