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Ura-dashi (tapping out)

Finished the latest project …
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And having a few days of relaxation to spoil daughter, who’s going to spend a few days with us from Tokyo, before I get started on the next major challenge.

A few people from one of the forums mentioned the
ura-dashi process after reading my piece on tuning my plane, so I thought this would be a good opportunity to describe how I carry out this important step in the correct use and maintenance of Japanese planes.

The overall process consists of two parts:
ura-dashi (裏出し) tapping out; and ura-oshi (裏押し) flattening the back.

Ura-dashi is normally done when the blade edge has been sharpened to the rim of the hollow (called the ura-suki 裏透き). It can also be done on new blades where the ura is not flat, either on one side or the other, or in the middle.

I’ll emphasize here that there are many ways of tackling this, and provided they work, each one is as good as any other. This is the method I use, and it works for me.

The first thing to do is check the cutting edge in the mouth. Eventually with sharpening over time the cutting edge will extend beyond the sides of the mouth (
ha-guchi), and if this is the case you will need to file back or grind the mimi (blue arrows). You will know if this needs to be done because the shavings will become caught up in the mouth.
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This plane is OK (green arrows), so on to the
ura-dashi.

This photo shows where to hit with the hammer.
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Too close to the edge will crack or chip the hard steel (
hagane). I only tap on the back half of the blade bevel (kireba 切れ刃). In the case of this blade, the left-hand side of the ura-suki (hollow) is slightly deeper, so I needed to tap slightly more on this side. The key point here is to look closely at the blade, and find out what its characteristics are, if any.

I use the edge of my workbench to support the blade, as shown in this photo.
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My workbench is Queensland maple, a reasonably firm hardwood. You can use any firm surface provided it gives good and solid support for the blade. I’ve rounded off the edge of the workbench for the blade, and filed the vertical corner to give a flat reference surface for my left forefinger. In Japan I used the end grain of a large board of
keyaki (zelkova). Anvils, and short cut-offs of railway track are also used.

I use the corner of my
gennou to tap out. This one is about 200 g, and it gives me good control. Any size or weight is OK provided you have control over where it strikes.

The point here is not to hit too hard. Many moderate taps is better than a few hefty smashes, which will almost guarantee a crack or chip. It also ensures better control of the hammer. You can tap slightly harder further away from the cutting edge, but the closer you get to the
hagane, the gentler the tapping should be. And always ensure the blade is fully supported at the striking point, so move the blade rather than the hammer. That is, always strike at the same point, but use your thumb to move the blade to the left, right, up and down.
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The next photo shows the result - an even distribution of tap indentations. And importantly, no chips or cracks. As I mentioned before, from this I continued to tap out with somewhat more bias towards the left-hand side because of the blade characteristics.
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That completes the
ura-dashi, now on to the ura-oshi.

I use my normal Shapton stones for ura-oshi. It is important to make sure that the head of the blade (atama) does not sag down as you flatten the back. So when doing this, you should feel as though you are lifting the head up slightly while applying the downward pressure to the edge, as shown in this photo.
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You shouldn’t actually lift the head up though, as this would give the blade an unwanted back-bevel, just feel as though you are. Regularly check the shape and adjust the pressure as necessary.

This is the blade from my first plane I bought quite a few years ago, and this is what happens if you let the head sag. This was a source of great embarrassment for me at the College, and complete humiliation when my instructor looked at it and simply said “Hmmmmm”.
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First, I give the back a few swipes on the 5,000 grit stone to check the condition of the ura-dashi, and make sure there’s an even protrusion. If that’s OK, I move to the 1,000 grit stone.
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Another point to take care with is that as you move the blade left/right on the stone while flattening the back, you should also move it forward and back 10-20 mm so that a ridge doesn’t form near the cutting edge.

Then progressively through the 2,000 grit…
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5,000 grit…
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and 8,000 grit.

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This is how it ends up. A clean even ito-ura.
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The geometry of the blade has been changed slightly, so I also have to check how the blade sits with the chipbreaker, and adjust the chipbreaker as necessary.
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From here it’s on to the normal sharpening process.
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