What am I doing wrong with my miter joints on a pine keepsake box?

[MichaelVR]

I’ve been trying to make a simple wooden keepsake box, but I keep messing up the miter joints on the corners. No matter how carefully I measure and cut on my miter saw, there’s always a tiny gap when I dry-fit the pieces. I’m using pine and I’ve checked that my saw blade is at a perfect 45 degrees, so I’m not sure what I’m doing wrong.

[GregoryH]

I’ve built a few boxes like that. The gaps showed up even when I had the saw dialed in, and I realized the stock wasn’t perfectly flat after letting it sit and move in the humidity.

[TimothyXW]

Maybe the culprit isn’t the cut angle but drift in the saw fence or blade; pine can close or open a hair with heat and glue, so a tiny wiggle shows up as a gap.

[James_J]

I once measured twice and still had a gap; I ended up trimming one piece a hair and pretending it was a trim, but that felt wrong.

[ZoeyMW]

Could be that the ends weren’t truly square to the faces, even if the angle looked right; try checking two different joints on scrap to see if it’s the stock or the setup.

[JosephNJ]

Is the real problem the box isn't flat on a bench? I found that if the box sits unevenly while gluing, the corners pull apart and you see gaps later.