Step 3: Inner Box Joinery
Alright, you've got 8 boards now. 4 of those are for your outer box to be joined with the legs and contain everything. Set those aside. We're going to deal with the smaller ones for now.
These are for the sliding storage box inside the footstool. Since there's downward pressure on this piece of the project, we're going to join these pieces together using dovetail joints. Working like this with pine or a similar softwood is pretty difficult, as it really loves to split and break and isn't at all like chiseling a hardwood.
I didn't sand these pieces after the joinery was done, and once a box is together it's hard to get in there and sand it. Since this is one of the main parts of the project that'll be interacted with, go ahead and sand those panels before you glue them together.
If you know...
Figure out your layout, mark and cut your boards, and chisel out the joints. Apparently it's recommended with softwoods to keep the joints pretty steep, which I was told later by a friend.
Test-fit the pieces together and see how it looks. I didn't keep this part of the project square, which didn't end up a problem, but was oversight and should be avoided. If they fit well, make some cauls that push the correct parts of the joints and glue 'em together. Measure your diagonals (corner to corner) to make sure they stay equal, ensuring a square box. Once it's dry, scrape and sand off any dried glue.
If you don't know...
This is pretty simple. A dovetail joint is a joint that can only be pulled apart outward, not in the direction in which any force on it will usually be pulling. Therefore, we'll have the tails shooting upward, caught in their pockets, keeping them from falling out. Obviously it'll all be glued, so there's really no cause for concern anyway, but it's cool to craft nonetheless :).
So, figure out which boards you want connecting where, keeping in mind about 2 inches will be hidden by the outer box when this one hangs out of it, so the top one doesn't need to be pretty. If you have an ugly panel, maybe use it there to keep it hidden. Once you're happy, I'd recommend marking and cutting out the pockets first (the top and bottom panels), because they're easier to scribe onto the other pieces. But either way is fine. 3-4 joints on each panel is very adequate.
Mark on the end of the board, and on either side, down as far as the thickness of the board it's connecting to. [image 2]
Saw down your lines, keeping as straight as possible. Then clamp the piece down onto a scrap piece of wood (so you don't ruin your work surface) and using a chisel and mallet, chisel out the area where the dovetail will sit. [images 3-5]
It's a good idea to chisel out half the pocket, then flip the board over and chisel out the other half from there [image 5]. You might even stay a bit away from your line to get the bulk of the material out, and then clean it up more carefully afterward.
Once that board is done, put its end onto its mating end in an L shape [image 9]. Keep all edges flush, and mark with a sharp pencil where they connect. Mark all the way around again, to the thickness of your boards.
Put the new board in a vise and cut on the inside of each of those lines (the lines are just outside of where the joint needs to be. If your cuts are too small, you've got something to shave away at until it fits). It might help to clamp the piece at an angle and cut vertically rather than cutting at an angle.
Chisel like before: half on one side, half on the other. There are router jigs for this, but it's good to know the hand tool method. Besides, it's easier to do at home.
Once you've got a rough version, check the fit and see where it needs work [image 10]. Be gentle, take off little bits at a time, and take your time. These joints will be seen.
Once everything fits, sand the things evenly until you're happy with their smoothness.
Cut some cauls from scraps that will clamp the piece only in the areas that will push the box together [image 13]. A straight caul will only push the pieces together until the edge is flush. Your joints might be able to be tighter than that, so take the time to get it clamped and glued snugly.
Once those are ready, do a dry run to make sure everything will work. Measure one diagonal once it's clamped and compare it to the other diagonal and see if they match. If not, you can angle the clamps a tiny bit to compensate and make sure the box glues up square. Get someone to help with this if you need it. Glue-ups can be stressful, and help is awesome.
If it works, brush glue into the pockets of the top and bottom panels, and the cheeks of the joints wherever things will be coming together. My documentation of this part isn't the best, but hopefully it's clear enough. If not, find someone knowledgeable who can help you.
Coat a corner and fit it together. What might help is dry fitting 3 panels, coating the other with glue and fitting it into the other 3, clamping everything square and waiting about 20 minutes while the glue sets. Then take the other end apart, coat that in glue and fit that together, clamping everything, making sure it's square, and leaving it to dry after that. It gives you less headache, and makes sure you don't rush through and miss something (like gluing the cheeks on half the box, like I did) or forget to square up the box before it dries (like I did).
Leave it overnight and come back the next day. Or take a break. Or, continue to the next step and work on another part of the project :).