The Branch Joint
The branch joint is one of the central visual and structural elements of this rocker (for better or for worse). I knew it needed to be strong. The individual components were certainly strong enough. I decided that all three of them needed to extend all the way down to the rocker (It doesn’t look that way on the outside, but you can see it on the inner faces of the legs. I’m still thinking about switching that around).
Inside view of the bending ply mock-up
Outside view of the mock-up. Still a bit of work to do to get everything centered.
And I wanted as much contact between parts as I could get. One last thought was that the outer parts would be stronger with an ‘L’ shaped cross section. What I came up with after much sketching and experimenting (and a number of unsuccessful jigs) was a complicated lap joint.
A sketch of what's going on in the joint
Because the faces of the bent laminations were not quite good enough to join, I started by re-shaping the front face of the back leg. I then reshaped the back face of the arm support to match it exactly. These two were then glued together, and then the front face of the arm support was faired. Next came a curved rabbet in the joined parts. To match this, I re-shaped the back face of the front leg, then cut the curved rabbet in the front leg. In the final assembly, I do shoot a couple of screws through the top of the joint for good measure. These get plugged, and a layer of matching veneer is laminated over the entire bottom of the ‘front leg’.
Altogether, it takes two sets of 6 jigs do both sides of the chair.
The hard part of all of this was getting all of the shapes and routed curves not only to match up, but to flow as well. It’s reasonably quick now that everything works, but it took forever to get it the way I wanted.
It wasn’t exactly a straight line, but there really was a path from the plywood prototype to the finished piece. I started by transferring the rocker and seat locations and curves from the prototype (see the previous post) to a piece of foamcore that a neighbor had discarded (1/4” plywood is my usual choice, but the foamcore was there).
Full-scale sketch based on the comfort prototype
It wasn’t anything fancy; I set the prototype down on its side on the foamcore and traced the rockers, then pulled a side off so I could transfer over the seat and back profiles. I had an overall concept of the piece with its branching components, and I played around for a few hours filling in the details until I had a basic sketch to work with. A good eraser really helped with this!
I then spent some time bandsawing rough parts out of some left-over 12/4 and some plywood, then screwed things together with drywall screws. A rough prototype emerged (I didn’t add rockers), but it gave me a chance to see if what I envisioned was going to work at all.
The first '3-D sketch'
It also gave me a sense of whether I could create the front/back shapes that I wanted if I kept going down this path. It wasn’t exactly pretty, and an awful lot was missing and/or wrong, but it was encouraging enough to want to move ahead.
My goal at this stage of any design is to figure out the important questions, and then to answer them as well (and as quickly) as I can. I had answered the most important question: ‘did this idea have some potential?’ Now came the tougher issues of refining the very rough visual elements and how to actually build this thing. I decided to laminate up the main components out of 1/8” bending ply. That meant building lamination forms. I took the curves off of my sketch and made up the four main bending forms: for the rockers, the rear legs, the arm supports, and – for lack of a better description – the front legs. Laminating the bending ply was easy. I added the front profile to the rear legs, but I left the front legs and arm support wide – I knew I was going to need to play with those.
And then I stopped. And thought. And thought some more. About the branching joint. I knew what I wanted, but this was going to require a very solid joint and some serious experimentation.
Next post: experiments in three-way lap joints.