I still need to do some shaping and sanding of the legs and arms before they'll be ready for glue-up, but I took some time to work out how to clamp the arm/legs assembly when it's time. I want to make sure that the pair of legs line up with each other (no relative twist at the leg/arm finger joints, and that the legs are parallel to each other and perpendicular to the ground.
I made this jig from scraps I had laying around. There's a plywood base with two right-angle gussets spaced to match the distance between a finger notch on one end of the arm to the corresponding notch on the other end. The gussets ensure that the legs will be parallel to each other and perpendicular to the ground. A jointed scrap of 2-by is attached on the far side to act as a rib to prevent the plywood base from flexing.
The leg ends rest on small cleats that are equidistant from the lower edge of the plywood base. This will ensure that the legs are aligned with each other.
To apply lateral clamping pressure as close to the finger joints as possible, I cut some small clamping cauls that can butt up to the edge of the protruding fingers.
I did find that even though the clamping force was close to the joint, the legs were being pressed tight to the gussets, so once glued, the legs may bend towards each other slightly. It wouldn't be much and would be corrected once the seat frame was installed. But, I wanted to see what would happen if I clamped directly on the joint. I cannibalized a scrap test piece from setting-up the finger joint jig to make a fingered caul.
I tested the clamping using the finger cauls, and found that the legs sprang outward, probably 1/8"-3/16" at the ends, which means the finger joint angle is slightly off. Not much though, and it would easily be pulled into place once the seat frame is installed. I'm not sure which caul I'll end up using.
The longitudinal clamps naturally apply pressure on their edge close to the joint due to the curve of the arm, so they don't really need a caul. This will also make sure that the legs are firmly pressed against the jig's plywood base. I measured diagonal distances of the dry fit assembly, and it was right on the money square.
So, I think I'm ready for a glue up when the time comes. I'll have to do one assembly at a time, but I'm sure I can find something else to occupy my time while the glue dries.
Heh, thanks Pottz. 😀 For this particular chair (where the glue-up won't be naturally self-aligning), time spent on a clamping jig will save me a lot of frustration later.
Nothing more stress relieving that a proper glueup jig. Especially with finger joints!
The fingered cauls are the proper technique for small boxes where an offset clamp can bow the part, but with your thick parts and having the ability to see the joints clearly for proper seating when clamped, I think your setup is super optimum!
I appreciate the perspectives, guys, thanks. I'm even considering hide glue for the finger joints just for the reversibility in case something goes wrong. And I haven't done too well with my hide glue attempts. Barring that, I'll at least use TB Extend to make sure I've got plenty of time to get the clamps in place.
