Tapping out the back hollow (ura-dashi) on Japanese plane blades, chisel blades, knife blades and carving tools....... Everything takes a hit. Ura-dashi is faster than just grinding away at the stones, preserves the all-important steel hagane, and also serves to maintain a proper fit between kanna blade and dai. And it's fun!
Ura-dashi is a necessary skill to learn if you want to get the best performance from these tools, but it has a reputation for being a process fraught with danger. It's scary the first few times, but (and I can't stress this enough) get the right hammer and it's a piece of cake.
I got this old kanna recently that showed evidence of having had some pretty serious work done to the bevel. It looked as though the blade had chipped out very heavily, necessitating a more extreme session of ura-dashi than typical. Rather than the edge coming to a point, it ended abruptly at an approximately 3mm blunt tip.
When a blade requires very forceful measures to correct it for warp, twist, or has had an unfortunate encounter with a nail/rock/concrete floor, the pros will often grind the edge back a few millimeters before performing ura-dashi. This gives more support to the steel, reducing the possibility of making the damage even worse. This looked like one of those cases.
Things are coming along nicely. I have already corrected the slightly mushroomed head of the main blade, and gotten a good start on forming a new bevel. This is a 65mm blade, BTW.
I was able to hammer the upper portion of the blade back into shape, but the perimeter was starting to fragment, so I had to resort to filing a decorative chamfer. Frustratingly close to the original profile, but no cigar.
The main blade had been cut back so severely, that the secondary blade needed amputation as well (or possibly the damage was so bad that BOTH blades needed shortening, yikes!). So, ura-dashi for the osae-gane, too.
This is perfect, to my eye.
You can't make a silk purse from a sows ear, as they say. This osae-gane was well forged, and the urasuki was nicely ground. I just try to keep things looking "proper". Sometimes you get lucky......
....and sometimes you don't (get lucky, that is). This is the main blade.
The tiniest remnant of the original damage. The crack only became apparent after starting the back-flattening process. What I *really* don't like is that the crack runs longitudinal to the length of the blade. It is possible that this will propagate right up the middle and make the blade unsalvageable, but my fingers are crossed.
I grind the blade back another 3mm, about 1mm past the visible damage.
Right back to where I started, haha!
Hopefully that gets all of it. I won't know until I've completely finished sharpening the blade
There is a bit of a push/pull interaction going on during this process, particularly in such an extreme case as this. The hard steel hagane is.... hard, and doesn't bend (well, it bends but it doesn't retain the bend. It is highly resilient and resists permanent deformation). The actual bending is due to the soft iron being bent and displaced/deformed by the hammer blows. The blade was made using a relatively hard wrought iron and it feels a little stiff. This isn't the easiest blade for ura-dashi. Maybe the steel is very stiff, too?
You hammer the bevel, displacing and expanding the iron, causing the cutting edge to be deformed downward. This induces tension into the blade.
|The little squiggle to the right of center isn't the crack, it's a piece of lint.|
I check for flatness using a #1000 grit diamond stone. You can see the barest hint of the flat beginning to form at the bottom edge.
I do 90% of ura-dashi using just a tiny (8 oz?) tack hammer, but this blade calls for the big gun!
A 32oz ball pein hammer is nearly the LAST type of hammer that I would choose for this process, but it is close to hand. A nice funate pattern genno would be perfect for this. Someday....
I am trying to achieve a perfectly straight, flat, land (the ura-ba, I think) at the edge.
This is looking good. It is a beginning.
The problem is that there is a lot of material yet to be removed from the bevel. I figure that doing this process in stages will minimize the stresses on the already stressed-out steel. Some of the pros advocate an even slower pace, and do just a little bit of ura-dashi over the course of days or weeks. I'm patient, but.....
Back to the bevel then. Heavy files remove the iron quickly.
And here is where the push-me, pull-you comes into play. The ura-ba was looking perfect before setting the bevel, but now the line has become uneven.
I target a very specific region to focus my attention on. Only strike where needed. Don't hit the steel part, only the iron (I know you've heard that one before). Strike heavily towards the base of the bevel to affect a broad area, lighter and closer to the cutting edge for a more localized effect.
The ura-ba is more even in width, but now there is a new condition to address. The corners are no longer making contact with the evidenc accentuated by the different directions of the scratch patterns.
I switch up directions occasionally, so the scratches act as an reference.
Back to my usual hammer. Small strikes, as close to the corners as I dare.
There is a small spot at the far right that is still not touching fully, but that will come out as I finish.
A note: As I look at all of these photos, I guess that it should be clarified that I am right handed. I hold the blade in my left hand, with the head of the blade close to me, the bevel of the blade furthest away. Nothing weird or original about my posture, pretty typical. I would take a picture, but I only have two hands (*sigh*..... If only.....).
When tapping, I keep my hammer arm elbow tight to my side, for a controlled/restrictive movement. If your elbow is flapping around, your accuracy goes out the window.
The blade is now shorter and thicker than it was, so the blade is now a tight fit in the dai. There had been a paper spacer...... Gone now.
There is still a long way to go before the blade reaches the proper position.
It's time to actually sharpen the blade, but before I do that, I take a few measurements to see if I need to remove more material from one side of the blade, or the other.
It looks like I will be favoring the left when I sharpen this guy. The left is closer to the surface, and would be the first side to protrude, all things being equal. I want the blade to protrude evenly of course so I will try to remove more material from the left of the blade.
I do that by applying more pressure to the left side as I grind away at this thing.
I've been singing the praises of this sandpaper, but got the name wrong, haha. I was calling it 3X (actually a Norton product, and a good one, too), but it is 3M sandblaster. This is very tough stuff, and lasts an incredibly long amount of time.
This single piece has been used on a number of blades, and doesn't feel sharp at all. It still cuts, though.
That dust is mostly iron. About 50% of the grit is missing by now, and it's long overdue for replacement. If you sweep off the paper occasionally, it cuts faster.
After the #60 grit sandpaper, I jump to a #400 diamond stone.
A full, even width bevel.
And one last time, I check the back side for flatness.
It looks good.
Well, maybe just one more.....
And one thing to note about the blade in the picture. The line of hagane is flat and even. If I had just ground down the blade without performing ura-dashi, the line would be thick in the middle, tapering to nothing at the corners.
Remember the chisel in the last post?
And, lest you think that all I do is sharpen tools.....
We have rebuilt almost our entire house, with the exception of one small area. The dreaded bathroom.
Just in time for us to move, haha!
I need a shower.