Fixing the piece that doesn't quite line up
Last time you clamped the whole thing together dry, no glue, just to see how it sat. For some of you it came together sweet as anything on the first try, corners tight, every piece flush, and you walked out of here standing a little taller.
For the rest of you, something wasn't right. A gap where two pieces meet. A shelf that rocks a little when you set it on the bench. A side piece that leans instead of standing up straight.
That's good. I mean it. This is exactly the lesson you needed.
First, figure out what kind of problem you have
Three things tend to go wrong at this stage, and each one gets fixed its own way.
The gap problem. One edge isn't sitting flush against the other. Usually that traces back to a cut that wasn't quite square, the thing we checked against the square a couple lessons back. If you waved that check off because your cut "looked fine," this is where it comes around to find you.
The rock problem. The shelf won't sit flat. Usually one leg or one side is a hair longer than the other, or your bottom shelf isn't parallel to your top shelf. This one's sneaky, because it stays hidden until you've got the whole thing assembled.
The lean problem. Nothing is horizontal or vertical the way it ought to be. This is almost always a measuring issue from way back, and it's the hardest one to fix after the fact, so we'll spend the most time here.
Finding out where the error actually lives
Before you fix a thing, you need to know exactly where the trouble sits. Don't guess. Get down at eye level with the joint and look at it straight on. Set your square right on the joint itself, not just on the individual boards.
People want to skip past this, because they already know something's wrong and they just want it fixed. But fix the wrong spot and now you've got two problems where you started with one.
Fixing a gap
A small gap, less than about a sixteenth of an inch, I'm not going to make you re-cut anything for, not in this class and not at this stage. A little gap on a shelf isn't the end of the world. It'll draw up some when you clamp it with glue, and if it doesn't close all the way, that's a lesson for another day, not a crisis for today.
If the gap runs bigger than that, the pieces probably need to come apart, and one edge wants a fresh cut, or a pass with a hand plane if you've got one. We haven't touched planes in this class, so if that's where you are, come find me and we'll sort it out together at your bench, not from across the room.
Fixing a rock
Set the shelf down on the flattest surface in the room, which is usually the workbench and not the floor. Find which corner or edge is riding high. A lot of the time no single piece is wrong; it's that two pieces, each fine on its own, don't agree with one another.
The fix here is almost always sanding the high spot down a little at a time, checking, sanding again. Slow. You're not trying to solve it in one pass. Go too fast and you'll turn a small problem into a bigger one, because once material's gone you can't put it back.
Fixing a lean
This is the one where I want you honest with yourself about that reference edge we keep coming back to. I'll say it once more because it earns the repeating: measuring twice does you no good if you measured from two different starting points both times. Every error needs to trace back to one edge, one reference, every single time. When a piece leans, nine times out of ten it's because somewhere in the cutting or the layout the reference point wandered.
Sometimes the honest fix is a fresh piece. I know that's not what you want to hear after all the work you put into cutting these. But a shelf you understand serves you better than a shelf built from pieces you're fighting the whole way through.
A word on patience here
This is usually where I slow the whole class right down, and I know that grates if you're itching to get to glue. I'd sooner you walk out today with a shelf that sits flat and square than one that came together fast and wrong. It's just how I teach. If it's driving you up the wall, tell me, and I'll probably still make you check it twice.
My neighbor Rodney came over once just to borrow a drill, and three hours later we'd built a birdhouse neither of us needed, mostly because the first two walls didn't line up and we kept pulling it apart and redoing it until they did. Neither of us was in a hurry that day. That birdhouse is still hanging in his yard. Not great, but it held, and it's square, which is more than I can say for some of my early stuff.
Before next time
Get your dry fit sitting flat, square, and gap-free before you leave today, even if it takes you a few passes. Next time we glue it up for real, and there's no fixing a lean once the glue has set.