Built not Bought: Hikouki Kanna
I first must fully disclose that this notebook entry and the process I followed could not have been without standing on the shoulders of some great giants. Both Brian Holcombe and Jon Billing wrote excelent articles on the Hikouki Kanna that were instrumental in helping me build my own.
A critical step when building a set of shoji doors is to prepare the Kumiko, the thin strips of wood that divide the light from the washi paper. These strips must all have an exact thickness so that thier lap joints fit snuggly with now gaps. Additionally, all surfaces should ideally be hand planed to bring out the luster of the wood.
To achieve this a special plan is used, the Hokouki Kanna, which has adjustable rails and a spring loaded pad which presses the thin wood strips down, ensuring they are all planed to a uniform thickness, while not rising up into the blade. While one can be purchased, they can be very costly, and making one's own tools is quite rewarding.
The general process for producing kumiko strips is as follows:
- Mill stock to a square and even thickness, such as with a power jointer and planer.
- Use a fine finish plane to take the board to final thickness and an even sheen. These faces will become the thin edges of the kumiko you see when facing the shoji.
- Finally use the special kumiko plane to take each strip to its final thickness in the wide dimension.
The remainder of this notebook entry will detail the construction on this plane.
Modifying an Existing Dai
To begin I took one of my older 65mm kanna and jointed the dai square and clean again. Then I built an angled saw guide attached to a perpendicular piece of wood so I could cut two angled cuts in the dai, approximately a half inch from the mouth. This will form the beginnings of a sliding dovetail to contain the pressure rail.
After the cross grain cuts established the sliding dovetail, I chiseled out the waste and cleaned up the groove.
Building the Pressure Rail
The sliding dovetail dimensions are largely arbitrary and so once that groove was complete I used its measurements to make a block with a triangular cross section that would slide easily into the dovetail, but not rattle. Note that it is important that this piece be sized to leave some space between the bottom of the sliding dovetail to contain the spring which will be inserted shortly. Also use a robust, dense hardwood for this to limit wear. I chose a scrap of Ipe.
Once satisfied with the fit, I trimmed the rail to the width of the dai. Using a spare piece of spring steel, I bent a rough arch with slightly curled ends, removing any burs. This is inserted into the bottom of the sliding dovetail:
Continue working the shape till the rail had a tight fit and could be compressed with determined thumb pressure. Finally, round over, sand smooth and burnish the pressure rail till it slides evenly and can be compressed inline with the sole of the dai.
Installing the Thickness Rails
I begin by cutting two rabbets into the side of the dai at a know vertical dimension. Between 1/4" and 3/8" should be sufficient as long as it is exact as this dimension will be used to determine the width of the guide rails.
Then I milled a few pairs of rails such that their width was the sum of my desired kumiko final thickness and the rabbet depth cut into the dai. For dimensions I chose 1/4" for shoji kumiko but also made 1/8" and 3/32" for smaller, decorative kumiko. A pair of countersunk screw holes finished them off.
Assembly and Use
Prior to final assembly of the kanna, I inserted a freshly sharpened blade as normal and checked the sole for proper relief and eliminated any twist between the leading edge and area just in front of the blade. The operation of this plane is different from a standard finishing plane due to the guide rails and pressure bar.
Turned over, you can see that during use, the pressure bar will force the kumiko strip down before the blade, preventing it from arching up causing tear out or variations in thickness.
Using a technique copied from John's article. I built a planning rail with a little barbed catch. This was made by inserting several sharpened nail ends into a small sliding dovetail rail, angled slightly backward. This allowed me to "hook" the kumiko on the nails and pull against them. Between the tension on the thin wood strips, and the pressure bar, these weren't goin anywhere during planning.
Having table saw ripped each of the kumiko strips to a slight oversize, I started with a few rough passes with a standard plan. Then with a very fine blade protrusion on the hikouki kanna, I took passes until I could no longer produce shavings, the side rails limiting the downward movement.
It was a rewarding experience to take the time and build a tool that make this process both much easier and resulted in an excellent surface finish on the kumko strips.