Continuing on from the last post about sharpening, it’s time to talk about the ura side of the blade. But first let me reiterate that there are a ton of different ways to sharpen a blade and the important thing with sharpening is to find a technique that is enjoyable, and gets you the results you’re looking for. These posts on sharpening are just one perspective on a very deep subject. They are a journal of sorts about my own evolving understanding regarding sharpening kanna as a carpenter/woodworker.
裏 Ura: Flat Vs. Curved
When you first buy a Japanese plane there usually is a curvature to the blade such that the ura side of the has a curvature down the length of the blade.
My first experience learning about sharpening Japanese planes, involved flattening the back of the blade considerably, and in so doing removing most if not all of the curvature.
Here is my first Japanese plane blade. It’s been sharpened such than any curvature is essentially gone, and for the most part it’s pretty flat in the areas where it has been polished.
Flat Ura Problems
One big problem that I’ve encountered with sharpening a completely flat ura, is that after a lot of planing the very edge of the blade gets worn back a bit. If you hold a worn blade in a good light and angle the blade so light is reflecting off the ura side of the edge, you can see the worn area as a fine white line. That worn area essentially becomes a low spot on the ura side of the blade, and needs to be removed to get back to a truly sharp edge.
When I sharpened my blades with a flat ura, removing the worn edge was something that frustrated me for a long time. I would often sharpen the bevel side of the blade until I felt a slight burr on the ura side, but when I followed through to the finish stone and polished the ura, there would still be a noticeable white line. In other words I wasn’t fully removing the worn area of the blade and was not getting nearly as sharp of an edge as possible. At that point you are left with either having to go back to do more sharpening on the bevel until the worn edge is removed, or just forge ahead and use the blade as is. For a lot of woods, especially harder woods, you can get by with a blade sharpened like this, but if you try to plane anything really soft like western red cedar or sugi you’re in for some frustration.
Hopefully these crude drawings can help illustrate the idea a bit more.
Imagine the red line in the picture above as the area of contact between the stone and the ura side of the blade. Obviously the drawings are exaggerated but the idea is that even if you have sharpened the bevel enough to feel a burr on the ura side of the blade, there’s a chance that the very edge may still be low compared to the flat ura.
Maintaining the Curvature
The method that I have learned from my friends here in Japan is to maintain the curvature of the blade, and to use it to your advantage to ensure that you are contacting the very edge of the blade when you hit the ura side on your final stone. With this method, whenever you initially setup the plane or when you are doing maintenance on the ura side and using a coarser stone, the idea is to work the blade off the stone. In other words if you start with the blade 2 cm on the stone (*for example* the actual distance depends on your situation) then you should stroke the blade back and forth maintaining pressure on the edge while working your way off the stone.
Sharpening the ura this way allows you to achieve contact on the stone across the edge, but also ensures that you aren’t removing the curvature of the blade along its length. By maintaining the curvature of the blade you are also reducing the points of contact between the blade and stone down to just the very edge of the blade and the area that is contacting at the very edge of the stone.
When I’m doing maintenance on the ura side of the blade using a courser stone I will work the blade off the stone to maintain the blades curvature, but when doing the final polish on a finish stone it’s not necessary to work the blade off the stone. On the finish stone all that’s required are a few light passes, working the blade straight along the length of the stone.
You can see in the picture below on the finish stone a line of contact just where the edge of the blade touched the stone, as well the areas where I polished the double bevel. I typically start with the double bevel and alternate a few times back and forth between double bevel and the ura side to remove any burr from the edge of the blade. If you want to see the process in action check out the video in the first post on sharpening.
The benefit of this method is that it takes much less time to ensure that you have polished right up to the edge on the ura side of the blade. Unlike a flat blade where a worn edge essentially becomes a low spot relative to the rest of the ura, with a curved ura blade you end up with more of a little hump behind the worn edge.
By using the curvature of the blade to your advantage, there is much less material preventing you from sharpening right up the the very edge. Essentially it’s just the little bump that needs to be removed in order to reach the very edge of the blade. In practice I’ve found that the little bump and burr pictured above is pretty easy to remove just after a few passes on a finish stone, but if I’ve really worn out my blade after a lot of planing I will occasionally work the ura side on a medium grit stone first.
It’s a bit difficult to see in the picture below, but there is a polished line right across the edge. You can see it most clearly on the right side of the blade.
You can also rejuvenate old blades using this method, just by tapping out the edge across the full width. Here’s an old used blade that was once ground essentially flat on the ura side, but I tapped out the bevel to start to put some curvature back in the blade and achieve the same line of contact just at the edge.
One final note about this method is that it does involve frequent tapping out of the blade (ura-dashi) to ensure that the area of contact at the edge doesn’t continue to get too large. Ura-dashi is a whole other topic to discuss so I think I’ll leave that for next time.
Hopefully this proves helpful to the Japanese tool fanatics out there. Let me know how it goes if you try it out. As I said in the previous post about double bevels, I had a fair share of skepticism about sharpening the ura this way, but once I tried it out of it was clear that it works really well, and my ability to get a consistently sharp edge has increased dramatically.
Happy planing.
The drawings are so helpful making everything much more clear.
Thanks! I’m glad to hear that. It’s difficult to take pictures of some of these details and even more difficult to explain.
Jon, this is the best explanation I’ve seen on this topic. Thank you. And makes me realize why my kez friends use tape. Do you ever use tape to allow yourself to reference off the corner of the stone without wearing a groove in the side lands on the back.
Hey Brian, I’ve never really used tape. I know tape works well for some people and probably allows you to keep the ura looking really clean.
But for general work when you’ve got to sharpen fast, it’s too fussy, so I’ve never really used it. That said with the method described here, since you work the blade slowly off the stone getting steps/grooves in the back isn’t really a problem. You are never grinding away on the stone in a fixed position where grooves can develop.
Hi Jon, i was wondering do we need to do tapping to the chip breaker as well and how often you tapping your blades
Hi Simeon,
When first setting up a plane I will tap out the chip breaker if it’s necessary. For example if its really far from flat, rather than grind away on my stones to achieve a flat surface I will speed things up by tapping in the appropriate areas to more quickly get even contact across the front edge. But rarely do you ever need to tap a chip breaker out once the ura is flat, since you won’t sharpen it nearly as much as your regular blade.
As for tapping the main blade, it’s hard to say how frequently I will do it, since some blades require more work than others. But I do usually tap out after hollow grinding, since sometimes the hollow grind will release tension in the blade and the ura will go out of flat. The other main situation where I tap is when I start the ura starts to get too thin near the edge.
Hope that helps a bit.
You are a very good writer, it’s easy to understand and I will try your way feels very logic, thanks
Tiu
Thanks for the kind comment.
-Jon