As with Sangaku, Yuugou features an interior frame that holds all the components and an outer frame that, well, looks pretty (and keeps out dust and such). This time, the exterior frame is made out of hard rock maple and Hawaiian curly koa. If you’re not familiar with koa, it is primarily only found in Hawaii and is one of the most beautiful and expensive woods in the world. The pieces used on the side of the case cost about $40 USD per square foot.
The main feature of the outer case is the eight joints that hold it all together. These joints proved challenging, because first I had to design them. The frame needed to be rock solid, use no glue, and be able to come apart. In fact, there is no glue used at all on any of the wood for this case. There are also no nails or screws that hold the wood together.
It took three tries, but eventually the joint design was found by experimenting with some cherry wood, which is what the case was originally going to be made of. The case fits through a series of interlocking mortise and tenon joints, each one holding the other in place. Once all four sides of the case are connected, they lock each other together.
One of the most important parts to any project dealing with wood is getting the wood straight, smooth and to the correct thickness. Unfortunately, I’m not yet ready to do this by hand and I had to use my 12” Delta Surface Planer for this. What you end up with is a finish smoother than what sand paper can accomplish and perfectly square pieces, ready for the next step.
Originally I was going to use the cherry for the entire frame and Sitka spruce for the koa parts. This would have given it a simpler aesthetic, but I had one problem -- my cherry wood was not thick enough after doing the full size drawings of the case. So, I had to go with the maple and koa combination. Personally, I think it looks a bit more classy and was much more fun to work with.
Since the tolerances with these joints were so tight, its was imperative that I made very accurate drawings of the joints to make sure there would be enough clearance for the everything. One drawing shows a top view of the top joint of the case, the other drawing shows a section cut of the lower front joint.
The next step was to mark everything out to cut the tenons using my marking knife. I cut the tenons themselves with my Mitsukawa rip dozuki saw and trimmed them up with my paring chisels.
After that, it was time to cut the corresponding mortises in the maple pieces. To do this, I first marked them out again with my marking gauge, then I drilled most of the material out with my drill press. From there, I cut out the waste with my Japanese bench chisels. This took a long time, because each tenon had to be specifically paired with a mortise (since they are each cut by hand, they aren't all exactly the same).
This extra step makes all of the mortise and tenon joints very tight, which is how they should be. I used the same technique for the mortises on the cross pieces themselves. These mortises accept the other cross pieces which will lock everything together.
The next step was to cut the second tenon on each of the existing tenons (confusing I know but the pictures show what I mean, I hope). The goal is to get the maple joints to look like the prototype cherry joint (the dark wood on the right).