Kanna Workshop – Sole Condition

In the last post I touched on the topic of kanna dai maintenance and the ideal condition of the sole of a finish plane. This time I thought I’d talk a bit more about that subject and also cover some of the problem areas that pop up when it comes to a kanna’s sole.

Typical Finish Plane – 2 Points of Contact

A finish plane is usually adjusted such that only two areas of the dai make contact with the workpiece. The area directly in front of the blade and the area closest to you when planing, which in Japanese is actually referred to as the rear or butt end of the plane (台尻). Reducing contact down to those two points does a couple important things. It greatly reduces friction between the body of the plane and workpiece, and it ensures that the area directly in front of the blade makes full contact with the workpiece just before the blade begins cutting which greatly helps to eliminate tear out and chatter. Also by hollowing out the back end of the plane, the last thing to contact the wood when planing is the blade, which elimates burnish marks being left from contact between the dai and workpiece. In reality achieving the perfect finish on a piece of wood requires more than just a perfectly set up kanna, but having a well adjusted plane will greatly improve your results.

Below is one of my favorite finish planes, which I pulled out and quickly tuned up for the sake of this post. Take the following photos with a grain of salt. It’s really hard to accurately capture the amount of hollow evident between the straightedge and the sole of the dai. That said, overall this shows a decent example of what I shoot for when tuning my kanna. A light tapering hollow between the contact point on the far left and the other near the blade, and a continuous hollow on the area behind the blade.

Here’s a look at each contact point, checking the level of flatness with the straightedge.

Using a pair of winding sticks, with one on each contact point, you can sight down and double check that the two are in plane with one another.

In the image below I brought the camera down to check the level of the winding sticks relative to one another, the same as you would do by eye in real life. You can just make out a faint line of light reflecting off the front stick which is nicely in line with the winding stick in back.

These days I rarely use winding sticks to check for warp in my kanna, largely because I use a sanding plate to “reset” my dai when doing any major tune-ups. The sanding plate does a great job of removing any potential twist, and I’ve come to trust that it will produce a flat consistent surface.

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Potential Problems

While the contact point at the butt end of the plane is important, it’s the contact point near the throat that tends to cause the most trouble when planing. The forces from planing wood are contanstantly pulling the blade down and the the workpiece up, and with the back end of the plane hollowed out, the contact point at the throat then becomes crucial for supporting both the blade and the wood you’re planing.

One common issue is when the throat contact point wears from use, and in extreme situations, it wears to the point that the area closest to the blade actually becomes hollow. This kind of situation is a recipe for chatter, as the point of contact moves further and further from the blade. With a single plane blade this kind of situation would also make stopping tearout nearly impossible since the contact point is no longer supporting the fibers of the wood being planed.

How quickly a dai will wear is really dependent on the hardness and abrasiveness of the material you’re planing; the width of the material you’re planing; the width of the contact points; and how much pressure you use when pulling your kanna.

Below is another kanna of mine that I tuned up and used to plane some narrow white oak stock. The stock was narrower than the blade which meant I could get a nice clean one-pass shaving, but that also resulted in the dai wearing unevely as the pressure from planing was concentrated on the middle of the dai.

Looks pretty decent from the from the side…

…but checking at the throat you can see how much the hard oak has worn the dai…

… yet it’s still pretty nice and flat at the butt end.

Even with the convex throat, this is still a perfectly useable kanna for general work, and I wouldn’t hesistate to use it as is. With continued use if it seemed like the convexity was getting worse and posing problems it would be easy enough to solve with a couple quick swipes from a scraper.

I share this just to show how quickly a dai can wear, especially when planing narrow stock which has the effect of amplifying pressure in a specific area. When planing a wide panel a dai will wear much more evenly.

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Another common situation that pops up is when a dai moves and the very front and back of the plane become the contact points, which you can see in the picture below. If you’ve even pulled a plane and found that you can only get shavings at the beginning of the cut, then a dai with this kind of condition is a likely culprit. With enough force you can press down while planing and make a kanna like this work, but it’s going to be a chattery cut and difficult to maintain even pressure, not to mention you’ll wear yourself out from all the extra effort.

Here’s the same kanna after a bit of a tune up, with the contact points and hollows reestablished.

I tuned this dai at my shop, and then brought it home to take these photos a couple days later and in that time the dai moved ever so slightly. You can see a slight covexity to the contact point at the butt end of the plane…

…and at the throat as well.

Again I wouldn’t hesistate to pick up this plane and just use it as is. For general work small imperfections in the dai are usually not going to prevent you from getting decent shavings. That said, for thin ultra thin shavings and kezurou-kai, the level of precision gets a lot more demanding. In other words, the thinner you go the more you have to perfect the condition of the dai.

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3 Points of Contact

One last example is a kanna setup with 3 points of contact; one point at both ends of the dai, and one at the throat. There are other variations on this theme, including an additional 4th point right behind the blade. Adding contact points behind the blade can help when jointing stock or when you have to do a lot of planing on a piece and are worried about maintaining the flatness of the stock.

I personally rarely setup a kanna with 3 points. Using machines to joint and plane stock means that 99% of the time a standard finish plane with 2 contact points is going to be adequate for finishing the surface of the wood. Also maintaining a 3rd (or 4th) point becomes more demanding as you’re now contending with another area of the dai that needs to be kept in plane both down the length of the dai and side-to-side.

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Kanna are fantastic tools, but they are also demanding and require constant upkeep. It can be frustrating to spend lots of time setting up and adjusting your dai, only to find that in short order some factor has changed and you’re no longer getting a quality cut. Many factors come into play when it comes to dai movement and wear, and these examples show just how finicky kanna can be and how much they can change in a relatively short period of time. But with practice and experience, the process becomes a lot less daunting as you develop an eye for what problem areas to look out for, and the skills to quickly tune and adjust your dai.

Happy planing!