Once the groove is planed into the sides, we have to make the bottom fit. Our bottom panel is 1/2″ thick, and our groove is only 1/4″ thick. So over a run of an inch or so, we need to bevel down from 1/2″ to 1/4″. I usually cut this with my #4 smooth plane, but there are various other ways to do it. Specialty planes for just such a task (known as panel-raising planes) are available…just. But they’re expensive and single use, so I just make do. Bevel across the grain first, so that if the ends spelch a bit, it will come out when you plane along the grain.
Once the bevel is cut down to the marks, the MOST IMPORTANT PART is next: the test-fit. Working to measurements or pencil lines is all well and good, but measurements lie (just like statistics!). Nothing can substitute for making sure everything will go together without trouble. If the panel seems like it’s too big, remember to plane off equal amounts from both sides, so that the bevel will remain centered. And speaking of the bevel, it may be the problem instead of the outside dimensions; if the bevel is too abrupt or too thick, it will bind up instead of seating in the groove, presenting with symptoms similar to the panel being too large for the interior of the box. Do check to see which is the problem before being excessively zealous in your correction. If you are making a box in spring or fall, leave a little wiggle room. In winter, leave a lot of wiggle room. In summer, make it pretty well tight. This will leave space for seasonal expansion or contraction depending on when you’re working. And this only applies for the sides. Since wood doesn’t get longer with the seasons (only wider), snug it up well against the ends. I usually sand the inside surfaces before I assemble the box; it’s easier to get at that way.
Finally, once everything dry fits to satisfaction, it’s time to glue. Do remember to put the bottom in before you put on the last side, or you’ll be quite cross. And don’t put glue in the grooves, only the dovetails. Gluing the bottom in the grooves would negate the effort we just put into allowing for seasonal expansion. Remember that you may be the problem if you attempt to square up and it just won’t adjust correctly, no matter how much you fiddle with the diagonal clamp. It’s possible, especially with the heavy-threaded screw clamps most of us use, to exert so much force on the sides that it bends the wood under the force of the clamp. It only takes a little pressure to hold everything until the glue sets. So be judicious in your efforts. Let your box dry overnight, and we’ll pick up with the obvious problem tomorrow.