After reading a few of the comments from my last post it is clear that I should have added a little background information pertaining to the drawings that I posted on kanna fabrication. I’ve been so immersed in this subject that what may seem obvious to me, may not actually present that way to the masses. So here we go:
Sheet 1 contains a basic overview of the kanna. A section view is presented with the basic parts indicated. I chose to use simplified western terminology for labeling the parts. I could have easily looked up and used the correct Japanese terminology, but I think that would have only made the understanding more difficult, both for myself and the majority of folks who may read this.
Sheet 1 also contains a couple of tables. One table lists the most common sizes of kanna blade with the corresponding dimensions for the required dai. While a basic guideline, the dimensions on this table can be modified to meet a specific need or application. There are no absolutes when it comes to kanna.
Another table lists the recommended blade angles for the general types of wood to be worked. It is important to note that these blade angles are very general guidelines. In reality, the blade angle can be greatly influenced by the wood, the sharpening angle of the blade, the finished surface desired and the personal preference of the user.
Lastly, there is a visual pertaining to the grain orientation for the dai block. Again, generally speaking, quarter cut will be the most stable and durable in areas where there is very little change in the relative humidity. The rift cut block will be most stable in areas where there are large seasonal swings in humidity leveled. While these are not absolutes, be prepared for the consequences. A quarter cut block will move, cup and possibly crack when exposed to large changes in humidity levels. Your safest choice will always by a rift cut block, although you sacrifice some durability. How much durability, I cannot say.
Sheet 2 deals with the layout for the dai block based upon the blade being used. I have used dimensions that will result in a dai block that is a little oversized with a slightly undersized opening for the blade. This will leave enough material shape the block as desired and to fit the blade to the user’s preference. This layout should also leave a little room for error and a greater chance at success.
I have located the chip breaker retaining pin two thirds the thickness of the block up from the sole of the dai, but have given no other locating dimension. Even in my limited experience I have found that the chip breaker as well as the pin vary widely. So it seems best to locate the pin case-by-case. The pin itself varies quite a bit. I have settled upon using standard nails for the chip breaker retaining pin. The size of nail being 16d-8d, depending upon the size of the kanna.
Sheet 3 tackles the conditioning of the sole based upon application as well as the fitting of the blade into the dai. This is where subjectivity is king and will drive some to the brink. Every source I have read pertaining to the fitting of the blade into the dai states that the fit should be “snug, but not tight”. How’s that for subjective? Some sources do try to quantify it based upon hand pressure, but that is only marginally helpful. To add to the confusion, some source discuss that some users prefer a loose or tight fit. The best I can add is that you should aim for tighter than you think it should be until you get a feel for it. My drawing shows that you should concentrate on maintaining positive contact at the side groove areas and the area just above the blade bevel. The tightest fitting portions of those areas should be closest to the blade.
The dai should be cut with no allowance for lateral adjustment. The 1mm per side of lateral clearance indicated can be created as you are fitting the blade down into the dai. Remember, generally these blades are hand-made and taper in width as well as thickness, so most likely the fitting will be asymmetrical. (Trust me on this)
Sole conditioning is yet another subjective aspect of the kanna. It varies widely between the application and the users preferences. The scraped areas can be as shallow as a single swipe with the scraper plane all the way up to 1mm deep. Generally speaking, the shallower the recess, the finer the cut. Like with most things kanna, conditioning must be experienced to be understood. The best advice I can give is to start with a flat sole and make small changes. Then see how those changes affect the kanna’s performance.
Sheet 4 covers the final touches. Most of the edges are eased over for comfort, but the finer points are noted on this drawing. To retract the blade the dai will be struck on the outer portion (never the center) of the upper front edge. The hammer blows should be parallel with the blade. To facilitate this, a wide bevel is created with its face perpendicular to the blade angle. The outer corners of this bevel recieve a heavy clip to prevent splintering.
The long edges of the sole should be beveled to reduce the surface area outside of the cutting edge. Traditionally this seems to be a 30deg bevel and should be brought close to the edge of the mouth opening. Why 30deg? I’m not entirely sure, but the angle doesn’t seem to be critical to function. The remaining area at the edges of the mouth are then removed. This makes conditioning the sole a little easier and ensures that these areas do not remain proud and prevent the blade from contacting the workpiece.
The front and back edges of the dai sole should not be chamfered. The thinking is that by leaving the edges “sharp”, shavings and detritus will be swept away from the surface of the wood being planed and prevented from finding their way under the plane. I see no reason to question this, but confess that I do break these edges with a very light pass of a sharp chisel.
I also chose to include a reference to the end-grain mouth insert. This insert can be added from the start or later to close up the mouth opening after repeated sole conditioning. The end grain presentation adds a measure of durability and the block is easily replaced when the need arrises.
Hopefully that adds a little clarity.