A rocking box or a mitered frame with a gap in the corner usually can be traced to the same root cause: the parts weren’t quite square. This isn’t about being compulsive about squareness; it’s about making pieces that fit together without having to force them. Square parts mean that the glue lines close properly and the pieces sit flat on the table. Check each cut edge with a square before assembly. If you can see light between the square and the edge of the wood, you’ve got a problem that will only become more apparent when you fit the pieces together.
How you make the cut often dictates whether the parts end up square. If you’re applying too much pressure on one side of the saw, you will be making tapered cuts that lean to the inside or outside, a condition you might not notice until you try to assemble your work. Try to position yourself so that you’re pushing the wood through the blade with a vertical motion. It’s helpful to practice cutting on scrap pieces to get a feel for a square cut. Check the cut immediately after you make it, rather than waiting until it’s time to assemble the parts, when it will be much harder to fix.
A simple exercise done on a daily basis will teach you how to be accurate most of the time. On a scrap board mark a small rectangle on the wood, then carefully cut all four edges, trying to keep them square. Then place a square in the corner to see how you did. If one of the corners is a little loose, check which cut caused the problem, and repeat that cut, paying special attention to your motion. If you spend just 15 minutes a day doing this exercise, you will begin to see flaws in your technique that you might not otherwise notice on larger projects.
An error that is easy to make is trying to fix a corner that isn’t square by clamping the parts together. While this might eliminate the gap, it will put the corner under a lot of stress that could compromise the strength of the joint. Check which edge is out of square and fix it with a plane or sanding block. It’s better to remove a little extra wood than it is to compress the wood fibers.
As your ability to judge when something is square improves, you will find it easier to make flat surfaces and square corners. Parts will sit flush in the right position without rocking, and assemblies will feel solid before you even nail them together. The joy of having corners fit up perfectly isn’t a matter of luck, it’s just a matter of checking, and making adjustments. And once you’ve developed that habit, you will find it much easier to tackle complex projects.