This project is about building relatively historical knock-down furniture for a medieval reenactment event. As far as I know, couches with backs aren't really historical, unless you're in the mid-east. However, they certainly are comfy.
These couches use through-mortises with tusk-pegged tenons to make for a sturdy, stable piece of furniture that can be assembled easily (and more to the point, disassembled easily at the end of two weeks).
Note: Thanks and kudos go to the various friends who helped and advised on this project, including the author of bloodandsawdust.com, the fine folks at lumberjocks.com, and of course friends like Orthox and Ludyte who actually got sawdust on them and took their lives in their own hands by being in the same room as me and power tools :-).
We ended up cutting 1.5” x 4.5” mortises through the wide face of a 2×6, and then cutting a secondary mortise (a "key mortise") in the tenon (another 2×6) to peg it in place, except with a slanted peg, aka a tusk. See this link for a wonderfully detailed description (with diagrams) of the structure and the mechanical advantages:
Here's how the couch came out:
It looked even better with the cushions and bear furs on it :-).
Overall it was a hit; rock-solid, comfortable and stable, yet extremely easy to knock apart at the end of the two weeks. Now we'll see how well it weathers a year in a storage trailer.
For next year, I plan to replace the hunk of plywood with slats, which will both make it not look so horribly modern without the cushions and coverings (usually we pack rain-sensitive stuff down ahead of time at the end, to avoid having to take it home wet and spread it out to dry) and make the seat have a bit more flex for comfort. I also plan to change some of the proportions (see below).
Here're a few photos of the initial assembly.
You can see in these next two, the back post is tilted 10 degrees back, for comfort. This worked out okay, but I'm definitely changing the proportions for the next version. For one thing, I'll probably make the back tilt a little more, maybe 12 or even 15 degrees, for a more laid back feeling.
For another, you'll notice in the final assembled photo, above, there's an extra back rail for lower back support. This was retrofitted on, by cutting a wedge into the armrest piece and adding a reinforcement. More on this below.
But another issue is that, because the back bottom rail is tilted, its edge ends up being slightly higher than the front rail, so the seat is actually tilted forward. Very slightly, but definitely noticably when you're sitting in it. I'm going to change that to make it tilt slightly back.
This was designed to double as a daybed if necessary, so we made it the size of a full-sized futon (e.g. 75" x 54").
The rails are 75" long from the neck of the tenon. Of course, once you add the two 2x6 arms and the last-minute addition of adding a 1/4" deep wider mortise to embed the tenon neck in, and you lose 3.5" (or more; the 1/4" depth was a bit, ahem, organic...). So next time we'll have to take that into account.
The full length is 85", an extra 5" on either end. We didn't have problems with people banging their shins on them, this year, but it did feel like they stuck out a bit and that would be a risk.
Meanwhile, depth is a bit contentious. 24" is pretty near to standard for couches, and it works out okay for a full-sized futon pad (75" x 54"), but it's a bit too deep for shorter people. Cushions can pad out the back as needed (and since one of those shorter people brought lots of cushions, I was quite happy :-).
Here's a closeup of the tusks and key mortises, before they were stained.
Here we are in the woods, getting ready to assemble the couches. I made two, but only photographed one. Here's the full set of parts at the start.
Note that the center rail has no tenons, it just drops into a slot in the armrest. More on this below.
Sanding this thing was as much time and work as everything else, put together. Of course, I had a single orbital sander and a limited number of disks with a fairly fine grit (100). Also, I may have been overdoing it with rounding the edges off.
For the next attempt, I'm going to try to make this easier by a) planing the edges before sanding and b) using a handheld belt-sander.
We ended up making a bunch of spare tusks, since they were fairly thin and would be a pain to replace while out in the woods. Even if we make more of these couches out of pine, I'll seriously think about making the tusks out of something stronger. However, as it turned out, the tusks mostly did quite all right. We only had two break, and that was when we were pounding them out at the end.
This next shot shows a pretty good close-up of the ugly added support for the drop-in lower back support:
This was a last-minute change (I'd been planning to have a second sheet of plywood for the back) but it worked out alright. I'm happier with this than I would have been with plywood, but it's one area where the design definitely needs some finessing.
My initial thought was to replace it in the next version with a fully tenoned rail, but somebody suggested that the two tenoned back rails provide enough structural support, I might as well take advantage of them and stick with the drop-in approach.
In any event, my next version will almost certainly not bother with trying to have the last 2" of the armrest provide support. The grain doesn't really support it, all of the armrests cracked when I screwed them down (after the first one, I preemptively reinforced them with screws, and added a second piece of wood with the grain perpendicular to the armrest to take the actual load). In the future, I'll just cut the corner out, and make the second piece of wood longer, to support the whole thing.
However, the placement and size of the back support will definitely have to change, too. As it is, the gap between the lower back support and the seat is just a little too large. I'm leaning towards making the back taller so you can rest your head on the back, and also make room for a third back support. But I might just lower the middle back support a few inches.
These are just here for reference and completeness:
Here's a closeup of the key mortises. Maybe you can see how they slant, it's hard to tell.
Here's the finished assembly again, though I left the plywood out of the subsequent shots, so it'd be easier to see the structure.